Parabolic FM antenna
Russ Button
FM band. I live in the San Francisco area and with all the
hills around the bay, we have a major problem with multipath
distortion. I've got a modified Radio Shack FM antenna that
gets plenty of gain, but it does a lousy job of rejecting
off-axis signals. The antenna has three receptor elements,
one reflecting element, and (with the modification) three
director elements.
I was chatting with a buddy of mine who's an old radio tech.
He's the one who helped me with adding the two extra director
elements. I told him what my problem was and he suggested that
I could build an antenna with a single receptor element and
a cylindrical parabolic array of reflectors where the receptor
is at the focus of the parabola.
Does anyone have any experience with this sort of antenna?
Aside from making the receptor approximately 1/4 wavelength
of the frequency I'm interested in receiving, do I just need
to make a parabola with about a 1/4 wavelength distance to
the focus? How many elements need to be in the parabolic
array?
Much thanks!
Russ Button
Fractenna
>From: Russ Button but...@idiom.com
>Date: 7/17/00 4:20 PM Eastern Daylight Time
>Message-id: <8kvpr2$2pkj$1...@news.idiom.com>
Russ--
Sounds like you are interested in a 'corner reflector'. Try searching that on
google.
73,
Chip N1IR
Leland C. Scott
Russ Button <but...@idiom.com> wrote in message
news:8kvpr2$2pkj$1...@news.idiom.com...
> I was chatting with a buddy of mine who's an old radio tech.
> He's the one who helped me with adding the two extra director
> elements. I told him what my problem was and he suggested that
> I could build an antenna with a single receptor element and
> a cylindrical parabolic array of reflectors where the receptor
> is at the focus of the parabola.
>
You could try a corner reflector instead. This antenna would be easier to
build than trying to make a cylindrical parabolic reflector. Check out the
URL below for more information.
http://www.cebik.com/corn1.html
--
Leland C. Scott
KC8LDO
RanHefner
>
>Sounds like you are interested in a 'corner reflector'. Try
searching that on
>google.
>
>73,
>Chip N1IR
>
>
Chip? Are you serious? He is looking for a FM broadcast band
(88-108 Mhz). Do you realize how big that corner reflector
would be? The driven element is a 1/2 wave dipole. That would
be almost a 5 foot wide corner reflector!
Randy A. Hefner
http://talk.to/frs
http://go.to/qx3
http://come.to/funnynews
-----------------------------------------------------------
Got questions? Get answers over the phone at Keen.com.
Up to 100 minutes free!
http://www.keen.com
Fractenna
>(88-108 Mhz). Do you realize how big that corner reflector
>would be? The driven element is a 1/2 wave dipole. That would
>be almost a 5 foot wide corner reflector!
>
>
>
>Randy A. Hefner
>http://talk.to/frs
>http://go.to/qx3
>http://come.to/funnynews
>
>-----------------------------------------------------------
>
>Got questions? Get answers over the phone at Keen.com.
>Up to 100 minutes free!
>http://www.keen.com
>
There is a wonderful picture in Kraus's BIG EAR of a simple corner reflector
built of a modest wooden frame and wire. This was for the 5 M band. That is
BELOW the FM BCB.
Mr. Button is looking for suggestions along a certain approach, which I
responded to. I note , just now, that I am not the ONLY one who suggested a
corner reflector..
Now shut your voicebox everytime I try to help out.
Chip N1IR
RanHefner
>Now shut your voicebox everytime I try to help out.
>
>Chip N1IR
>
>
My, my. Ok Phil.
Richard Harrison
"Chip? Are you serious?"
Chip`s suggestion of a corner reflector in place of a parabola is
logical. A parabola would likely be about 24 feet across to rival a
corner reflector with 10-foot reflector bars.
Here is Arnold Bailey`s corner reflector scaled for 100 MHz:
Folded dipole 52 inches long, 1/4-inch or 1/2-inch rods or tubes.
Mechanical considerations are important here. Space 2 inches between
axes.
Reflector made up of (17) 10-foot long 1/4-inch or 1/2-inch rods or
tubes spaced 15 inches apart. The corner is a right angle. The apex
should be 5 feet behind the folded dipole. Reflector bars must parallel
the dipole bars to function properly.
Use 300-ohm balanced feedline.
The gain is 10 dbd and the pattern is cleaner than a parabola gives.
Best regards, Richard Harrison, KB5WZI
horses...@my-deja.com
RanHefner <ranhefne...@yahoo.com.invalid> wrote:
> frac...@aol.com (Fractenna) wrote:
>
> >Now shut your voicebox everytime I try to help out.
> >
> >Chip N1IR
> >
> >
>
> My, my. Ok Phil.
Randy! Fancy meeting you here. I've been getting information on
acquiring a building permit for my tower on this newsgroup - and what
do I find? Buzz Lightyear on RRAA! You gotta lighten up, man. Things
don't always go by the book - especially in radio.
> Randy A. Hefner
Stewart - N0MHS
--
Wireless High-Speed Networking Information:
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Sent via Deja.com http://www.deja.com/
Before you buy.
Fractenna
Hello Stewart--
Your friend's 'hobby' is to harass me. Perhaps you can dissuade him from this
venture...for his own good.
73,
Chip N1IR
RanHefner
>
>Hello Stewart--
>
>Your friend's 'hobby' is to harass me. Perhaps you can dissuade
him from this
>venture...for his own good.
>
>73,
>Chip N1IR
>
>
Yes Phil.
Richard Harrison
"It`s not so much signal strength I need as directivity and rejection of
off-axis signals."
Directivity and gain are products of the same antenna characteristics.
It is possible to waste the gain and keep the directivity.
If you wanted a simple, single direction, narrow beamwidth pattern, you
could build a rhombic. It is a diamond shape about twice as long as it
is wide that has 4 or 5 wavelengths in each of its 4 sides. It only has
to be a few meters above ground. It needs a 600 to 800-ohm termination
resistor at the end pointed to its target to make it unidirectional. Add
up all those wavelengths at 3 meters per, and you see that it is quite
large. But the construction is cheap and simple.
You might find that your RS antenna worked well with a different radio.
Richard Karlquist
Russ Button <but...@idiom.com> wrote:
>I'm interested in making an antenna for receiving just the
>FM band. I live in the San Francisco area and with all the
>hills around the bay, we have a major problem with multipath
>distortion. I've got a modified Radio Shack FM antenna that
>gets plenty of gain, but it does a lousy job of rejecting
>off-axis signals. The antenna has three receptor elements,
>one reflecting element, and (with the modification) three
>director elements.
>
>He's the one who helped me with adding the two extra director
>elements. I told him what my problem was and he suggested that
>I could build an antenna with a single receptor element and
>a cylindrical parabolic array of reflectors where the receptor
>is at the focus of the parabola.
>
Tom W8JI
On 17 Jul 2000 20:20:50 GMT, Russ Button <but...@idiom.com> wrote:
>I was chatting with a buddy of mine who's an old radio tech.
>He's the one who helped me with adding the two extra director
>elements. I told him what my problem was and he suggested that
>I could build an antenna with a single receptor element and
>a cylindrical parabolic array of reflectors where the receptor
>is at the focus of the parabola.
You would need a very large antenna to use either a corner reflector
or a parabolic Russ. I'm afraid neither are pratical at FM
frequencies.
For reasonable size, your yagi is the best you can do.
Maybe the yagi you have is not properly designed? A yagi for FM can
only be optimized over part of the band. If someone just guesses and
uses approximations, the antenna could be far from optimum.
>Does anyone have any experience with this sort of antenna?
>Aside from making the receptor approximately 1/4 wavelength
>of the frequency I'm interested in receiving, do I just need
>to make a parabola with about a 1/4 wavelength distance to
>the focus? How many elements need to be in the parabolic
>array?
You'd need something ten or twenty feet across to equal an easy to
handle and build yagi for FM.
Dishes and corner reflectors are used when the array has to have
extreme gain, and they just really substitute a single feedpoint for
many many elements. The physical size of the array has to be very
large to justify using a corner reflector or a dish. I'm afraid you
are getting bad advice.
73 Tom
Fractenna
I'm sorry that your reasonable query is turning into another one of the sordid
affairs here.
Richard Harrison and myself has some experience on these matters and we
answered your question in the best possible ways.
For its size, the corner reflector will provide superior rejection of unwanted
multipath across the FM band. You have already encountered the limitations of
the Yagi-Uda, which will not work as well for your problem, as it is a
narrow-band unidirectional antenna.
A corner reflector is not small, but based on your query, that is not the big
impediment. A paraboloid will have to be too big to acheive the same or better
performance.
Good luck!
Chip N1IR
Russ Button
: You would need a very large antenna to use either a corner reflector
: or a parabolic Russ. I'm afraid neither are pratical at FM
: frequencies.
Bummer.
: For reasonable size, your yagi is the best you can do.
I'm sorry to hear that.
: Maybe the yagi you have is not properly designed?
Probably not. It's not so much signal strength I need as
directivity and rejection of off axis signals. The notion
of a parabolic antenna intuitively suggests that.
The kludged Radio Shack antenna I have doesn't tune out
some stations hardly at all, no matter where you point
the damn thing. And then there's the problem of what
seems to be multipath distortion. I get a fuzz or slight
buzz to a number of stations, which I am assuming is
multipath distortion. I've got what seems to be sufficient
signal strength.
I thought 1/4 wavelength was the key to antenna design.
I'm primarily interested in the lower end of the FM band,
92 Mhz and below, which is where most of your public stations
are. The great majority of stations above 92 Mhz are commercial
junk.
So if I were to optimize for 90 Mhz, that would be for a wavelength
of about 3.3 meters, right? And a 1/4 wavelength of about .82 meters
or about 32 inches. I thought that a single receptor element of
this length with a parabolic array approximately the same distance
from it would be the idea, not that I know shit about this really.
From what you guys are suggesting here, it seems like this is waaaaay
off the mark.
Maybe I should just go buy that Winegard antenna...
: You'd need something ten or twenty feet across to equal an easy to
: handle and build yagi for FM.
Twenty feet? What happened to 32 inches?
Q. What has 151 feet and 7 teeth?
A. The front row at a Merle Haggard concert...
: Dishes and corner reflectors are used when the array has to have
: extreme gain, and they just really substitute a single feedpoint for
: many many elements. The physical size of the array has to be very
: large to justify using a corner reflector or a dish. I'm afraid you
: are getting bad advice.
That's why I asked here.
Thanks!
Russ
K1BQT
I haven't been following the back chatter on this thread, so I don't know much
about what's been discussed so far. But, if you're primarily interested in
88-92 MHz, a long-boom yagi is not an unreasonable choice. Were you interested
in the whole FM band, that would be a totally different matter--and perhaps
demand a whole different solution.
If you want to buy your antenna, obtaining a custom-made long-boom yagi for the
NPR/EDU segment gets into a "head-end" grade product--an expensive proposition.
However, if you want to scale a small K1FO 2-meter yagi or use a YA/YO
template to design a 90-MHz antenna from scratch, it could perform well (don't
rule out a scaled quad, for that matter).
I've lived in Marin County (San Rafael to be specific), so I know a little
about what you're up against. With hills on one side, and San Pablo Bay plus
more hills on the other side, it's a very tough signal-congested area.
Some of the interference you describe is likely multipath--as you suggest.
But, from what direction? Is it coming in at 30, 45, 90, or 120 degrees off
axis?? Almost any angle is possible in that terrain.
Also, much of the interference around the San Francisco/Richmond "metro" area
may be from adjacent channel activity (let's face it, the IF filtering in even
some high-end stereo receivers leaves a bit to be desired--and some stations
are "cleaner" than others).
In either case, an antenna with narrow beamwidth and deep off-axis nulls might
actually work better than a relatively smooth and broad frontal lobe with
ultra-deep F/B. Turn a sharp yagi a few degrees, and the adjactent chatter or
multipath can drop into a null and vanish!
If 4 MHz is all the pattern BW you need, my choice over other options would be
a long yagi. As I recall, Dave Olean, K1WHS, (Directive Systems, Inc.) solved
a very nasty and similar problem in the FM band for his brother Arnie (a
classical music DJ in Portland, ME) using the same approach. Hence, my
suggestion.
73
Rick K1BQT
Russ Button
: Russ-
: I'm sorry that your reasonable query is turning into another one of the sordid
: affairs here.
I hang out a lot in the audio newsgroups. Now THAT'S where you're
going to find some flame fests! I don't know how prima-donna you
ham guys can get, but believe me when I tell you there are a lot
of audiophiles who need to get a life!
: For its size, the corner reflector will provide superior rejection of unwanted
: multipath across the FM band. You have already encountered the limitations of
: the Yagi-Uda, which will not work as well for your problem, as it is a
: narrow-band unidirectional antenna.
: A corner reflector is not small, but based on your query, that is not the big
: impediment.
I took a look at the link for the corner reflector design. That
looks like a real possibility. Should the receptor element be
a 1/4 wavelength of the desired frequency?
Russ
Tom W8JI
(Richard Harrison) wrote:
>Russ Button wrote:
>"It`s not so much signal strength I need as directivity and rejection of
>off-axis signals."
>
>Directivity and gain are products of the same antenna characteristics.
>It is possible to waste the gain and keep the directivity.
>
>If you wanted a simple, single direction, narrow beamwidth pattern, you
>could build a rhombic. It is a diamond shape about twice as long as it
>is wide that has 4 or 5 wavelengths in each of its 4 sides. It only has
>to be a few meters above ground. It needs a 600 to 800-ohm termination
>resistor at the end pointed to its target to make it unidirectional. Add
>up all those wavelengths at 3 meters per, and you see that it is quite
>large. But the construction is cheap and simple.
Rhombic's and other longwire arrays have the poorest directivity of
almost any style antenna for the spatial area required.
The nose of the pattern is sharp, but spoiled by many large minor
lobes.
A long properly-tuned yagi is the best solution to a directivity
problem. That's why they are so common at VHF as to almost exclusively
be the antennas used.
73 Tom
Richard Harrison
"How many elements need to be in the parabolic array?"
Usually only one balanced 1/2-wave element is used at the focal point of
VHF dishes.
Rays of energy from a distant point to which the dish is aimed appear as
arriving parallel to each other, and perpendicular to a plane across the
rim of the dish.
No matter what point on the concave dish is struck by one of the
incoming rays, it is reflected back through the "focal point", in front
of the dish.
There is space for only one antenna element at the focal point, so an
array would be redundant, except for cross-polarized elements for
diversity, etc.. (Then, you need a criss-cross or solid dish reflector.)
Fractenna
to get a useful form of corner reflector with a hypotenuse less than 1/2 wave.
Since you were entertaining a parabola--and people here LOVE antenna projects--
I stuck to your train of thought on it.
However, if your objective is to get reasonable multipath rejection over the
bottom part of the band ONLY, you can get away with a Yagi--and should. It does
have many 'sidelobes' but, as K1BQT points out, you can turn the antenna a bit
and notch them out.
Lyle's suggestion is a good one. There is a description of a DL6WU Yagi design
on the Cebik link, as well as other designs. Probably the easiest to start
from.
If you want to have some fun with it, you can model your OWN using a program
called EZNEC (see http://www.w7el.com) which has a limited, free download.
Worth purchasing IMHO.
73,
Chip N1IR
WB3KUM/9
something about this size.
http://www2.jpl.nasa.gov/files/images/browse/333-6338ac.gif
A corner reflector is can be done, but a large Yagi or a combination
Quad/Yagi would be more practical. A fullwave loop for the reflector
and driven element, Yagi directors. If you want to use a Yagi, see the
ARRL Antenna book. If large enough, you may want to consider a "Tygon"
reflector as described in the book.
BTW, for reference, that is an early 1970's, full size Ponitac Safari
Station Wagon over to the right. I have more images of this antenna is
any one is interested.
73's Dennis WB3KUM/9
Lyle Koehler
>
>
> I took a look at the link for the corner reflector design. That
> looks like a real possibility. Should the receptor element be
> a 1/4 wavelength of the desired frequency?
There have been a number of good answers to your questions, but I
haven't seen one that specifically addresses your assumption about 1/4
wavelength element size. 1/4 wavelength antennas are used when a large
ground plane is involved, such as a vertical whip on a vehicle. However,
the driven element (which is what you refer to as the receptor in a
receiving antenna) in a Yagi, corner reflector or parabolic antenna is
1/2 wavelength long.
--
Lyle, K0LR
RanHefner
>I took a look at the link for the corner reflector design. That
>looks like a real possibility. Should the receptor element be
>a 1/4 wavelength of the desired frequency?
>
>
>
No, the "receptor element" as you put it will be almost 5 feet.
It will be a 1/2 wave dipole. The reflector itself will be
somewhat bigger. This will be a HUGE antenna compaired to the
one you already have.
RanHefner
>>
> All of these designs are big, Russ. Perhaps prohibitively so.
It would be hard
>to get a useful form of corner reflector with a hypotenuse less
than 1/2 wave.
>
No kidding, Chip. That is what I told you from the start but
you just jumped all over me for it. NOW you are seeing it my
way. Instead of pouncing first, you should have thought out
your answer.
Fractenna
>you just jumped all over me for it. NOW you are seeing it my
>way. Instead of pouncing first, you should have thought out
>your answer.
>
>
>
>Randy A. Hefner
>http://talk.to/frs
>http://go.to/qx3
>http://come.to/funnynews
>
>-----------------------------------------------------------
>
>Got questions? Get answers over the phone at Keen.com.
>Up to 100 minutes free!
>http://www.keen.com
>
didn't ask a specific question on a simpler SOLUTION--just one entailing a
paraboloid.
If Kraus can do it so can anyone else Randy.
Now buzz off OM, as I asked you already.
No kidding.
Chip N1IR
RanHefner
>No kidding Randy; he asked about a paraboloid and I gave him an
alternative. He
>didn't ask a specific question on a simpler SOLUTION--just one
entailing a
>paraboloid.
>
>If Kraus can do it so can anyone else Randy.
>
>Now buzz off OM, as I asked you already.
>
>No kidding.
>
>Chip N1IR
>
>
Look FRAC. The gentleman was looking for a viable alternative
to his beam antenna. Someone suggested a parabolic dish...We
all know that was bad advice. YOU suggested a corner
reflector. That is almost as bad as a dish! The size of the
entire antenna is just too big! I questioned your "corner
reflector" suggestion stating that the antenna would be too
big. You just told me to shut up...YOUR WORDS.
Now that several others have stated that the corner reflector is
not a good idea, you have conceeded that the idea was not a good
one. YOU have finally agreed with my ORIGINAL message. You
have shown your true colors Phil...Chip...or whoever you are.
I guess if someone shows that your idea or ideas are not sound
you CRY harassment. Go ahead and cry Chipster or whoever, it
just adds to the entertainment.
Fractenna
>Look FRAC. The gentleman was looking for a viable alternative
>to his beam antenna. Someone suggested a parabolic dish...We
>all know that was bad advice. YOU suggested a corner
>reflector. That is almost as bad as a dish! The size of the
>entire antenna is just too big! I questioned your "corner
>reflector" suggestion stating that the antenna would be too
>big. You just told me to shut up...YOUR WORDS.
>
>Now that several others have stated that the corner reflector is
>not a good idea, you have conceeded that the idea was not a good
>one. YOU have finally agreed with my ORIGINAL message. You
>have shown your true colors Phil...Chip...or whoever you are.
>
>I guess if someone shows that your idea or ideas are not sound
>you CRY harassment. Go ahead and cry Chipster or whoever, it
>just adds to the entertainment.
>
>
>
>Randy A. Hefner
>http://talk.to/frs
>http://go.to/qx3
>http://come.to/funnynews
>
>-----------------------------------------------------------
>
>Got questions? Get answers over the phone at Keen.com.
>Up to 100 minutes free!
>http://www.keen.com
>
I stand by my statements as truthful; helpful; and cogent to the question
asked.
If you had read my statements, you would realize that a Yagi-Uda is a very poor
solution to a multipath problem for the FM BCB. The corner reflector is a
superior antenna in this circumstance.
Is it build-able? Of course! You are confusing Richard Harrisons's post and
mine. Kraus shows a 5m(that's 60 MHz--far longer in wavelength--by 50%!--than
the lower end of the FM BCB) wavelength, wire corner reflector which is far
smaller than what you are thinking of. Easy to build, and, on a 'dolly', easy
to turn.
See page 90 of BIG EAR by John Kraus.
Now: if you are only interested in a small part of the BCB, then the Yagi-Uda
option makes more sense. It makes no sense if one desires the entire band.Even
4 MHz is pushing it for good multipath rejection.
Basically, when Russ decides what he wants, he has options.
If Russ only needs the lower partof the FM dial--which he later specified--
then a long Yagi-Uda should work.
A corner reflector will always work.
Hope this helps; if it doesn't then try going outside and screaming into the
wind.
Reading and paying attention to what is said works, too.
73,
Chip N1IR
Fractenna
Actually, if SIZE is the issue, then a 'corner director' will do. The
director-grid has lengths of about 0.28 waves (approximately 4 feet at FM BCB)
and they are built up to about 6 feet. I checked my notes and the sidelobes are
down at least 20 dB over , say, a 20% bandwidth. The hypotenuse is thus about 8
feet. That's a lot more do-able than 14 plus feet from Richard's design (which
is a good one for a corner reflector).
Hope this helps.
73,
Chip N1IR
Richard Harrison
"---he asked about a paraboloid and I gave him an alternative."
Yes, if Russ only had a strong interfering station, he might point the
end of a dipole at the interfering station to solve the problem. Chip`s
suggestion was aimed at a more manageable size than a dish. We still
don`t know why Russ has a problem. He may need a radio.
We said that directivity and gain were sides of the same coin. Here are
the gains of four directive antenna choices:
1) 24-ft dish = 12.5 dbd
2) 10-ft/14-ft corner reflector = 10 dbd
3) 3-element yagi = 7 dbd
4) rhombic from (2) 72-ft wires = 14 dbd
Fractenna
each a little more than the 1/4 wave you asked about; they should be about 0.28
to 0.3 waves. Call it 4 feet long for your FM problem. Take 7 and space them
parallel to make one side each. By the numbers at under 100 MHz, I get then
spread over 6 feet on a side. Then combine the two sides in a right angle 'V'.
STick the driven element about 3 feet away from the apex (corner).
Good luck!
This sounds do-able.
73,
Chip N1IR
Fractenna
Yes; very nice Richard:-)
BTW I'm having fun with the 'corner director'; dug it out of my notebook.
Almost had forgotten about it!
Kraus describes it briefly in ANTENNAS(I just saw)...must have been why I
looked at it way back.
Best 73,
Chip N1IR
Goldreged
design
on the Cebik link, as well as other designs. Probably the easiest to start
from.>>
How about stacking a bunch of TV antennas... Bandwidth, Gain & Rejection...
& you can get paid by your neighbors to remove those unsightly antennas...
Ed G.
KB1DQX
(Please Remove "JUNKBLOK" to E-mail.
Unless it's a Flame.)
Richard Harrison
"I`m interested in making an antenna for the FM band."
If Russ` problem is interference from azimuths other than towards his
desired station, he could try a broadside array of dipoles, or yagis.
Two antennas give about 3 db gain. Four antennas give about 6 db gain,
etc.
Often, antennas are placed side-by-side, with their tips almost touching
This works, but wider spacing is better. 0.94-wavelength
center-to-center is optimum.
Equal length feedlines are used to get same phase operation of all
antennas.
Matching depends on choices. For example, you could use (4) folded
dipoles connected with equal lengths of 300-ohm twinlead to the
paralleling point where you connected to a 75-ohm line to your radio.
Tom W8JI
(Richard Harrison) wrote:
>If Russ` problem is interference from azimuths other than towards his
>desired station, he could try a broadside array of dipoles, or yagis.
Hi Richard.
If the problem is QRM, you use null depth to response level.....not
gain. That's what makes your Rhombic solution poor. A Rhombic, even if
you igore the fact it has the lowest gain for a given physical size of
almost any array, has far too many minor lobes and they are far too
strong.
>Two antennas give about 3 db gain. Four antennas give about 6 db gain,
>etc.
That is meaningless when considering QRM rejection.
I can't believe you and Chip are suggesting such poor solutions, when
a simple array of one or two stacked yagi's would do a better job in
less area.
73 Tom
Tom W8JI
Chip must have a degree in unworkable solutions.
73 Tom
Kent
little large.
What I might suggest is stacking multiple (2 perhaps) antennas. That
narrows the beamwidth and give you more gain.
Kent, K9EZ
Russ Button <but...@idiom.com> wrote in message
news:8kvpr2$2pkj$1...@news.idiom.com...
> I'm interested in making an antenna for receiving just the
> FM band. I live in the San Francisco area and with all the
> hills around the bay, we have a major problem with multipath
> distortion. I've got a modified Radio Shack FM antenna that
> gets plenty of gain, but it does a lousy job of rejecting
> off-axis signals. The antenna has three receptor elements,
> one reflecting element, and (with the modification) three
> director elements.
>
> I was chatting with a buddy of mine who's an old radio tech.
> He's the one who helped me with adding the two extra director
> elements. I told him what my problem was and he suggested that
> I could build an antenna with a single receptor element and
> a cylindrical parabolic array of reflectors where the receptor
> is at the focus of the parabola.
>
> Does anyone have any experience with this sort of antenna?
> Aside from making the receptor approximately 1/4 wavelength
> of the frequency I'm interested in receiving, do I just need
> to make a parabola with about a 1/4 wavelength distance to
> the focus? How many elements need to be in the parabolic
> array?
>
> Much thanks!
>
> Russ Button
Fractenna
I ran the 'corner director' on EZNEC and tweaked. If you decide to go with it,
let me know and I will download the model file.
Really nice rejection!
73,
Chip N1IR
Richard Harrison
"Two antennas give about 3 db gain. Four antennas give about 6 db gain,
Gain and directivity are descriptions of the same phenomenon. Remember,
squeeze the balloon in one place and it baloons out where it`s not
constrained? The only way to get gain in one direction is by depriving
other directions.
Gain is also a measure of spatial selectivity. Gain means a response in
the desired direction at the expense of responding in the undesired
directions. When you get the idea, then gain won`t be meaningless to you
when considering QRM rejection.
Richard Harrison
"What I might suggest is stacking multiple (2 perhaps) antennas."
That was what I was suggesting too with (2) antennas give 3 db, etc.. My
suggestion was side-by-side "stacking", as vertical stacking
concentrates the vertical radiation angle, while horizontal stacking
concentrates the horizontal beamwidth. Russ had described his
interference as coming from other azimuths. as I read it.
Fractenna
>squeeze the balloon in one place and it baloons out where it`s not
>constrained? The only way to get gain in one direction is by depriving
>other directions.
>
>Gain is also a measure of spatial selectivity. Gain means a response in
>the desired direction at the expense of responding in the undesired
>directions. When you get the idea, then gain won`t be meaningless to you
>when considering QRM rejection.
>
>
This is perfectly true.
The problem with 'stacking' these antennas--to add to Richard's earlier
comment-- is that it makes them even more narrow banded arrays from the point
of view of the desired side and back rejection. The sidelobe pattern will
change dramatically with frequency also.
A corner reflector is a well-behaved antenna for side and back rejection, over
a fairly large bandwidth. Landstorfer, and Cohen dipole-arrays (so-called) are
another type. A conventional planar Yagu-Uda array is a hi Q device from the
point of view of side and back lobe rejection.
73,
Chip N1IR
Tom W8JI
On Tue, 18 Jul 2000 20:34:27 -0500 (CDT), richard...@webtv.net
(Richard Harrison) wrote:
>Tom, W8JI wrote:
>"Two antennas give about 3 db gain. Four antennas give about 6 db gain,
>etc.. That is meaningless when considering QRM rejection."
>
>Gain and directivity are descriptions of the same phenomenon. Remember,
>squeeze the balloon in one place and it baloons out where it`s not
>constrained? The only way to get gain in one direction is by depriving
>other directions.
Gain and directivity are NOT descriptions of the same thing.
Directivity is the directional properties, while gain is only the
strength at a given reference point compared to some other antenna.
In receiving, unless signal level is so weak it is into system noise,
gain is absolutely meaningless. Directivity is the only concern,
unless the signal is down in the receiving systems internal noise.
>Gain is also a measure of spatial selectivity.
Nope. Not all all.
Directivity is the "directional selectivity", and Rhombics are very
poor compared to other antennas with the same GAIN. If you want to
reject off-path signals, you want an antenna with a wide null-area
with virtually no minor lobes compared to the response in the desired
direction.
>Gain means a response in
>the desired direction at the expense of responding in the undesired
>directions. When you get the idea, then gain won`t be meaningless to you
>when considering QRM rejection.
Gain is meaningless when considering "QRM rejection" unless the QRM
arrives equally from absolutely every direction. Even then,
directivity is still 100% useful.
On HF, my receiving arrays have -10 to -20 dB gain. These arrays have
seval dB more S/N ratio, sometimes up to 40 dB more S/N ratio, than my
transmitting arrays with more than 7 dB gain. The reason is simple.
My transmitting arrays have minor lobes that pick up undesired signals
and noise. My receiving arrays have clean, wide deep nulls with
virtually no minor lobes and very narrow front lobes....even though
they have almost 25 or 30 dB less GAIN than the transmitting antenna.
73 Tom
Tom W8JI
>>Gain and directivity are descriptions of the same phenomenon. Remember,
>>squeeze the balloon in one place and it baloons out where it`s not
>>constrained? The only way to get gain in one direction is by depriving
>>other directions.
>>
>>Gain is also a measure of spatial selectivity. Gain means a response in
>>the desired direction at the expense of responding in the undesired
>>directions. When you get the idea, then gain won`t be meaningless to you
>>when considering QRM rejection.
>>
>>Best regards, Richard Harrison, KB5WZI
>>
>
>This is perfectly true.
Chip,
Please read my rely to Rich. Feel free to point out any errors in what
I said.
73 Tom
Fractenna
I suggest you ignore Tom Rauch, W8JI. He is acting as a troll.
He is famous for taking a comment and re-writing it so it sounds different from
what you said. Then he makes a pedantic statement, often correct, that has no
meaning relevant to your statement. The intent is to get you to react and
respond, and ultimately, to those who do not have the knowledge, to make him
look important in the eyes of others. Most of the time he re-writes to
customized, to make you look foolish for saying what you did--even though it is
correct.
You made an accurate statement concerning gain and spatial rejection. It is
especially relevant with multipath rejection because there are often many paths
coming from a variety of solid angles. The 'balloon' allusion is an excellent
one which I have used many times myself.
I do not recall , by the way, that you discussed directivity at all. Nor needed
to.
He appears frustrated that I will not respond to him, and has chosen you as his
next 'target'. I suggest ignoring him.Or, in the event (or is it 'vent'? )that
he starts getting personal, consider litigation--that's what I am doing.
73,
Chip N1IR
Tom W8JI
>I suggest you ignore Tom Rauch, W8JI. He is acting as a troll.
No, actually I'm trying to undo some technical damage.
My posts are clearly technical in nature, unlike yours.
73 Tom
K1BQT
you necessarily wind up with a lot of forward gain. However, this is usually a
liability rather than a benefit. Consequently, there's one very important
system element I neglected to mention--the attenuator!
In metro areas, signal levels are already extremely high, with additional gain
being the last thing needed for quiet distortion-free reception. This means
the installation of a good front-end attenuator is essential! Our ham rigs
have them, but (other than perhaps a "local/distant" switch), most stereos do
not. Yet, these radios are subject to the same ills and limitations as any
other superhet. Mixers go into gain compression, have intercept problems, and
generate intermod responses. IF filters have skirts with finite shape factors
and limitations on ultimate rejection that allow adjacent channel energy into
the signal path. L/C oscillators and PLL synthesizers generate spurs that may
become significant LO sources when RF input levels are extremely strong. All
of this may add unwanted bleeps to your Beethoven.
In order to truly enjoy the rejection benefits of a narrow-beamwidth yagi in a
strong signal environment, the incoming signal level MUST be attenuated to the
lowest level needed for noise-free reception. Otherwise, it may be wasted
effort!
73
Rick K1BQT
Fractenna
W8JI's harassment gems . Chock full of lies and hypocrisy and arrogance and
abuse and..well, the very worst in human attributes.
Witness from our resident 'guru' in trying to get Chipster to talk to him::
>My posts are clearly technical in nature, unlike yours.
>
>73 Tom
>
----------------------------------------------
and from the HUGE historical archive we also have Mr. Rauch say--
"When I entered into an agreement ...with Chip..I learned my lesson, never
bargain with a psychopath.
73 Tom"
That's right the VERY best of BEST REGARDS from CHARLES THOMAS!
Stay tuned tomorrow when we present another gem! Collect'em all! All 949 and g
r o w i n g! Not a 'technical in nature' in the lot!
Best,
Phil N1ZKT
Richard Harrison
"Yet, these radios are subject to the same ills and limitations as any
other superhet."
Yes, but one notable exception is the nature of an FM signal. The radio
thrives on overdrives. It has a built-in signal clipper called the
limiter with the assignment of making the signal into square-waves.
Signal distortion has little effect on modulation if it doesn`t affect
the phase/frequency transistions of the signal. Severe overload may mean
stations appearing at various dial locations, but if one of the
strongest found at some spots is the one you want, no problem. The
presence of some spurious signals tends to add a tad of background noise
which, as Rick noted, can be reduced by wasting some of the overall
signal.
Russ mentioned multipath as a problem which I took to mean reflection of
the signal from buildings etc. which combined with the direct signal in
a way tending to cause cancellations.
A good directional antenna aimed at the right source should fix that
problem.
Tom W8JI
(Richard Harrison) wrote:
>Yes, but one notable exception is the nature of an FM signal. The radio
>thrives on overdrives. It has a built-in signal clipper called the
>limiter with the assignment of making the signal into square-waves.
I think Rick was addressing unintentional overload, which applies to
FM receivers as well as other types.
One limiting is reached on the desired signal, that's all you need.
Excessive gain can cause a deterioration in reception.
More is not always better, even on FM.
73 Tom
K1BQT
Your comment is well taken, but a bit off the mark. The limiter, as Tom
suggests, limits what comes at it--including IMD, spurs, adjacent channel, and
all the rest.
Don't know how many of these things you've designed, but poop in equals poop
out. If you don't belive it, just pull a IMD test and watch the SINAD meter!
73
Rick K1BQT
Richard Harrison
"The limiter, as Tom suggests, limits what comes at it--including IMD,
spurs, adjacent channel, and all the rest."
Most of this stuff falls outside the passband and scarcely affects a
good FM receiver at all. As Rick previously noted, the simple cure is a
loss-pad in the receiver input should noise appear on the demodulated
signal.
Too much signal is a non-issue.
Richard Karlquist
K1BQT <k1...@aol.com> wrote:
>In metro areas, signal levels are already extremely high, with additional gain
>being the last thing needed for quiet distortion-free reception. This means
>the installation of a good front-end attenuator is essential! Our ham rigs
>have them, but (other than perhaps a "local/distant" switch), most stereos do
>not. Yet, these radios are subject to the same ills and limitations as any
>other superhet. Mixers go into gain compression, have intercept problems, and
>generate intermod responses. IF filters have skirts with finite shape factors
I have lived in the SF Bay Area for 25 years and done a lot of testing of
FM tuners. The RF environment here is so brutal that the only tuner
I have ever tested here that was essentially free of RF overload
artifacts is a 1978 Technics top-of-the-line tuner with 6 ganged air
variable caps in the preselector section and 2 ganged air variable
caps in the local oscillator section. Brute force! Unfortunately,
unless you are one of the few who own this collectors item, you
will probably be limited in what you can receive. Of course a
directional antenna is somewhat helpful in reducing the amount of
RF you have to handle.
Another thing about tuners is that the goods ones (such as my Technics)
generate less audio distortion for a given amount of multipath.
This requires an amplifier/filter/amplifier/filter/.../amplifier
chain with LOW gain in each amplifier. Not a single cheap ceramic
filter followed by a gazillion dB of gain. Furthermore, you need
a scope to properly evaluate multipath vs various antenna configurations.
The scope connects to the outputs of a high end tuner (again such
as my Technics).
As you can see, good FM reception in the SF Bay Area is actually
a very complicated proposition, and a frustrating one because you
can't buy the tuner you really need. They don't make 'em like
they used to because of the cost issue. (My tuner was $450 in
1978, nearly a whole semimonthly paycheck). The tuners available
today are universally junk, no matter what you pay for them.
I agree with the previos poster that about all you can do is
put an attenuator in front of them and don't try to listen to weak
stations.
Rick Karlquist N6RK
Russ Button
any one design. From what I've seen here, Chip seems to have
a doable corner reflector design, and that a pair of stacked
yagis might be a great choice as well.
My tuner is an Onkyo T-4055. It's a solid state design from
about 1976, and was considered to be a fine tuner in its day.
It even has a built-in multi-path detector, not a common item
on consumer FM tuners.
In fact, it was this specific tuner that established Onkyo as
being a cut above the rest of the Japanese manufacturers of the
day. It was roughly equivalent to Pioneer's top line tuner at
the time, at about half the price.
One of the very finest tuners ever made was the Onkyo T-9090
and T-9090 MK II, built in the mid-80s. I think they discontined
building them because there really is little market for very
high quality tuners.
I agree that a signal attenuating device may well be needed.
I've been eyeing some better tuners on EBay, and a buddy of
mine in Berkeley owns nearly a dozen different high-end tuners,
so I can try out a number of other options there. But I came
to the realization that the best tuner is only as good as
the signal it gets, so I figure that the first thing to do
is to work out this antenna thing.
I'm going to look closely at the designs suggested here and
may well be back with more questions.
Thanks!
Russ
PS. If you'd like a good yuk-yuk, I've got a web page up
of musician's humour at
K1BQT
Yup, you got a good 'un there alright (wasn't there a Marantz tuner in the
'60's with a scope built into it??) Anyway, about the best you can do in some
locations is set up the pipeline, then turn down the faucet!
73
Rick K1BQT
K1BQT
Too much signal is a non-issue, I agree. Too many signals showing up at the
same time is a big fat hairy issue, as Rick Karlquist's post aptly points out.
Rick K1BQT
Kent
Fractenna <frac...@aol.com> wrote in message
news:20000719144646...@ng-fm1.aol.com...
Fractenna
>
EXACTLY!
Please tell W8JI.
Phil
N1ZKT
Richard Harrison
"Too many signals showing up at the same time is a big hairy issue as
Rick Karlquist`s post aptly pointed out."
Yes, on 2-meter net check-ins, doubles and triples cover each other at
times.
But, with good-quality entertainment FM radios, I experience no
interference problems, even up close to multiple FM transmitters. Do
you?
K1BQT
problems, even up close to multiple FM transmitters. Do you?<
Ummm, yeah! Especially if I want a weak one next to a strong one. But I doubt
you live in the Bay Area or the Northeast Corridor in space between major
markets where the ground is ultra lumpy. Not to sound snotty, 'cause I'm only a
"fake" engineer, but have you ever designed an FM receiver and looked at this
stuff on the bench under controlled condtions?? It can get uglier than you
might believe!
Rick K1BQT
Tom W8JI
(Richard Harrison) wrote:
>But, with good-quality entertainment FM radios, I experience no
>interference problems, even up close to multiple FM transmitters. Do
>you?
I sure have. It was a major problem near Atlanta. Airplanes, two-way
radio, and even multiple FM stations overloading the receiver and
wiping out what would otherwise be clear stations.
73 Tom
Richard Harrison
"---have you ever designed an FM receiver---?"
No, not as a product, but I`ve built several of my own design and
converted surplus equipment to entertainment FM.
I`ve worked on FM from my naval service in WW-2 to the present.
I`ve worked for FM/AM broadcasters.
I`ve worked decades for 2-way FM land/mobile users and checked FM
performance.
I`ve installed countless hops of microwave with FM modulating the
repeller of a klystron and measured the performance. I was a telecom
manager.
Our baseband was usually conventional SSB frequency-division multiplex,
but I`ve done video and Motorola FM frequency-division multiplex. I`ve
monitored the baseband intermod with a permently installed system which
sounded the alarm on a high level in a 600-channel system.
We had a substantial tower on the roof of our company headquarters. We
leased space to several FM broadcasters and I`ve worked with them on
their problems.
Thinking back on trapping interference in 2-way and on various BCI
problems, the most interesting to me happened at Radio Free Europe.
Our Portuguese studios were in Lisbon. Our transmitters were about 30
miles away, across the Tagus River Valley. We used UHF STL for program
relay. We had 10 links. My predecessors had decided that the fading
sometimes produced by stratified valley air could be eliminated by
dropping the STL into the FM broadcast band. The government gave them 10
assignments in the low end of the FM broadcast band for STL. There were
only a couple of FM broadcasters that nobody was listening to, at other
spots in the band.
!0 new Gates broadcast transmitters arrived, each with a couple of
matching monitoring receivers, one for each end of the path.
We had a great line-of-sight path. So, we tried replacing one of the UHF
paths with the new VHF stuff. Ye gods! The receiver was in a building
with 15 HF transmitters opreating at powers up to 100 KW. These had
"low harmonics" but they also had proximity and beat notes. No problem
at UHF, but a big problem at VHF.
We had about 500 acres and a portable FM receiver. We found a "quiet
spot" almost a mile from the transmitters and it became the site of our
new STL building. I was back a few years ago and everything was still
working fine.
We did use corner reflectors on our new STL tower. Not 10 of course. We
were accustomed to using preamps with multiple isolated outputs for our
TDR program relay. This works as well at VHF as it does at HF. I no
longer remember whether we had 3, 4, or 5 corner reflectors. Probably
the latter if the order were placed near the end of the fiscal year and
probably the former if the order were placed early in the year. I don`t
remember the frequency separation either. It probably was 400 KHz. The
Portuguese probably didn`t want to give away the whole band and we
probably didn`t want 200 KHz spacing, just in case.
It was nice that I could check the program relay from my apartment in
Lisbon or my villa in the country. We would normally have a couple of
channels carrying one of three to five languages almost 24 hours a day.
The kind of problem of receiver overload imagined by some as a result of
high antenna gain is highly unlikely from my experience. If it were to
happen the solution is as simple as inserting a loss-pad.
Best regards, Richarf Harrison, KB5WZI
Richard Karlquist
Richard Harrison <richard...@webtv.net> wrote:
>Rick, K1BQT wrote:
>"---have you ever designed an FM receiver---?"
>
>No, not as a product, but I`ve built several of my own design and
>converted surplus equipment to entertainment FM.
>
>I`ve worked on FM from my naval service in WW-2 to the present.
>
>high antenna gain is highly unlikely from my experience. If it were to
>happen the solution is as simple as inserting a loss-pad.
>
>Best regards, Richarf Harrison, KB5WZI
With all due respect for all your experience, you don't have
any experience in the SF Bay area. Drive a car with an average
or even "premium" radio south from San Francisco on US101 and
just outside the city limits, your receiver will suffer severe
overload, with loss of numerous stations. This is because
there are many transmitters on San Bruno mountain overlooking
101. I have tested dozens of tuners and only ones with a great
deal of preselection come through relatively unscathed. I
am not aware of any currently available receivers that are within
20 dB of my old Technics tuner, regardless of price. BTW,
all these tests were done with a whip or dipole, not a high gain
antenna.
Rick Karlquist N6RK
Richard Harrison
"---you don`t have any experience in the S F Bay area."
True, I`m unfamiliar with current radio reception conditions in the
area. It is the port we sailed into at the end of WW-2. I decommissioned
one ship there and took another upriver to Stockton and decommissioned
that one in Srockton. Business has returned me to San Francisco several
times since. But that has nothing to do with antennas.
Just outside San Francisco, south on US 101, may be one of those spots
where radios don`t work well. There will always be such places. Tapes
and CD`s can inform and entertain in these instances.
If such undortunate receiving locations were prevalent, radios would be
better or replaced with something better.
Today`s radios select reograms by their carrier frequencies. Tomorrow`s
radios may choose a program by its assigned time slot for sampling.
With frequency division program selection, success is a function of
tuned circuit quality and application. If you have the right selectivity
you can subordinate the unwanted signal. On top of that, FM is better at
dealing with subordinated competing signals than some other forms of
modulation.
The original poster probably doesn`t occupy as unfavorable location as
the southern outskirts of town on US 101. Not being mobile, a
directional antenna may improve his situation. Being serious about
solving a problem instead of imagining obstacles, he can either make his
receiver work, or replace it with a receiver that does.
Richard Harrison
"Especially if I want a weak one next to a strong one."
To receive a weak FM signal next to a strong FM signal requires
selectivity sufficient to shift the balance to favor the desired signal
by at least 6 db. This can give the desired recovered audio a signal to
noise ratio of about 35 db.
Nothing like this happens with AM. Experiments have placed the FM to AM
advantage at 25 db.
This does not happen at minimum detection levels. Below the the "FM
improvement threshold", relations are db per db for either AM or FM. So,
if you are struggling to hear a weak signal, it makes no difference
whether it is AM or FM.
Roy Lewallen
(rather than an interfering signal) it does make a difference. At poor
signal-to-noise thresholds, the signal is approximately mulitplied by
the noise, rather than simply being added(*). This severly distorts the
signal. I believe this is the reason aircraft radio is AM.
Roy Lewallen, W7EL
(*)cf. Carlson, _Communications Systems: An Introduction to Signals and
Noise in Electrical Communication_, McGraw-Hill, 1968, p. 258.
Richard Harrison
"If you`re struggling to hear a weak signal in the presence of noise
(rather than an interfering signal) it does make a difference."
I learn a lot of new things on this newsgroup. Always accepted the oft
repeated "no difference below the FM improvement threshold". It seems
logical. The carrier and audio recovery have a one to one relationship
with AM. You can modulate your generator then tune a radio on the tone
output as long as nothing saturates and the relationship is linear.
I`ve also witnessed the same one-to-one relationship while measuring
sensitivities of all types of FM sets. The generator pads are calibrated
in db and in microvolts. The audio or composite video output meters are
also calibrated in volts and db. So, it would be very easy to check.
It`s been about 40 years, but I have some recollections of injectimg
noise and examining the results when Motorola introduced its "Extender"
noise blanker. I don`t recall anything that shook my lomg-held beliefs
back then. I just never have looked for any differences in noise
performance between AM and FM at the detection threshold.
I thought aircraft radio was AM due to tradition and the difficulty of
getting the whole world to change to FM in spite of the advantages of
level audio output over great changes of r-f strength and much quieter
operation with normal signal levels. There is also a perception that FM
is only good at VHF and higher frequencies. Aircraft still use HF for
some long distance communications, but this may soon be history too.
I`ll look for Carlson`s "Communications Systems".
K1BQT
sufficient to shift the balance to favor the desired signal by at least 6 db.
This can give the desired recovered audio a signal to
noise ratio of about 35 db.<
FM TV satellites duplex on the same channel using just this approach--you
simply flip polarity to capture the desired signal. However, a weak signal
situated NEXT to a strong one is a different matter that cannot be compared to
two weak ones on the same or on adjacent channels.
1. The adjacent strong signal desenses the front end of the receiver, making
the weak one appear even weaker.
2. Signals from several strong stations may form a new mixer product capable
of sucessfully competing with the desired weak one.
3. The skirts of the receiver's IF filtering system may prove inadequate in
shape factor and/or ultimate rejection to maintain the required
channel-bandwidth needed to maintain separation. This is especially true if
the earlier stages use cheaper "roofing" filters while the later ones use more
narrow "channel bandwidth" filters.
4. FM modulation artifacts spreading beyond the strong FM station's channel
(even if suppressed to FCC limits) may "spew" into the weak station's
allocation--sometimes with sufficient strength to "break capture" and generate
a type of loud interference reminiscent of AM-band splatter on the recovered
signal.
If your premise were true, I suppose we could pull all the cavities off our
repeaters and throw 'em down the mountain side--or mount all our uplink
receiving antenna next to the biggest paging transmitter on the site!
Rick K1BQT
K1BQT
>The generator pads are calibrated
in db and in microvolts. The audio or composite video output meters are
also calibrated in volts and db. So, it would be very easy to check.<
If you're making FM recovered audio readings, don't forget to pack along your
SINAD and THD meters.
Rick K1BQT
Richard Harrison
"If your premise were true, I suppose we could pull all the cavities off
How would you then make the undesired signal inferior by at least 6 db?
Obviously, selectivity must be used to separate signals which are
selected from a group by their unique frequency band. If this doesn`t
precede non-linear circuit elements, mixing may result in hetrodynes
within the desired passband.
But, none of the above makes frequency-division multiplexing impossible.
This is the overwhelming history of radio, and it works!
Skirt selectivity, filter flanking, or whatever isn`t what the question
was. It was: How can the multiple reflections of the desired signal be
kept from interfering with my reception. This surprises me as an FM
question. It would be an expected TV question as it can produce ghosts.
So we have a user dis-satisfied with the FM antenna he is using.
I believe that an antenna which receives adequate desired signal
strength can be improved enough to operate satisfactorily in the
presence of other signals outside the desired signal`s passband, no
matter how intimidating the interference may be.
Frequency selectivity such as your cavities may be needed in addition to
directivity. Or, as I said in my first posting, the user may need a new
radio. Russ, the user, said he might borrow other radios for trial. That
might be a good move.
K1BQT
My point, exactly.
My "non-electronic" training may dicatate a slighly approach to this issue than
yours. I'm far more interested in how Russ's radio actually "behaves" while
doing its job than what it "should do" in order to be a perfect radio. Thus, my
prescription is (1.) to reduce the number of choices the mixer has to
discriminate between by installing a narrow beamwidth antenna, and (2.) to
minimize the radio's pre-limiter "stress level" by using a front-end attenuator
to reduce input levels.
Why introduce 1 dBm more wideband energy into the mixer than needed to get
Beethoven full limiter and noise free?
Rick K1BQT
Tom W8JI
>Why introduce 1 dBm more wideband energy into the mixer than needed to get
>Beethoven full limiter and noise free?
>
>Rick K1BQT
Any active stage can be a mixer.
And why suggest an antenna with many spurious lobes and/or one that is
impossible to rotate or one that needs to be exceptionally large for
the amount of directivity it produces compared to other easier to
handle designs?
73 Tom
Richard Harrison
"And why suggest an antenna with many spurious lobes and/or one that is
impossible to rotate or one that needs to be exceptionally large for the
amount of directivity it produces compared to other easier to handle
designs?"
Rotatable antennas are not needed for point-to-point use. A broadcast
station doesn`t often move, and the listener may not have to move his
antenna until he relocates.
Spurious lobes are not a goal, but don`t automatically disqualify a type
of antenna.
A large and simple antenna may br cheaper and easier than a smaller and
more complicated antenna. It also may be more tolerant of great
variations of frequency.
The best compromise depends on the user`s circumstances and needs. In
this case, they are poorly defined. So many suggestions result, even
about the receiver design. Many suggestions are off-target because we
don`t know where the target is.
Which, and how many, directions does the interference come from?
Another simple suggestion. If a simple twinlead dipole gets enough of
the desired signal, and a reflective sheet can be inserted between it
and the interference, mount the dipole 15 inches from the sheet. If the
conducting sheet is a lot larger than the dipole, it will block all
interfering signals from the rear, and give 4 or 5 dbd gain. Dipole
impedance will drop from 300 ohms to 150 ohms, but there are ways to
accommodate 150 ohms.
K1BQT
Excellent question! Even if we could know that information for desired station
A, it would surely not hold valid for desired stations B, C, or D. Russ's
interest is in a 4-MHz span of frequencies in huge multi-city metropolitan
market (San Fran/Oakland) where potentially every channel is occupied by a
station running either high or low power. Further, almost the entire Bay Area
is surrounded by hills and small mountains. It's an audiophile's chamber of
horrors with virtually every ill imaginable to overcome (including coastal
ducting). I suspect antenna rotation is, at minimum, essential for his
application.
73
Rick K1BQT
Tom W8JI
It think a lot of this can be avoided by putting ourselves in the
listener's postion, and thinking about how workable solutions really
are.
First, I'm sure we all agree now that gain is pretty much unimportant.
Only directivity matters.
On Sat, 22 Jul 2000 10:54:21 -0500 (CDT), richard...@webtv.net
(Richard Harrison) wrote:
>Tom, W8JI wrote:
>"And why suggest an antenna with many spurious lobes and/or one that is
>impossible to rotate or one that needs to be exceptionally large for the
>amount of directivity it produces compared to other easier to handle
>designs?"
>
>Rotatable antennas are not needed for point-to-point use. A broadcast
>station doesn`t often move, and the listener may not have to move his
>antenna until he relocates.
And he knows how to point the antenna exactly where? Think about it.
The problems with a Rhombic are major.
First, it has poor gain and directivity for the physical area
required.
Second, it has a large near-field and Fresnel zone area. It doesn't
like to be within a few antenna sizes of other re-radiating structures
Third, it has so many minor lobes that you'd play hell aligning a null
on undesired stations...even if you could rotate it.
>Spurious lobes are not a goal, but don`t automatically disqualify a type
>of antenna.
They sure aren't a goal, and the generally disqualify any receiving
antenna. The last thing you want in a receiving array that is facing
QRM from multiple or unpredictable directions is multiple spurious
responses!
Now if I was in Podunk, Iowa on a large farm trying to receive St.
Louis stations on FM and low-band TV I might consider a Rhombic, or
if I was in Cape Cod trying to receive Europe on shortwave. So it
could have its place.
>A large and simple antenna may br cheaper and easier than a smaller and
>more complicated antenna. It also may be more tolerant of great
>variations of frequency.
It sure is. That is one of the only true advantages of a Rhombic. It
is broadbanded and easy to build...if you have lots of clear space.
>Another simple suggestion. If a simple twinlead dipole gets enough of
>the desired signal, and a reflective sheet can be inserted between it
>and the interference, mount the dipole 15 inches from the sheet. If the
>conducting sheet is a lot larger than the dipole, it will block all
>interfering signals from the rear, and give 4 or 5 dbd gain. Dipole
>impedance will drop from 300 ohms to 150 ohms, but there are ways to
>accommodate 150 ohms.
If a yagi didn't do it, neither will a roll of tin foil and a dipole.
15 inches of spacing, eh? Better use more than that.
73 Tom
Richard Harrison
"Russ`s interest is in a 4-MHz span---."
I reread Russ`s original post in which he said he was interested in an
FM band antenna. I had confused his request with another questioner`s
who only wanted to receive a PBS station. So, I`ve been suggesting
point-to-point solutions.
For a full range of stations, a simple yagi on a rotator should produce
a usable signal on some azimuth (not necessarily aimed at the station)
across all 100 channels. Adding directors narrows the usable frequency
range and may hurt the nulls. A radiator and reflector should be enough.
Gray Frierson Haertig
Most of the parabolic antennas I've worked with use a directive feed.
In the case of a nominal 100 MHz parabola, it would consist of driven
element backed by a reflector element - like a 2 element Yagi. Of
course the reflector is farther away from the dish than the driven element.
The reason a unidirectional feed is used is the you don't want direct
radiation from the feed to interfere with the reflected radiation from
the dish in the direction of the major lobe (or to spill over the sides
of the dish and cause unwanted side lobes).
A perfect feed would illuminate the dish evenly over its whole area and
have zero radiation elsewhere. Because of practical feed designs, the
illumination is usually tapered as it approaches the edge of the dish.
Gray
Richard Harrison wrote:
>
> Russ Button wrote:
> "How many elements need to be in the parabolic array?"
>
> Usually only one balanced 1/2-wave element is used at the focal point of
> VHF dishes.
>
> Rays of energy from a distant point to which the dish is aimed appear as
> arriving parallel to each other, and perpendicular to a plane across the
> rim of the dish.
>
> No matter what point on the concave dish is struck by one of the
> incoming rays, it is reflected back through the "focal point", in front
> of the dish.
>
> There is space for only one antenna element at the focal point, so an
> array would be redundant, except for cross-polarized elements for
> diversity, etc.. (Then, you need a criss-cross or solid dish reflector.)
>
> Best regards, Richard Harrison, KB5WZI
--
Telecommunications Engineering
Gray Frierson Haertig & Assoc.
820 North River Street, Suite 100
Portland, Oregon 97227
503-282-2989
503-282-3181 FAX
g...@haertig.com
Richard Harrison
"Most of the parabolic antennas I`ve worked with used a directive feed.
In the case of a nominal 100 MHz parabola, it would consist of a driven
element - like a 2 element Yagi."
I`ve never worked with parabolas below 960 MHz. But, I agree, all the
dishes I`ve worked with used a directive feed too. Some coax fed dishes
had exactly the dipole with reflector you described. Others had a disk
reflector at the end of the feed. The reflector improves the radiation
pattern.
The benefit of shaping the illumination of reflectors is proved by
measuring energy distribution on the dish surface and in the radiation
field.
I mis-spoke by dismissing a question by saying there is only room for a
single element in the focal point. It is also important to screen the
radiator from direct radiation. High performance dishes install
additional shields to avoid stray energy.