cylindrical parabolic reflector calculations
linear_shift
How do you calculate gain and pattern (namely -3dbi beamwidth) for a
cylindrical parabolic reflector (parabolic trough)? I would like to
know because I bought two of these antennas,
http://www.p4c.philips.com/files/s/sdv9201k_17/sdv9201k_17_pss_aen.pdf
on sale (for $30 each!) at Circuit City after being turned on to them
by a SARA member on my IRC channel (irc.freenode.net 6667
#radioastronomy). I am going to make them into an interferometer (w/ a
8 meter base line for ~2 degree resolution), attached to the outside
of my house. The 8 bay antennas I was going to use broke my budget
after a $100 shipping quote, so I could not buy them. anyway I have
calculated with QuickYagi 4 the gain of the yagi array (w/ normal yagi
reflector element the width of the parabolic reflector) to be ~7.5dbi
and the -3db BW to be ~62 degrees for 610mhz (49.1cm). Now the
remaining problem is the parabolic trough. I used the SETI excel sheet
for a parabolic dish to calculate it and came up with ~5.1dbi and ~75
degrees for the -3dbi BW with a 50% efficient dish. Is this correct?
The SARA member I chatted with said a gain of 9-12dbi could be
expected (though he wasn't sure either) for a parabolic trough (w/
half-wave dipole) extending a short bit of the sides of each end of
the dipole feed. If this is correct I would either expect a 12.6dbi
net gain in the former case, or a 16.5 to 19.5dbi gain in the latter
case. So which one is it (if either at all), and what is the combined
pattern (most importantly -3dbi BW)? Or, is it something else
entirely?
Keep your ears on the sky, ls.
Fiel...@aol.com
How do you calculate gain and pattern (namely -3dbi beamwidth) for a
cylindrical parabolic reflector (parabolic trough)? I would like to
know because I bought two of these antennas,
http://www.p4c.philips.com/files/s/sdv9201k_17/sdv9201k_17_pss_aen.pdf
on sale (for $30 each!)
Get fantasy football with free live scoring. Sign up for FanHouse Fantasy Football today.
Hans Michlmayr
linear_shift wrote:
> Hey guys, another question:
>
> How do you calculate gain and pattern (namely -3dbi beamwidth) for a
> cylindrical parabolic reflector (parabolic trough)? I would like to
> know because I bought two of these antennas,
>
> http://www.p4c.philips.com/files/s/sdv9201k_17/sdv9201k_17_pss_aen.pdf
>
The antenna depicted does not constitute a "cylindrical parabola" , it
is simply a reflector element
for the Yagi beam. A cylindrical parabola surface has to be far enough
away from the linear feed
element of such antennas to be practically phase-decoupled from the
feed, the same applies to
paraboloids of revolution, "normal" dishes . In the antenna shown the
reflector is tightly coupled
to the driven element and acts like a phase coupled reflector element,
it does not "focus" anything.
Hans Michlmayr
Jim Sky
money. I have a stressed design in The Radio Astronomy Teachers Notebook
but you can probably design your own with a little effort. I used wood
lattice ribs on a 2x4 backbone. The reflector was aluminum window screen
glued to the ribs. 3 broom sticks supported the PVC that housed the coaxial
collinear feed. It worked great on 600 Mhz.
Jim Sky
Jim Sky
http://radiosky.com
Radio-Sky Publishing
PMB 242 P. O. Box 7063
Ocean View, Hawaii 96737
USA
Jan Lustrup
fool people think they get more gain out of the yagi antenna. This type of
close and smal reflector whether cylindrical, parabolic or corner reflector
type in a yagi antenna will NOT add much more gain...just "good looks". A
parabolic reflector must be very large and farther away from the dipole.
Some years ago when home satelilte Tv just started out you could see for
sale a fantastic "Indoors Rabbit ear TV antenna" with so called "super
powerful parabloic satellite reflector element". The so called "parabolic
reflector" was only a few inches in across. They claimed you could see free
satellite TV with this antenna....Well, maybe you could...if your neigbor
has a satellite dish and his cable was leaking!
Well you can't have the best of both worlds....J.Kraus said that although a
dipole with a couner reflector will give good gain, it has to have the
correct distaince and angle. But you can't just add directors in front of it
and then get the same gain as a yagi antenna + the same gain as a courner
reflector setup! Run MiniNec program and see for yourself, you will end up
with almost just the "yagi" type of gain...
German amateurs used long yagi antennas for e.m.e. and added 6 or more
reflectors behind the dipole element, and got only 0.2dB more gain...but the
front to back ratio did inprove a bit , letting one to have less ground
noise inproving the Y ratio (less ground noise).....
For an interferometer setup go for good front to back ratio and a nice clean
front lobe and don't worry to much about forward gain...
Good luck,
Jan Lustrup - LA3EQ,
Norway
linear_shift
huge uhf tv yagis (something like 40-60 elements) I was also taking a
look at:
http://www.summitsource.com/antennacraft-mxu59-uhf-antenna-extreme-deep-fringe-maxus-refurbished-59-electronic-elements-directional-tv-outdoor-television-signal-local-offair-digital-hdtv-reception-aerial-blue-zone-part-mxu59-p-6565.html
and
http://www.summitsource.com/antennacraft-mxu47-uhf-tv-antenna-maxus-47-electronic-elements-directional-deep-fringe-outdoor-television-signal-rooftop-local-offair-digital-hdtv-reception-aerial-blue-zone-part-mxu47-p-6309.html
They're pretty cheap too. What do you think?
Keep your ears on the sky, ls.
linear_shift
to mention. Wouldn't the yagi pick up some signal, and the reflector
pick up signal independantly but in phase (like a yagi/corner
reflector), adding together (I am not expecting huge gain from a small
reflector, but it should at least be on par with a corner reflector
which is supposed to get 7dbi gain optimum)? It should also behave
like a corner reflector in that it acts more like the reflector at
certain wavelengths (longer lambda) and more like a yagi at others
(shorter lambda). This is how the TV antennas achieve bandwidth,
though the para reflector is supposed to have less bandwidth than the
corner reflector. The reflector has 0.4m aperture (doesn't look like
it from the pic), and when assembled it is parabolic (again, doesn't
look like it from the picture).
Here is the site I am getting my info from:
http://www.hdtvprimer.com/ANTENNAS/types.html
I have emailed the author (he is a ham, so I am assuming he's
experienced) with my question and a link to this thread so it can be
further discussed.
Keep your ears on the sky, ls.
Randall family
Fancy reflectors have very little effect on the gain of a yagi antenna
because the antenna already has a lot of aperture spread out in the
direction of signal arrival. To get an idea of the aperture or capture
area, look at the recommended stacking distance for a pair of yagis. The
proper stacking distance will cause the capture areas of the yagis to just
touch. If the proper stacking distance is 1 meter that the capture area is
roughly a 1 meter diameter circle. On smaller yagi antennas the capture
area will become more elliptical in shape.
Unless a parabolic structure is at least 3 or 4 wavelengths across, it is
just a piece of metal. It will be another element in an antenna and not
much more.
The fancy reflectors are a great marketing gimmick for sales of antennas.
They "distinguish" a product. They also collect ice in winter weather
resulting in the mechanical failure of the antenna and the need to purchase
another one!!
I noticed the Phillips yagi antenna mentioned earlier had a "patented"
reflector. To get a patent, your "invention" must only be different, not
better! I hold several patents: A few are usefull and some are well....
Bruce Randall, WD4JQV
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>
>
Marcus D. Leech
> I noticed the Phillips yagi antenna mentioned earlier had a "patented"
> reflector. To get a patent, your "invention" must only be different, not
> better! I hold several patents: A few are usefull and some are well....
>
> Bruce Randall, WD4JQV
>
>
generally follow
Sturgeons Law, which is to say that 90% of everything is crap, and the
rest is
suspect :-) Patents are *supposed* to adhere to a number of
doctrines, which include
uniqueness, and "non-obviousness to one skilled in the art". The
problem is that the
patent office is generally too swamped to do enough due diligence, so
there are an
awful lot of truly egregious patents out there! Sounds like this
"parabolic trough reflector
for a yagi" thing belongs among the truly egregious.
My colleagues generally subscribe to the "patent your bowel movements"
doctrine, since there are
modest financial incentives to do so. Where they frequently file 5
or 6 patents a year, I file one
or two, since I like the Sturgeon Filter to filter stuff out *before*
it's filed :-) :-)
Cheers
Marcus
Principal Investigator, Shirleys Bay Radio Astronomy Consortium
Tom Crowley
commercial antennas with minimum results. Looking at commercial antennas,
go for the largest you can find or consider making your own. 15 dB gain
would be the minimum for RA, 20 or more is better. Just hard to find in a
commercial antenna.
You may want to look at the back issues of the SARA Journal, over the years
there have been some excellent article published for the UHF bands. Go to
www.radio-astrronomy.org for instructions on ordering all of the back issues
on a CD.
Cheers,
Tom Crowley
"Not only is the universe stranger than we imagine, it is stranger than we
can imagine." -- Sir Arthur Eddington
----- Original Message -----
From: "linear_shift" <linear...@yahoo.com>
To: "Society of Amateur Radio Astronomers" <sara...@googlegroups.com>
Sent: Thursday, July 31, 2008 3:44 PM
Subject: [SARA] Re: cylindrical parabolic reflector calculations
>
linear_shift
return it but there isn't much else in my price range that is small
and light enough). I have hooked one up to a tv and it seems very
directional (pointing it at a semi-distant tv station and sweeping it
side to side, and using a compass with azimuth markings to measure),
seems to have a overhead beamwidth ~20-15 degrees (by determining
where the picture quality starts to degrade) and seems to get good
gain (judging by picture quality). Maybe theres more than what meets
the theoretical eye. So, how can I test them for gain and pattern in
the real world? I was thinking actually pointing it at the sky and
drift scanning something like Sagittarius A to find the half power
points and level improvement with the receiver/dc recorder I have
ordered. As far as gain goes I am only shooting for a 13dbi minimum
for each antenna (phasing two together in a 8m baseline
interferometer). I am not going to go to too deep into modeling the
antenna, real world's the final word, and I'll derive a model from
that ("experimental polar plot" if you will). Anyway, I will report on
results after.
Keep your ears on the sky, ls.
> Size does matter when it comes to RA. Many of us have tried small
> commercial antennas with minimum results. Looking at commercial antennas,
> go for the largest you can find or consider making your own. 15 dB gain
> would be the minimum for RA, 20 or more is better. Just hard to find in a
> commercial antenna.
>
> You may want to look at the back issues of the SARA Journal, over the years
>
> Cheers,
>
> Tom Crowley
>
> "Not only is the universe stranger than we imagine, it is stranger than we
> can imagine." -- Sir Arthur Eddington
>
> ----- Original Message -----
> From: "linear_shift" <linear_sh...@yahoo.com>
> To: "Society of Amateur Radio Astronomers" <sara...@googlegroups.com>
> Sent: Thursday, July 31, 2008 3:44 PM
> Subject: [SARA] Re: cylindrical parabolic reflector calculations
>
> > Ok, glad I only paid $60 for both of these. There is are a couple of
> > huge uhf tv yagis (something like 40-60 elements) I was also taking a
> > look at:
>
>
> > and
>
> >http://www.summitsource.com/antennacraft-mxu47-uhf-tv-antenna-maxus-4...
David Ocame
been where you are and I've tried the low cost alternatives and there
really are none. In the end, by the time I got a decent sized dish (and
I'd still like it to be bigget...8 ft diameter still isn't great, but it
was free), I found that I had spent so much on going cheap that I
could've saved the money in a savings account and had the money to get
what I wanted. If you feel you can't wait, and I can't blame you for
that, then put up a couple dipoles and play with that until you've saved
the cash. There's no getting past the laws of physics (and EM), bigger
is not just better, it plain just best...at least where RA is concerned.
73!
--
http://www.qsl.net/n1yvv
***********************************
****I'm not dead, yet!************
***********************************
Dave Ocame, WS1ETI
Awards Chair
The SETILeague, Inc
www.setileague.org
Stony Creek Observatory
FN31og
-72.834 longitude
41.272 latitude
Member: The SETILeague, Inc. and,
The Society for Amateur Radio Astronomy (SARA) and,
The Planetary Society
Jim Sky
them as an interferometer. I used simple dipoles 1/8 wave above ground for
my 38 Mhz interferometer. The spacing was 500 feet or about 19 wavelengths.
With this I could see fringes from at least Cass A and Cygnus A. A
similar spacing for 610 MHz is only 31 feet. You can distinguish the
sources by their fringe rates. This is an instructive exercise for someone
just starting out. Of course more gain is better for most things, but the
low gain antennas do not have to be steered so much. Mine were just
pointing straight up. I have had nice success with a couple of 7 element
yagis at 144 Mhz with 20 lambda spacing. Beautiful source fringes,
especially of the active sun. You don't need great antennas to have fun
with an interferometer.
Jim Sky
linear_shift
(probably a red flag right there, but elsewhere they were selling for
much more. Besides which I am getting decent results from them as a tv
antenna. The beamwidth is very narrow for an antenna of its size
(judging by tv reception of a semi distant station, the was about
20-15 degrees as measured by a compass attached to it as I scanned
side to side). Also, I have combined the patterns (0.41m diameter
parabolic reflector and 6 director yagi) by hand on polar plot paper
and gotten, what I believe to be a similar pattern (beamwidth at
least). The net gain was about 13dbi as combined (7.9 for the yagi and
5.4 for the 50% eff. reflector == 13.3dbi), but the real test will be
the sky itself. It would be wondeful if these things work, I have a
tight size and weight limit for the side of my house's roof (my
parents aren't too keen on 8 meter dishes, 2.5 meter long yagis, or
even 2x2 meter dipole arrays). Anyway got the gain figures from
QuckYagi 4 and the SETI parabola.xls spreadsheet. The rough pattern
was from QuickYagi 4 and http://www.satsig.net/pointing/antenna-beamwidth-calculator.htm
(very rough, plotted by hand; this calc actually gives my a higher
gain value, for a 65% eff reflector, which I don't think this
reflector is). The pattern is rough, especially since its done by
hand, so a real software and field test analysis is needed.
As for the reason why it works I am thinking that because the
reflectors pattern is so broad, its actually picking up radiation
outside the yagi's beam (much like a optical scope with a central
obstruction, the object in the center does not affect the light
collected by the primary mirror). It affect the reflector *has* to be
low gain and have a broad pattern to achieve the final pattern!
Anyway, its just a thought (and me trying to be optimistic about my
purchase ;) ).
Keep your ears on the sky, ls.
David Ocame
experimenting. The reason for my advice was not meant to dishearten you.
Far from it. It was simply to suggest you save rather than spend any
hard earned cash you have.
Good luck!
Dave
--
Jan Lustrup
together as if they where two antennas!
The capture area from dipole and directors is allready spokenfor! 'You don't
get much extra from the reflector. (ecept noise reduction from the back, not
much more forward gain.
Try you own reallife polarplot and see. Use this program
http://www.g4hfq.co.uk/download.html ...it's free to download and use.
You will need a test transmitter (simple xtal oscillator and receiver with
agc "off" or RF gain backed down a bit so not to overload.
Then you can check out for your selg how much gain youre getter and see the
polar-patern too!.
I've made long yagi's with NO refelctor that have fantasic front to
back.......
73 LA3EQ Jan