Best FM antenna
warren...@gmail.com
installing a rotator on a tripod on the roof. Size is no major issue,
as I currently have the biggest channel master antenna up on the roof
(deepest fringe, huge) I have been looking at this fm antenna
http://www.starkelectronic.com/spr98a.htm (top), but I am really not
sure. I am looking for 150 mile+ reception, and a very directional
antenna. Any ideas?
Noon-Air
got. If you do the quad, make sure that you use a good, high velocity, low
loss feed line and cut it to multiples of the electrical ½ ~
<warren...@gmail.com> wrote in message
news:1140916959.9...@z34g2000cwc.googlegroups.com...
warren...@gmail.com
tha channel master? I know I can do better than the channel master, as
it is really a tv antenna and struggles at longer FM distances.
Brenda Ann
<warren...@gmail.com> wrote in message
news:1140921294.2...@t39g2000cwt.googlegroups.com...
>I am trying to avoid building one, does the link above compare well to
> tha channel master? I know I can do better than the channel master, as
> it is really a tv antenna and struggles at longer FM distances.
>
Never seen a VHF TV antenna that didn't also have good FM specs.. since the
elements for Channel 6 also cover FM well.
Only thing I know of that can really beat a good super fringe TV antenna for
FM DX is a 10 element yagi. (or larger)
Here's a monster for you that should pull pretty much anything that's
hearable...
Keep in mind that FM is LOS (Line of Sight), so your 150 mile away station
better be on a hill that sits higher than you and your surrounding terrain.
(there are exceptions... some stations can be heard via reflection or
refraction.)
nelson
"Noon-Air" <Noon...@comcast.net> wrote in message
news:4sOdnVDM6rbIkZzZ...@comcast.com...
> Either build a about a 4 element quad, or use the channel master you
> already got. If you do the quad, make sure that you use a good, high
> velocity, low loss feed line and cut it to multiples of the electrical ½ ~
>
Altering the length of coax to fool the RX into seeing 75ohms is no good as
it would still work if there was a completely detuned aerial on the end of
it, although you wouldn't hear much.
That's why people get confused with ATUs, they don't tune the aerial or make
it resonant, they just fool the input/output stage into seeing the correct
impedance.
Noon-Air
"nelson" <nel...@prv.net> wrote in message
news:u7jMf.52458$494....@newsfe2-gui.ntli.net...
Assuming that the antenna is built to the exact design specs and correctly
tuned, the 4 element quad that I build is 14dbd, with 28db front to back,
and the correct length of feedline minimizes signal loss for both transmit
*and* recieve.
As a real time example, take a random length of RG-213 a little over 50ft
and run say...50 watts @ 450mhz into one end and see what comes out the
other, then cut a simular length of 9913 to the exact multiple of the
electrical ½ ~, and see what you have coming out with the same power and
freq.
I did this in real life with the antenna cut to the *exact* specs and tuned
to the exact freq...the only variable was the feedline.... with the random
length of RG-213, I had 50watts in and a measured 28 watts out at the
antenna.....with the correct length of 9913, and 50 watts in, I had a
measured 46 watts at the antenna.
The same signal loss with random length of feedline is also there in recieve
mode.
The channel master is not much more than a log periodic, where as a quad is
a frequency specific, "purpose built" antenna, and with a lot better
results.
Bob Bob
Okay so lets tackle this from a slightly different direction...
You are trying to get the absolute best/optimum setup for your system so
you must consider more than just the antenna. Pls consider the following
points;
- The antenna you gave us the link for is probably okay as a starting
point. Keep in mind though that it is a wideband design covering the
entire FM band and as such the gain and reject figures are not optimum
for a specific frequency. I saw another post about building one specific
to the job but you dont want to do this. I doubt you'll find a
commercially built antenna off the shelf that has the gain etc that you
could more effectively build for. For the ssme length as the Stark
antenna you might get another 3-4dB of forward gain for a specific few
channels.
- You can obviously stack antennas to provide more gain. If for example
you put 4 of the Stark ones at an optimum spacing you could get 5dB or
so better forward gain. Unfortunately the optimum spacing will be
channel/frequency dependent, even to the point of breaking the main lobe
up into multiple smaller ones
- Dont be fooled into thinking that the antenna will perform perfectly
wherever you mount it. Nearby objects will detune it and skew the
radiation pattern. Make sure it is clear of other objects including
other antennas. If you intend mounting it for vertical polarisation make
sure the mounting mast is behind it as this will add considerably to the
detuning. There will also be an optimum height above roof/ground where
the main lobe of radiation will come from the lowest angle above the
horizon. What I am trying to get at here is that by varying height you
can adjust for best signal.
- Indoor FM radios are notoriously low in sensitivity so it is
worthwhile adding a masthead preamp at the antenna. If I was doing it
I'd be tuning it for the frequency in question to reduce the chance of
overload from other services. Having a good preamp also negates the loss
problem from the coaxial run. A warning here, dont get a cheapy RS or
equivalent thing for this. They can radiate by themselves and will have
a worse noise figures than a better device.
- Antenna directivity implies gain. ie you cant have one without the
other. Gain is also function of usable bandwidth. ie the wider the
bandwidth the lower the "best" gain. Depending on your situation the
ability of your antenna to reject unwanted signals may also be
important. If for example you have another FM station within a few miles
it will likely desense or block your receiver to the weaker signals. If
you arrange an antenna such that it is pointing at 90 degrees/right
angle to the closer station you will attenuate its signal by a great
amount (50+dB wouldnt be unrealistic but depends on multipath
alternates). This is often a better option even if the antenna isnt
pointing right at the station you want. The F/B figure of the Stark
antenna is a way of measuring what attentuation you will get off the
rear of the antenna for the same reason.
- Since you think the path maybe marginal go and model it using UKWtools
or Radiomobile. You'll need the FM transmitter location/gain/power etc.
Boltzsman puts a FM thermal noise bandwidth at around -122dBm so add
maybe 5dB noise figure to that for your preamp and use this in the path
calc. If you are then more than 15dB shy it probably isnt worth playing
antennas with but special diffraction/bending modes in your area (eg
inversions over mountains) may make it usable
(Noise BW from http://www.tscm.com/TSCM101noise.html)
Realising that this os probably way more than you are looking for. Sorry!
Cheers Bob W5/VK2YQA
Noon-Air
Bob Bob
only 4 watts in 50ft exceeds the manufacturers loss spec by a factor of
around 9.. How did you do it?
Methinks you introduced some other variables, like output power change
in the presence of a reactive load. I am not going to bother spouting
why and making lots of noise. You are wrong however in you assertions of
power loss being reduced by tuned coaxial lengths. The situation you are
thinking of is multiples of half wave lengths in the coax itself. ie
allow for the velocity factor (0.66 in polyethylene dielectric cable -
it varies. Foam for example is about 0.86) What happens is that the
complex impedence at the antenna is then exactly the same as presented
to the terminals of the transmitter. You can for example use any
impedence coax in this situation.
If the antenna impedance is off though you can use tuned lengths to make
the TX happier and present a lower return loss. A 100 ohm antenna for
example would benefit from a 1/4 wavelength section of 75 ohm coax in
the line just before it meets the antenna.
If however it works for you, and you are happy then who am I to argue
against you! I suggest thought that if inclined you should do the
experiment with better control and based on distant received field
strength rather than an inline power meters. Oh and "exact specs" are no
substitute for complex impedence measurements.
Cheers Bob VK2YQA
Noon-Air
> That attenuation at 450Mhz with the 9913 is truly astounding! To lose only
> 4 watts in 50ft exceeds the manufacturers loss spec by a factor of around
> 9.. How did you do it?
>
> Methinks you introduced some other variables, like output power change in
> the presence of a reactive load. I am not going to bother spouting why and
> making lots of noise. You are wrong however in you assertions of power
> loss being reduced by tuned coaxial lengths. The situation you are
> thinking of is multiples of half wave lengths in the coax itself. ie allow
> for the velocity factor (0.66 in polyethylene dielectric cable - it
> varies. Foam for example is about 0.86) What happens is that the complex
> impedence at the antenna is then exactly the same as presented to the
> terminals of the transmitter. You can for example use any impedence coax
> in this situation.
>
> If the antenna impedance is off though you can use tuned lengths to make
> the TX happier and present a lower return loss. A 100 ohm antenna for
> example would benefit from a 1/4 wavelength section of 75 ohm coax in the
> line just before it meets the antenna.
>
> If however it works for you, and you are happy then who am I to argue
> against you! I suggest thought that if inclined you should do the
> experiment with better control and based on distant received field
> strength rather than an inline power meters. Oh and "exact specs" are no
> substitute for complex impedence measurements.
Point taken.... At the time, I had access to a machine shop but not to the
anetnna analyzer, so I made due with what I had. I will say this... the
measurements on the antenna were accurate within .001inch, and the feed line
physical measurements were almost as close.
And there was a marked improvement in the performance. I was able to work a
UHF repeater almost full scale/full quieting with it 150 miles away and my 4
element quad was only up 38ft above sea level.
FWIW... same height, same feed line, same transmitter, same everything
except using a 1/4~ ground plane would barely bring the machine up and was
only about S-1 on recieve.
> Cheers Bob VK2YQA
>
> Noon-Air wrote:
>
>> As a real time example, take a random length of RG-213 a little over 50ft
>> and run say...50 watts @ 450mhz into one end and see what comes out the
>> other, then cut a simular length of 9913 to the exact multiple of the
>> electrical ? ~, and see what you have coming out with the same power and
Sal M. Onella
< snip >
> If you can't use a non-conductive mast, then try get the mast behind the
antenna and
> space it from the mast using this formula for > the distance;
> (f(mhz)/12360)= distance in inches
>
> 12360 =distance in inches
> f(mhz)
>
> This will turn the mast into a reflector and help increase gain and lobe
directivity.
< snip >
Tried it both ways with 98 MHz mid-band freq. Mast is either 8-thousandths
of an inch or 10.5 feet away from the antenna. Say what?
Noon-Air
"Sal M. Onella" <salmo...@food.poisoning.org> wrote in message
news:wKtOf.418$wp.140@fed1read12...
Sorry... got new glasses now....looking at the correct page and formula.....
Should be .....
12060 = wavelength(in inches) X 0.2 for the distance from the mast
f(mhz)
@ 98mhz = 123.061 inches(wavelength) X 0.2 = 21.612 inches.
Chuck Forsberg WA7KGX N2469R
I once made an excellent two meter beam by trimming the elements
on a channel 2 single channel Yagi.
So find the longest single channel Yagi and trim it to the FM band.
Since few channel 2 stations kept that frequency with the digital
transition, you may be able to find a channel 2 yagi cheap.
For really good reception, try 20-50 feet length and/or stacking.
--
Chuck Forsberg c...@omen.com www.omen.com 503-614-0430
Developer of Industrial ZMODEM(Tm) for Embedded Applications
Omen Technology Inc "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231 FAX 629-0665