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Discussion Starter #1
I'm wondering how good a GH or even a CM4221 would work for FM if it was scaled up? I have built various GH's with excellent results. I'm now looking at FM antenna builds and the gains seem disappointing by comparison. I am aware of the size of the antenna i am contemplating, but I have the hight and width required for this in my attic.

..I know: crazy idea.. but I would love to see this modeled.

Cheers!
 

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Why bother?

Why bother re-engineering the already excellent GH for FM radio use?
Just offer to remove a neighbor's "unwanted" VHF TV antenna. They may even pay you to remove and "cart off" the antenna! This antenna will work quite well (especially if you have a rotor) for FM radio use. I use an old Wade / Delhi VHF TV antenna for FM radio and it lets me do some very good DX to boot!
 

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Discussion Starter #5
I found the "Quadix" design on the net: Crazy gain, and great F/B ratio but it is almost 19 feet long and won't quite rotate in my attic. Looks like it's an evil weapon designed to drop birds from the sky. :D ..pretty cool!
 

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hehe i was just playing around today and yes the GH if done for FM is big so i Opted for a simple only 9 db gain max, SWR little high, between 2 and 5 ,but im not done playing with it yet. it will be 2 elements reflector and drive and both made the same. 104 inches across by 46 inches high,there are 8 lenghts at 34 inches long with a 2 inch gap for balm oh wire radus #14. reflector is about 7.5 inches back. Make my anteena with out seeing it hehe 300ohms i know you can do it.

nikiml
I mean to say a GH scaled up not a NAROD tech job
 

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Discussion Starter #8
wildwillie,

Did you happen to run a scaled FM GH? If so what was the gain, F/B, and SWR?

Cause if there's enough gain, I'm partially insane.. and I'll build it. :eek: :D
 

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Wouldnt you want it to be bi-directional ? Which case, no reflectors and a simpler build. :)
 

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300ohms should we pull out one from area51 and maybe go omni . well if your crazy enough , the jeds gh8 would be nice it has a high gain short band area but she even be higher and wider.

i finshed playing with a new design something of a yaga look at 10 feet long 10.5 db with a very nice swr below 1.8 and just about 1 across some of the bands. with out the diretors shes does about 6 db
 

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Lineloss
Did you happen to run a scaled FM GH? If so what was the gain, F/B, and SWR?

Cause if there's enough gain, I'm partially insane.. and I'll build it.
yes i did and with out reflectors 12 db peek at 102 with swr at

BAND/SWR 88=2.9 90=3 92=3 94=2.9 96=2.8 98=2.6 100=2.3 102=2 104=1.8 106=2.14 108=2.9

Band/gain
88=10.54 90=10.75 92=10.97 94=11.21 96=11.41 98=11.72 100=11.93 102=12.02 104=11.86 106=11.28 108=10.6

thats with a GH4 i had to reajust the nibs down to 20 and I split the center feed in 3 and ajusted it to a 3 inch gap

total hieght 240 inches and width 145 i want to see this build :cool:
 

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total hieght 240 inches and width 145 i want to see this build
All you need is a side of barn, some nails and some fencing wire. Any farmer would have all those materials on hand. :p

Hmm, at those frequencies, barbed wire could be used and would have little effect on performance. I do have a 1000' coil of that for some reason. Now I just need a side of a barn, heh.
 

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Lineloss, keep in mind that FM Radio propagation is different from VHF-HI TV and very different from UHF (for which the GH was developed) so the choice of a GH for FM Radio is overkill when there are smaller alternatives that achieve great results. Also FM Radio is broadcast in circular polarity, so the GH's horizontal design would not be optimal in certain situations.

As a modeling exercise I'm all for it though. :)
 

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Discussion Starter #15
Stampeder,
As I understand it most FM channels use circular and oval polarization (TX) but I have read that not all stations use right polarization. I know that there is a loss to pay when the RX is horizontal or vertical like in my case, but the loss would be very high if TX was circular left and my antenna was circular right. Unless I can find out if circular right is a standard for FM TX, I will sadly have to stay away from the potentially optimum circular polarized antenna .
 

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Technically, its elliptical polarization, weighted to the horizontal. Screwy huh ? Heh.

In the early days of FM radio in the 88-108 MHz spectrum, the radio stations broadcasted horizontal polarization. However, in the 1960's, FM radios became popular in automobiles which used vertical polarized receiving whip antennas. As a result, the FCC modified Part 73 of the rules and regulations to allow FM stations to broadcast RHC or elliptical polarization to improve reception to vertical receiving antennas as long as the horizontal component was dominant.
http://www.astronwireless.com/topic-archives-antennas-polarization.asp

A bi-quad gives a nice ball pattern, and a reflector less bi-quad for FM (just quickly rescaling my WiFi bi-quad) would basically be 2 squares about 29.5" X 29.5" each and would give about 5 to 6 dbi gain in two directions with a wide beamwidth. It could be constructed with PVC pipe and two crosses, notching the pipe for stringing the wire. Total size of about 84" wide by 43" high, fairly managable.

Odd that Ive never seen a crossed Yagi in use for FM, as it sounds like a good idea too. :)
 

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Discussion Starter #17
300ohm,

I have built a “cubical quad” for FM and funny enough I was considering an array of 4 bi-quads with reflector since the total height is almost in the ballpark for me. I'm guessing we probably don’t see the crossed yagi on our end due to the frequency specific delay network used on one of the two antennas in order to approximate circular polarization. :confused:
 

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FM TX Rototiller Antennae

Lineloss if there is ever a chance to drive by the antenna tower you would be able to eyeball what polarity your favourite station is using. There are some great photos of FM TX "rototiller" antennae in Posts #121 and 122 of the following thread, with older FM TX antennae in Post #124 and a different type of new rototiller in Post #128:

TV Broadcast Towers & Transmitters (photos, vids)

Question for all: any point in having a rototiller-style FM RX antenna?
 

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Discussion Starter #19 (Edited)
Cool pics!

On the net somewhere, I came across someone who wrote that the antenna pair compromising a rototiller better approximates a perfect circular polarized wave due to its symmetry while the helical design (no delay circuit needed) is not symmetrical and does not approximate a perfect circular polarized wave. Now I don't know, but I would assume that the rototiller used as a RX antenna would be band limited due to the fixed delay circuit (non issue for TX), but on the plus side we could change from left to right polarization just by swapping the delay circuit to the opposing antenna in the rototiller.. interesting thought for my dilemma. FM is a narrow band by proportion so a fixed delay may not be so bad.
 
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