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FM helical antenna DIY
FM helical antenna DIY
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Old 31st December 2017, 04:14 AM   #1
70s Veteran is offline 70s Veteran
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Default FM helical antenna DIY

Maybe this forum is the best one. Or are the readers only interested in vynal that in my humble opinion is now as obsolete as a tape deck.
I in the 80s had a good record player and an impressive collection of LPs. But the scratchy noise irritated me as did the hiss on tapes.
CD sound is better, MP3 sucks the ambience out of music. So I tuned into FM radio.
A Pioneer car CD, FM receiver and a modified antenna tuned to be a 5/8 in length simply cut off the 1/4 wave antenna with 0 dB gain and then in the mid band of 100 MHz or 1.875m using the inner part of the coax helicaly wound around a wooden dowel rod. Gives you 6 dB of low noise gain.The improvement of pure analogue FM sound is easily obtained.
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Old 31st December 2017, 01:19 PM   #2
DF96 is offline DF96  England
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6dB gain is unlikely from a normal-mode helical "5/8" antenna. Even if achieved (and in the right direction - towards the horizon) it will probably have narrower bandwidth.
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Old 31st December 2017, 02:12 PM   #3
AuroraB is offline AuroraB  Norway
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Why helical?? FM transmission is either horisontally or vertically polarized, so you will have polarization loss...
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Old 31st December 2017, 06:45 PM   #4
PRR is offline PRR  United States
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Quote:
Originally Posted by 70s Veteran View Post
...Or are the readers only interested in vynal that in my humble opinion is now as obsolete as a tape deck....
Perhaps not best to start with an insult.

Anyway, it depends. In My Humble _Local_ Opinion, FM is useless. Way up the Maine coast, there's only about four stations will come in, three poorly even with a 6dB gain. And three of them play the *same* MOR play-lists and same rotations. Same songs several times a day for a month, and then different songs for a month. (The fourth station here is nominally owned by Steven King, and is, uh, different; but does not come in well over the mountain.) (A fifth station is community anarchism but super low power.)

I got a small kick out of finding a *GOOD* AM radio. A 1948 Zenith *with an RF stage*. That buzz/hash I get on all AM here turns out to be filterable. I can bring in NYC 500 miles away clean enough to listen to.

But thanks for the whip-tip. Basically you replaced the metal car ground-plane with a stiff-whip counterpoise so you can use it anywhere (like high in the house above ground clutter). I bet the wrapped dowel is not a "helix" but a low-Q whip. And of course such a dipole can be turned this way and that to catch the multi-path as good as possible.

And I thought FM broadcasters used combined H/V polarized antennas? At least last time they would send me free magazines, that was a thing. Claimed to throw well to either living room dipoles or car whips.
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Old 31st December 2017, 10:48 PM   #5
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Absolutely as posters have noted the helical antenna due to its length and mounted vertically can easily capture ME stations as well. It's very compact a mere 20cm in length.
I live on a farm and in comparison my car radio cannot get the same reception. Try it it is a low cost DIY antenna and unlike a Yagi antenna it's not directional.
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Old 1st January 2018, 03:19 PM   #6
DF96 is offline DF96  England
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There are two modes for a helical antenna. I am guessing that the one used here is a normal-mode helical; this is used when it is desired to shorten a monopole. Commonly seen as a 'rubber duck' antenna on a handheld transceiver. It gives linear polarisation (probably vertical in this case), and narrow bandwidth. In some cases a poorer impedance match too. Best avoided unless small size is a definite requirement.

There is also the axial mode helical. This gives circular polarisation and fairly broad bandwidth. Usually used with a rear reflector to get a narrow beamwidth. Can be used for satellite reception. Some VHF broadcast transmitters may use a short axial mode helical to send circular polarisation, but I suspect that this is less common now than in the 1970s.
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Old 1st January 2018, 09:30 PM   #7
PRR is offline PRR  United States
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Originally Posted by DF96 View Post
There are two modes for a helical antenna....
Wikipedia covers the difference. Helical antenna - Wikipedia

If the helix diameter is small (compared to wavelength) it can function as a side-fire. Put it vertical and it picks-up all around the horizon (the usual case for earthly broadcasting).

If the helix diameter is *large*, it can function end-fire. A larger reflector on the back helps the pattern. However at 100MHZ FM a wave is a couple yards long, so that becomes a very big rig. 1GHz+ it becomes practical and economic.
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Old 1st January 2018, 11:48 PM   #8
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OK I will explain my fashioned with the 5/8. Way back in the 70s was the CB radio craze and to reach my buddies on the radio, I built a super stick tuned to the 27MHz bandwidth. An old fishing rod with the antenna also helical wound. With four quarter wave ground plains.
Which extend the range to a neighbor country.
In the sound system I am building for my daughter's wedding present I made a Boom Box with two side wiring 6x9 3way speakers and four of those lovely 2" PC inverted dome midrange. The box height was limited to be 12" so as I didn't want a fragile whip antenna I mounted the antenna inside it. It's definitely vertically polarized but so are most FM transmission antennas.
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Old 2nd January 2018, 12:00 AM   #9
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by 70s Veteran View Post
Maybe this forum is the best one. Or are the readers only interested in vynal that in my humble opinion is now as obsolete as a tape deck.
I in the 80s had a good record player and an impressive collection of LPs. But the scratchy noise irritated me as did the hiss on tapes.
CD sound is better, MP3 sucks the ambience out of music. So I tuned into FM radio.
Where do your FM stations get their program material? The best stereo SNR is far worse than CD and many independent stations that actually play more than 20 songs still spin LP's.
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Old 2nd January 2018, 11:32 AM   #10
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Fair enough CD sound is the best. But FM radio is actually very good for background music. Far better than music on Sound Cloud.
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