Rabbit ears air antenna experiment leads to question about antenna length . . . help needed

KB2GOM

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Someone here on RR posted that they had repurposed an old TV rabbit ears antenna for monitoring air frequencies and that it worked pretty well (If I could find the post again, I would reference it). If I recall correctly, they adjusted the rabbit ears until each was about 18 inches long. I look up online, and a quarter-wave of 128 MHz is around 19 inches. Close enough.

Hmm, I thought, I've got an old rabbit ears antenna somewhere . . . I found it; a ham friend (thanks, Steve!) whipped up a jumper cable to connect the rabbit ears to the BNC connector on my SDS200, and -- voila! -- we're in business. I figure the ATIS signal at Albany Country Airport will work just fine as a reference signal, so I lock the SDS200 onto that. The indoor antenna I am using for the SDS200 is a Comet W100RX, extended to its full length, about 40 inches. The signal strength meter is showing just one bar.

I hook up the rabbit ears (set to 18 inches long) to the SDS200, and the ATIS signal is immediately worse; no bars. Making sure the switch on the antenna is set to VHF, I try clicking the dial for various VHF settings on the rabbit ears antenna. No improvement, and some positions are noticeably worse.

As I am putting the Comet antenna back on the SDS200, I notice that there are markings on the black barrel of the first segment of the telescopic antenna. One of them says "VAIR," and it indicates that the W100RX should be extended 5 1/2 segments to receive VHF air frequencies. So I try that and -- ta da! -- the SDS200 shows two bars on the ATIS signal. Cool! I measure the length of the Comet antenna at 5 1/2 segments, and it's 27 inches to the right-angle bend where the BNC is connected.

So maybe the rabbit ears need to be adjusted to 27 inches. I try that, and it works no better than before.

That 27 inches looks kinda familiar, though, so I measure the stock telescopic antenna that comes with the SDS200, and it is almost exactly 27 inches. I connect that antenna to my Uniden 125AT, and it, too, shows two bars of signal strength on the ATIS frequency (and that's better than my Diamond 77 antenna). Very nice . . . an improved setup for listening to civilian air frequencies on two scanners.

But now here is the question: why should a 27-inch antenna work better? It's not -- as near as I can tell -- quarter wave, or half wave or five eighths wave. It is approximately 3/8 wave.

Can someone please shed some light?
 

ka3aaa

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why does it matter if it works leave it along and scan on and by the way the length of antenna you would need for the 119 mhz to 125 mhz frequency range is about 25 inches. 19 inches is for the 146 mhz frequency range
 

KB2GOM

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why does it matter if it works leave it along and scan on and by the way the length of antenna you would need for the 119 mhz to 125 mhz frequency range is about 25 inches. 19 inches is for the 146 mhz frequency range
Well, it does work, but it matters to me because I want to know how to calculate the proper length of antenna in case I want to "tune" one in the future.
 

KE5MC

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Antenna length calculations are a starting point. Many factors not part of the frequency calculation effect results. Make it long and cut back. For receive only it is not as critical as transmit. For receive I would adjust for best reception without much regard to length and listen on...
 

TomLine

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I would hook it up to my Nano NVA and see what the frequencies of resonance are.

The length of the leads could have something to do it with it as well. Flat wire?
 

popnokick

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If you're using a 300 ohm (flat lead) to 75 ohm (coax) transformer, setting both of the antenna elements to 18" is going to make a dipole - which will not have a 300 ohm feed point. Try this: Keep the transformer, flat 300 ohm lead, and 75 ohm coax. Adjust one of the antenna elements to 18 inches, and the other as close as you can get it to 48 inches. You've just made an Off-Center Dipole, a very widely used and popular antenna here on RR... and it should be fine for both civilian and military air bands. More details are here (scroll down to see the wire version, which is what you basically have, except using rabbit ears)-
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
If you can't get 48 inches on one of the elements, pull it out to full length but then shorten the other element proportionately. To tune the antenna further, use the ATIS signal that you have.
 

KB2GOM

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If you're using a 300 ohm (flat lead) to 75 ohm (coax) transformer, setting both of the antenna elements to 18" is going to make a dipole - which will not have a 300 ohm feed point. Try this: Keep the transformer, flat 300 ohm lead, and 75 ohm coax. Adjust one of the antenna elements to 18 inches, and the other as close as you can get it to 48 inches. You've just made an Off-Center Dipole, a very widely used and popular antenna here on RR... and it should be fine for both civilian and military air bands. More details are here (scroll down to see the wire version, which is what you basically have, except using rabbit ears)-
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
If you can't get 48 inches on one of the elements, pull it out to full length but then shorten the other element proportionately. To tune the antenna further, use the ATIS signal that you have.
Interesting . . . I'll give it a try . . . stand by for "film at 11."

Do I still keep the antenna elements vertically oriented?
 

popnokick

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Yes - keep them vertical (just as described in the RR Wiki for the Homebrewed Off-Center Fed Dipole). And I'm hoping also that your 300 to 75 ohm TV transformer is electrically OK, i.e. has not been damaged by being previously used outdoors or in damp / wet conditions, experienced a lightning surge, etc. If the rabbit ears antenna was always indoors then that probably is not the case. But you should be able to receive something with an OCFD and working transformer.
 

popnokick

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Aircraft comms ... including ATIS.... are vertically polarized. If your antenna is horizontal you attenuate the signal you're trying to receive by 20-30dB. Polarization is crucial in line of sight / near line of sight (VHF / UHF) communications.
 

KB2GOM

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If you can't get 48 inches on one of the elements, pull it out to full length but then shorten the other element proportionately. To tune the antenna further, use the ATIS signal that you have.
Okay. I couldn't get 48 inches; I got 41, so I shortened the other one to 13 inches (I can't get it any shorter). With the right one long and the left one short, it didn't help any, but with the left one long and the right one short, I'm getting 1-2 signal bars after fiddling with the tuner wheel on rabbit ears antenna.

Any additional suggestions? I can't make the longer one any longer, and I can't make the shorter one shorter.

I just did the A/B comparison, and the rabbit ears are receiving a stronger signal than the Comet 100 "tuned" to 27 inches.
 

popnokick

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I'm getting 1-2 signal bars after fiddling with the tuner wheel on rabbit ears antenna.
I just did the A/B comparison, and the rabbit ears are receiving a stronger signal than the Comet 100 "tuned" to 27 inches.
Tuner wheel? So that implies that there are some other components in that rabbit ears antenna set (coil / cap maybe). May be helping, but might also be reducing your signal. You write that the rabbit ears give you better (stronger) result than the Comet 100, so you might benefit by following the instructions in the RR Wiki for the Homebrewed Off-Center Fed Dipole (linked previously). Use the 300 to 75 ohm transformer (or get another... they're inexpensive) and make the "wire version" shown in the Wiki. Hang it vertically in the highest window you can (or attic). Run the coax away at a 90 degree angle to the two vertical antenna elements.
 

majoco

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Throw away the 300ohm ribbon and the transformer (4:1 balun ?). Disassemble the rabbit's ears and join the coax cable to the telescopic elements at the base. Re-assemble. Run the coax to your receiver. Set the lengths of each rod to a quarter-wave for you frequency of interest.

A 300ohm antenna is usually a loop like mine. The 300:75 balun is in the orange box . Works well on the 118-136MHZ air band.

airband dipole sml2.jpg
 

wtp

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i took one of those old antenna selector things apart and if i remember it just swapped the antennas back and forth or just to use one side.
 

PDXh0b0

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for me, the ATIS signal was the hardest to catch, I was receiving lots of airband traffic, got the antenna 28ft up in the air and am able to receive the local ATIS , which is less than 5 miles away as the crow flies
 
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