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Spaceclam

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May 31, 2021
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Hello,
I have a New-to-me flybaby that was fitted years ago with a whip antenna mounted behind the headrest, and connects to a hand held radio. This airplane is about 30 years old, and one of its issues is radio noise. I have to keep the squelch all the way up, and sometimes it causes me to miss transmissions. It’s not engine noise. It’s more like aliens screaming in my head, and the noise is far louder than the transmission level. The radio works just fine by itself, but there isn’t really much room in the cockpit to use the handheld radio antenna and I like my radio mounted as it is

Seeing as there is corrosion at the base of the antenna and the connector is visibly discolored, I’d like to just replace it all and see if it helps. In looking at antennas, there appear to be nearly identical models ranging from $75 to $1500+. Is there some compatibility issue I am not aware of? Does it need to be sized to work with the lower-power handheld unit? What should I buy?

Thanks in advance,
-Clam
 
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Rhino

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Is this noise all the time, during all received transmissions, or only when you key the mike? If it's the latter, it may be a simple matter of the external antenna being too close to the radio. Fortunately you can put an antenna anywhere in a Fly Baby. It doesn't need to be externally mounted, because the wood/fabric construction won't affect your signal. I would try something as cheap and simple as this, just to test the possibilities:

 

Spaceclam

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Messages
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Is this noise all the time, during all received transmissions, or only when you key the mike? If it's the latter, it may be a simple matter of the external antenna being too close to the radio. Fortunately you can put an antenna anywhere in a Fly Baby. It doesn't need to be externally mounted, because the wood/fabric construction won't affect your signal. I would try something as cheap and simple as this, just to test the possibilities:

The noise is not when the mic is keyed. It just happens randomly, even when the engine is off. I’ll give one of those a try. Also, any thoughts/comments/suggestions on the connector and wire? How long can the wire be? Do they make 90degree connectors so it doesn’t put so much strain on it?

Thanks,
-Clam
 

rv7charlie

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If it's breaking squelch with no transmissions in the area, then either there's something wrong with the radio or you've got excessive RF or electrical noise in the a/c.

If it were me, before spending money I'd try to find the source of the problem. Things to look for/think about:
If it's an ancient engine, are the mags & plug wires shielded?
Are your P-leads properly shielded and terminated? (Whole chapter there...)
Is there a generator/alternator? If so, does turning it off improve things?
Is the shield on the antenna coax properly terminated *on both ends*?
Is the antenna grounded properly to the airframe?
Not directly related to a noise problem, but does the antenna have a ground plane? (doubtful if it's on the fuselage top behind the headrest)

Just a few things that came to mind in the time it took to type them.
 

Spaceclam

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Messages
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I probably should have mentioned -this aircraft has no electrical system. So given the noise when the engine is off... that pretty much rules everything out besides antenna and connections. I think?
 

rv7charlie

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A comm antenna can be as simple as stripping about 22" of shield off a piece of coax, and attaching 3 or 4 22" long radial arms attached to the shield at the strip point: 1/4 wave antenna Now it it will only be maximally efficient at one frequency, but it will 'work'.

If the radio is breaking squelch without the engine running and in receive mode (not transmitting), and there's no electrical system, it's hard to understand where the noise is coming from, except from the radio itself. Does it have the problem on multiple frequencies, or just your local active frequency? Are you doing the test out in the open, well away from a hangar or other building that might have fluorescent or LED lights, or other noise generators? The fluorescent lights in my hangar drive my handheld crazy, unless I get 20-30 feet away from them and outside the hangar. Have you verified the integrity of the coax, from the connector at the radio end to the connector at the antenna? Is the noise the typical 'white noise' that you hear from any comm radio when you disable the squelch, of does it have any different characteristics? 'aliens screaming in my head' does not sound like normal open squelch. If you're getting a howling or squealing sound, that almost sounds like it's interacting with another radio that's transmitting; the sound you get when one pilot 'steps on' another's transmission.

Kinda hard to 'remote troubleshoot'; just trying to give you some ideas on things to check.
 

Rhino

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The noise is not when the mic is keyed. It just happens randomly, even when the engine is off. I’ll give one of those a try. Also, any thoughts/comments/suggestions on the connector and wire? How long can the wire be? Do they make 90degree connectors so it doesn’t put so much strain on it?
Have you tried a different radio? Maybe borrow one from a friend? It'd be a shame to mess with wiring if the radio is the problem. From what you're saying though, replacing the antenna/wiring is probably a good idea either way. Make sure it's a 50 ohm cable. TV antenna cables can look identical, but they aren't 50 ohm, which is what you should use for the aircraft band. Make sure you have good ground connections at both ends if you fabricate your own cable. Normally you'd make the cable run as short as practical, but don't go overboard trying to save every inch. Prepackaged antenna/cable combinations might be longer than you need, but they're probably acceptable for your application, especially since you have no electrical system.

Might I also suggest you get a copy of Bob Nuckolls' Aeroelectic Connection if you can? Much of it won't apply to you since you don't have an electrical system, but it's still a great reference book to have. Much of it applies to vehicles other than airplanes too.

EDIT: This isn't bad to have either:
 
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Rhino

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Oh, and the specific cable type depends on the application. RG 400 is fantastic antenna cable, but it would be incredible overkill for what you're doing. RG 142 is good, but RG 58 will probably do you just fine.
 

Map

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I still have a few pieces of RG-400 antenna cable for sale, 17 & 16 ft pieces no connector, and 1 x 17 ft with TNC connectors, $1.10 per ft & $8 for cable with connectors.
 

tallank

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Joined
Sep 22, 2020
Messages
77
Hello,
I have a New-to-me flybaby that was fitted years ago with a whip antenna mounted behind the headrest, and connects to a hand held radio. This airplane is about 30 years old, and one of its issues is radio noise. I have to keep the squelch all the way up, and sometimes it causes me to miss transmissions. It’s not engine noise. It’s more like aliens screaming in my head, and the noise is far louder than the transmission level. The radio works just fine by itself, but there isn’t really much room in the cockpit to use the handheld radio antenna and I like my radio mounted as it is

Seeing as there is corrosion at the base of the antenna and the connector is visibly discolored, I’d like to just replace it all and see if it helps. In looking at antennas, there appear to be nearly identical models ranging from $75 to $1500+. Is there some compatibility issue I am not aware of? Does it need to be sized to work with the lower-power handheld unit? What should I buy?

Thanks in advance,
-Clam
Cheap antennas are fine. The antenna needs a ground plane. The size of the ground plane's radius is the length of the antenna. The only thing I can think of the antenna causing your noise issue is if the cable is shorted somewhere. From what you are saying about corrosion at the antenna I would think that is the area that is the problem.
 

Rhino

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The ground plane is a good point to address. If you're using a rubber ducky type antenna, like the type that come with handheld radios or the stick-on antenna I linked above, you don't really have to worry much about a ground plane because they're designed to operate without one. But if you use a traditional aircraft antenna, a ground plane is more important. Fortunately, since you're just using a handheld radio, you don't need to go theough all the hassle to create a ground plane that owners of composite or other wooden aircraft typically have to. A ground plane disc will work just fine. An example is linked below. It doesn't need a flat piece of fuselage, because like I said before, you can mount it inside a wooden/fabric aircraft. Be aware though that aircraft comm antennas are vertically polarized, so the antenna should be as close to vertical as possible or your signal might suffer a bit. I always get a laugh out of seeing people hold their handheld radios sideways when they use them (Hollywood!). Personally, I would mount it upside down internally, or on the bottom of the aircraft, to minimize what energy the ground plane might interfere with, but that isn't all that important either.

 
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