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ELT question. Oh no not another!

R

Rivethead

Well Known Member
I'm not out to start another ELT war :eek:. I've been doing some searching tonight and I see it's been a well hashed out subject. What I would like to know is what would happen if a person were to use an ordinary comm antenna for the ELT? 121.5 does seem to fall right in the middle of the frequency that the run of the mill comm antenna is useful at.
 
From a physics standpoint, it would probably work fine on 121.5 but I'm not sure how well it would work on 243.0. If it's a 406 MHz ELT that could be another issue.

It wouldn't be as survivable in a crash.

There might be TSO issues.

My .02,
Paige
 
Will a normal com ant survive the "crash"?

It should work fine for transmission. The ELT is not very frequency sensitive and will bleed over onto several frequencies. The biggest problem is withstanding the impact. The ELT antenna is designed to withstand a pretty severe impact. And the other point is that the ELT is TSOd and as such must be installed per the manufacturer's instructions which usually call for a "specific" antenna.
Oops. Looks I just said what Paige already said.
 
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Dope slap moment

I actually gave myself a dope slap this morning while reading my ELT install manual. Right there in black and white it tells ya that other antennas you can be used. It reads, other antennas may be used so long as they meet the stated minimum VSWR requirements and any other FAA requirements. Nothing like reading the manual over a cup of coffee in the morning.
 
OK some one has to say no

Yes that is true 121.5Mhz is the ELT freq (one of 3). Yes its right in middle of the aviation VHF COM band, but so what. :rolleyes: :D But what about 243Mhz and 406Mhz?

Each ELT antenna is made for that particular ELT unit. If its an older 121.5/243 Mhz ELT there might be some compromise (in the transmitter circuit) to make it work well on both freqs with one antenna. There might be some "tuning" circuit, filters or balun, optimized for their specific antenna. I'm guessing but what I know about radio circuits, when you try a new antenna on any transmitter you should analyze it.

A COM antenna is a COM antenna for a "Broad Band" of Freqs. An elt antenna is for two specific freqs.

Since 121.5 and 243 are almost multiples of two, so one antenna worked out OK for both. Still there might be some "optimizing" for a very specific antenna they designed to match the ELT.

Now with 406Mhz things are different.

Most manufactures of 406Mhz & 121.5 Mhz ELT's use two antennas, one for each freq. The answer would be a big NO to using a COM antenna with a 406 Mhz elt.

Artex and their basic unit is the only one off hand I know of that does this, but there may be others. I understand EBC, Ameriking and ACK (if and when they come out with 406Mhz elts) will use two antennas. Of course the 406Mhz antenna is pretty short.

One last rant and lecture. Of course using your own antenna, not the one that came with the ELT is a no-no, illegal, forbidden, since its all part of a STC system, components and installation. Experimental? Yes ELT is one of the only STC items needed on an experimental.

Sorry for being nit picky, but I think COM antennas, home-brew, DIY and rubber-ducky antennas are a bad bad idea.

By the way those cute rubber ducky antennas on hand helds work terrible, even the ones that come with them. Test have shown that a real honest to goodness full 1/4 wave or 1/2 wave antenna puts out 6 to 9 times power!!!!! So you're talking about a measly 100mw ELT on 121.5Mhz. You need all the antenna you can get. Not saying a COMM antenna can't or woun't work (better than a rubber ducky for sure). The point is who knows with out testing it.

With that said. To use a different antenna from the one that came with it, you really need to know what you are doing. You should have antenna analyzing gear: SWR, power and spectrum analyzer. Do you really know the impedance, resonate freq of the original antenna? Why do manufactures warn that you must use the matching antenna?

An ELT on 121.5Mhz is only 100mw. That is 0.10 watts of POWER!!!!!! Do you want to compromise your antenna, making your ability to be heard worse? :rolleyes:
 
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Since 121.5 and 243 are almost multiples of two, so one antenna worked out OK for both.
Click to expand...

George,

You may already know this, but I thought I'd throw it in for completeness.

I have no idea how ELT manufacturers do it, but as far as using antennas on harmonic frequencies, here are the general rules of thumb:

Current fed antennas (i.e. low feed point impedance) may be used on fundamental and odd harmonics. Current fed antennas include the 1/2 wave dipole, and 1/4 wave vertical.

Voltage fed antennas (i.e. high feed point impedance) may be used on fundamental and even harmonics. An example of a voltage fed antenna would be a 1/2 wave vertical.

If you try to use a plain old 1/4 wave vertical on its second harmonic, you're going to get a very high SWR (it will have a high feed point impedance). It would be interesting to check the SWR of an ELT antenna at 243.0. If it is low, they must have done something in the antenna design. If it is high, then they must have compensated in the transmitter design (which would also imply that changing the coax length would de-tune the system).

Long post, but the bottom line is the same as before, I don't know how it would perform on 243.0.

Paige
 
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