ELT antenna VSWR ??

pmanton

pmanton

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Has anyone bothered to check out the SWR of an ELT antenna used as a comm antenna? Are they broadband enough to be usable on 122.9 ?

I have an extra that would be a dandy hangar radio antenna IF the SWR wouldn't be out of sight.

I'm in the process of moving right now and don't have access to my Bird Thru-line to make up a test myself.

We're moving to a bigger house and hangar on the airpark. :)

Thanks

Paul
Salome, AZ
 
SWR is generally not frequency dependent. Matching impedance will reduce or eliminate SWR. What is important is the wave segment of the antenna vs the frequency. I'm not going to calculate the wave segment of the comm unless you measure the ELT antenna for me.
 
I'm not sure how you can "eliminate" SWR. It will never go below 1. It's a ratio.
The impedance does indeed vary with frequency.

Depending on what the antenna is will depend on what it's resonant frequency is as well as how broadbanded it is.
 
Since SWR is a measure of reflection, if the ratio is 1:1 then there is a perfect impedance match, the entire power supplied is dissipated on the antenna and the reflection coef is zero or null. In other words, eliminated.

Seems wiki agrees: http://en.wikipedia.org/wiki/Standing_wave_ratio
 
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I do not think you would have an SWR problem but a performance problem. ELTs antennas normally are shorter with a coil in series to allow operation at 121.5 and 243Mhz. Unlike regular COM antennas that uses a 1/4 wavelength whip.

José
 
Yes, frequencies below 121.5 would be a cutoff metric of the fixed antenna length. I can calculate the wavelength cutoff loss if I know the length of the antenna exactly.
 
docmirror said:
Yes, frequencies below 121.5 would be a cutoff metric of the fixed antenna length. I can calculate the wavelength cutoff loss if I know the length of the antenna exactly.
Click to expand...

Most ELT antennas have a coil in series to allow operation at 121.5 and 243 MHz. So they will be shorter than a 1/4 wave length monopole. So the tradional 234ft/freq.Mhz equation will not be accurate due to the coil effect.

José
 
Piloto said:
Most ELT antennas have a coil in series to allow operation at 121.5 and 243 MHz. So they will be shorter than a 1/4 wave length monopole. So the tradional 234ft/freq.Mhz equation will not be accurate due to the coil effect.

José
Click to expand...

Yes, I know. It's still possible to calculate the losses. once I have the length of the antenna. The coil will affect the resonance and impedance, but as it's a coil, it won't affect the wave propagation as much as you seem to think. The coil will have no effect on frequencies below 200Mhz. Also, the OP is looking for the value of 122.9Mhz, which is a lower wavelength than 121.5, so he should be ok, provided the SWR is low enough.
 
For transmitting, or just receiving?

If it's for receiving only, I wouldn't even waste time worrying about SWR. If it's high it might increase your feedline loss a decibel or two, but it won't affect your reception in any noticeable way.

-Rich
 
RJM62 said:
For transmitting, or just receiving?

If it's for receiving only, I wouldn't even waste time worrying about SWR. If it's high it might increase your feedline loss a decibel or two, but it won't affect your reception in any noticeable way.

-Rich
Click to expand...

Thanks Rich, exactly why I asked the question a few posts ago.
 
Building a quarter wave antenna for your hangar at the right resonant frequency would be trivial. Cut five pieces of wire at 22 7/8" Mount the one vertical to the center contact on the antenna connector, use the others for radial (attached to the shield).
 
EppyGA said:
Thanks Rich, exactly why I asked the question a few posts ago.
Click to expand...

I see that now.

I sort of miss this kind of thing, actually. The math, mainly. I never had any desire to get a ham license because I don't care for actually talking over the radio. I also have a CB in my car that I think I've turned on three times since I installed it.

But I've always enjoyed the tinkering part of it all.

-Rich
 
flyingron said:
Building a quarter wave antenna for your hangar at the right resonant frequency would be trivial. Cut five pieces of wire at 22 7/8" Mount the one vertical to the center contact on the antenna connector, use the others for radial (attached to the shield).
Click to expand...

That's exactly what I would do for a transceiver if I only needed one single frequency. Why compromise in that situation when you can build an exquisitely-tuned antenna for pocket change?

-Rich
 
I presumed just the other way, that the OP would be transmitting. Since SWR has no relevance or meaning for a reciever-only antenna.
 
SWR may not be meaningful, but that doesn't mean you can stick any hunk of wire on the antenna terminal and expect it to work reasonably.

On the other hand, 122.9 is busy enough around my field, that I don't need the best sensitivity. I'd settle for only hearing the guys in the pattern and not the announcement from the jump plane 30 miles away.
 
docmirror said:
Yes, I know. It's still possible to calculate the losses. once I have the length of the antenna. The coil will affect the resonance and impedance, but as it's a coil, it won't affect the wave propagation as much as you seem to think. The coil will have no effect on frequencies below 200Mhz. Also, the OP is looking for the value of 122.9Mhz, which is a lower wavelength than 121.5, so he should be ok, provided the SWR is low enough.
Click to expand...

I would say that the coil will have a reduced effect at frequencies below 200 MHz, not no effect. For example, the reactance of an inductor at 121.5 MHz will be half the reactance at 243 MHz. In any case, the inductor will reduce the bandwidth over which the SWR would be acceptable.
 
Yes I can, and I have. Give me a hunk of coat hanger, tape measure, the freq and a dab of solder and I'll get the sensitivity(Si) down around -90dBm or better. Impedance matching helps the S/N ratio on the recieve side, but it means very little when not transmitting.

It's not rocket surgery, just takes a little math and maybe a matching resistor.
 
pmanton said:
Has anyone bothered to check out the SWR of an ELT antenna used as a comm antenna? Are they broadband enough to be usable on 122.9 ?

I have an extra that would be a dandy hangar radio antenna IF the SWR wouldn't be out of sight.

I'm in the process of moving right now and don't have access to my Bird Thru-line to make up a test myself.

We're moving to a bigger house and hangar on the airpark. :)

Thanks

Paul
Salome, AZ
Click to expand...

Here's an inexpensive SWR meter:

http://www.mfjenterprises.com/Product.php?productid=MFJ-812B

The specs are a little squishy. For example, the description says it's for 144 - 220 MHz, but the front panel in the photo says 114 - 220 MHz, while the manual says it will function down to 21 MHz with reduced sensitivity.

If you know any ham radio operators, maybe one of them has an SWR meter that you could borrow.
 
I will use it to transmit on occasion. The ELT antenna appeals to me because I'm lazy and I could install it in minutes without having to make an antenna up from scratch.

If the SWR isn't so high that it shuts down my final it should work. I have a Bird thruline watt meter and could test it but then again I'm lazy and wondered if anyone had used one as a comm antenna. All my stuff is between two hangars and is a real mess right now. It might be 6 months before I find my watt meter again. :)

Paul
Salome, AZ
 
I would think the ELT antenna would be absolutely fine and w/ very little reflected. Heck, your TX time is relatively short burst anyway. Not a big deal and it's not like your going to smoke the PA!

You can verify reflected w/ your Bird (when you find it :) Make sure you have an adequate groudplane and you should be good to go! Frankly, if you can mount it on your hangar roof (metal?) that would be ideal.

If you have a piece of LMR-400 laying around that should suffice for feedline.

122.9 MHZ:

Half wave: 3.80 Feet (45.7 inches)
Quarter wave: 1.90 Feet (22.8 inches)
 
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As several have pointed out, for this receiving antenna application the ELT antenna will work fine. But if you want to invest maybe $10 for some coax (RG-58 will work and is cheaper than RG-8) and a piece of plastic pipe or a broom handle, try this: http://www.hamuniverse.com/vertbazooka.html

If happen to have a nonmetallic hangar, you could just tape the coax to an inside wall, the higher the better. No need for supporting it outside.
 
airdale said:
As several have pointed out, for this receiving antenna application the ELT antenna will work fine. But if you want to invest maybe $10 for some coax (RG-58 will work and is cheaper than RG-8) and a piece of plastic pipe or a broom handle, try this: http://www.hamuniverse.com/vertbazooka.html
Click to expand...
We call that a dipole. The only thing is you're using a few feet of the braid for one of the radiators.
 
The OP doesn't care. The point is that it is cheap and easy. I made my first one about the time I got my general class ticket in the seventh grade, having not much in the way of skills or money.
 
If he doesn't give a hoot about sensitivity or transmit ERP, the ELT antenna will work fine. Frankly, just touching the center terminal with his finger will probably give him enough to hear aircraft in the pattern.
 
flyingron said:
Building a quarter wave antenna for your hangar at the right resonant frequency would be trivial. Cut five pieces of wire at 22 7/8" Mount the one vertical to the center contact on the antenna connector, use the others for radial (attached to the shield).
Click to expand...

I made a 1/4 wave antenna in this fashion for 2meter ham and it worked great. Used an old vinyl record at the base to hold the radials in place. Its still hanging in my parents attic.

2m is FM voice and its pretty close to the frequencies of aircraft radios. I think it would work very well for a hangar radio
 
Report back: I found my watt meter in a milk crate in the new hangar. Cobbled together a test rig and at 122.9 I had 12.5 watts forward and 1 watt back. Close enough for government work.

I got me an antenna by crackey.

I'll mount the antenna on a small square of aluminum with enough RG-58 attached to reach my bench. I'll use the man cage on the fork lift and take a battery drill up and drill a hole in the roof large enough for the coax and connector. Feed the coax down through the hole and pop rivet the plate to the roof. Seal the whole works up with RTV and I'll be good to go.

I was using a "bazooka" antenna made by peeling the braid back on a piece of RG-58 hanging from a purlin in my old hanger. This was good enough to get planes close by in the pattern. My new rig should really work well.
(If I ever get move in that is)


Cheers:

Paul
www.Indianhillsairpark.com
Salome, AZ
 
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