Use a modified ELT Antenna for APRS?

Could we use a modified ELT antenna for an APRS antenna? An ELT antenna would be around 22", could it be trimmed down to 19"?

This is the type of antenna I have for my ELT that would look nice to install on the bottom of the plane for APRS purposes. It's pricey at $69. Aircraft Spruce has one for $45 that might work (11-17961). I don't know the internal construction of these to know if they could be "cut".

N523RV said:

Could we use a modified ELT antenna for an APRS antenna? An ELT antenna would be around 22", could it be trimmed down to 19"?...

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That should work just fine.

Why buy, when you can make.

Take a female bulkhead BNC and mount it where you want the antenna. Then take a crimp type male BNC and stick a solid wire (stainless would probably be better) in the center pin crimp end and crimp or solder it (silver solder would be preferred) and cut it to ((468 / frequency in mhz) /2) * 12 (468/freq is 1/2 wave in ft, divide by 2 is 1/4 wave and times 12 gives you inches). Then push the pin of the crimp BNC into the connector. Take some stripped off wire insulation (larger diameter) and slide it down the wire so that it goes inside the shield crimp cavity and cut it flush with the end of the crimp barrel so the center wire can't ground to the shield crimp. Then cover the connector in heat shrink (glued would be best).

If you want you can mold it into a "bent whip" (note it will change the resonance slightly).

You also could fabricate a mount that is angled with the female bulkhead BNC if you like.

And there ya go, a perfectly good antenna for APRS (btw, the aprs freq is 144.390 mhz)

APRS antenna tests

N523RV said:

Could we use a modified ELT antenna for an APRS antenna? An ELT antenna would be around 22", could it be trimmed down to 19"?

This is the type of antenna I have for my ELT that would look nice to install on the bottom of the plane for APRS purposes. It's pricey at $69. Aircraft Spruce has one for $45 that might work (11-17961). I don't know the internal construction of these to know if they could be "cut".

Click to expand...

Great timing, Matthew!

I finished flight testing this week the exact antenna shown in your photo. As a baseline and to address the post about building your own antenna, here are photos of the homebuilt 1/4 wave antenna I have been using with great success:







I used a PL-259 with 1/8" stainless welding rod potted into the connector and this along with the 3"x3" doubler carrying an SO-239 bulkhead connector has held up well in the slipstream and turbulent prop wash under the plane. I would be concerned about a BNC connector tolerating the brutal conditions on the outside of the plane. The whip has to be stiff enough not to deform under aero loads so this takes a pretty substantial element.

Now, for the little ELT antenna. The original antenna was pulled off my plane and the ELT unit went in its place. I trimmed 1.2" off the element to try to put it more or less near 144.390. After a couple of test flights, it was obvious the antenna was working, but not as nicely as the original 1/4 wave whip. Beacons near the ground were not getting to the network nearly as well as with the larger whip, and I was occasionally losing enroute beacons as well.



Having said that, the little ELT antenna looks very nice and does work well enough to be an option for someone who wants an unobtrusive antenna and can tolerate less than optimum performance. But I'm spoiled to having nearly every beacon hit the net so the original 1/4 wave job is back on my plane.

ELT antennae

Sam Buchanan said:

....
Now, for the little ELT antenna. The original antenna was pulled off my plane and the ELT unit went in its place. I trimmed 1.2" off the element to try to put it more or less near 144.390. After a couple of test flights, it was obvious the antenna was working, but not as nicely as the original 1/4 wave whip. ......

Click to expand...

Sam... don't the commercial ELT antenna have some sort of loading coils in them to make them resonate on 243.0 as well as 121.5 MHz?

Isn't there something inside that big rubber lump at the base?
Are they really straight whips?

az_gila said:

Sam... don't the commercial ELT antenna have some sort of loading coils in them to make them resonate on 243.0 as well as 121.5 MHz?

Isn't there something inside that big rubber lump at the base?
Are they really straight whips?

Click to expand...

Gil, I assume there is "something" in the lump. Trimming the antenna a little was purely a shot in the dark since I don't have an SWR meter handy. With careful tuning the ELT unit might work a little better than I observed. The little antenna was so neat looking I had to give it a try.

n5lp said:

That should work just fine.

Click to expand...

Oops, well maybe. Reading the original post I thought it meant that the ELT antenna was a full length 1/4 wave antenna. If not, that does complicate things but if tuned with an SWR meter or antenna analyzer there is still a good shot that it should work well.

n5lp said:

Oops, well maybe. Reading the original post I thought it meant that the ELT antenna was a full length 1/4 wave antenna. If not, that does complicate things but if tuned with an SWR meter or antenna analyzer there is still a good shot that it should work well.

Click to expand...

Note, 121.5 and 243.0 are exactly 2x of one another, so I suspect there is no loading coil in the ELT antenna, it most likely resonates at the 2nd multiple just fine. Now the 3 band ones do have a loading coil and you can usually see it about mid span in the wire of the antenna. The coil just acts as a way to electrically shorten the antenna for the other frequency(ies). However, if the antenna is 1/4 wave at 121 (should be able to check that by measuring it's length), then at 243, it would just be a 1/2 wave...

Sam Buchanan said:

Great timing, Matthew!

I finished flight testing this week the exact antenna shown in your photo. As a baseline and to address the post about building your own antenna, here are photos of the homebuilt 1/4 wave antenna I have been using with great success:

I used a PL-259 with 1/8" stainless welding rod potted into the connector and this along with the 3"x3" doubler carrying an SO-239 bulkhead connector has held up well in the slipstream and turbulent prop wash under the plane. I would be concerned about a BNC connector tolerating the brutal conditions on the outside of the plane. The whip has to be stiff enough not to deform under aero loads so this takes a pretty substantial element.

Now, for the little ELT antenna. The original antenna was pulled off my plane and the ELT unit went in its place. I trimmed 1.2" off the element to try to put it more or less near 144.390. After a couple of test flights, it was obvious the antenna was working, but not as nicely as the original 1/4 wave whip. Beacons near the ground were not getting to the network nearly as well as with the larger whip, and I was occasionally losing enroute beacons as well.

Having said that, the little ELT antenna looks very nice and does work well enough to be an option for someone who wants an unobtrusive antenna and can tolerate less than optimum performance. But I'm spoiled to having nearly every beacon hit the net so the original 1/4 wave job is back on my plane.

Click to expand...

Yep, you can use a PL/SO 259/239 just fine, however, they are not constant impedance so there is a 3db loss across that connector where as a BNC (or you could use a TNC) is constant impedance and does not have the loss. I'm sure you could pot the BNC or TNC and because you can use a much smaller wire, I suspect the loading could be minimized such that it would work just fine. But I haven't performed any experiments with the concept either.

That last option would be to try an SMA version with a bulkhead SMA and adjust the depth such that the base of the SMA antenna formed a mate to the fuselage, that way you'd get the strain relief of the connector. I"m thinking those commercial type SMA antennas for HT's and because they have a very flexible antenna, they are probably even less of a windloading issue... However, they may de-tune being bent in the slipstream as well. They would be a better candidate for under some fiberglass. But then we did start this thread talking about cost effective antennas.

More food for thot

Can I have (buy) your little ELT antenna since it is off the plane? I'll get an SWR and check it and put it on my plane.

Also, given you fantastic luck with your existing antenna, can you give us the length and where the bend is at? One cannot argue with something that appears to be performing so well, regardless of the engineering data.

Also, which MicroTrak are you using? Would the power output have any affect on which antenna was being used?

Sam Buchanan said:

Great timing, Matthew!
Having said that, the little ELT antenna looks very nice and does work well enough to be an option for someone who wants an unobtrusive antenna and can tolerate less than optimum performance. But I'm spoiled to having nearly every beacon hit the net so the original 1/4 wave job is back on my plane.

Click to expand...

Black Magic....

aadamson said:

Note, 121.5 and 243.0 are exactly 2x of one another, so I suspect there is no loading coil in the ELT antenna, it most likely resonates at the 2nd multiple just fine. Now the 3 band ones do have a loading coil and you can usually see it about mid span in the wire of the antenna. The coil just acts as a way to electrically shorten the antenna for the other frequency(ies). However, if the antenna is 1/4 wave at 121 (should be able to check that by measuring it's length), then at 243, it would just be a 1/2 wave...

Click to expand...

...comes into play here....

... but, being a digital injunear not an RF one... I always thought a 1/2 wave antenna should be center fed, not end fed, to keep impedances somewhat correct?

Is this not the case?

gil A - who had a center-fed 1/2 wave antenna in the vertical tail of my wooden sailplane...

aadamson said:

Note, 121.5 and 243.0 are exactly 2x of one another, so I suspect there is no loading coil in the ELT antenna, it most likely resonates at the 2nd multiple just fine. Now the 3 band ones do have a loading coil and you can usually see it about mid span in the wire of the antenna. The coil just acts as a way to electrically shorten the antenna for the other frequency(ies). However, if the antenna is 1/4 wave at 121 (should be able to check that by measuring it's length), then at 243, it would just be a 1/2 wave...

Click to expand...

The little ELT antenna is not 1/4 wavelength, it measured ~12". That is why I assume there is some sort of loading in the base.

Or.....is it 1/8 wavelength at 121.5??

N523RV said:

Can I have (buy) your little ELT antenna since it is off the plane? I'll get an SWR and check it and put it on my plane.

Click to expand...

Sounds good Matthew, send me a PM and we'll get it in your hands.

Also, given you fantastic luck with your existing antenna, can you give us the length and where the bend is at? One cannot argue with something that appears to be performing so well, regardless of the engineering data.

Also, which MicroTrak are you using? Would the power output have any affect on which antenna was being used?

Click to expand...

144.390 works out to 19.45" for the element length, at least for me. I bent the element so it would look right.

Refining the length due to bend, element diameter, connector used and all that other eng-i-neering stuff is beyond me.

I fly the MT-8000.