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-   -   GS Ant Issues (https://vansairforce.net/community/showthread.php?t=182005)

lr172 05-02-2020 09:31 AM

GS Ant Issues
 
In my 6, I used a home made dipole ant for GS (~18" long) mounted horizontally on the lower cowl and it works very well (composite prop). I did the same on the 10 and am having issues getting a stable GS signal. It is a bit jumpy and will occassionally move a full dot or more without aircraft vertical movement off the GS (it does eventually come back and it never flags and no issues picking it up at the intercept point). Both have 430W's and the only real difference is the Aluminum prop on the 10.

I have not yet swapped the 430's to eliminate a receiver issue, but will do so soon. I have also ordered a diplexer to try splitting the archer signal instead of the GS ant.

Have others seen issues mounting a GS ant behind an Al prop? I am wondering if the spinning chunk of metal is distorting the signal some how.

Larry

swordtail 05-02-2020 09:42 AM

I believe that your antenna should be mounted vertically for the best reception. The vhf transmissions are not horizontally polarized and you are not receiving the signal at full strength. That's why the antenna are up or down, not sideways. Ask a Ham radio operator about that!

lr172 05-02-2020 10:32 AM

Quote:

Originally Posted by swordtail (Post 1426949)
I believe that your antenna should be mounted vertically for the best reception. The vhf transmissions are not horizontally polarized and you are not receiving the signal at full strength. That's why the antenna are up or down, not sideways. Ask a Ham radio operator about that!

From my research, VOR & GS antennae are horizontally polarized, unlike VHF comm.

dpansier 05-02-2020 10:48 AM

Quote:

Originally Posted by lr172 (Post 1426960)
From my research, VOR & GS antennae are horizontally polarized, unlike VHF comm.

That is correct, the localizer, glide slope and marker beacons are horizontally polarized.

Carl Froehlich 05-02-2020 12:00 PM

Two thoughts:
- I suspect you have a massive engine ignitions noise issue feeding into your GS receiver. One way to check this is replace the 18? antenna with one cut for VOR frequencies (about 51? total length and fed in the middle). You can angle the ends forward along the sides of the cowl as the first third of the antenna does most of the work. When done, test for noise by listening to a VOR.
- A wingtip antenna and splitter is a preferred way to go. Ignore the ?but you loose half your signal? argument as in practice it makes little difference. Remember you are interested in signal to noise ratio, not simple signal strength.

I experimented with an under the cowl VOR antenna - ignitions noise made it useless.

Carl

RV7A Flyer 05-02-2020 02:18 PM

Quote:

Originally Posted by dpansier (Post 1426961)
That is correct, the localizer, glide slope and marker beacons are horizontally polarized.

I thought marker beacons were pointed straight up... :)

BobTurner 05-02-2020 04:33 PM

Quote:

Originally Posted by RV7A Flyer (Post 1427004)
I thought marker beacons were pointed straight up... :)

Horizontally polarized, parallel to the localizer direction...
Back to the OP: When I owned a 182 with windscreen GS antenna, the POH had a caution about interference due to the prop at certain RPMs. Next time try changing RPM, see if issue goes away. That said, I agree with Carl. Why go looking for trouble? A GS-LOC splitter off the Archer works fine. Shoot, my SL30 (still, IMHO, the best nav ever made) doesn?t even have a GS antenna input. It splits the LOC signal internally.

lr172 05-02-2020 09:32 PM

Thanks for the input. I don't think it is ignition, as I have the same EI on both planes. The prop made the most sense. I thought about RPM, but it was after the fact and haven't been up since to test that, but that could explain why the problem was somewhat intermittent.

I'll wait til the diplexer gets here and hope that works well. I have the archer in the wingtip. When I had an sl30, it worked well off the archer so hope the diplexer will solve it. I can't imagine that the comant unit, at .5 db, has much more insertion loss than the SL30 circuitry.

Larry

dpansier 05-03-2020 08:14 AM

This splitter is widely used in Commercial / Military applications and is less expensive and works great.

https://www.minicircuits.com/pdfs/ZFSC-2-11.pdf

lr172 05-25-2020 09:56 PM

Quote:

Originally Posted by lr172 (Post 1427101)
Thanks for the input. I don't think it is ignition, as I have the same EI on both planes. The prop made the most sense. I thought about RPM, but it was after the fact and haven't been up since to test that, but that could explain why the problem was somewhat intermittent.

I'll wait til the diplexer gets here and hope that works well. I have the archer in the wingtip. When I had an sl30, it worked well off the archer so hope the diplexer will solve it. I can't imagine that the comant unit, at .5 db, has much more insertion loss than the SL30 circuitry.

Larry

Installed the comant diplexer and the GS is now rock solid. Apparently the diy cowl ant only works with a composite or wood prop. Haven?t yet checked to see if it degraded the vor reception.


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