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GS Ant Issues

lr172

Well Known Member
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
 
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!
 
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.
 
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
 
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.
 
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
 
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.
 
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!
FWIW unless they are pilots, most amateur radio operators are lost in the aviation world and this situation is an excellent example of that. Also, glideslope frequencies are from 328.6Mhz to 335.4Mhz which basically places them in the low UHF band. ;)

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.
Great to hear. BTW, good call in using a diplexer instead of a splitter which are two different things.

A splitter "splits" the power in two so each side gets all the frequencies but at 1/2 the total power. A diplexer (not to be confused with a DUPLEXER) "divides" the frequencies so some frequencies go one way while other frequencies go the other way. The power is not divided between the two outputs as they are isolated from each other, and the output is frequency dependent. Basically each output gets almost 100% of the power at the correct frequencies (slightly less due to internal loss) but almost nothing at the other frequencies. You have to make sure to connect the correct output to the right equipment.

Since VOR/LOC frequencies (100Mhz range) and glideslope frequencies (300Mhz range) are so far apart, diplexers do an excellent job as you found out.

:cool:
 
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