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Antenna Question

 
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tomcostanza



Joined: 19 Oct 2008
Posts: 49

PostPosted: Sun Mar 13, 2022 11:17 pm    Post subject: Antenna Question Reply with quote

Building an RV-7a with fiberglass wingtips. Had planned to use one Bob Archer VOR antenna (AeroElectric connection chapter 13) in each wingtip. For reasons I won't get into, I'm going to use a uAvionics tailBeaconX transponder (250 W). For those that don't know, it has an integral antenna.
It's meant to replace the tail light, but again for reasons I won't get into, I'm going to put it in one of the wingtips.

1) Would I be better to put the 2 Bob Archer antennas in the other wingtip?
2) Put the transponder and one Bob Archer antenna in the same wingtip?
3) Neither?

If #2, although the frequency of the xponder is 10x the freq of the VOR, the 2 antennas would only be inches apart. Seems like a bad idea to have a 250 watt transmitter that close to any antenna regardless of the frequency, but I defer to those who know. I'm sure the two Bobs have forgotten more than I'll ever know about antennas.

Thanks.


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wsimpso1



Joined: 04 Nov 2018
Posts: 33
Location: Saline MI

PostPosted: Tue Mar 15, 2022 3:06 am    Post subject: Re: Antenna Question Reply with quote

Two VHF antenna on one wingtip is generally bad practice.

RST Engineering (Jim Weir) talks about the de-tuning effects of having any piece of metal near antennas. Specifically, Jim Weir tells us that any piece of metal with a dimension bigger than 1/8 wavelength within 1/4 wavelength of the tips of antenna will mess with radiation patterns. The exception is the ground plane normal to and at the base of antenna arm or even the vertical stabilizer between the two legs of a NAV dipole. But put that metal anywhere else... Two VHF antenna and their cabling in the same space will probably give you dead spaces in some directions and poor range and reception in the directions where it works at all.

Much better to split the VHF antenna to the opposite wingtips...

Now the miniaturized antenna in the uAvionics Tailbeacon is an order of magnitude smaller than the VHF antenna, so you might be tempted to hang it in the wingtip with one VHF antenna, but the wiring is also a big piece of metal that will be too close to the tips of the VHF antenna. Then there is the matter of the VHF antenna messing up the Tailbeacon, leaving you non-compliant on ADS-B Out.

If you pair up some of these items that usually need to be apart from each other, you could have so much fuss over trying (and likely failing) to get them all to work nice that figuring out how to put the Tailbeacon where uAvionics intends for them might seem a real bargain.

Billski


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wsimpso1



Joined: 04 Nov 2018
Posts: 33
Location: Saline MI

PostPosted: Tue Mar 15, 2022 3:20 am    Post subject: Re: Antenna Question Reply with quote

Another temptation is to drive two VOR receivers off one VOR antenna. This might be acceptable. What happens is splitters divide the received power in half between the two output cables. (Again, from RST Engineering and Jim Weir) No receiver even needs to be in the airplane for this split to occur. That makes the range of each of the VOR receivers 70% what it would have been with a dedicated antenna to each receiver. In an airplane as fast as an RV, this is likely to deny you VOR reception until closer than you might like.

The counter argument is that many spam cans are running around with a single VOR antenna on the vertical fin with a splitter. Yeah, I have one of those, and the range on the VOR can be pretty poor. Sure would be nice to have that 100% instead of 70%... The up side is GPS in both the panel and the iPad may make the VOR range issue ... less important.

Billski


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merlewagner2



Joined: 01 Jun 2016
Posts: 18
Location: Spring Hill, FL

PostPosted: Wed Mar 16, 2022 4:59 am    Post subject: Re: Antenna Question Reply with quote

Billski, you can get the Best efficiency with two VOR receivers by using a receive multicoupler. Not typically seen in an aircraft as they are expensive. I just removed mine since I removed both my NAV receivers. Not at the hangar at the moment but will get the vendor info for you. Around $130 if memory is close.

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nuckolls.bob(at)aeroelect
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PostPosted: Wed Mar 16, 2022 7:33 am    Post subject: Antenna Question Reply with quote

At 07:59 AM 3/16/2022, you wrote:
Quote:
--> AeroElectric-List message posted by: "merlewagner2" <wagnermerle(at)gmail.com>

Billski, you can get the Best efficiency with two VOR receivers by using a receive multicoupler. Not typically seen in an aircraft as they are expensive. I just removed mine since I removed both my NAV receivers. Not at the hangar at the moment but will get the vendor info for you. Around $130 if memory is close.

you can get pre-owned couplers off eBay for cheap.

here's one example

https://tinyurl.com/y7p7spd5

these are passive devices that do not 'age' or
wear out. purchasing an 'old one' does not
pose any risk to performance.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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Peter(at)sportingaero.com
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PostPosted: Sun Mar 20, 2022 6:43 am    Post subject: Antenna Question Reply with quote

Only one Archer antenna is required, use antenna splitters.
I don't see a huge problem with a VOR/ILS antenna and a transponder antenna
close to each other, but would avoid if possible.
Because the frequencies are reasonably far apart and not on a harmonic there
should be little interaction. Txpdr duty cycle is very short so any
interference should be minor, but using different wing tips would minimise.
Peter

--


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