Antenna Locations

[Archer39J]

Researching antenna locations and wanted to discuss options. I have dual nav/comm radios and still want VOR for a backup if nothing else. Can't use Archer VOR in the tips due to carbon fiber so I'm looking at putting whiskers under the belly between the skin and fuse, if that's even feasible. Transponder and comm 2 bent-whip on the belly, comm 1 (maybe bent-whip?) and GPS on a plate secured between the top stringers aft of my skylight. ELT mounted in space aft of baggage bulkhead. Read somewhere GS can just be a bare wire nestled up the edge of a windshield? Still looking into that though. That's the plan so far...

[Julian]

Your com antenna need a ground plane equal to the length of the antenna in diameter

[Archer39J]

Yeah been reading that some folks use wore mesh in areas for that. Still looking into it.

[Archer39J]

Looking at some VOR whisker drawings it really seems like you could fit one between the ventral stringers and structure. Has anyone tried this before?

[Archer39J]

Just an update of my antenna needs and locations. I'm at 14 nominally, though a few might be combined (Nav 1, 2, GS).

Comm 1
Comm 2
ELT Comm
Nav 1 (VOR/LOC)
Nav 2 (VOR/LOC)
Glideslope
Marker Beacon
GPS 1 (GNS430w)
GPS 2 (GPS-250 Dynon backup)
GPS 3 (Mini-X)
GPS 4 (Stratux)
Transponder
Adsb-In (Stratux, 1090)
Adsb-In (Stratux, 978)

I know some folks mount the ELT antenna internal aft of the cargo bulkhead but I’m going to go ahead and put it between the dorsal stringers along with comm 1 and gps 1. For a ground plane I was thinking some copper foil strips under the skin will do well, might be a pain to install though.. Comm 2 is a bent whip on the Ti tunnel which will work well as a ground plane per the manufacturer (Deltapop).

Currently my plan is to put a single VOR whisker between the skin and fuse aft of the belly pan and use this to feed Nav 1, 2, and glide slope with a splitter that came with my radios. I’ll be making my own out of some coax, super simple and with an antenna analyzer I can check my work. Originally I wanted some Bob Archer VOR antennas inside the wingtips but them being carbon fiber makes this option not viable, would have been nice to have separate antennas for each NAV since the splitter reduces reception by about 25%. I’m still looking at putting some Archers between the ribs of the horizontal stabilizer, but I’m debating if I’d ever need to access them there. If I go the 2 antenna route I’d probably do a separate glide slope as well rather that split one of the navs.

Marker beacon is a high power beam shot straight up 1000-1500ft so it doesn’t take much to receive, so I'll probably start by attaching it to a stringer, zero drag and minimal effort.

Transponder will be a stub on the boot cowl access panel.

I’m certainly flush with GPS, but since that’ll be my primary means of navigation I wanted more than one high-integrity signal coming into my EFIS in case there was ever a disruption from the 430, so that gets 2. I’ll have a GRT mini-x as a backup EFIS and it has its own WAAS antenna. The Stratux I made is my adsb-in (2 antennas currently), 4th ahrs, and final GPS (non-WAAS). My iPad will actually be GPS #5 but it’s not permanently installed.

[Bcone1381]

My sense is that Marker Beacons are becoming obsolete...at least in the transport category world they are.

Have you considered internal Archer VOR antennas with fiberglass wing tips?

[Archer39J]

My sense is that Marker Beacons are becoming obsolete...at least in the transport category world they are.

Have you considered internal Archer VOR antennas with fiberglass wing tips?

I agree the MB is on the way out, but my intercom has the connection so I figure for the penalty of 4 or so feet of coax I might as well hook it up.

As for the Archer VORs I mention that, given the premium I've already paid for CF wingtips and the hassle of trying to swap them out for glass ones (shipping mostly) I'll just stick with what I have and work around it.

[spinningwrench]

Comm antennas on the belly are not the best idea as you will have directional shadowing when on the ground and likely blocking reception from a tower depending on your orientation. Best to mount on the top of the fuselage with a real ground plane. Using wires or mesh for a ground round plane is not nearly as effective as a plate of metal. Just been through all of this on my Citabria.

[Archer39J]

Comm antennas on the belly are not the best idea as you will have directional shadowing when on the ground and likely blocking reception from a tower depending on your orientation. Best to mount on the top of the fuselage with a real ground plane. Using wires or mesh for a ground round plane is not nearly as effective as a plate of metal. Just been through all of this on my Citabria.

That's why comm 2 is on the bottom and comm 1 is on top, others have done this and reported good results. I also question the effect of shadowing in our rag-n-tube planes from a transmission source in the same airport environment. While I've heard this complaint before I suspect it is an exacerbation of an already marginally performing comm antenna.

There is no "real" ground plane on top of the fuselage, you'll have to fabricate something. Unless you've used the fuselage as your ground plane, again a recipe for marginal performance since we're trying to design good ground plane geometry for resonance between it and the antenna. Providing independent comm grounds will also protect against noise from stray currents. Properly designed wires, mesh, or foil strips are RF equivalent to solid sheet for 1/4 wave antennas at comm frequencies, as recommended in AEC and supported by my own testing. Higher frequency 1/4 wave antennas like transponders benefit from the "infinite" radials a solid sheet provides and need only be on the order of 5.5" in diameter, so boot cowl it is for me.

[spinningwrench]

Well I have a bit of experience with antennas and also what didn’t work on my Citabria but you seem to already have determined what you want to do so I wish you the best.

[Archer39J]

Anecdotal experiences aside what I'm planning is consistent with other Bearhawker's design, my own experiments measuring SWR of different configurations, decades of proven AEC design, and antenna physics.

[jim.mclaughlin924]

This is a dipole comm antenna I made for my S7 about 25 yrs ago. I plan to use it in my Patrol. It seemed to work really well and does not require a ground plane. It is easy to mount using adel clamps on the ground end to connect to fuse tubing. The whip end just goes through a grommet in the fabric. The whip is just cut from a stainless car antenna and the ground is an aluminium tube.

[jim.mclaughlin924]

forgot the attachment

[whee]

I recognize you are just looking for ideas and thoughts about your plans but I found that opinions about antenna placement are just that, opinions. I’d suggest following AECs recommendations if you are looking for maximum performance.

I’m sure you recognize that SWR is just an indicator of antenna match not a guarantee of performance. The antenna we built has an excellent SWR (1.1-1.2) but early indicators are showing it is going to be pretty marginal.


My antennas:

GPS antenna stuck on top of front carry though tube. Many other builders have don’t this with zero issues. Eventually I’ll have three up there.

Com antenna is made of foil tape stuck to the inside of the fabric behind the rear bulkhead. If it doesn’t work I’ll install at bent whip on the belly mounted on the tunnel.

ELT antenna is inside the tail.

Transponder is on the belly between the gear legs.

Nav antenna will be foil tape stuck to the inside of the belly fabric below the rear passengers.

Haven’t thought about ADSB antenna yet. I won’t be adding it till I get my instrument rating.

[Archer39J]

Right, you can have a perfectly matched antenna but if you put it in a metal box it defeats the purpose. This is why I'm suspicious of internally mounted comm antennas, belting out the watts inside a steel cage like that it seems reasonable to me it would interfere. I'd love to experiment with that, or a half-wave dipole (I think that's what Jim's was but I'm not sure), but I already have my retro Mooney comm and bent whip antenna so I'd rather not start yet another project.