Belly Comm Antenna Placement Tips

[AndyWW]

I'm trying to decide where to place my RV-7 belly-mounted comm antenna. (It's a DeltaPop bent whip antenna if that makes any difference.) My current thinking is to place it with a doubler out of the exhaust (and oil mist!) flow, so off to one side (passenger), and either just aft or just forward of the main spar carry-through.

Arguments in favour of Aft placement:
1) Cable will be out of the way of feet and carpet forward of the spar.

Arguments against Aft placement:
1) Radiation will be directly under the passenger?
2) Aft belly skin is thinner than forward belly skin,
3) Cable will need to go through the spar carry-through area,
4) Cable could interfere with aileron push-rod.

Arguments in favour of Forward placement:
1) Forward belly skin is thicker than aft belly skin,
2) Less radiation for passenger?
3) Easier cable layout,
4) Cable won't interfere with aileron push-rod.

Arguments against Forward placement:
1) Cable could get damaged by feet?
2) Cable will interfere with carpet and require a hole to be made?

Obviously if the forward placement could have the cable emerge into the area forward of the spar but aft of the F-782A-R cover plate then it would be an easy choice, but my feeling is that probably isn't possible - can anyone comment on this?

Any pointers or thoughts gratefully received! Thank you.

 

[wirejock]

Anyenna placement

Maybe SteinAir drawings will help.
My COM is between the seats in the tunnel.

 

[terrye]

I placed mine (two comm antennas) aft of the main spar carrythrough about 1/3 of the way to the rear spar carrythrough. I made a doubler for the belly skin (0.063") and added aluminum angle (1" x 1" x 0.063) reinforcements on each side that were riveted through the doubler and riveted to the seat ribs. I wanted the "weak link" to be the wire whip antenna rather than the belly skin. If you have made your coax connections properly and with RG-400 there is no issue with "radiation". In most Cessna's the comm antennas are above the wing and in the case of the two seat models within about a foot of the occupant's head.
Yes, cable will have to go through the spar carrythrough. See Vans downloads for a diagram of the size and location of holes through the web. The cables will need to be properly secured to the seat ribs to prevent potential interference with the controls. Again, no problem.

 

[wirejock]

Antenna doubler

Vans sells an antenna doubler for the ADSB & Xponder.
The COM came with a doubler.
Definately need something to protect the skin.
I put the Xponder and ADSB antennas in the corners forward of the spar but far enough to clear the towers and covers. Also enough to allow the doublers to be riveted.
I used 90 degree BNC connectors instead of a straight BNC with a 90 adapter. One less connection. Expensive but worth it.
I was able to get all my wires through the existing spar snap bushing holes. I didn't want to drill new ones.

 

[Mel]

Both com antenna on my RV-6 "just" in front of the main spar carry-thru.

One would have to be "trying" to step on the antenna because it is so close to the spar. Flying since 1993 and no problems what so ever.

Radiation not a factor in the cockpit as people are on the opposite side of the ground plane.

 

[PilotjohnS]

Comm Antenna

I placed mine just in front of the front spare underneath the landing gear supports.

This location minimizes the shielding from the wings on the ground since they are even with the leading edge of the wing; if I am pointed down the runway at the tower, the antenna has a clear view to the tower's antennas. If they were further back, the wings and forward fuselage could shield the antenna, possible making for poor reception on the ground.

I also put them as far outboard as possible to maximize the spacing between the two for better isolation of the two radios.

Lastly this position does not require pushing the coax through the spar; my spar through holes are already full of other harnesses.

After 50 hours, seems to work well.

 

[CF86301]

Doubler part number F-00020

Vans sells an antenna doubler for the ADSB & Xponder.

The doubler that comes in the RV-14(A) Fuse Kit is Van's P/N F-00020. It works with the Delta Pop mounting arrangement.

One thing to take into account is you may need to be able to reach the connector after you finish the airplane, so make sure you put it someplace you can reach with your hand without drilling out rivets.

The RV-14(A) has a transponder antenna that's located aft of the main spar carrythrough and offset to one side, but you can reach the connector through a baggage floor rib lightening hole if you remove one tunnel cover, which is attached with screws. Hmmm, need to think about the location a bit more, because I'm installing a YIO-390-EXP119, which has two exhaust pipes.

 

[mburch]

I placed mine just in front of the front spare underneath the landing gear supports.

This location minimizes the shielding from the wings on the ground since they are even with the leading edge of the wing; if I am pointed down the runway at the tower, the antenna has a clear view to the tower's antennas. If they were further back, the wings and forward fuselage could shield the antenna, possible making for poor reception on the ground.

I also put them as far outboard as possible to maximize the spacing between the two for better isolation of the two radios.

Lastly this position does not require pushing the coax through the spar; my spar through holes are already full of other harnesses.

This is a great idea for an A-model, for all the reasons stated. I think it would be a little trickier on a taildragger, because there is a brackety thing in the way (F-7114) which would complicate mounting an antenna there. You could definitely place it nearby and out of the way of F-7114, but I'd want to do something to avoid having the coax and connector mashed by passengers' feet, since there's no protective weldment to prevent that.

 

[sahrens]

Under the seats

On my 7 the antennas are aft of where the aileron push tubes enter the fuselage. One on each side, with doublers on the skin.

 

[Biggiesauls]

Antennas

On my 7 the antennas are aft of where the aileron push tubes enter the fuselage. One on each side, with doublers on the skin.

On my 9A my 2 comm antennas are in the same place, just aft of aileron push tube, doublers inside skin. Transponder and ADSB are aft of baggage bulkhead.

 

[RV7A Flyer]

On my 9A my 2 comm antennas are in the same place, just aft of aileron push tube, doublers inside skin. Transponder and ADSB are aft of baggage bulkhead.

Ditto on my 7A. Works peachy, although I placed them a bit far back so it makes removing the coax a bit of a pain (reach under the seat pan), but I've had not need to do that since it was painted (9 years now).

 

[Thermos]

Arguments in favour of Forward placement:
1) Forward belly skin is thicker than aft belly skin,
2) Less radiation for passenger?
3) Easier cable layout,
4) Cable won't interfere with aileron push-rod.

Your aluminum fuselage is an excellent shield, so I wouldn't be concerned about RF exposure from any antenna under the passenger compartment. Out of interest I ran an RF exposure model for a 10 watt comm radio driving a dipole-like antenna and the min safe distance for direct, uncontrolled exposure (no shielding) was 9 inches. I wouldn't worry.

HTH

Dave

 

[AndyWW]

Thanks for all the replies

This is a great idea for an A-model, for all the reasons stated. I think it would be a little trickier on a taildragger, because there is a brackety thing in the way (F-7114) which would complicate mounting an antenna there. You could definitely place it nearby and out of the way of F-7114, but I'd want to do something to avoid having the coax and connector mashed by passengers' feet, since there's no protective weldment to prevent that.

This is something I hadn't thought of since my wings aren't fitted and the F-7114 isn't there yet! So thank you very much Matt.

Your aluminum fuselage is an excellent shield, so I wouldn't be concerned about RF exposure from any antenna under the passenger compartment. Out of interest I ran an RF exposure model for a 10 watt comm radio driving a comm antenna and the min safe distance for direct, uncontrolled exposure (no shielding) was 9 inches. I wouldn't worry.

This is very reassuring - thank you Dave. Data always trumps opinion!

I'm leaning towards the aft placement for the above reasons. I have the Vans F-00020 doubler but it is very thin so I will probably make a bigger one to go with it and spread the load to the seat ribs via some angles too. I'm not too worried about the wing shielding the antenna when I'm on the ground because my other comm antenna is on top of the aft fuselage - the only orientation where I could have a problem is if the tower was directly behind and the top antenna was shielded by the VS.

 

[rapid_ascent]

My 2 comm antennas are just aft of the spar. I used 90 degree BNC connectors (Amphenol 112725) to keep the coax low profile. I added a hole in the spar per Vans location directions. Place the hole on the lower end of the allowed area to keep the cable low. Then I made a little service loop routing the coax first aft and then forward through the spar. I also added a cable tie mount to ensure the coax couldn't get in the way of the push tube. I played around with the exact arrangement a little bit since initially I didn't like how I had the coax routed, but this approach looks pretty clean.

 

[BillL]

My -7 got the two antenna mounted just aft of the spar. If done again it would be one station back and connect the doubler to a stringer and rib, it will be much easier to torque the spar bolts on annual and not cross the skin overlap. And . . there is room to use nut plates so it can be removed solo from the bottom. (think painter)

2 cents . . .

 

[RV7A Flyer]

Then I made a little service loop routing the coax first aft and then forward through the spar..

Excellent idea. Just watch that you maintain at least minimum bend radius for the coax (RG-400 is 1" radius).

 

[MikeinPhoenix]

Distance between dual com antennas?

On my 9A my 2 comm antennas are in the same place, just aft of aileron push tube, doublers inside skin. Transponder and ADSB are aft of baggage bulkhead.

I’m also looking to locate my dual comm antennas in the same location (just aft of the aileron push tube). On the 9A, it measures about 28 inches between the 2 antennas (not the 36 inches Garmin recommends). Just wanted to confirm others have similar spacing without any negative results.

Many thanks!

 

[Mel]

I’m also looking to locate my dual comm antennas in the same location (just aft of the aileron push tube). On the 9A, it measures about 28 inches between the 2 antennas (not the 36 inches Garmin recommends). Just wanted to confirm others have similar spacing without any negative results.
Many thanks!

A general rule of thumb is that no other antenna should be within the length of the antenna.
i.e. If your antenna is 21" long, no other antenna should be closer than 21" from it.