Belly Whip antenna mounting locations

Hey All, I am working on installing the doublers for my twin belly whips and am thinking about putting them in the exact same location(s) as this guy did on his -7 that I saw while down at SnF this year.



Comments?

Agreement?

Your thoughts?

I need to start planning for my ICOM A-210 (shameless plug for the ICOM A-210 group buy that will be ending on October 12, 2007!) and need input.

CJ

My question is how many builders mount their #2 comm antenna on the top fuselage for ground communications?

Good question...

That location (the top of the fuse) would be best for talking to ground from the ground.

None too many RV's have antennae there. The big name belly whips do most of that type of thing pretty good and are even better in the air, where we need them the most.

That is my thought path anyways.

CJ

...started working on it already.

Kinda like putting these:



Right here:



Howzit look? Whatcha think?

CJ

That location is fine, but I put mine just in front of the spar (on the -6). It is easier to access for maintenance since you don't need to remove the seat pans. No that this is an area that should require much attention.

Mine are just forward of the spar also. When you are talking to "ground" from the ground, you are usually very close.
99% of your communication is from the air where belly mount antennas work better. With a low wing, you get a better ground plane with belly mounted antennas.

Captain, consider putting them in trail front to back. The com antennae like to be in the middle of their ground plane (the belly skin). If they are to the side, there will be non symmetries in their performance from one side to the other (don't have a clue if it is significant or not, but why experiment?). Putting them all in a line in the middle might save some drag, but I don't know.

My avionics guy (many decades of experience) also said that there is a suggested separation between transponder antennae and the transponder unit (I believe the Garmin install manual concurred). Additionally, you want maximum separation between com antennae to minimize cross talk (hearing noise on one com when transmitting on another). I don't recall exactly what he said (7 years ago), but my setup is one bent whip about 6" forward of the main spar and one somewhere in the middle of the baggage area (6A), perhaps giving about a 48" fore/aft separation. The transponder antenna is somewhere in between. Note that there is a nice "circle" of aluminum around both com antennae to a radius of maybe 24". You can see them here:

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I put mine there...

John,

I put mine exactly where you're contemplating, using the same antennas. I had the local radio shop verify this was an ok location, considering interference, etc. The RV isn't a big airplane (surprise ) so the antenna locations are a slight compromise from their 'ideal' placement. But - being a realist, there are lots and lots of RV's out there flying with this exact arrangement having no problems talking to ground, working with the GPS antenna, interfering with the transponder, etc.

Captain_John said:

...started working on it already.

Kinda like putting these:

Right here:

Howzit look? Whatcha think?

CJ

Click to expand...

Well, I would bend up a couple of braces that would take the load back to the rib. A couple or right triangles with the adjacent sides bent to make flanges, rivet to your antenna plate, and the rib, fore and aft.

Gents, you all have some very eloquently stated points!

Thank you all for your input. I like everything everyone has to say.

I am glad that I am at least tracking correctly. These signal propagation characteristics are works of the Devil!

Thanks again!

CJ

I don't see much need for any more doublers - you wouldn't find them on a Cessna and they seem to last 50 years or so !

The doubler that came with our Miller aerial is all I am going to use - 2 belly mount whips, Txp up front by the firewall, GPS under the cowling.

Don't forget that you need to mount them a set distance from the transponder antenna.

I bolted mine to the spar and just used a doubler on the front two bolts.


This picture was taken facing aft, in front of the main spar. You "A" builders out there should note the lack of brake lines and gear tower.

Hey John,
You may want to make 90 degree bends on the outbtrd edges. It will stiffen it some, and keep it as close as possible to a flange i.e. rib, spar, stringer etc.

which size of doubler did you use to install the antenna? .062 or less

Same as most with the supplied doubler used, Van's has the best price BTW on the RAMI AV17

[/url]RAMI antennas installed in outermost bays behind control column opening by Dave Hock, on Flickr[/IMG]

AV17 by Dave Hock, on Flickr

in front of spar or behind, your choice

I see people putting the com antenna on the belly either in front of the spar or behind the spar.
I chose to put mine in front of the spar because I wanted a little bit to peak out in front of the wing; I was hoping this would improve ground communications by having the tower's antenna directly "see" a portion of the antenna.

Not sure it matters, that is just what i did. YMMV