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Com antenna locations

Freemasm

Freemasm

Well Known Member
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Boundary Conditions =
2 com radios ->
2 com antennas
Small belly area

Tried looking at Rocket pix that I’ve taken, forum pix, etc

For two antenna installs, it appears most are parallel, spaced as wide as physically possible (less than three feet). Most appear to be in the bays behind the spar. At least one appears to be in the bays forward of the spar.

Let me know where yours are and:
Any problems with re-radiation?
Other issues?
Any novel approaches?
Would you do it differently if you could?

Sorry and thx. Always get nervous when about to cut holes in fuse skin.
 
I'll attach aa PDFof placement on my F1. I tried for the greatest amount if separation on the belly. I haven't had any interference issues, but tried to stay out of unpaved fields since the aft one is closer to the ground.
 

Attachments

Some options:
- Mount the antennas off center and fore/aft to gain separation.
- Consider a wingtip mounted Comm 2 antenna.

As we know, the Archer wingtip comm antenna is a poor performer. I can say you can make your own antenna that works halfway decent. It will not be as good as a bent whip on the belly but will be adequate for Comm 2. I ran such an antenna for my single comm on my first build for a couple of years and was able to communicate as needed. The data point I can offer is flying side by side with another RV using the Archer wingtip comm antenna I had to relay ATC comms for him as ATC could not hear him.

The approach is to take the basic wingtip antenna design out of the AeroElectric bible, lengthen the arm that moves out from the wing rib as much as possible and get as much vertical drop (or rise) in that arm as the wingtip allows. As in mounting the base on the bottom of the outboard rib and angling the arm to the upper, outboard wingtip edge. Remember the first third of a quarter wavelength antenna is the high current area (the part that does most the work) and vertical polarization is preferred. The arm going fore and aft is shorten to compensate for this extra length. Find a ham radio guy to help you on this.

Carl
 
skirting_virga said:
As someone with an 8 who appears to have no good topside mounting options, I'm also interested in the answer.
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2 belly mounted comm antennas, or as Carl said in post #4 an antenna in the wingtip remembering that your comm antenna needs as much vertical component as possible.

I’m just guessing , but I don’t know how possible it might be to mount a comm antenna on the top of the vertical stab. It might need some additional structural support as part of that sort of installation.l, but that could be a good location.
 
rockyfatcat said:
2 belly mounted comm antennas, or as Carl said in post #4 an antenna in the wingtip remembering that your comm antenna needs as much vertical component as possible.

I’m just guessing , but I don’t know how possible it might be to mount a comm antenna on the top of the vertical stab. It might need some additional structural support as part of that sort of installation.l, but that could be a good location.
Click to expand...
Top of VS is not a good location - not enough ground plane @ 0° to 45° (90° - 135° below) the feed point — as a result the impedance (Z) will be too high (R or jX - I forget which) so the antenna will not radiate properly.

edit: expanding a bit -- the above statement applies to your typical COBHAM/COMMANT CI-12x bent whip, RAMI, DP antennas which are "monopole" antennas and designed to be bottom/top mounted with a very large ground plane (the skin) beneath.

You *could* fashion a 1/4λ dipole antenna and mount it to the top of the VS, the length would be ~22" above the VS top rib. The problem with this approach is achieving a match to the 50Ω feed line. The bottom portion of this dipole would be much "longer" electrically than 22" (still need a matching device C or RL/C). Add a gamma match at the feed point and turn the whole VS + top element into antenna/radiator and then play around with your VNA / SWR meter cutting/moving the gamma match to get to 50Ω and VSWR < 2:1 for the entire Air band.

Viewed from the side, the tail would resemble a Manta Ray :).
 
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bjdecker said:
Top of VS is not a good location - not enough ground plane @ 0° to 45° (90° - 135° below) the feed point — as a result the impedance (Z) will be too high (R or jX - I forget which) so the antenna will not radiate properly.
Click to expand...
You obviously know about this than I do. Does the wingtip have better mass for a ground plane? I know the vertical component is key.
 
rockyfatcat said:
You obviously know about this than I do. Does the wingtip have better mass for a ground plane? I know the vertical component is key.
Click to expand...
Not mass but area.

The wing tip rib is sufficient fore / aft of the feed point & radiator for a ground plane (counterpoise) at the frequencies we're worried about (118 - 137MHz), but the radiator (antenna) would be aligned with the horizon - thus making it more sensitive to horizontally polarized signals (VOR, GS) (receive) and transmitting creates a field/wave that's aligned horizontally.

As Carl alluded to, the radiator would need to be positioned close to the bottom (or top*) of the wing tip rib/skin interface (flange) and then traverse to the opposite point within the wing tip volume.

Now imagine a torus field (donut) around the radiator with the minimums (nulls) at the tip/end of the radiator and at the feed point. The fat portion represents the area of highest gain (the most energy emitted and received), the "funnel" shape is the lowest gain (nulls).

As the donut moves relative to the horizontal or vertical plane, the efficiency changes accordingly. Ergo - a vertically aligned "donut" (tip at the top, ground plane beneath) will be more efficient at receiving signals/waves moving vertically, and vice-versa.

*Top to bottom orientation would present a field optimized for reception/transmission toward and above the horizon. Bottom to Top moves the field from horizon and below. Understanding this, you could use one for in air ops and the other on the ground...
 
bjdecker said:
Top of VS is not a good location - not enough ground plane @ 0° to 45° (90° - 135° below) the feed point — as a result the impedance (Z) will be too high (R or jX - I forget which) so the antenna will not radiate properly.

edit: expanding a bit -- the above statement applies to your typical COBHAM/COMMANT CI-12x bent whip, RAMI, DP antennas which are "monopole" antennas and designed to be bottom/top mounted with a very large ground plane (the skin) beneath.

You *could* fashion a 1/4λ dipole antenna and mount it to the top of the VS, the length would be ~22" above the VS top rib. The problem with this approach is achieving a match to the 50Ω feed line. The bottom portion of this dipole would be much "longer" electrically than 22" (still need a matching device C or RL/C). Add a gamma match at the feed point and turn the whole VS + top element into antenna/radiator and then play around with your VNA / SWR meter cutting/moving the gamma match to get to 50Ω and VSWR < 2:1 for the entire Air band.

Viewed from the side, the tail would resemble a Manta Ray :).
Click to expand...
You might still end up with a crappy antenna. Just because you have a good 50 ohm match does not necessarily mean it is going to work well, regardless of what the VSWR meter indicates or the what the VNA shows.
 
Norman CYYJ said:
You might still end up with a crappy antenna. Just because you have a good 50 ohm match does not necessarily mean it is going to work well, regardless of what the VSWR meter indicates or the what the VNA shows.
Click to expand...
There is, of course, masking effects of the wing/fuselage between the station and receiver. What else?
 
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