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Top antenna reinforcement

az_gila

Well Known Member
Guys,
Does anyone have a slick method for creating an antenna stiffener plate for a top mounted antenna - comm #1 and ELT in this case.

Unlike my -6A, the -10 fuselage has a central stringer down the center of the tail cone and any large hole that is centered would seem to remove most of this stringer.

If I offset the antennae a bit from the center line I think the slight curve in the top surface would make them look crooked....:)


Starting on equipment selection and initial conduit/wiring installation.
 
Because there is a slight curve where you are mounting it, if you make a stiffener it will flatten the area above the stiffener and it will show. Ask me how I know. What I ended up doing is creating a stiffener that is fastened to each stringer, I then made shims and installed washers between the aircraft skin and the stiffener so the shape of the outer skin is unaffected by the stiffener. You will never notice the slight leaning of the antenna. The antenna is extremely stiff doing it this way


38153121472_d32bc6fc79_c.jpg
[/url]IMG_1262 by Bill Peyton, on Flickr[/IMG]
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[/url]IMG_1263 by Bill Peyton, on Flickr[/IMG]
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[/url]IMG_1264 by Bill Peyton, on Flickr[/IMG]
 
Probably a dumb question from the electronically challenged, but what's wrong with putting it on the flat bottom? Maybe under the wing at the airfoil high point (or low point as it were)?
 
Probably a dumb question from the electronically challenged, but what's wrong with putting it on the flat bottom? Maybe under the wing at the airfoil high point (or low point as it were)?

I like one on top - comm 2 for ground use - and one on the bottom - comm 1 for airborne use. Sort of standard on certified planes.

My two lower VHF antennae on my -6A have already shown problems at one local towered airport. It was not TUS but check the note at the bottom of their taxiway PDF diagram -

http://155.178.201.160/d-tpp/1711/00430AD.PDF
 
Guys,
Does anyone have a slick method for creating an antenna stiffener plate for a top mounted antenna - comm #1 and ELT in this case.

Unlike my -6A, the -10 fuselage has a central stringer down the center of the tail cone and any large hole that is centered would seem to remove most of this stringer.

If I offset the antennae a bit from the center line I think the slight curve in the top surface would make them look crooked....:)


Starting on equipment selection and initial conduit/wiring installation.


Mine is off center as well. I couldn't quickly find any pictures of the doubler, but the top is still rather flat. Yes, the antenna isn't parallel with the vertical stabilizer, but nobody seems to notice.

FP29062013A0000Q.jpg
 
Another consideration

The location shown in the photos of mounting a top antenna on an RV-10 happens to be a prime piece of real estate for mounting ADHARS modules. I made a small box with aluminum sides and fiberglass pre-peg for the bottom, suspended from the two longerons on either side of the center line. The box came out light but very rigged - perfect for the ADHARS modules.

On my RV-10 the two comm antennas are on the belly and the ELT antenna is under the empennage fairing.

Carl
 
Thanks Bill/Bob,

Offset it is. :)

I like Bill's method, and if I put the VHF antenna mounting holes in the stringer and put the larger ELT hole just touching the stringer, the offset will be quite minimal.
 
The location shown in the photos of mounting a top antenna on an RV-10 happens to be a prime piece of real estate for mounting ADHARS modules. I made a small box with aluminum sides and fiberglass pre-peg for the bottom, suspended from the two longerons on either side of the center line. The box came out light but very rigged - perfect for the ADHARS modules.

On my RV-10 the two comm antennas are on the belly and the ELT antenna is under the empennage fairing.

Carl

My present plan (subject to change like most construction plans :) ) is to put my Dynon ADHRS one bay back from the antenna location shown by Bill P.

I think I will put two alum. angles across the fuselage fastened to the longerons. An offset down of a coupe of inches should keep the cross pieces below the path of the two rear seat belt cables. The center of the fuselage at that location should be clear of the battery, pitch AP motor and the elev. trim stuff magnetic effects, as well as far enough below the upper antenna.

See my earlier comment on not wanting two belly antennae.
 
SNIP
See my earlier comment on not wanting two belly antennae.

Gil,

Build as you wish - I offer however that there is little, if any measurable difference between a top and bottom mounted comm antenna on our airplanes while on the ground. If we are flying 747s then maybe I could see the logic, but not RVs.

Line of sight comms are not really line of sight.

Again, build as you like but top vs bottom comm antenna performace is not, in my opinion, a factor.

Carl
 
Gil,

Again, build as you like but top vs bottom comm antenna performace is not, in my opinion, a factor.

Carl

I have my #1 com on a belly antenna; #2 on a wingtip (I know, many report awful performance there. But mine seems okay - not as good as the belly, but generally useable). I have had 3, but only 3, occasions where the tower either heard nothing, or said ?weak and unreadable?, in response to the belly antenna. Twice on the ground, once in the air. In all three cases the tower reported the wingtip as loud and clear. And, in all three cases, they reported the belly antenna as loud and clear after turning 10 or 20 degrees. So I think there may be a small advantage to a top mounted antenna. But small enough that I?m not going to rush out to the plane and put one on.
 
I have my #1 com on a belly antenna; #2 on a wingtip (I know, many report awful performance there. But mine seems okay - not as good as the belly, but generally useable). I have had 3, but only 3, occasions where the tower either heard nothing, or said ‘weak and unreadable’, in response to the belly antenna. Twice on the ground, once in the air. In all three cases the tower reported the wingtip as loud and clear. And, in all three cases, they reported the belly antenna as loud and clear after turning 10 or 20 degrees. So I think there may be a small advantage to a top mounted antenna. But small enough that I’m not going to rush out to the plane and put one on.

Then it simply becomes atheistic since I have none of the reinforcing plates made yet. An upper antenna does not bother me.

However, ask any RF engineer what he/she would think of an inverted (or bent) 1/4 wave antenna at 120 MHZ with the tip of the antenna a few inches above a ground surface. :)
 
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As a point of reference the C172 I used to rent had 2 comm antennas on top. I never had an issue in the air or on the ground. On my RV10 I put COM 1 on top and a bent whip for COM 2 on the bottom. So far I have detected no difference. My audio panel permits split TX which requires the antennas to be separated or I probably would have put them both on top.

I mounted the top one offset from the center with a flat doubler. No way anyone would know (much less care) unless I told them.
 
Then it simply becomes atheistic since I have none of the reinforcing plates made yet. An upper antenna does not bather me.

However, ask any RF engineer what he/she would think of an inverted (or bent) 1/4 wave antenna at 120 MHZ with the tip of the antenna a few inches above a ground surface. :)

Gil, I was going to put one on top and one on bottom. My CFO vetoed that plan on appearance. So I have both on belly. Only place I have ever had a little issue was Prescott at the north end of 12, which is depressed below the runway. I've had no issues with range, including over the NW corner of the TX panhandle, which is usually difficult at lower altitudes. Even at Prescott, just switching to #2 with antenna on opposite side of belly took care of the issue. Both my antennas are under rear seat, near the front of the compartment, as far apart as possible laterally.
 
As a reminder

If you haven't gotten this far in your build yet,
I see no reason why this wouldn't work for Com 1. My Com 2 will be a belly-mount actually on a an inboard wing inspection plate. Obviously not yet flying - but in theory a quarter wave stuck to an RF-transparent radome should get out just fine. That's a quarter-wave of copper foil in the left foreground. Feedpoint will be grounded on the right longeron.



While we are at it, why not put the VOR/ILS Vee in a similar spot and feed it with a coax balun? :D No one will ever poke their eye out on this dipole, nor get tangled with it while cleaning the belly. It's a resonant half-wave dipole and extends a ways down the doorway B-column areas. 26" per leg.


I haven't yet trimmed the comm "whip" to resonance, needing an antenna analyzer to do so, and with the cosmetic paint in place I hope it won't need trimming (yeah, right). The antennas are completely invisible and zero-drag.


One thing I'd do differently: cut the antenna shorter than formula length and bring it to resonance with a little base-loading inductance once installed. As it it, if I'm long, I will have to shorten it with a razor knife through the paint "by feel," or with a series capacitor at the base. Finding a suitable silver-mica cap to electrically shorten this thing could be a bit of work, and I wish I'd thought to go too short to begin with :(
 
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