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twoton
12-02-2009, 03:20 PM
Looking for input on what folks have been using as a ground plane for their Com antennas. Comant suggests a 24" x 24" metal ground for the antenna. I see most folks using the top root fairing as a ground plane which much less. I just about to begin covering and if need be can add a ground plane to the upper baggage compartment of my PA 12 rebuild.

HydroCub
12-02-2009, 04:16 PM
Machine a spacer out of aluminum.... about 1" thick and shaped to match the base of your antenna to the top of the tank (slightly curved)cover. Mount it on your fuel tank cover and you will get max performance from your VHF Comm antenna. Mine would give me 40+ miles using a hand-held radio. You will need a 90 degree coax adapter to connect the cable to your antenna.

kevin
12-02-2009, 04:38 PM
If you use the dipole antennae, you will not need a ground plane. You can install inside the airframe. From what I have been reading, it works really good with reception reported as far as 50 miles out. This is the antennae that is 3 feet long by 1 inch wide and is very flexible. I called the factory on the best mounting plane. They suggest to epoxy it on a thin piece of wood. I can't remember the name of the antennae. I know that Spruce and Chief sell them. If I heard the name of it, I'd know but my mind is drawing a blank.

There was a member here who installed it in his vertical stabilizer. He posted pictures of the install and had the last 3 feet of his belly removable so he could get to it.

Hope this helps.

mike mcs repair
12-02-2009, 04:47 PM
wing fairing......

seen all sorts of weird attempts at other things....

biggest thing is to buff screw holes with scotchbrite so you bond the antenna ground to whatever amount of ground plane you got....or so I was taught by the old radio guy...

so a question for those who know, is the bonding for receiving or I think more so for transmitting????

sjohnson
12-02-2009, 06:52 PM
I have a dipole antenna mounted in my vertical stabilizer. Love it, works great. Eliminates all the problems with ground planes and cracked fairings.

The dipole antenna is made by Advanced Aircraft Electronics, at
http://www.advancedaircraft.com/

AAE was very helpful in answering my mounting questions. The top part slides between the fabric and the vertical stabilizer frame. The bottom is mounted on a piece of wood inside the fuselage tubing. Use high quality coax, as the length is pretty long.

sjohnson
12-02-2009, 07:02 PM
so a question for those who know, is the bonding for receiving or I think more so for transmitting????

Both. The math works the same both ways. Poor bonding may distort the antenna pattern and/or result in poor gain for both transmission and reception. You may notice it less for reception since the radio's gain control circuit will compensate for a weak signal, up to a point. You may miss really weak signals altogether, but not know what you're missing.

Darrel Starr
12-02-2009, 08:10 PM
Bob Schefter mounted our antenna on the right fuel tank cover -- works great.
Darrel
http://www.supercub.org/photopost/data//500/medium/8032416.JPG

Scooter7779h
12-02-2009, 08:48 PM
Mike Skup (Mike MCS Repair) did my -12 and put the com whip on the wing root fairing. I can get 80 mile transmit in open area (like Susitna Valley) at 200 agl. Not that I ever fly that low except for when I need to. With the big tank cover connected by screws to the fairing, that is a great ground plane.

Steve Pierce
12-02-2009, 09:02 PM
Here is a good thread with some good information on ground planes. http://www.shortwingpipers.org/phpbb3/viewtopic.php?f=4&t=1205

fobjob
12-02-2009, 11:05 PM
The problem with antenna placement is: too many variables.... put your antenna inside the structure, and you have too many variables to predict the outcome...mount it on top of the wing with a nice big ground plane under it, and you can predict the outcome....
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You can also find your bud, who just crashed, by flying in a circle, and hearing his ELT peak in the direction of the low wing, and null in the direction of the high wing... :-?

sjohnson
12-04-2009, 09:56 PM
... put your antenna inside the structure, and you have too many variables to predict the outcome
Yes, there are some performance tradeoffs when putting the antenna inside the fuselage - primarily blanking forward. The dipole works better than a monopole, because the dipole antenna is not referenced to fuselage. If you mount the antenna away from vertical members, it seems to perform pretty well. If the top arm of the antenna is inserted into the vertical stabilizer, it should see forward pretty well. It seems to work ok in my installation, YMMV.


...mount it on top of the wing with a nice big ground plane under it, and you can predict the outcome....
Well... maybe not. Often true for a large ground plane, like on a Cessna wing. But the Cub fuel tank covers aren't large, and the wing root fairings don't create a symmetrical ground plane and generally aren't bonded well to the fuel tank covers. Cub antennas generally work remarkably well given the crappy installation options, but it is far from certain.

StewartB
12-04-2009, 10:23 PM
I know doody about ground planes, but I can offer this. My 180 has the comm antenna mounted on top of the cabin and the King KY97A has worked perfectly, not unlike the Cessna radios before it. My PA-12 antenna is mounted on the wing root fairing. My wings are equipped with Dodge tanks and the fuselage has a full skylight if that makes any difference to anyone. The 12's Becker comm works perfectly and seems to have equal range for reception and transmission that my Cessna has. While I can detect some differences in sound between the two the range and performance are so close it's a dead heat. Take it for what it's worth.

SB

fobjob
12-04-2009, 11:01 PM
Sjohnson:
Quote myself:
You can also find your bud, who just crashed, by flying in a circle, and hearing his ELT peak in the direction of the low wing, and null in the direction of the high wing... unquote
I guess without tank covers to reflect off, you'll have to install an interferometer to locate your bud's ELT.... did you measure your VSWR?

Fortysix12
12-05-2009, 08:28 AM
Both my 12 and 18 have the wing root with the proper channel style reinforcing backing plate. I've talked to folks 65 miles away while in the air. I prefer 10 watt radios myself. I went to a lot of trouble to meet the ground plane for the new style ELT. That's one radio needs to work. Be sure to add the grounding wire strap to an etched and alodined aluminum surface. Be sure to add a grounding strap to the whole Assembly.

sjohnson
12-06-2009, 10:14 PM
...did you measure your VSWR?
My SWR varies from 1.2 to 1.7 across the band, measured at the radio. The SWR at the antenna is higher, since cable losses cause it to appear lower at the radio end of the cable. I calculate the peak SWR to be about 2.1 at the antenna.

fobjob
12-06-2009, 11:56 PM
You kinda made my point. You pretty much need a meter to expect satisfactory results. Proximity to structural members gives the standing waves too many places to sit down. Hope it works out...it would probably be good in a rollover....??.... :o

sjohnson
12-07-2009, 12:01 PM
Fob: I'm not sure I understand your point. You asked for the SWR measurement and I provided it. Of course I measured the SWR with a meter.

If you're trying to say that SWR does not provide a measurement of antenna pattern, you're right. It is quite possible to have a highly distorted antenna pattern with an SWR near 1, a directional antenna is deliberately designed to be so. SWR is at best an indirect measurement of impedance matching and is primarily of interest to insure that the voltage at the drivers does not exceed their breakdown voltage. Most comm radios have a max SWR limit (usually about 3:1), which is why we're required to measure the SWR at the radio during installation.

I haven't measured my antenna pattern, although I haven't noticed any operational problems. I've talked to an RCO about 80 miles distant. Measuring the pattern has been on of those things I've been meaning to get around to, but haven't done it.

I'm not saying that mounting the antenna on the wing is wrong - most Cubs have them there and work just fine. I'm just pointing out that an imbedded dipole is a viable option, for those who don't want the antenna on the wing for any number of reasons. Personally, I like the clean look, and a previous installation on the fuel tank cover cracked the cover (over thousands of hours - the plane was used as a trainer.)

fobjob
12-08-2009, 12:18 AM
Nah, all I was trying to say was that some areas have a higher probability of good results than others. If it works for you, then knock yourself out. What I would be curious about, would be if you could make one work well inside the vertical stab; it might help with the forward blanking....
The tank cover cracking implies that there may not have been a doubler installed...??....to me, the tank cover ground plane permits really convenient ELT location using the wing null method. I had an interferometer half built, and decided to toss it into the back of the work bench when I discovered how well it works. One tends to want to simplify as you get older... :-?

Northern flyer
09-03-2012, 03:56 AM
I'm having COM issues with my experimental cub. The top of my fuselage is mostly lexan, hence no ground plane, which provided limited and inconsistent operation with my SL40. I acquired AAE antenna and am having marginal success with installing it aft portion of the fuselage. It mostly vertical, with about 1/3 angled horizontally. My last resort is to place it in the vertical stab, but am concerned with the metal tubing causing too much interference. Did you experience any? What kind of range are you getting with the AAE?

NunavutPA-12
09-03-2012, 09:56 AM
If you're close to ANY structural airframe tubing and can connect a short ground lead to that tubing without compromising its strength (hose clamp?), then the entire airframe becomes the ground plane. You don't need a solid sheet of metal under the antenna. Take a look at so-called "ground-plane" vertical antennas - three or four short wires as a ground plane.

I'm coming from the experimental camp. Certified, I'm sure, has all kinds of restrictions.

I've been told that the performance of an antenna inside the fuselage will be compromised by the aluminum U-V blocker (silver coat).

NunavutPA-12
09-03-2012, 10:17 AM
Forgot to mention: mine is on the wing root fairing and works well in that location. That's where I'd put it if I had a mostly Lexan top.

mike mcs repair
09-03-2012, 10:20 AM
wing root fairing is where they go.....

kevin
09-03-2012, 12:46 PM
Check with Steve Johnson from ID here who has an AAE antennae in his vertical stabilizer. I think he has posted a photo of it in his album. I was going to use this antennae but decided not to.

Installing an antennae inside a covered airframe will get good reception.

Northern flyer
09-03-2012, 01:09 PM
Where can I find Steve Johnson photo album?


wing fairing......

seen all sorts of weird attempts at other things....

biggest thing is to buff screw holes with scotchbrite so you bond the antenna ground to whatever amount of ground plane you got....or so I was taught by the old radio guy...

so a question for those who know, is the bonding for receiving or I think more so for transmitting????


Check with Steve Johnson from ID here who has an AAE antennae in his vertical stabilizer. I think he has posted a photo of it in his album. I was going to use this antennae but decided not to.

Installing an antennae inside a covered airframe will get good reception.

Alaska_Rallyer
09-03-2012, 02:35 PM
Tank bay covers are a terrible place for antennas. How the hell do ya get wing covers on?!

sjohnson
09-03-2012, 05:54 PM
I don't have an actual picture of the internal AAE antenna installation, but here is a drawing. The upper part of the antenna fits between the tubing and the fabric; the lower part is supported by a 1x2, clamped to the frame with Adel clamps. This is in accordance with the AAE installation instructions. Note that the horizontal airframe members don't have much effect. You want to try to space the antenna away from vertical members.

Note that this is a dipole antenna; you aren't using the fuselage as a ground plane. Don't ground the antenna to the fuselage or scotchbrite the tubing.

sjohnson
09-03-2012, 06:04 PM
... a previous installation on the fuel tank cover cracked the cover (over thousands of hours - the plane was used as a trainer.)
If you're going to use an external antenna, put it on the wing root fairing. Antenna on the tank cover causes cracks around the hold-down screws at the edge of the cover, even with a doubler under the antenna.