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Ground ops w/underbelly antenna?

sbalmos

sbalmos

Well Known Member
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Planning out my wiring runs has me reading a boatload of old VAF threads. I know there are mentions of certified planes having a topside straight stick antenna, for better ground and clearance ops reception. But I slightly jumped the gun a bit and ordered two bent whip antennas, both for the underbelly.

In all practicality, aside from maybe having problems at a mega-large airport (what are the chances of flying in there? Doubtful), or problems with clearance out at a middle of nowhere airport before getting off the ground (two words - cell phone. Yet to fly in anywhere I didn't have at least /some/ signal)...

How bad would underbelly ground ops reception /really/ be at your typical airport, especially a Class D or even C towered one? I'm betting not absolutely optimal, but good enough.

... Unless I wanted to mount a bent whip on top, which would just look weird. :p
 
The belly whip antenna on my -6 has had impeccable transmit/receive performance over the last 14 years on both the ground and in the air. I can't think of a single time I couldn't hear or be heard. I've been to Indy, Cincy, Daytona, and several other relatively large airports in addition to all of the smaller fields I've visited.
 
That close to the tower you could put it in your suitcase and the tower would read you just fine! ;) Seriously, no worries. I've never even seen an RV with a top mounted comm antenna, although I'm sure there is one or two.
 
A belly mounted bent whip antenna generally works perfectly fine for airport ground ops. I wouldn't worry at all about it.
 
Even on the rare occasion that you do hit a shadow from the gear leg or some other structure, it doesn't take much to turn a few degrees and get the reception back. That's been my experience anyway.
 
I'm in my garage a mile from the airport, and unicom and people in the pattern hear me fine...Bent whip RAMI

Chris
 
Neal@F14 said:
A belly mounted bent whip antenna generally works perfectly fine for airport ground ops. I wouldn't worry at all about it.
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Not always... there are some older ramp areas at Tucson that must have a lot of grounded rebar in the concrete that screws up the GND communications.

See the note at the bottom of the FAA Airport Taxi Diagram.

http://155.178.201.160/d-tpp/1506/00430AD.PDF

I had problems on the West Ramp, which might have been a runway in the past.
 
The bent whip antenna on my taildragger belly is all I have. I have no issues with ATC transmissions and reception on the ground at all, anywhere.
 
If you bent whip is stainless, i.e. not covered with fiberglass, try taking as much of the bend out as you can. The straighter the antenna, the better it will perform. The com antenna is meant to be vertically polarized.
 
dutchroll said:
The bent whip antenna on my taildragger belly is all I have. I have no issues with ATC transmissions and reception on the ground at all, anywhere.
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Same here, I have communicated with a friend in flight from Cuba, MO to Troy, MO, some 83 statute miles. The bottom bent antenna is ok.

No problems on ground anywhere so far.
 
Yup, thanks all, not nearly the doom and gloom I was reading. I figured as such, dredging up really old knowledge from days I played in ham radio.

Mel, I'll have to look and play with some scrap first. The antennas are standard Comant CI-122's, so they could be bent. I'd be slightly worrisome about ground clearance then.

And Sam, yeah, us sliders have to worry about that pesky open canopy bubble. :D I not even sure I could mount GPS pucks on the top side of the back.
 
Ground ops and top v. bottom antenna

At the Newark International airport I saw a sign on the taxiway directing aircraft to switch to the top antenna for ground communications. I had never seen such a sign before.
 
CI 122's are what I have on my taildragger mounted just forward of the main spar. I have them almost straight and they still have sufficient ground clearance.

They've been working great for 22 years.
 
Mel said:
CI 122's are what I have on my taildragger mounted just forward of the main spar. I have them almost straight and they still have sufficient ground clearance.

They've been working great for 22 years.
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Even though my com antenna is mounted topside on the turtledeck, the APRS antenna is "straight" and mounted forward of the spar on the bottom of my RV-6. Visually it appears close to the ground but has never had any clearance problems.
 
That answers another q for me. Thanks

Mel said:
CI 122's are what I have on my taildragger mounted just forward of the main spar. I have them almost straight and they still have sufficient ground clearance.

They've been working great for 22 years.
Click to expand...

I have worried about interference from mounting on floor just forward of spar. In the front corners I have a transponder lollipop on one side and ADSB lollipop on the other(both about where fuel vents would have been). I have 2 bent whips to mount (1 on each side) and I'm glad to hear they work ok there. Also keeps from squeezing 2 Rg400 cables through spar along with my other wires. I am nearing the end on my -6A build.
 
Sam Buchanan said:
But you won't see them on slider RVs.
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Never say never, Sam. I have one on my slider 9A, and the reason I put it there is that I saw several one year at OSH that did as well. No interference with the slider at all.

To the OP: I did the same thing initially - bought two bent whips from Delta Pop. I called Don and he was very gracious allowing me to exchange one bent whip for a straight. Both work great.
 
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Use caution

Mel said:
If you bent whip is stainless, i.e. not covered with fiberglass, try taking as much of the bend out as you can. The straighter the antenna, the better it will perform. The com antenna is meant to be vertically polarized.
Click to expand...

The bent part 'sees' a little exta capacitance to the ground plane, and a well designed antenna will have an impedance matching network taking that into account. If you straighten it, you may see the SWR go up a bit. So it may be better, or maybe worse.

BTW, bent whips work as well as they do because most of the current is in the vertical part. All vertical is better, but not as much as you might think.
 
BobTurner said:
The bent part 'sees' a little exta capacitance to the ground plane, and a well designed antenna will have an impedance matching network taking that into account. If you straighten it, you may see the SWR go up a bit. So it may be better, or maybe worse.

BTW, bent whips work as well as they do because most of the current is in the vertical part. All vertical is better, but not as much as you might think.
Click to expand...

However, all of the above applies to an antenna in free space.

The real question here is "how does the antenna work when there is a true ground plane inches from it's tip."

Perhaps straightening the bent antenna may improve it slightly while flying, but may make it far worse when on the ground above a concrete ramp containing grounded rebar...
 
Belly antenna location

Most common mounting place for bent whips is just before or after the spar. Lately I have been installing them just before or after the Rear Spar to accommodate ATC grumbling at our airport. This location still has good ground clearance & improved broadcast coverage than the forward location. Also communication with planes flying above me has greatly improved.
 
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