Bent whip belly antenna reflected pwr?

Putting a question out there for those who might have tested the SWR of a bent whip on the belly of their RV. I am measuring one for a friend and get high reflected PWR. Almost 90%. I removed the antenna and found paint under it. Cleaned that all off and it is well grounded now. The SWR went down, but not by much. I tested a top mount antenna on same airframe and got just a slight reflected power reading. So, does a bottom mount read differently naturally, due to ground proximity? Or am I missing something? It is a good quality Comant and looks OK physically. Any observations appreciated.

Most, but not all, of the com gear in aviation is 50ohm stuff. 50ohm and 75ohm coax cables and connectors all look the same, but impedance wise, they are not.

Check the coax cable RG-xxx numbers and be sure 50 ohm cabling is used.

It is almost impossible to visually check the BNC connectors for 50ohm compliance.

If you checked for reflected power near the transmitter, disconnect the belly antenna and insert a 50ohm termination resistor in the coax at that point. Then retest for reflected power.

You can remove the belly antenna and temporarily mount it on a sheet of aluminum. The aluminum will now become the groundplane. Now, connect the antenna with known 50ohm coax and retest for reflected power.

One of the above test should point to the problem component(s).

My test gear has 50 ohm coax attached to it. I tested by connecting directly to the BNC on the antenna. Have you found that with a belly mount, when all other factors are correct..... that you can get a good SWR? In the range of 2.0 to 1 or less at band center????

Bent Whip Comant

Nick, Never had much luck with a bent whip for SWR. If it's on the belly
and a tail dragger the SWR was about 4.5 on a Bird, when working.
An RV friend asked for his to be checked and got the same as you for SWR, turned out he used crimp connectors and forgot to crimp the one at the antenna. Hope this helps....
John

The tip of the belly mounted antenna is probably only 6 inches or so above the natural ground surface. That would probably affect the measured SWR.

If it's on a taildragger, lift the tail and re-measure.

The Comant bent whip antenna you described grounds through the screws, not the base of the unit. IIRC, there is a rubber pad between the aluminum base and the aircraft structure.

Remove one screw at a time, give the opening one or two turns with your deburring tool to remove any light aluminum oxidation, put the screw back in with some dielectric grease and move on to the next screw until you have done that for all four.

I did that and my transmit and receive issues went away.

The challenge to a aircraft antenna designer is to achieve reasonable RF performance across the VHF 18 MHZ bandwidth, minimize the drag, and have a robustness that will standup to hurricane force winds.

The bent element VHF antenna is a compromised design, not capable of the VSWR?s of a straight element VHF antenna in broadband conditions. Bending the element back near parallel to the ground plane, the element proximity to the ground surface and metal objects in the near field, all add to poor VSWR?s results.

Automatic antenna tuners do offer the antenna designer options of optimizing antenna performance over a very wide band but they add complexity, cost and little realized enhancement to the communication process in line of sight operation.

With that said, the key to 1/4 wave aircraft antenna performance is good grounding, most if not all, VHF antennas ground thru the mounting screws. Many use star type washers under the nuts to insure good contact from the mounting screws to the aircraft fuselage.

Sealing the antenna base to the fuselage can keep moisture out and minimize the effects of corrosion. The EAA has a good video linked below describing methods to seal the antenna.
In addition to the info in the video, I suggest masking the antenna base and fuselage so the sealant forms a neat 1/8? wide bead when the tape is removed.
http://www.eaavideo.org/video.aspx?v=1136841468001

Thank you

Don, Thank you for taking time to write on this topic.... and to the others who replied.
I see such high reflected power, I am going to suggest that the owner purchase a new straight whip and mount it above the fuse.
He is on his second radio after first was damaged. I suspect heat may destroy the final amp transistors in time.
I am curious now though.... about my Icom and belly mt. on my 9A. Have never put the SWR meter in line to see what it reads. Will try that this week.
Thanks again for all input.

flightlogic said:

...
I am curious now though.... about my Icom and belly mt. on my 9A. Have never put the SWR meter in line to see what it reads. Will try that this week.
Thanks again for all input.

Click to expand...

Try it on the ground and in flight and see if there is a difference.

Good idea.... I can leave the meter inline during the flight. Thanks Gil

I have the same problem in my -4. I have a belly mounted bent whip antenna (RAMI AV-17) and have never had great performance. This past annual, I removed the antenna, replaced the star washers, scuffed the interior surface of the belly skin and reinstalled everything. After all that, I measured the SWR at 7-8.
The person I spoke with at RAMI thought I should be able to get a much better installation (2-3 SWR) given my aircraft and antenna, though they could provide no specific means to attain this. One thing that was suggested was to measure the SWR using a handheld radio and a different coax (and the installed antenna) to eliminate radio or wiring issues.

I have yet to try this, but it is on the list... Please keep us posted. I would love to improve my installation.

Dean
Bolton, MA

The VSWR on a VHF belly mounted antenna will change when going from ground to flight conditions, I have confirmed this thru testing.

The ground plane where I test antennas has the ability to vary the height over the ground surface, the antennas can be mounted on top or the bottom to simulate top or belly mount.
With a belly mounted antenna on the ground plane the VSWR will change as the ground plane distance to the ground surface is changed.

The freedoms of ?Experimental? offer us much latitude on the type and we select to mount our antennas.
Reflect back to the 60?s and 70?s when most of the Certificated aircraft were produced, Cessna and Piper would call in the RF engineers with their expensive test equipment and work with the structures guys to find suitable antenna positions.
Once the positions were determined, they were standardized and all the antennas were mounted in their place. Most if not all VHF antennas were mounted on the top of the aircraft with good reason.

A long winded explanation to say, if users are looking for optimal antenna performance, they will not see it with a belly mount, bent element antenna.

az_gila said:

Try it on the ground and in flight and see if there is a difference.

Click to expand...

I worried about belly performance on the ground. However, I found having a type certified aircraft with all of the com antennas originally on the top of the airframe, lined up in a row caused problems with ELT transmitter excitation that interfered with regular com radios. So I replaced one com antenna with a Comant CI-122 on the belly. Not only was the above problem mostly eliminated, but the belly antenna worked equally well on the ground and in the air. No problems calling ground or getting ATIS while taxiing.
So I installed two CI-122 antennas on the belly of my RV-10, and they both work great on the ground and in the air. Using GTN-650 and SL-30 for com radios. Never had any problem communicating with ATC. Have not attempted to find someone to check SWR. I used all RG-58 with crimped BNC connectors for nav and com equipment, RG-400 for GPS and transponder.

Good data. Thanks Kelly. Comant has robust construction as well.

This is a "YMMV" reply ...

I tried DeltaPop "bent" whip on the belly of my RV-8 and had less than stellar results. I have "enough" clearance that I run the DeltaPop "straight" whip (aka slightly angled) on the belly and do so with better results.