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  #1  
Old 02-04-2017, 05:13 PM
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flightlogic flightlogic is offline
 
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Default 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.
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Old 02-04-2017, 06:02 PM
noelf noelf is offline
 
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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).
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  #3  
Old 02-04-2017, 06:35 PM
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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????
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  #4  
Old 02-04-2017, 06:54 PM
Flyingleap Flyingleap is offline
 
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Default 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
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  #5  
Old 02-04-2017, 08:27 PM
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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.
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  #6  
Old 02-04-2017, 08:46 PM
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N941WR N941WR is offline
 
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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.
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Last edited by N941WR : 02-04-2017 at 09:40 PM.
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  #7  
Old 02-04-2017, 09:32 PM
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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
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  #8  
Old 02-05-2017, 03:41 PM
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Default 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.
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  #9  
Old 02-05-2017, 03:53 PM
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Quote:
Originally Posted by flightlogic View Post
...
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.
Try it on the ground and in flight and see if there is a difference.
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  #10  
Old 02-05-2017, 04:36 PM
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Good idea.... I can leave the meter inline during the flight. Thanks Gil
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