Best SWR achieved?

[AJ85WA]

Hi Guys

I have a Delta pop, bent antenna for VHF mounted on the belly of my RV6 just before the spar.

I am interested in what the best SWR you have achieved in that location.

I was able to get the following SWR on a test rig at home (bench testing)

118.0 1.39
127.0 1.09
136.0 1.41

I am unable to get the same results on the belly of aircraft approx 1.8 SWR

I have been advised that belly mounted VHF is not acceptable and would always have issues.

*All my testing was done in open area away from any buildings/hangars.
*Ground plane is connected to antenna mounting screws and checked with ohm meter.
*Same coax used in both tests (Dummy load used to confirm coax is not at fault)

[n5lp]

I don't see any issue with SWR even at 1.8. I have my VHF com antenna on the belly and recall one airport at one location on the ground where I had a problem. That was from shielding by the wing and fuselage, not SWR. No problems ever in the air.

Larry Pardue N5LP

[Trevor778]

from the gnc 255 manual "The VSWR should be less than 2:1. A VSWR of 2:1 will cause a drop in output power of approximately 12%. " You're good.

[AJ85WA]

I have also thought that 1.8 would be ok, but for some reasons I am expierencing "talking in bathroom" effects on a number of frequencies. eg 127.6 and 118.1 all the others are pretty good

This is what my radio supplier has advised me asking the same question on SWR

SWR of 1.8 is not very good. It's just good enough to not blow up the transmitter.
Any reasonable installation should be able to get 1.4 or lower on any frequency.
What are you using as antenna ground plane ? In my experience practically all TX issues I have so far worked on myself (with V6 and V10 radios) are due to incorrect ground planes. These sometimes can lead to interesting resonance effects at certain frequencies ( you can hear this in your headset during TX - the side tone - in mild cases it sounds like you are talking in a bathroom, in very bad cases you get howling, squealing or distorted voice (sometimes only if you speak loudly - which means high carrier modulation).

[Walt]

Anything less than 2:1 is fine. Sounds like a radio problem to me, the vendor obviously prefers to blame the installation rather then his equipment, can't imagine that!

[RVDan]

The VSWR on the airplane is less than the bench for a bent whip because the antenna element is bent parallel to the ground plane. I straight (swept) antenna typically performs slightly better.

However, the "bathroom effect" is something else. Mic/headset/radio?

What transmitter you using?

[Bill Boyd]

You can only do so much about SWR with a fixed-tuned (non-adjustable) antenna and no tuner in-line. It's going to have a minimum SWR at resonance (which you should set to be at the middle of the comm band by trimming, if necessary) and higher SWR at the edges. The shape of that SWR curve is determined by the style and lossiness of the antenna (its "Q"), and the actual value of SWR at resonance is determined by the mismatch or ratio between the transmitter output/coax characteristic impedance (typically 50 ohms) and that of the antenna feed point.

A well-build and reasonably-shaped antenna of quarter-wave radiating element over a generous, conductive ground-plane will have half the impedance of a half-wave dipole in free space (72 ohms). If your bent-whip quarter-wave comm is 36 ohms at resonance (it should be, give-or-take a few ohms), you're not going to see 1:1 VSWR with a 50 ohm feed line. I'd have to look up the math, but 1.4:1 sounds very reasonable to me. Starting at 1.4:1 means that the SWR will necessarily be higher at the band edges than it would be if you were able to match to 50 ohms +0j (perfect) at the resonant frequency.

A manufacturer who says his radio will blow up above 1.8 SWR (rather than fold-back the output to protect the final) has no business marketing his product or giving customer support.

Bottom line, IMO, you will not see better than what you currently have without a matching device at the antenna feed point, and you shouldn't need such a thing anyway. (Unless you develop a lossy, corroded connection, which will add resistance and make the SWR look better - at the expense of true performance, which is a whole 'nother story.)

-Stormy

[AJ85WA]

Thanks for the input guys

Well this is my DIY testing rig made to confirm the antenna itself was able to achieve a good SWR (Trimmed it about 10mm to get SWR of 1.05 at 118.0 to 1.4 @ 127.0)



The radio is a MGL V6 and the supplier is working very closely with me to resolve the problem (Support has been excellent) but I didn't want to set myself an unachievble goal if SWR of 1.8 was acceptable, hence asking the question to you guys.

I am going to experiment with the antenna on the aircraft and see if I can resolve the problem.

BTW: I used cheap Coax on the rig not to waste money..... RG400 used in the aircraft.

[wirejock]

I haven't slept in a Holiday Inn recently, but why would the vendor have you cut and change from 1.05 to 1.4? Maybe I'm missing something.

[RVDan]

What is serving as a ground plane in your test rig?