measuring com power at the antenna

[n502pd]

Recently I had reason to check power from com radio at the antenna, which is a 'COMANT" brand comunications antenna. Since the tower reported me as weak, I was very concerned when i found my old NARCO com 11B was measuring only 1 watt, vs what it was suspoised to be at 8 watts. That is a long story, in itself, for another time. Question, for those of you who have a COMANT brand antenna, have you checked the SWR ? My swr appears quite high, at least from what I expected, at almost 3:1, and gets a bit worse as the frequency goes higher, and lower, to the band ends. There appears to be very little 'sweet spot' for lowest swr, meaning swr less than 1.5:1. These antennas are susposed to be broad banded enough to cover the entire aviation com band, and work reasonably well. My very low power did make communications difficult to say the least. ( Getting radios repaired!) but I was wondering if anyone had any input on the 'usability' of COMANT antennas, and what you are using instead. I looked into the old style whip, but they are only usable over a small area of the band, however they can be tuned for swr of nearly 1.1:1, very good , but very narrow bandwidth. Any info would be greatly appreciated!

[GraemeSmith]

I can tell yo uI had one of those COMANT's "go bad".

Nominal resistance check with a meter showed it was OK

But power loading it and it went infinite resistance and was not putting out any meaningful output.

It looked fine and undamaged - but a replacement was the answer.

[gammaxy]

Joe,

I wouldn't be concerned about a 3:1 SWR, especially since it sounds like you do have some "sweet spot" within the band. Remember that as long as your cable is reasonably lossless, much of that reflected power will actually make it out the antenna after being reflected back from the source end of the cable. It's unlikely you'll be able to tell the difference. https://www.arrl.org/files/file/Technol ... 106037.pdf

Did you perform the measurement both connected directly to the antenna and through the coax? I think typically you want to connect directly to the antenna, but going through the entire cable can also yield useful information if there's an issue with a connector or issue on the line. Did you use a high quality coax? I used RG-400.

Do you feel like your radio was still working but only to about 1/3 the range of before? In this case, I'd expect it probably got no complaints while you're on the ground or in the traffic pattern, but would have been weak out past 5 miles. This would be very consistent with only transmitting at 1 watt.

[BRS]

Joe,

I wouldn't be concerned about a 3:1 SWR,...

... I think typically you want to connect directly to the antenna,...

Not so sure I agree with the above. But differing opinions is what makes theses forums so interesting.

-The reason to check the SWR is to make sure your radio is happy AND to radiate as much of that signal as possible AND to reduce stray signals from causing issues with other equipment. Check the SWR at the radio end of the coax as that is the SWR your radio is going to see.

-Even with a 'lossless' (no such thing) coax, if the SWR is above a certain level, the (most) radio will dial back the Xmit power to protect the final transistors thus your radiated signal will be low. There are several simple tricks to improve the antenna match. Such as ferrite beads and coax choke.

-Where is your antenna mounted? If bottom mounted then the ground (earth below the wheels) can negatively affect your antenna match but in the air the results might be much different. Be sure the connection from the antenna base to the airframe is clean and solid. If top mounted, make sure your canopy is closed when taking readings. Nothing should be within ~3 feet of the antenna except airframe.

[n502pd]

Thanks very much for all rhe replies! In my own defence, I have been a ham operator since i was 12, or so, years old, and builder of receivers, triansmitters, antennas, and am very comfortable with testing most everything. The 'COMANT' products appear to have some sort of internal network to provide broadbanded-ness, usable on the aero band.Maybe I will cut this one open and see? at times, those items internally do expire and the antenna becomes unusable, IMHO. yes, 3:1 might be a reasonable swr, for tube radios, but, I agree, not so much for transistor radios. coax length, as a multiple of 1/2 wavelength can locate a sweet spot, as well as physical environment. most formuals use a consept called 'Free space', which does not concern itself with environment, to make the math easier and way nless complicated. the results are a close estimate of the real world environment, and allow one to tune the antenna system properly. swr is a primary indicator, and with field strength meter data, one can get proper results, as in max radiated power, even with a bit of elevated swr. this is just one part of our wonderfull 'experimental' endeavors!

Unfortunately, for me as a retired engineer in field operations (making things that used to work, work properly again), I often get embroiled is situations that are a bit too exasserbated to easily finish out to my personal limits. what i would want is a very low swr across the aviation band, high radiated energy, very clear audio and an uncomplicated physical installation. so far, i have found a radio that outputs rated power, 8 watts, into my dummy load, and is acceptable on all other aeras. Maybe changing the coax from RG 58c/u to the r400 type might help,or even to go to rg8/u foam, low loss type, but I think the real problem is/was the COMANT antenna. changing to a whip antenna is a distant possibility, but will most likely need some sort of network for broader bandwidth. The antenna is top mounted. Still a work in progress!

Again, thanks for all the replies!

[GordonTurner]

I hope my radio just plugs in and works. I have no idea what you guys are talking about :)

[gammaxy]

Good points everyone.

I agree that 3:1 is probably more concerning than what I wrote in my previous post. Interestingly, however, the COMANT antennas sold by Aircraft spruce list maximum SWR of 3:1 for bent and 2.5:1 for straight. So maybe in the ballpark of what you're seeing? https://www.aircraftspruce.com/catalog/ ... ant122.php Unfortunately, I never measured my antenna across the full band so don't have any numbers to compare (but now you're making me curious). I guess I don't understand what you mean by changing to a whip antenna. Isn't your antenna already a whip?

It's also interesting that it sounds like you do see less than 1.5:1. Is that roughly centered in the band? If so, it sounds like you might be close to their spec. My suggestion about measuring at the antenna was more about removing your coax from the equation incase you have an issue in the cable.

My comment about cable loss had to do with how much power is lost in the cable due to the reflections which is probably ~1dB for good cable for our short cable runs. I didn't consider whether your radio intentionally scales back power due to the 3:1 SWR or is otherwise damaged by it.

[n502pd]

Hi Chris! whip antenna is made out of stainless and has narrow bandwidth, but can be adjusted to nearly 1:1 swr at a single frequency... not ideal. the COMANT antenna and clones are fiberglass rod antennas and are inflexasble in slip stream. I have had both on Sirpeedee, and want the extra bandwidth, but not elevated swr and potential transmitter damage. mine do not pull back power. so I am back to the rod style, and experimentally determined a coax length that minimized swr over most of the aviation band, to 2:1 or so. this is ok with me, so far. I did mesure the power at the transmitter, and at the antenna end, and was almost identical...could not really see any differences that mattered.. next step is to retest battery capacity since it is 5+ years old, and I am at a 9000 ft runway now...long taxi at almost idle rpm, and no charging available at that rpm.

sorry for delayed responses. Thanks to all who have chimed in! Fly safe, and have a good turkey day and Merry Christmas!

[gammaxy]

Awesome. Sounds like you've got it sorted out.

Interesting that you were able to minimize SWR by adjusting coax length. This is surprising to me. How much of a difference did it make? I'd expect SWR to get a little worse as you shorten the cable due to less attenuation. If it was making a big difference I'm concerned you've got a different problem like RF radiating from the coax or an uncalibrated SWR meter. Meters are calibrated to the characteristic impedance of the line so even a few ohms calibration error will measure noticeable fictitious VSWR variation along the cable. Is varying cable length how you were able to get down from 3:1 to <2:1. If so, something doesn't seem right to me.

[gammaxy]

I got curious and bought a NanoVNA in order to measure the VSWR of my RAMI AV-10 antenna. I performed the measurement at the radio end of the cable.

RAMI_N256CM_VSWR_80.png (25.21 KiB) Viewed 3914 times

My measurements seem to match RAMI's claimed performance of 2.0:1 over the voice band. I also did some experiments with the canopy open and under the plane port. Both of these made the VSWR at the low end several tenths worse.

I imagine there's some tuning circuitry in the base in order to achieve the SWR over the whole band.