NanoVNA V2 Plus4 Measurements of A Wellbrook Loop - Is this Correct?

Merovingian

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I finally got one of those NanoVNA's people have been talking about. I have never used one before so I have been playing with mine for the last hour or so, just seeing what I am getting as far as SWR and Impedance goes on my three SDR/scanner antennas. As I expected, my Diamond discone and Siro UHF discone do better on some frequency ranges and a little worse on others. The surprise came when I hooked up my Wellbrook loop antenna and did a sweep from 0.1 MHz to 30 MHz. The SWR and Impedance was bad to terrible on all frequencies and some parts it was horrendous (see the attached screenshot).

I have been using my loop for about a year now and in my limited "year" experience with receiving SW with the loop I thought it had been doing well. I am not an expert with all of this but I consider an SWR of 2.0 or less to be "good" and 1.5 or less to be "very good". Anything above 2.0 I would think the antenna was stepping into "bad matching" territory, maybe I'm being overly conservative, I'm not sure. . . When the Loop's best SWR measurement of 3.54:1 at 22 MHz and an impedance of 14 ohms and its worst SWR of 11:1 at 18.47 MHz and impedance of 710 ohms, being fed through a 50 ohm cable makes me wonder how it can receive anything at all. I wasn't expecting the range of frequencies the loop can receive to be perfect or anything nor to have an SWR below 2.0 for everything but 11:1? and an impedance well below 50 and well above 50?

So, is this being measured correctly? The loop seems to work so maybe all of this isn't very critical at these low frequencies? I'm also guessing this is how all loops like this are?

Thanks
 

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Billy
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I seriously doubt the loop can be measured this way. What you are seeing in the chart is the various half-wave multiples of the coaxial cable. This is an "active" antenna and there is a lot going on between the coax and the antenna itself. Assuming a 66% velocity factor it looks like the coax is about 100' long. This could be skewed by whatever is going on at the feed point. I'm going to assume that the very tiny signal that an analyzer produces won't be harmful to the active circuits within the antenna's feed system.
 

Merovingian

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I seriously doubt the loop can be measured this way. What you are seeing in the chart is the various half-wave multiples of the coaxial cable. This is an "active" antenna and there is a lot going on between the coax and the antenna itself. Assuming a 66% velocity factor it looks like the coax is about 100' long. This could be skewed by whatever is going on at the feed point. I'm going to assume that the very tiny signal that an analyzer produces won't be harmful to the active circuits within the antenna's feed system.
I see. . . I think the coax is only about 30-40' long or so. That makes some sense I guess. I learned something new. . .
 

Merovingian

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Was the loop powered? You can only look at the VSWR of the bias T and the output of its preamp unless you break into the box and measure just the loop before the preamp.

Yeah, the loop is powered. That may be why the signal looked so funky.
 

JerryX

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SWR of receive-only antennas like the Wellbrook are essentially meaningless. Even if the SWR is relatively high, that's not going to be an issue because modern receivers have so much gain at HF frequencies as to not make a difference. As long as the noise level with the antenna connected is around 6 dB higher than the noise level with the antenna disconnected, you're not going to see any improvement by lowering the SWR. It's SNR, not absolute signal strength, that matters in receive systems.
 

Merovingian

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SWR of receive-only antennas like the Wellbrook are essentially meaningless. Even if the SWR is relatively high, that's not going to be an issue because modern receivers have so much gain at HF frequencies as to not make a difference. As long as the noise level with the antenna connected is around 6 dB higher than the noise level with the antenna disconnected, you're not going to see any improvement by lowering the SWR. It's SNR, not absolute signal strength, that matters in receive systems.
I guess I was thinking the circuitry did the "translating" from the loop to the coax similar to a transformer and regulator turns AC into smooth DC. The circuitry would take care of the translating of the magnetic part of the RF and be transparent to the VNA. . . . I guess even when I think I have a few things figured out I don't really have them figured out. . .

Thanks for the info.
 

wowologist

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But just think now you have a most important item to then make your own antennas and hang them and check them against each other and what your trying to recieve..I have 15+ antennae around my shack, not one is commercial O, I take that back I did hang a chicomm mini whip.....suxx. Next up I'd make yourself a 16 port antenna switch or even better a remote one powered and driven by an arduino! Now your cookin'!
 

Merovingian

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But just think now you have a most important item to then make your own antennas and hang them and check them against each other and what your trying to recieve..I have 15+ antennae around my shack, not one is commercial O, I take that back I did hang a chicomm mini whip.....suxx. Next up I'd make yourself a 16 port antenna switch or even better a remote one powered and driven by an arduino! Now your cookin'!
I have been waiting on the NanoVNA V2 to come out since last year, when it finally came out earlier this year I heard that they were working on a better one that goes up to 4 GHz and has greater dynamic range, not to mention a 4" screen. I had to wait another 3-4 months for that so now I can at least do some rudimentary testing, finally. I have to play with it and learn more about what I'm doing so I know I'm doing it correctly. I've been watching some YouTube videos in the last couple months.

I do have one major antenna project I want to do, to build a forward scatter meteor detection system using VHF TV channels 2-6. I am hoping to build a simple Yagi for that maybe a log periodic, I'm not sure yet. I was also thinking to build an air band antenna as well. At least I can now do a little testing. . .

Also in my building queue, Prcguy wrote a paper on receiving Satcom signals with a home made antenna. I would like to build that antenna one of these days as well, the NanoVNA should be able to help test that as well.
 

wowologist

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Dont be shy of that little box! It's pretty accurate and seriously versatile! And don't forget you can design and real-time adjust filters, TDR, distance to short your coax etc etc, I have the 2.8 in the white shell and the 4.3 - the 2.8 I generally only use when its attached to my laptop running the nanosaver software, can't really see that little screen!
 

dlwtrunked

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Yeah, the loop is powered. That may be why the signal looked so funky.
You definitely cannot measure the Wellbrook this way. The pre-amp is at the base of the loop and potted making it not possible to get to the loop. The separate box is a bias -T to provide power up the coax to that pre-amp. The measurement made is totally meaningless.
 
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