Steve a quick question, as per the title, just trying to confirm what I'm looking at, if x=0 is the resonant point of the antenna, do this plot indicate that the antenna has two resonant points:

That's what the charts show! It's quite unusual for a simple antenna like a dipole or full-wave loop to do that, but not unusual for something like a beam.

What is it?

Steve G3TXQ

Looks like it will be ok for 555

It's the much vaunted Antron A99, an end fed 1/2WL CB antenna which I'm trying to test a couple of theories on, I had it on a 10' mast today and attached my analyser directly to the feedpoint and isolated the antenna from the mast, I then added an 18" section of coax and then tried it with 24' of coax, this plot above is with the 24' length of coax and whilst the resonant point and readings alter with each test, the antenna has two resonant points between 26 and 29MHz with no feedline and with the long feedline, if I extended the lower scan point to 25MHz then I'm sure the 18" section of coax would exhibit the same phenomena

ps. FAO M0SDM, I won't be using the antenna on any of those frequencies, I borrowed the antenna for the sole purpose of testing some theories.

Maybe something to do with using the feeder as part of the antenna. I bet if you put a RF choke at the feedpoint it'd not show the same result.

Now wouldn't that be a thing if the traces are the 5/8 antenna >and< the length of coax feeding it which just happens to correspond to the associated wave-length.

You may wish to read my reply above, two points of resonance when no feeder attached


Here's the trace with no feeder, this is analyser attached directly to the base of the antenna, forget marker points, however the vertical black lines correspond to the approximate 2:1 VSWR points, this hardly changes if I have no coax attached, 18" of coax attached, or 24' of coax attached:

I haven't had the time to analyse it thoroughly, but if I believe the schematics I'm seeing of the matching network - a couple of inductors plus a couple of capacitors - it's perhaps no surprise that together with the end-fed antenna wire impedance there are a couple of resonant points close together.

73,
Steve G3TXQ

I wonder if that was the intention of the antenna designer so as to give the antenna a very wide and forgiving bandwidth, my surprise was that the antenna was not only resonant, but had a low complex impedance, without a counterpoise, across such a broad range of frequencies. I think further tests are in order.

how did you isolate the antenna from the mast?

I wonder if there could be some capacitive coupling to the mast?

If I simulate the matching network using SPICE, I can get a couple of resonant frequencies even with the antenna wire represented as a pure resistance. Now throw in the fact that the antenna impedance actually has reactance which changes with frequency, and it's pretty obvious the antenna wire plus matching network could easily show two - or even more - resonant frequencies.

You don't need common-mode braid current or mast-coupling to explain the results, although of course they may also contribute.

Steve G3TXQ

The image below is how the antenna was mounted and I too don't believe I can discount capacitive coupling to the pole, I'd check the capacitance if my meter wasn't boxed away along with the rest of my shack contents. I had originally intended to mount the A99 on a fibreglass pole, I have a strong good quality 8m roach pole and the bottom two section are very sturdy, however with the compression force required to mount and stabilize the A99 to the top of the pole I'm sure it would damage the poles even with the slightest breeze, so I had to default to alloy poles and well insulated.

Whilst you can't see the base of the antenna, you can just see the analyser sticking out from the back, I went for the miniVNA PRO because it has a built in bluetooth device, perfect for mounting at the antenna feed:

The whole point of my tests were to view the effect of CMC on this particular antenna. For the CB fraternity this antenna has been the next best thing to sliced bread for many years, I'd venture they've sold tens of thousands of these over the years across the EU and US, there's many a discussion, fable and folk lore about how it works, what is need to make it work including having to use specific lengths of coax, radial kits, mast isolation and common mode chokes, I just wanted to understand some of it's finer points, and you've filled in some of the gaps with your analysis of the feed arrangement, many thanks