AIM 4170, first time user, readout questions

[2E0ILY]

I have had my 4170 for a while, but last weekend was the first chance I have had to try it out. My setup:

Horizontal quad loop of about 490 feet in length, at a height of about 30 feet or so, varying. fed into bungalow at a corner with what I believe is 450 Ohm ladder line, about an inch wide, windowed black plastic. the ladder line comes through a hardwood window frame, and is about 20 feet long, all but 2.5 feet being out in the fresh air. It then goes to my Yaesu FC-902 ATU, either direct to the screw terminals on the back, or via a 4:1 current balun, depending on what band I want to work. Some bands TX loading is VERY peaky. I can always get under 1.5 to 21 SWR as shown at the ATU meter though, and mostly 1 to I . I fitted a PL239 to BNC adaptor to the 4170, then on the BNC adaptor I fitted a two post banana plug post adaptor. With this assembled I did a custom cal, using a short wire for shorted, just leaving the posts bare for open, and a 49.9 Ohm close tolerance resistor, checked on a calibrated resistance meter as correct, across the posts and put 49.9 Ohms in as the resistor value. I then connected the ladder line direct to the posts and did some scans. i got bizarre results... I next retried with the balun between the 4170 and the ladder line, and got more sensible looking results. I selected various band limits for the scans. I found no points in any of the amateur bands where, to me, the results looked good. Should I have done a cal with the balun connected to the 4170, without the ladder line connected to the balun first? I took screen shots and zipped them up, the names of the files should be pretty self explanatory. I wonder if someone familiar with this device would have a look at them and critique my methods used, and the results please, to help me make a start in using this device, and analysing the results? The file is at http://www.chriswilson.tv/AIM/aim.zip the cal file is in there, too, if it helps.

As an aside, I was on 15 meters until late last night, digi modes, 25 watts, and the antenna was loaded up 1 to 1 SWR, working fine. This morning after a very wet and windy night, I turn the rig back on, no settings having been changed, and on TX the SWR was 3 to 1. Resetting the ATU tune and load brought it back down to 1 to 1 SWR. Is this the effect of rain on the loop? Normal? Thanks, if someone has the time to look at the shots I'd be very grateful.

[G7DIE]

I can't help with the antenna as I can't open the files on my works PC, however the shift in VSWR will more than likely be the effect of rain on the ladder line.

[G3TXQ]

I then connected the ladder line direct to the posts and did some scans. i got bizarre results...

What do you mean by "bizarre results"?

Was one side of the analyser grounded via the PC?

Your screen shots show you were using the "standard" calibration file; is that the custom terminal post calibration or the basic instrument calibration? You really need to save the custom calibration files for future use under a meaningful file description.

It would help answer your questions if your charts showed Rs and Xs, as well as SWR, and if you put the cursor mid band so we can see the values there.

Steve G3TXQ

[4L4AGU]

"Standard calibration" is the process user performs with specific calibration loads included together with device.

AIM outputs also data as .csv files, which can be used in almost any editor to visualise scan results.

[KO6WB]

The feedline being wet accounts for some of the SWR change you saw. You would still have an SWR change even when the feedline is nice and dry. This is due to the ground under the antenna has changed. With added moisture it became more conductive.
Hope this helps
73
Gary

[2E0ILY]

OK, thanks for the replies. I think I may have inadvertently been using the standard cal file and not my custom one... ;( Having said that, my custom one would have been wrong anyway, as I should have been doing the cal with the 4170 connected to the balun. I have a question before trying again please. When I do a cal with the 4170 connected by a short coaxial cable to the SO239 on the balun, does it matter if the shorting wire and resistor joining the feedline terminals on the balun is quite long? the feedline terminals are on either side of the case that balun is in, so the shortest connector wire / resistor lead length will be about 5 or so inches. There's not much I can do about that, but thought I should ask!
Thanks.

[G3TXQ]

The long lead lengths during the calibration phase will introduce some inductance which is not then taken into account during the AIM4170 calculations. The magnitude of the error will increase with frequency. At low HF it's probably not significant.

Edit: I just did a test on my AIM. At 30MHz a load resistor, which measured 218-j1 Ohms after a correct calibration, measured 212-j41 after calibration with "long lead" calibration loads.

Steve G3TXQ

[2E0ILY]

OK, thank you Steve, I#ll try again. There is a dedicated forum for the AIM stuff, on proboards.com but I have never had any replies to questions there, so I have to pester you guys!

[2E0ILY]

I have done 2 more calibration files. One with the 4 to 1 balun attached to the AIM via a short commercially made co-ax with PL259 plugs on and using a 49.9 ohm close tolerance resistor as the cal load resistor, and one similarly connected but using a 470 ohm close tolerance resistor, as per a link from the AIM site suggests when using a 450 ohm ladder line feeder to a balanced loop antenna. I have put the bitmap files at http://www.chriswilson.tv/AIM/aim2.zip Do these make any more sense please? The cal files are with the bitmaps. Thanks.

[G3TXQ]

I took a quick look at the 80m, 40m and 20m results with the "non 470 Ohm" calibration - they are reasonably close to predictions from EZNEC.

It would make things easier to analyse if you scaled the charts so that traces didn't disappear top or bottom!

Calibrating with a 470 Ohm resistor is not really helpful in this application; the ladderline is not matched, so the one impedance you will never see at the balun is 400 Ohms.

Edit: forgot to say that the 160m results also look close to EZNEC predictions. For example, EZNEC says system resonance at 1.98MHz with Rs=65, Xs=0

Finally: when I switch between results for the same band but the two different calibrations, I see almost identical Rs and Xs figures but very different SWR figures; have you changed the SWR reference impedance between the two?

73,
Steve G3TXQ