AIM4170: Custom calibration of open-wire feedline

[g3nrw]

A friend of mine wants to use his AIM4170 to custom-calibrate his open-wire feeder for the HF bands. At first I thought it would be a simple matter of terminating the feeder in a short, open and resistive load as directed by the AIM program. With coax feeder this is straightforward, in that you can do all this with the feeder fairly close to the ground, but with open-wire I assume you would have to suspend the feeder in its final position, but not connected to the antenna.

Has anyone here done this? Did you take any special precautions to ensure there was no coupling between the (disconnected) antenna and the feedline? Or did you, in fact, find it necessary to remove the antenna altogether while doing the calibration, with the feeder suspended by string, maybe?

--
73
Ian, G3NRW

[Bob]

Hi Ian,

The output of the AIM is unbalanced, so you'll get more meaningful results by using a balun to feed the transmission line. The calibration short, open and resistor can be placed at the output of the balun to compensate for its imperfections.

Since the wave does expand in the air around the wires of a balanced transmission line, it should be spaced well away from conducting materials.

--73/Bob

[g3nrw]

Hi Bob

I should have included the balun in my description. But, assuming the balun is located at the TX end of the open-wire feeder, surely the short/open/resistive load should be connected at the *antenna* end of the feeder when custom calibrating, not at the output of the balun? As I understand it, we are talking here of calibrating the *whole* of the feed system; i.e. coax + balun + open-wire.

And OK on keeping the feeder away from other conducting materials. That was the real point of my question.

--
73
Ian, G3NRW

[k9fv]

Ian, Glad you posted the question, it does sound like Bob is calibrating so the whole antenna system will be checked, feedline included in order to see the info (SWR, Impedance, etc) at the end of the ladder line since that is what the tuner has to deal with. That is what I'm interested in. I think you are wanting to see info at the antenna? I think you have the right idea.

Bob, should the balun be a 4:1 or 1:1? I'd guess 1:1? Current or voltage balun?

I just got my AIM4170 Friday and it sure is neat - now I need to learn how to use more than the SWR scale

73 de Ken H>

[Bob]

Hi Ken,

You can calibrate right at the AIM RF port in order to see what the transmitter sees. Alternatively, to see the antenna itself, you put the cal loads at the feedpoint to the antenna. If the transmitter drives the antenna through a balun or balanced tuner, that should be included in the hookup. That way, imperfections in the balun will be compensated by the cal data. The balun itself can be 4:1 or 1:1. The main thing is to have a good quality balun in the frequency range you're interested in. The AIM can deal with any value of Zo. The specified value of Zo does not affect the collection of the raw data (which is the complex impedance at the input connector). It's only used as a reference value for calculating SWR and the reflection coefficient.

Stray capacitance to ground can be a problem in getting repeatable data in a high impedance circuit. If possible, operate the computer and the AIM on battery power and keep all the connecting cables as short as possible.

--73/Bob

[jayt]

Seems like this is a related question to the one I just posted too. Here is a copy.

Regarding using the AIM or VNA for ladder line. I suggest you first need a balun that is rated at the frequencies of interest. A small MFJ balun may not work well at below 3 MHz if it is just a few turns of coax around a ferrite core. Your balun can be 1:1 or 4:1. I use one of the Array solutions 4:1 baluns that are rated to go below 1.5 Mhz and up to 60Mhz on a ladder line fed antenna. Calibrate OUT the balun using the custom calibration technique over the frequency range of interest and make sure you put in lots of cal points. If using a 4:1 balun (200 ohm balunced to 50 ohm unbalanced) Set your Zo to 200 ohms and your VSWR plots will be made relative to 200 ohms IE the end of the ladder line hooked to the balun. If you want to see whats on the other end of the balun just set Zo to 50 ohms and its like reading the result of what the 4:1 balun is doing to your transformed impedance. Simple as that, and thats what your tuner will see if you have a ladderline fed dipole or loop. Have fun!