Custom Cal at 4:1 Balun Terminals

[ad5vs]

In an attempt to analyze the conditions on the transceiver end of my 450ohm ladder line I performed a custom cal at the 4:1 current balun terminals where the ladder line normally terminates. The custom cal completed but with alerts of high impedance (> 3000ohms) during the short and known resistor steps. My questions are (1) can you truly achieve a zero ohm short circuit on the high impedance side of a 4:1 current balun or achieve a pure known resistance? (2) what causes the high impedance alerts during the custom cal? (3) would a 1:1 current balun yield a better analysis result?

[Bob]

At the start of each calibration step, the program does a quick check to see if the specified load is attached. It's possible the incorrect load could be attached and the resulting cal data would be incorrect and it might not be obvious at first that the results were incorrect (been there, done that). This check for load consistency prevents a lot of trouble. There have been cases when the 50 ohm load is damaged and it reads 100 ohms instead of 50 ohms. The bottom line is, this alert message is only a warning, not a fatal error. If you're sure you have the correct load attached, then you can continue the cal.

With some baluns and filters, the results may be extreme at some frequencies outside the range of interest and you can ignore the results. For example, it may be way off at 10 MHz but you are only interested in 40 meters, so 30 meters doesn't matter.

After doing the cal, do a scan with the cal resistor still attached to see if the results are ok in the region you are interested in. Note, the cal resistor does not have to be 50 ohms, it can be any value such as 200 ohms or 400 ohms. Enter that value when prompted for the Standard Resistor load during cal.

One trick to take into consideration is the coaxial line used for a lot of baluns is a distributed transmission line with some length.   If the AIM is calibrated with the loads attached directly to the RF connector, then the balun will act like a transformer, even though the length appears to be short, it may be not negligible.  If you have a 1:1 balun terminated with a 200 ohm resistor, the 200 ohms is transformed to other values over the HF band. That is, the input to a 1:1 balun does not read the exact impedance that is attached to it's output (except for the ideal case of a 50ohm load), even if it's a perfect balun and the coax it's made with is essentially loss less. It's the length of the line that is causing this transformation effect.

When the calibration loads are attached to the output of the balun, this transforming effect is canceled out and the attached test load will read the expected value all across the band.  This is a special case and it is not what your transmitter will see but it's interesting to evaluate the various components separately to see what may need to be changed or adjusted.

Keep in mind: the balun, the transmission line and the antenna itself form a system and it should be evaluated as a whole for the final assessment  since that is what your transmitter will see. 

-73/Bob

[ad5vs]

Thanks very much for the quick reply Bob. I have achieved very good results with my AIM 4170C when I evaluate the entire antenna system based on a standard cal at the AIM 4170C input connector. I have been able to adjust the two unbalanced legs and now have reasonably low SWR on 80,40,20,12, and 10 meters. In fact I can almost operate without a tuner on all these bands. 10 meter SWR is still a little high at band edges so tuning is required there. I normally use the tuner anyway on all the bands but it is a piece of cake to achieve a match since I am reasonably close on all bands of interest.

I continue to evaluate my system because I know the results I am seeing at the transceiver are not necessarily representative of what is happening on the 450 ohm transmission line. I could still have mismatch and high SWRs on the transmission line while seeing good matches and low SWRs at the transceiver. In fact I likely do have this situation. That is why I am using low loss 450 ohm line. When I use the custom cal to analyze the 450 ohm line I am seeing resonances and low SWRs outside of the ham bands and of course high SWRs and low return loss inside the ham bands. I just wanted to make sure I could trust the custom cal and know that the data I am seeing on the 450 ohm line is accurate.

The AIM 4170C is a fine instrument. In a matter of minutes I have all the data it would take me a couple of hours to collect with a hand held instrument and the improved accuracy against hand held instruments is well documented.

I'm sure I'll have more questions. Thanks for the customer support you provide on this forum.

[w4dnr]

So, if you put a 200 ohm cal resistor after a 4:1 transformer, do you tell the AIM that the load is 200 ohms or 200/4 = 50 ohms ( what a transmitter would see ).

Don W4DNR

[Bob]

Hi Don,

When you put the cal resistor after a balun (or a filter), tell the program it is 200 ohms (or whatever value it is). Also the short and open loads have to be placed at the same terminals as the resistor.   This will take care of imperfections in the balun (within reason).   You can test the cal data by putting other resistors at the terminals and you should get nearly a flat line in the HF bands.

73/Bob