Discrepance between Manual and the on line Help

[ea7qd]

Dear Bob,
I have found a discrepancy between the manual and the on line help. When calibrating a coaxial cable using the CALIBRATE – CABLE menu, first you are asked to connect a small resistor. On line help explains that the value of this resistor should be between 20 to 100 ohm. The same can be found at the manual (see page 22).
Once this have been done, you are asked to connect a large resistor. On line help establishes that the value of this large resistor should be between 200 to 500 ohm. However, the manual recommends a value between 200 to 2Kohm (see page 23). This seems to be a discrepancy between the manual and the help on line.
Therefore, which one should be the correct value for the second resistor?

Another question:

If I would like to analyse a multiband antenna - e.g. a 80 to10 meter vertical one - should I calibrate the feeding coaxial cable for each band or would it be enough just to perform an unique calibration on the 3’5 to 28 Mhz range? Which is the best option?
Thanks,
José, EA7QD

[wa3off]

As currently implemented, the scan limits and step size for your antenna-with-cable scan need to match the limits and step size used for the cable calibration. While you can do a calibration of the cable over the whole 3.5 to 28MHz range, at the maximum allowable 1000 points the step size is about 24kHz. That's a pretty big step for some purposes.

Once you have the far end of the cable in the air, it's a bit difficult to do any more cable scans, so it's best to do all the variations you can think of beforehand. I would scan the cable across the entire range with 1000 points ("1000p" entered for the step size) and also scan each individual band of interest at higher resolution. Since scanning time is generally not the issue, I would probably do 1000-point scans for all of them.

Dave, WA3OFF

[Bob]

The value of either the large or the small resistor isn't critical. For the large resistor I would use 200-500 if my antenna will be fairly well matched and maybe 1K ohm if it's seriously mismatched. The value of 2K is pretty extreme, although it may be useful in some cases.

The main thing to consider is this procedure only applies to transmission lines that are symmetrical. That is, they look the same from either end. Usually this is ok if you have a single kind of coax and you don't have any transfomers in the line. If you have a matching transformer or a combination of coax and ladder line, this may not give accurate results. If you have a complex transmission line, check the results using known loads at the far end of the line (with the antenna disconnected).

-- 73/ Bob