Would it be of any value ?

[w4dnr]

This is a somewhat related thread to my "Long Lengths of Heliax" question.

If I send a tower climber up the tower with a shorted termination, I could sweep the coaxial cable from 88mhz to 108mhz and at "some" frequency,
I should see a dead short. If I cut the heliax to the exact multiple 1/2 wavelength at the transmitter frequency, or add a same impedance jumper to equal an exact multiple 1/2 wavelength, would there be any advantages from a measuring stand point ? Maybe the transmitter might be happier ? Or a recheck several years in the future might be easier if the transmission line length was documented to be an exact 1/2 multiple ?

I realize that I would need to do a "standard calibration" at the AIM open/short/50 ohms before doing the short at the end of the heliax to get the length just right, but other than making it easier to use other analyzers that can't do a "custom calibration" at the opposite end of the heliax run, would I be accomplishing anything that would make troubleshooting this installation simpler / more precise ?

Thanks

Don W4DNR

[Bob]

Hi Don,

If your cable length is such that you have a dead short or complete open in the band of interest, the AIM will work when using a custom cal but it's actually better to have a cable length that does not have these extreme Z values in the band of interest.

First calibrate the AIM with the cal loads at the RF connector to prepare for a basic impedance measurement.
Then scan the cable of interest with the far end open. Look for a zero or infinite impedance value in the FM band. If possible, adjust the cable length to shift the extreme values outside the band.

When doing the custom cal, make the freq steps small enough to get plenty of samples around the impedance valley (or the peak). The AIM uses linear interpolation between the sample frequencies. Cable loss tends to smooth out the peaks and valleys in the impedance scan, so the samples can be further apart for a lossy cable than for a cable with a low loss.

Using an analyzer that does not have a custom cal feature may not be too accurate because the cable parameters like Zo and loss may differ from the catalog specifications. These parameters can vary with frequency too.


--73/ Bob

[smokchsr]

I can't answer about the value of doing it, but I can say I do have a test set up to find the half wave point. I stole the idea for it from a continental engineer, setting up two combined 25's. Basically I made up 6 or so chunks of RG-400 each is half (or double depending on which way you want to figure) of the next one, all barreled together. Just add them to the end of the line, and add or remove jumpers until you find the desired length. Then I take the correct length cable I just found and use the AIM to measure the electrical length in degrees. Then just make up a jumper of the correct electrical length appropriate size line.
I did this primarily for working with FM channel filter cans that some transmitters have a problem with.
Alan CSRE KD4QOF