Long Lengths of Heliax ( how to estimate calibrations ? )

[w4dnr]

I'm installing a low power 96.1mhz FM translator and forgot to do a custom calibration on a 450 foot run of 7/8 heliax.

I forgot to do a custom cal on the length before it was installed.

The antenna is a DC ground -antenna, so somewhere off frequency it should show a dead short , but how to determine exact length ( tdr function ? )
and calibrate the line where a meaningful presentation would be created.

I know I should see better than a -35 db return loss on the antenna on-frequency ( checked the antenna on the ground ... AIM at feedpoint )

But to record the measurements for future reference, how should I proceed ?


Thanks in advance


Don W4DNR

[mel]

I have not done the TDR function yet, but the old way was to sweep the feedline for a wide bandwidth and you will see a ripple of the mismatch and the distance between the nulls is the electrical 1/2 wavelength of the coax. Then you can verify it with the TDR and get the length.

Mel, K6KBE

[w4dnr]

Thanks Mel,

We ordered 450 feet of heliax and that's probably correct plus or minus a foot or so.

I was wondering if the newer software's TDR function was as precise as looking for the nulls ?

Probably not.



Don W4DNR

[smokchsr]

Don,

I would think there is a problem with your methodology. Calibrating to the end of the cable on the ground is likely to no longer be valid once the cable is up on the tower. If you add any ground kits, have any bumps or dings to the cable as it goes up the cable calibration is going to change.
The normal way to do that is to install the cable and then send the calibration loads up the tower, and calibrate in position.
I've never tried what you are doing, so it may work reasonably well, I don't know.

Alan CSRE KD4QOF

[wa9vez]

A back of the envelope calculation shows that for 450' of AVA5-50 Heliax, open or short circuit, the average SWR should be 5.74 at 88 mhz and it drops to about 5.17 at 108 mhz, assuming enough cable grips were used, no kinks, or dents. In the 1970's I had a 300+ foot run of smooth 7/8 LDF hardline pull its center pin out of the center pin connector fingers during erection. Not enough cable grips. Only saw about 2:1 SWR on a Birdwatt at 159 mhz. Took a while to realize why the 110 watt VHF had no range (not even at the tower parking lot). Of course, the rigger drops the center pin off the top of the tower while reworking the connector with a dull hacksaw blade, a pocket knife, and a pair of crescents. The center conductor had dropped down about 3" over 200' of tower. My point is cable attenuation reduces measured antenna SWR mismatch. No antenna at all measured a under 2:1 SWR at the TX end.

Jim / wa9vez

[smokchsr]

Don,
At this point I think the main thing you would want to do is do a precision FM calibration on your AIM. Then do a system sweep across the entire FM band, as well as one that's only 2 Mhz either side of your center frequency, followed by a TDR sweep. Of course save each for future reference. You might also want to do a TDR sweep out of band and save that. That should give you a pretty good indication of the as installed parameters. If you happen to have the tower climbers headed up for some reason then you can send up the test loads. But realistically there isn't much reason to do that if you don't need to prove that the installed feed line met specification. The reason I say that is, the calibration you do to the end of the feed line is only valid for a short while. So even if you save that calibration, over time as the temperature changes, the cable ages and all that good stuff it's not going to be the exact same. Not being the exact same future measurements using that calibration file may be misleading indicating a problem where none exist.

Alan KD4QOF