[TowerTalk] More on trimming the 75 ohm hardline

[Steve Zettel]

Thanks to all who responded to my request for help with cutting some 75 ohm 1/2 hardline to a electrical half-wave multiple at 40M, to be used to feed stacked tribanders on 20, 15, and 10M. As most pointed out, using the Z function of the Autek RF-1 gives a sharper minimum than the SWR method I had been using (although, my interpolations of the midpoint of the SWR dips turned out to be surprisingly close). So, I went out in the rain today and started measuring looking for minimum Z. I am measuring 232 feet, terminated in a PL-259 at one end and shorted at the other end. Here is what I got:

Freq in mHz		Min. Z in ohms

1.808			2
3.601			3
5.401			3
7.213			3
9.007			3
10.81			4
12.62			4
14.43			5
16.23			6
18.05			7
19.84			6
21.65			7
23.47			7
25.28			7
27.07			7
28.88			7

So, in retrospect it appears I underestimated the velocity factor of the hardline and my first cut at 232' actually left me a tad short. In the words of my favorite carpenter, "Damn, I've cut this board three times and it's still too short!" So I guess there is no purpose in trimming anymore from this length unless I want to end up as Tom described, with lots of 2' lengths, none of which are a half-wave multiple at 40 meters.

I'll use Pete Smith's method of using a length of RG-11 for tuning/trimming, if the existing length gives me problems. My tribanders are all tuned lower in their repective bands than the Min. Z points of the hardline. This whole exercise was to replace about 200' of good RG-213 that currently runs from shack to Stackmatch.

I am curious about something I observe when I analyze the frequencies of Min. Z:

I understand where 1.808, 3.601, 7.213, 14.43, 21.65 and 28.88 mHz are coming from (even harmonic multiples of the basic half wave length, in this case corresponding to 1.808 mHz). 5.401 mHz must then be the 3/2 WL point, correct? This would be consistant with the other phenomenon of the series of frequencies above:

Adding any two adjacent numbers gives a product that also appears in the series (yeah, I KNOW, just bear with me as a rediscover all this stuff that I am SURE I once knew before I started working for the government. . .).

So now, if anyone has any tips on stretching hardline (probably a call for a little "leg stretching"). . .

Thanks again,

Steve Zettel  KJ7CH
near Libby, MT       "Where the only thing short is our hardline."			 

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