[TowerTalk] measuring/cutting CATV cable

[w8ji.tom]

Hi Ted,

you wrote:
> i get an approximate length of 224.5 ft for a half wave on 1.775mhz....
> my problem is that the distance from the top of the tower (where relay
> box is)
> back to the shack is only about 150-160ft.....can i coil up the extra
> 65-75ft
> at the house wall before it enters the house?? 

Keep in mind the bandwidth of the cable, where it acts like an exact 1/2 wl
section, goes down as the length of the cable increases. Because of that,
you want to "TUNE" the cable on 28 MHz.

An electrical length in feet of 278 feet just about optimizes the cable.
That would give you only two main (non-WARC) bands where you'd have more
than 1.5 VSWR at band edges. Ten meters would be 28 - 28.6 MHz and 80
meters would be 3.28 to 3.88 MHz for VSWR<1.5. That assumes a very low loss
cable.

If I were you, I'd cut the cable a safe amount longer than expected to get
an electrical length of 278 feet, terminate it in 50 ohms, and measure SWR
on an accurate meter. You want the lowest dip in SWR at 28.3 MHz, and every
1.76875 MHz below that. Check it on several frequencies (like 14.15 MHz,
7.075, and 3.5375) to be sure you cut it right! 

The MFJ-259B will allow you to trim it within a foot reading directly off
the dial! Remember, always measure SWR performance at the highest frequency
for the most accuracy.

73 Tom

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