[TowerTalk] more CATV questions

[w8ji.tom]

Hi Ted,

> hello everyone....first, i wish to thank everyone who has responded to
> my earlier msg concerning the measuring/cutting of CATV cable....
> I guess at this point I have a choice to make....my dilema is that when
> i calculate the physical length of a run of CATV , the two numbers that
> i end up with are : 112 feet +/- and 224 feet +/- (dependant on VF of
> cable)...

With 112 feet, you won't have a half wave on 160. A length about that long
would be OK for the other bands. Just be sure the VF is what you think it
is, because a very slight error can totally screw up ten meters.

For example, the difference between around  .84 VF and  .81 VF on ten
meters is enough to make a 50 ohm pure resistive impedance seen by the rig
change to almost 112 ohms! With a cable 8 wavelengths long, you absolutely
MUST measure the highest frequency performance and trim the cable while
watching the electrical length. Many foam cables vary much more than that
in VF, since VF is a function of air/dielectric ratios in the dielectric.

> 2. cut for the shorter length (good for .5 wl on 80m) and make up rest
> with
>     rg-213....(this will mean 40-50ft of rg-213...doesnt this defeat the
> purpose
>     for using low-loss cable?? 

Not on 160 through 40 or so meters. Loss is so low you'd never know the
difference.

> 3. use matching transformers ....do you still need to cut CATV to
> resonant
>     lengths when using these transformers???

No, but I'd bet good 1.8-30 MHz matching transformers with a 1.5:1 ratio
are tough to make.
Not only that, you'd never measure the difference in performance over the
cheap and dirty system of cutting the cable for ten meters.. unless ten
meter bandwidth of only 600 kHz or WARC band performance are concerns.

Some rigs "dislike" 2:1 VSWR's. You know what your rig "likes" and what you
want. It's better you make your own decision on what to do.

73 Tom

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