At 08:19 AM 10/8/98 -0400, w8ji.tom wrote:
>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.
Yes, be careful!!
I measured 0.87 for VF in the hardline I just installed (3/4" CATV).
A friend assumed 0.81 plus a little in his CATV and was way SHORT.
---
Chad Kurszewski, WE9V e-mail: WE9V@qth.com
The Official "Sultans of Shwing" Web Site: http://www.QTH.com/sos
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|