[TenTec] OT: coax versus parallel wire

[Dr. Gerald N. Johnson]

On Wed, 2008-06-11 at 15:07 -0700, Jim WA9YSD wrote:
> Spacing for power handling right.  I believe 6 inch was used because of long distant runs say 300 feet or more. To the antennas back in the horse and buggy days,  I mean Model T days.  They used High impedance antennas because they had to run feed line for long distances.
> 
Until the spacing gets to radiating, a wider spacing for a particular
conductor size has less loss because the higher impedance leads to lower
current, providing the line is matched. I've heard (once) of a 300' run
of couple inch (I don't remember that detail precisely) space open wire
used at 2m going up a cliff to an antenna on top the cliff. I know that
W0PFP (now SK) tried 1" open wire line on 6m and found it radiated a lot
and received one of the first 50 WAS on 6m AM. So that was a long time
ago.

Most of us use open wire as a tuned feeder with a significant to high
SWR and so don't necessarily achieve the low loss it could give, but
still the loss is acceptable. If we ran the same multiband antenna
through coax, the low impedance points on the line might be a load Z of
just a few ohms (worst case, full wave wire center feed, perhaps 1K
impedance through a quarter wave of 50 ohm coax is transformed to 2.5
ohms and at that impedance, even 100 watts is a lot of current: 15.8
amps) which can required much larger C in the tuner than available.

> You might want to float a 10M loop on the inside of that and/or what ever band your having a hard time tuning it on, and see if it improves things.  Put it in like a quad element hanging on ropes tune it but do not tie it to the feed line.  That way you will not have to build an ex-stream tuner.
> 
>  Keep The Faith, Jim K9TF/WA9YSD
> 
73, Jerry, K0CQ