[TenTec] OT: coax versus parallel wire

[Dr. Gerald N. Johnson]

On Tue, 2008-06-10 at 14:24 -0700, Jim WA9YSD wrote:
> doc, I am missing something here  you said for 160M .001 - .01 Wave length right?  For 80M I come up with 3.4 to 24 inches spacing.  That does not sound right, or is it?
> 
>  Keep The Faith, Jim K9TF/WA9YSD
> 
> 
Well, I figure I couldn't say 6" was too wide on 160m while I allowed
1/4" on 1296 or even 1/2" on 2m, though I know 3/4" is probably actually
too wide at 6m. The closer the spacing, the better. And remember that
the ARRL hand book formula for characteristic Z is WRONG for low
impedances, below about 250 ohms. Their formula has the conductors
overlapping at about 160 ohms, yet the correct formula allows even 1 ohm
Z for balanced pairs, though it does require large conductors spaced
only a few microinches.

The best reason for wide spacing is power handling, but with 1/8" space
in the tuner capacitors, its unlikely to flash over a transmission line
with 1" space. Just maybe that sometimes a wider spacing allows sturdy
conductors and a high enough impedance to come close to matching the
antenna (such as a full wave zepp). I think VOA used 12" spacing at
Dixon, Delano, and Bethany with 250 KW output on AM (1 MW peak), maybe
18". At Collins we used two 150 ohm coaxes in push-pull, made of 1/2"
copper water pipe center conductors in separate 8" aluminum (irrigation
tubing) outer conductors. That made the closest spacing we could support
about 8". We did occasionally flash over those lines, about the time the
SWR on the dummy load was rising as it burned up again. A MW peak at HF
can get unruly.

The trade off is that the wider spaced line radiates (or receives) more.

73, Jerry, K0CQ