K7ITM wrote:
On Feb 11, 8:02 am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Oh, and to more directly answer your question, the formula generally
given for accurate description of the impedance of two parallel, round
conductors of the same diameter, in air, is:

Zo = 120 * invcosh(D/d)

where D/d is the center-center distance and d is the conductor
diameter. So...

D/d for 50 ohms = cosh(50/120) = 1.088

For 1" diameter wires, you'd have to space them 0.088 inches apart,
which is tough to maintain unless you embed them in a solid
dielectric.


or you go away from round wires...

On Feb 11, 8:02*am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

And something else for you to ponder: Coax is generally available
commercially at about 50 ohms and about 75 ohms, with a few lines at
about 95 ohms and some specials of other low impedances. What would
~500 ohm coax look like (assuming a straight center conductor, not a
helix)?

Cheers,
Tom

On Feb 11, 4:02*pm, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Some simple experimental work he

http://www.karinya.net/g3txq/twin_feed/

73,
Steve G3TXQ

steveeh131047 wrote in news:cd964a18-6932-407b-86fd-
:

http://www.karinya.net/g3txq/twin_feed/

An interesting read.

I think that if you had measured the loss, none of the options tried would
be very attractive. Measuring a short stub with your new AIM4170 will allow
calculation of the effective loss of the line.

I don't understand the detail of the connection neatness thing, but one way
to construct a 50 ohm 'balanced line' line is with four 50 ohm coax lines.
Cut equal lengths, tie all of the shields at each end together, tie pairs
of inner conductors together, and use the now paired inner conductors as a
50 ohm 'balanced line'. Of course, the line is only balanced (in current
terms) by virtue of its usage rather than its construction.

Owen