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.

I suspect the formula above is in error for very close spacings
because of proximity effects...I suspect it assumes a uniform thin
cylinder of current...but I'm not sure about that.

Cheers,
Tom

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...