On Mon, 08 Dec 2003 17:52:27 +1300, MikeN
wrote:

Thanks Jacques, George and Richard for you replies.

I've got in mind building an HA8ET return loss bridge
http://www.pollak.sulinet.hu/elektro/hidak/hidak.htm, if necessary
separating it from the antenna by a couple of wavelengths of coax,
expecting the antenna load to repeat at half-multiples of a
wavelength.

This resistive bridge is physically small and specified at 40 dB
return loss at 70 cm, so why shouldn't it give some meaningful
although not necessarily exact results for the impedance?

Would the separating cable add to the imprecision.of the results.

Any views?

MikeN ZL1BNB

Hi Mike,

Resistive Bridges (in other words, those not using directional
coupling through ferrites) are pretty robust up to this region. You
definitely want to keep the power down because it will cause the
reference load to change resistance if it heats up appreciably.

Also, use odd 1/8th wave cable lengths so that the reactance change
that is being transformed is not on the edge of a steep slope
(resonance causes things to change fast making accurate measurements
difficult). Reference Walt Maxwell's "Reflections" to delve deeper
into this matter. Half waves for casting back a repeated Z only if
you can insure you can measure a short at the connector (not as easy
as it may seem - hence the 1/8th wave recommendation).

73's
Richard Clark, KB7QHC