Larry Coyle wrote:

I have been thinking of an antenna impedance measurement setup using a
HF bridge of some kind, driven by a high-level generator to swamp out
the ambient junk. My 100 wattHF rig followed by a power attenuator
(say, 10db?) would serve to drive the bridge, I would guess.

Anybody have some thoughts on this approach - pro or con? What kind of
bridge would be good?


You've got two basic approaches to dealing with strong interference:
1) Make the signal you're measuring huge, so the interference is small..
any detector works
2) Use a tuned narrow band detector.


I would think you could use a simple resistive bridge with option #1,
although the tricky part is getting phase measurements. The current
crop of PC based VNAs and the like rely on having a handy quadradure
source. You could probably use a TAPR VNA, with a linear amplifier on
the Tx port and pads on the Rx port, after calibrating out the amp/pads.

A narrow band detector would be easier. This is sort of how the "noise
bridge" schemes work.. they use your HF receiver as the narrow band
detector. Again, almost any bridge would work for scalar (non-phase)
measurements.

Another approach is the old "three voltmeter meter" technique, which is
essentially a broadband detector and half a bridge, and lends itself to
high power.

The real challenge isn't in making the measurement, but in automating it
so that you can do a "sweep" conveniently. For spot measurements,
almost anything works well enough, but if you want to make 100
measurements it gets real tedious.