On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards"
wrote:


You must have been reading what I've been writing for the last 6 or 7
years.

Reg, Yes, I have probably read and learned a good deal from stuff you
have written, but I have skipped over a lot of what you have
written... probably most noticeably when you and others trade kicks to
the groin to see who is the last man standing.

Back on topic:

A point that you hinted at, but might have been overlooked by some is
that it can be relatively unimportant that the SWR bridge's sampling
line has the same characteristic impedance as the impedance at which
its detector has been nulled.

For example, a typical SWR meter designed originally for 75 ohms, with
a 0.1m long ideal 75 ohm sampling section, but with the detector
adjusted to read nil reflected power with a 50+j0 load on the
"antenna" terminals of the meter, will in most cases operate just as
well as a 50 ohm SWR meter on 7MHz, as the detector will truly show
when it has a 50 ohm load, the indicated VSWR for other loads will
substantially correct (ie within typical accuracy for the type of
instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of
75 ohm line will be insignificant in practice.

In many amateur reflectometer designs (and in some commercial
implementations), very little attention has been given to the
characteristic impedance of the sampling section, and in some cases to
the insertion VSWR (that results).

I recall testing a relatively expensive SWR meter rated from 1.8 to
150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a
good dummy load a Bird 43 ahead of it indicated an insertion VSWR
1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its
"antenna" terminals, it was not very capable of delivering that load
to its "transmitter" terminals.

Owen
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