"Suzy" not@valid wrote in :

....
Thanks for that. I really want to emulate a throughline (like the Bird
43 I once had) and am thinking more in the terms of a stripline on PCB
(using BNC), with parallel lines to sniff the RF and diodes to convert
for the meters. But what I'm usure of is the sort of dimensions I
should use for 70 cms, and wther these are critical. How long should

If you make the coupling line very short wrt wavelength, you can analyse
it with a lumped constant approximation.

You can think of the coupled line as located in the electric field of the
main line, and it will have a voltage difference to ground.

Similarly, the coupled line will be cut by magnetic flux due the the
current in the main line, and so a voltage will be induced end to end in
the coupled line.

By adjusting the resistor at one end of the coupled line, you adjust the
contibution of these current and voltage derived samples, and can adjust
them so balance each other (ie no meter deflection) when V/I on the main
line is 50.

When the coupled line is short, the characteristic impedance of the
coupled line is not very critical.

You should be able to achieve sufficient sensitivity for 5W pwr on 70cm
with 20mm of coupled line.

The Zo of the though line is more important, it is the main determinant
of the insertion VSWR of the instrument, so you need to strive to achieve
close to the desired Zo, presumably 50 ohms.

Next, don't put the coupled line to close as it will load the main line
and degrade the insertion VSWR.

Then adjust the R at the end of the coupled line to null the DC output on
a 50 ohm load.

Repeat for the other coupled line (if you use one).

Check for symmetry, ie that reversing the instrument gives exactly the
same readings on the dummy load.

Tom suggested Schottky diodes. Dick Smith has 1N5711 Shottky diodes (at
exhorbitant prices), or you could get 1N34 germainium diodes from Jaycar.

BTW, you did mention the Bird 43, the above is not frequency compensated
like the Bird slugs, deflection will be frequency dependent.

Owen

PS: I have only met one VK YL ham named Susan, I sometimes wonder what
happened to her, haven't heard her on air in decades.