On Jan 29, 6:01 pm, Owen Duffy wrote:
"Suzy" not@valid wrote :

...

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.

Owen's comments reminded me that I always used to think of coupled-
line hybrids as sampling the magnetic and electric fields at a point,
and of course you can adjust the ratio by adjusting the load as he
suggests. But then something began nagging me: his example of a 20mm
(2cm) line isn't exactly short compared with a "full length" quarter
wave coupled line. Because of the relatively slow propagation in
stripline over FR4 PC board material, a quarter wave is only about 10
cm for 450MHz signals. But thinking of it in terms of a distributed
line system, you're just terminating the coupled line in the proper
impedance to not get reflections off that end; so this same thing
works even with full 1/4 wave lines to adjust the directivity.

However, if you have one through line and one coupled line, and you've
made them very symmetric so you can swap the two and not see a
difference, then if you have to terminate the coupled line in other
than 50 ohms to get perfect directivity (no reflection from that
port), it also says that the through line is not 50 ohms. As Owen
points out, you'd really like that through line to look like 50 ohms
(or other system Z0 if you wish), so it doesn't disturb the system
it's installed in. There's the incentive to make the coupler
symmetrical and tuned so the coupled line terminates properly in 50
ohms, to get best directivity.

I'll readily admit that the details of coupled lines from a fields
perspective is a bit beyond my full understanding, so there may be an
error in my thinking about this, but I believe it's pretty accurate.
The thing that comes first to my mind is that for stripline, the
propagation velocity for even and odd modes is different, but still, I
think if the lines are lightly coupled, the paragraphs I wrote above
are a valid way to look at the situation.

Geez, Schottky diodes should be dirt cheap these days. It's germanium
that are hard to find around here, unless they are old stock.

Cheers,
Tom