On Thu, 05 Jan 2006 22:24:54 -0800, dansawyeror
wrote:

I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a
different way. I don't understand "two 50 Ohm loads in parallel with the stub".

Parallel two 50 ohm loads with a tee piece, then use another tee to
put the stub in parallel. The tees will be imperfect, but at HF, the
impact should not be major.


Tests of stubs are now clearly reading correct for various lengths across
frequencies. That is once the 1/4 wave is determined the next odd and even
is very predicable.

Do you get the correct answers for 1/8, 3/8 wave stubs in parallel
with a 50 ohm load? (1/8 wave s/c stub has an impedance close to
0+jRo, and you need to put it in parallel with the dummy load, so
series equivalent is 25+j25. 25 ohms in parallel with 1.8 wave s/c
stub should be around 20+j10. Put a 25 ohm load at the end of a metre
of coax and check its transformation at different frequencies
(equivalent to quarter wave, eight wave etc) against your Smith Chart
prog.

They are some examples, work some out for what you have at hand.

Prove that your measurement system works on predictable loads.

Owen


Thanks,
Dan


Don't you need to measure some "known" loads. Why not try a 50 ohm
load tee'd to a s/c stub (quarter wave at a known frequency) and see
if you get the predictable results at different frequencies around
resonance. Then try two 50 ohm loads in parallel with the stub. (339mm
of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at
half that frequency it should be 0.85+j50, etc...


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