Tim wrote:
Hi Kevin,
On the notch depth, using Wavetek 2410 for signal source,
HP 8590B spectrum analyzer.
Start @ 0dBm on sig gen.
1st 'can'. hook input to sig gen, output to SA. Using RG-142
with PL259s. Using a T at the can. Tune for minimum signal
display on SA. Hook up 2nd in series, - tune for minimum.
Hook up 3rd in series. Up sig gen to +20dBm. Hook up 3rd can.
Tune for minimum. Looks to be about -80dBm. 80+20=100.
I have some experience with this duplexer and about 80 dB of rejection
(or so - some are slightly worse than others) is all that can be
afforded with three stubs of "inch and 5" in cascade. I'd say something
is amiss with your test set-up. Some reactance is likely occurring
which is skewing your test results. You should be seeing some real
world numbers between -70 and -84 dB for the notch depths depending on
your construction.
You said you used RG-142 for the interconnecting cables. What method
was used to calculate the lengths - or did you simply cut them to the
lengths called out in the article/website/diagram? As I remember, RG-58
was used in the original article. The velocity of propagation (velocity
factor or VF) is different between RG-58 and RG-142 therefore making the
lengths of cable different than what is called out for RG-58.
Kevin
