Hi Bill...
I agree with both Vic and Carl, but to add a few tidbits: 1) I prefer
Carl's approach of checking the frequency with a receiver, as that
eliminates one variable (the coupling loop to the freq counter moving
around and possibly pulling dip meter freq.); 2) Use a wide strap or braid
to short the ends of the choke so as not to affect it's actual series
resonant frequencies more than necessary; 3) I try to couple off the top
end of the choke, as far away as the dip is still detectable - it tends to
minimize coupling to other parts of the circuit (e.g., tank coils) which
may mess up results; 4) the choke must be "in situ" BUT its hot end should
be disconnected from the anode circuit, also to avoid confusion from
unidentified couplings; 5) the first (lowest-f) series resonance almost
always is quite high-Q and therefore couples well to the dip meter - you
may be able to detect the dip with your dipper coil even 3 or 4 inches
away! - and with typical closewound solenoid chokes of roughly 75-100
microhenries I've found that it tends to be in the 24-27 MHz region; 6)
with enough RFC inductance to do a good job on 160m, the first series
resonance may be as low as 15-20 MHz, and the second series resonance also
may possibly be below ~35 MHz where it can be a problem; 7) you wouldn't
believe (without trying it) how nifty it is to look directly at the choke's
complex Z with a vector impedance meter or similar device. It's the only
way to really know what's going on --- but I used a dipper (my favorite is
the military AN/PRM-10, often available surplus for under $100) for more
than 30 years before being introduced to the magic of a vector Z meter.
Good luck!
Dick W0ID
-----Original Message-----
From: Bill Murphy [SMTP:zuluhe@hotmail.com]
Sent: Thursday, March 11, 1999 11:44 PM
To: amps@contesting.com
Subject: [AMPS] Chokes and Dipping chokes?
I have just completed several homebrew chokes, the question is what is
the most effective way to use a dip meter to check these chokes for
resonance. I have tried several coupling methods, but the dips change
depending on how you couple the dip meter to the choke. I seem to get
very sharp 30>200mhz dips, but the <30 mhz seem weak. I am using 2 turn
coupling on the end of the choke, there is a 5" lead between the
coupling turns and the choke base. I noitice that if i direct couple the
dip meter to the base of the choke the dips change freq. Maybe its just
my pathetic Jap Dip meter. Any suggestions would be appreciated.
Bill Murphy.
yes still alive and well after my 4cx5000 conversion!
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