--- In ham_amplifiers@..., Bill Turner <dezrat@...>
wrote:
ORIGINAL MESSAGE:
On Mon, 23 Oct 2006 00:20:30 -0000, "craxd" <craxd1@...>
wrote:
If a dip meter shows a
resonance, there is a resonance at its tuned frequency. Forget
reading the freq off the dial, simply couple it to a known
accurate
freq counter, it will show the truth if you want precision.
------------ REPLY FOLLOWS ------------
One needs to be careful with this. The typical single-tube or
single-transistor dip meter can be pulled considerably off the true
resonant frequency if coupling is very tight to a high-Q circuit.
Just
physically move the dip meter away far enough so the dip is barely
perceptible and the accuracy will improve greatly.
If you have used a dip meter much, you have no doubt had the
experience of tuning slowly across the dip, and as you continue
tuning
have the dip suddenly disappear as the pulling effect disappears.
Loose coupling will prevent this from happening.
### Highly agreed. Loose coupling is as good as it gets.... for
acuracy anyway. Mike Stahl, K6MYC warned me about this 25 yrs
ago.... he didn't trust em.. when tweaking yagi's.... I wouldn't
either.
### For a real laff.... I bought the "mating" grid dip osc
coils for my MFJ-259 analyzer. Two coils cover the entire
spectrum. I can't grid dip ANYTHING with em... even simple
stuff.. like a coil in parallel with a cap... on the test bench.
Then I find out nobody else can grid dip anything either! Then Rauch
sez they don't really work. They still don't. It was
suggested not to buy the optional coil(s).
### The inductance measurement is not to be trusted in a MFJ-
259-B either. Inductance measurements drop like a rock with
increasing freq. Even at 1.8 mhz.... coils will show no where
near what my B+K 875A/B does. Per Tom.... the reason is the
the stray C between turns on a coil will subtract from the coils
inductance.
### To prove that theory one way or another.... Using the
various PI spreadsheets available..[like GM3SEK's... which will
factor in the stray L of the parasitic suppressor, and all stray L
between tube and C1 cap... and also factor in the UH/XL of the RFC
[which will require an equal amount of XC... coming from the
C1 cap]. His sheet will also factor in all stray C... plus anode
to chassis of tube... in the socket.
### When the sheets say C1 should be XXX and L should be
ZZZ, and C2 should be LLL. When I use my B+K 875... and
actually measure C1 L C2... and then tie the interconnecting
straps together.... then use a resistor between anode to chassis
[to simulate the plate load Z]... and a MFJ-259 on the output....
guess what? Flat swr, or at the most... just a very tiny teak
on the C1/C2 cap to resonate.
Now the B+K operates at 1 khz... and has never let me down
yet.
### Known coils from various manufacturer's always measure dead
on with the B+K. [rare exception was this 14 uh multronics
coil used on the last project.. which measured 12.2 uh]
## If I had used the inductance measuring function of the 259-B...
at the freq under test for the simple PI [say 80/40/20m], the
resulting coil would be WAY smaller.... and of course, the C1 and
C2 values had to be INCREASED by a huge amount to compensate....
then the loaded Q is WAY up... and the resulting circulating
currents are way up.
### Moral of the story.... use a MFJ-259B to measure inductance
for anything... like PI nets, LC networks, you name it... and you
will be guaranteed 100%, to have complete failure. Get a real LCR
meter. If you can't measure L + C accurately..[that's 90% of
radio].... you are dead in the water.
Later...... Jim VE7RF
Bill, W6WRT
