In a message dated 9/16/01 6:54:54 PM Pacific Daylight Time, firstname.lastname@example.org
This is what I did to make a *USEFUL* "dip" circuit for
my MFJ 259-B. Of course it should work with other
RF generators too, but the MFJ (or equivalent) is
hard to beat for convenience and reasonable accuracy.
Here is the schematic of my "grid" dip meter.
(I hope it came through OK.)
100 OHMS (non inductive)
COAX TO "HOT">----------/\/\/\/--------*------
RF SOURCE | | | LOOP
(259b, "GND">---| | | *------
ETC.) | V | |
| --- 1N34 | |
| | | |
| |-----|(----| |
| | .02 | |
| | | |
| |____+ -___| |
| UA METER |
Not shown is a pot across the meter for level control.
I used a large 50 microamp meter, but one could use
other meters or even a VOM as a substitute for the
meter. The loop is a single turn. Mine is about
3" diameter of solid, insulated wire. I used a twisted
pair of the same wire for a "transmission line". I
installed every component on the back of the meter using
a terminal strip. The loop line connects to a pair of
binding posts soldered onto the terminal strip. I didn't
bother building it into a case (yet). The coax can be
almost any convenient length.
To set up, connect the 259B, adjust the pot (again, not shown)
for MINIMUM resistance. (The pot can be a screw driver
adjust type.) Turn the 259B on and adjust the pot
for a nearly full scale deflection. Place the
loop close to the trap coil you want to measure then
vary the frequency until a sharp "dip" or null is indicated
on the meter (not the 259B's meter). Start at a frequency
higher than what you might expect the trap to be resonant
at to avoid reading a null at some harmonic of the RF source
such as the 259B. Move the coil further away from the trap
and dip again. The most accurate reading is when the loop
is loosely coupled to the trap.
On Sun, 16 Sep 2001 04:28:40 -0400 Frank Norton KB8XU
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>Ur email is most helpful. I was wondering if u might help with
>detail? I have an MFJ 269 but I don't know where to get or how to
>"outboard dip circuit". I used to own a GDO but it was lost in a move
>several yrs ago. I sure liked that HeathKit Dipper! I like my MFJ
>but I don't know how to check things like traps etc. that used to be a
>with my Dipper.
>Any advice wud be appreciated. Thanks again for ur help Charlie.
>73 es God bless, Frank kb8xu
>PS (I apologize if u are reading this in a very large font, I have a
>problem so I have to use it to see what I write, most email systems
>the formatting at the other end)
Increasing the sensitivity of the MFJ is a "great concept." I plan to try
this when I get thime this winter. I'll need a better schematic. The
accurate frequency reading is a great concept. Tell MFJ to add it. I've
used the Boonton which is very sensitive and a few others like the Millen but
one always has to measure the frequency in the receiver. I can do it quicky
but its a nuisance.
The SWR bridge and the grid dip are my most used instruments and only a few
really know how to the the GD. Without knowledge on how to use the GD there
are many things you just can't do. The Grid Dipper will make a comeback.
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