Modification to Bird 43 Slugs
                 D. Mascaro WA3JUF (now W3KM) - April 1986


Bird Slugs can be modified to read different power levels at full scale.
For example, a 25 Watt slug can be made to read 50 Watts full scale. Changing
the calibration resistor inside the slug is all that is needed in most cases.
Adjusting the pick-up loop may also be necessary. These slugs can also be
modified for different frequency bands. Low power VHF slugs can be used to
make UHF or microwave slugs. There are many odd frequency and power level
slugs floating around at HAM-fests that are perfect for conversion. All these
slugs are basically the same inside. They have a pick-up loop that samples
RF, which is rectified. A calibration resistor sets the F.S. reading on the
30uA meter movement.

The following example describes using a 5C element (5W, 100-250 MHz) to make
a 5W, 2304 MHz slug.

Either pry off the model # plate with an awl, or heat up the top of the slug
on a hot plate to soften the glue. Remove the screw inside the cover. This
exposes R2 and D1. Next remove the two screws holding the Teflon cover on.
This cover encloses the pick-up loop L1 and a load resistor R1. Remove L1
(2-turn coil) and the plastic coil form by cutting it out with a diagonal
cutters. Remove calibration resistor R2. Replace L1 with a brass strip.
Replace R2 with a 3.3K resistor. R2 can be adjusted by using a resistor of
lower value and filing a groove in the carbon to raise the resistance.

The size and placement of L1 and the value of R2 was found experimentally as
follows: I replaced the calibration resistor R2 with a 10K, 10 turn trimpot
to find the nominal resistor value. Then different sizes of brass strips were
tried. The slug is partially assembled each time and checked in a Bird 43 which
is connected to a 2.3GHz source, calibrated output load and a HP435 power meter.
The shape and size was adjusted until both full scale and linearity were correct.
Reasonable tracking can be achieved by trial and error.

Lastly, the directivity is checked with the slug in the reverse position. The
accuracy and linearity depend on how long you spend adjusting L1 and R2.
Performance is at least as good as my other slugs. Re-assemble the slug and
check its performance again. Re-adjust R2 if calibration is off. Change the
Model # plate to indicate the correct frequency band and you are done.

Many inoperative slugs can be repaired rather easily. Dropped slugs usually
have a broken L1 coil. Some have bad solder joints. Don`t be afraid to open
a defective slug, it is broken anyway.

Note (4/2004):
The mechanical construction of new slugs is different - so the above disassembly
procedure may be different.