Additional Info on Modifying Bird Slugs

The following information is an excellent discussion on modifying Bird slugs.  I originally saw it on the web page for PA3CSG and I wrote to the original author for permission to add it to this collection of Bird slug information.  The author, David Mascaro was WA3JUF when he wrote the article back in 1986 and his call now is W3KM.  You can visit his web site at:

Again, with his permission I include his article below.  Thanks very much, David.



4.28 Modification to Bird 43 Slugs D. Mascaro WA3JUF - April 1986

Bird Slugs can be modified to read different power levels for 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 describes using a 5C element (5W, 100-250 MHz) to make a 5W, 2304 MHz slug. To open a slug: 1. pry off the model # plate with an awl or 2. heat up the top of the slug on a hot plate to soften the glue or 3. Drill and tap a 2-56 hole in the cover and pull it off. Remove the screw inside the cover. This exposes R2 and Dl. Next remove the two screws holding the Teflon cover on. This cover encloses the pick-up loop LI and a load resistor Rl.

Remove L1 2 turn coil) and the plastic coil form by cutting it out with a diagonal cutter.  Remove calibration resistor R2. Replace L1 with a brass strip as shown. Replace R2 with a 3K3 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. 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 calibrated load and a HP power meter. The shape and size was adjusted until both the F.S. 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 LI and R2. Performance is at least as good as my other slugs. Reassemble the slug and check it's performance again. Readjust R2 if calibration is off. Change the Model # plate to indicate the correct frequency band on 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 one up. It is no good if it's broken anyway.                                      

Modifying Bird Slugs


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