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Retuning the VHF Bandpass Filter By Kevin K. Custer W3KKC

If you find you cannot get rated power from a VHF station or mobile transmitter when converting a highband unit to 2 meters you will probably find that the bandpass filter that exists between the exciter and power amplifier needs to be taken apart and retuned.

The high band mobile or station power amplifier requires about 200mW of drive for full output. Other web sites and books say that it takes about twice that. Here's why: The high band exciter is specified for 400 mW of power output. However, in a stock station or mobile there is always a band-pass filter between the exciter output and the amplifier input. These filters have about 2 to 3 dB of loss, so the actual drive reaching the amplifier is about 200 mW. Ironically - that's exactly the same input requirement as a GE MASTR II power amplifier.

Follow these instructions to retune the bandpass filter to the 2 meter ham band.

Tune the exciter as per the manual on your 2 meter frequency, but into a Bird wattmeter and dummy load.  Normal power output is 400 mW or slightly more even with a 3 foot test cable of RG-58.  Note the power output of the exciter.  Connect the filter to the exciter's output, and the wattmeter and dummy load to the filter's output.  Tune the filter for maximum power on the Bird (minimum insertion loss).  The case will affect tuning so I have found it beneficial to mark and drill holes in the casting to gain access to the tuning slugs.  Here is a picture example of a drilled high-band BPF:

Drilling the cover allows the unit to be tuned with the cover on.  There will be some insertion loss, this is normal, just tune for minimum insertion loss, or maximum transferred power.

** NOTE **  The Station BPF is *usually* glued with epoxy to prevent it from being retuned.   If you find this is the case, get a mobile and steal the BPF from it.   It is physically and electrically the same and Motorola tended to not glue the mobile filters.

Reinstall the tuned filter into the radio set and retune the last stages of the exciter, and then the PA as per the manual.

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Text and HTML Copyright 11-26-1999  Kevin Custer  W3KKC
Page updated by W3KKC on March 18 2005 by adding the picture of the drilled case.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.