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Probably QRT tonight and in the morning due to storms in the area

Beware of modifying amateur transceivers for 472 kHz or 137 kHz!

– Posted in: 630 Meter Instructional Topics, 630 Meters

There have been a number of questions recently received via email asking about modifying existing amateur rigs for use on 137 kHz or 472 kHz.  While I cannot provide you information on doing this for your specific rig, there are a few warnings that need to be stated and heeded by the prospective operator.

Often times following the modification of an amateur rig, the operator sees anywhere from 15 watts to 50 watts of output into a load and there is an assumption that the RF is at the frequency of interest.  This could not be further from the truth!  The reality often is that  a mod results in 0.5 – 1 watts of actual RF at the frequency of interest while the balance of power is harmonic content.  This fact becomes apparent when an external low pass filter is used as the RF output will drop considerably.  I’ve actually known operators that removed the low pass filter because power dropped and they did not understand that the filter was doing its job by attenuating the higher frequency harmonics.

Dirty signals can often be observed on a scope while transmitting into a 50-ohm load.  You will either see multiple waveforms or a waveform that is ragged and not smooth.   A properly filtered sine wave output with be smooth and show symmetry.  Modified rigs do not have low pass filters at these frequency and assuming that a high-Q antenna system will perform the filtering function is asking for trouble.

If you are planning on using a modified rig on 630 meters, be sure to add an external low pass filter using values like this one or this one.  DON’T SKIMP ON THIS AND DON’T THINK YOU ARE EXEMPT!

Additionally don’t push the power on these modified rigs.  While some are designed to work for MARS and CAP with modification, those frequencies are much higher in spectrum than those of 630-meters.  Persistent high power operation at 472 kHz will likely result in PA or other component failure.

Also don’t forget that HF SWR meters are essentially worthless at 472 kHz and 137 kHz and persistent operation with such meters can sometimes result in permanent damage.  Consider an MF or LF-specific meter or consider building a scopematch system.

73 and good luck!