A long-wave SWR meter

Posted on Feb 6, 2014

This document describes a simple SWR-meter suitable for long and medium waves bands (137 and 500kHz). The design is nothing new and very similar to common short wave meters, but it has been designed to work with low frequencies. The majority of amateur radio SWR-meters are specified for short waves or higher frequency and it`s very hard to find an

A long-wave SWR meter
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instrument suitable for 137 or 500kHz use. Many are rated 1. 8 to 54MHz, some can span from 1. 8 to 150MHz, but I never found a commercial meter specified below 1. 8MHz. Of course, professional equipment like a Bird directional watt-meter can do the job, but this page is about "cheap" amateur meters. The first idea is trying available meters to see if the can do the job below 1. 8MHz even if they are not designed for, but the results are very disappointing since none of the ones I had on hand worked at 137kHz. The test is very simple: just connect your long-waves transmitter to a suitable 50 © dummy load via the SWR-meter under test and look at the reflected power: if you gat a non-zero reading, your meter is not suitable for long waves. Being below the minimum specified working frequency of the instrument, one could tolerate some percent of error in the absolute power readings. But the fact that the reflected power is not zero when connecting a dummy load cannot be accepted at all, because it will be impossible to match the antenna with such an instrument. The large majority of amateur SWR-meters are bridge circuits based on current transformers (at least those designed for frequencies up and including short waves). Current transformers are limited in the upper end of the band by their stray capacitance and by the length of the wire composing the windings. In the lower end they are limited by the inductance of the...

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