[BC] Antenna matching tuners: if from scratch

[Richard Fry]

>Can you beg, borrow, or steal a 500 to 1000 foot roll of 50-ohm coax? 
>....You connect the output of your sweep generator to a coaxial tee. The 
>through-connection connects to your 500 ft cable delay-line, and the tap 
>connects to your RF diode. The BNC from the diode goes to an ordinary 
>scope. It's set to auto-trigger. The detected signal will be in the 
>kilohertz range....Start by leaving the antenna end of the delay open. Set 
>your sweep generator so you can see the tone envelope on your scope. ... 
>This tone is generated because the reflected signal will be mixed with the 
>incident signal from the generator and detected by the diode. Because the 
>signal is being swept, its frequency will have changed by the time it 
>travels the delay-line, the produces the beat signal you observe on the 
>scope. etc
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While this approach is rather primitive compared to modern test equipment, 
it can show the distance from the source to a system mismatch, as well as 
allow the calculation of SWR and return loss.

If the length of the delay line is 492 feet times the v.p. of the coax used 
for it, then the round-trip transit time in the delay line will be 1 
microsecond, and the scope will show one cycle of "tone" for each one 
megahertz of the swept spectrum, when the delay line is unterminated.

So the number of cycles seen in the scope trace when the delay line is 
connected to the antenna system via a low-SWR coax adapter will be an 
indication of the distance to the mismatch.

For example, if the v.p. of the transmission line used in the antenna system 
is near unity (rigid coax), then two cycles of the "tone" per megahertz 
swept would indicate that the mismatch is about 492 feet away from the input 
to the main transmission line.  That distance would be reduced by the v.p. 
of the main transmission line.

A complication exists in that the scope waveform seen when sweeping the 
antenna system can consist of numerous small reflections from different 
locations in the system.  The tone is no longer shaped like a sine wave. 
This can make it difficult to determine how many cycles of a given rep rate 
exist across a given frequency span.

Operator skill, experience and a bit of luck are important in getting the 
right answers with this method (BTDT).

RF