Standing waves and SWR

At the meeting on 1/2/19 I described how we might use the club’s antenna analyser to best effect.

An important part of using the analyser is to understand the idea of standing waves.

I found this good illustration on youtube:

Hang in there, it’s a little slow moving but the illustration towards the end gives a really good visualisation of standing waves when there is a correct load and an open circuit.

How to tell if your SWR bridge is misleading you.
Have on hand an extension length of coax. It should be a ¼ wavelength long for the band you are experimenting with. After measuring your SWR, insert the extra length in the line – this moves your testing point 1/4 wave along the coax to a different point on the standing wave.

Here are some useful lengths:

6M band use a length of 1 metre, 2M band use 340 mm,70cm band use 13cm.

It’s not too critical. Now if your two SWR readings are similar (e.g. 1.2 and 1.3 then you probably don’t have a big problem but 1.2 and 2 would be unacceptable and require more investigation.

To actually calculate the length of the 1/4 wave section the formula is:
75*0.67/freq in MHz and the answer is in metres.

e.g. for 2M it’s 75*0.67/144.0 = 0.348 metres

I’ve used 0.67 as the velocity factor, this is true for many coax cables. Heliax is more likely to be 0.8.

John VK5DJ

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