SWR using twin feeder and bulbs ?

[Gi4CZW Cliff]

Just heard that in older days SWR indication of forward and reverse power of a 'ham radio transmitter could be shown, using a length of twin feeder and two torch bulbs. My older books fail to show this device and so I'm not sure, if the twin line has to be terminated, or how the distinction between forward and reverse is effected. Mr goggle not finding it for me. Your thoughts appreciated. Cliff.

[ms660]

http://www.rfcafe.com/references/qst...tober-1947.htm

Any use?

Lawrence.

[G8HQP Dave]

SWR (standing wave ratio) tells you about peaks and troughs of voltage or current on the line. Anything which can sense voltage or current can thus be used to detect standing waves. A rough estimate of SWR may then be possible. A properly terminated line will have SWR 1:1 so nothing much to see.

[G6Tanuki]

Part of me wonders if you were to try building such a thing these days whether a pair of modern 'high output' LEDs might not be a better, more sensitive indicator than old-fashioned incandescent bulbs?

I've got a homebrew 'Wheatstone Bridge' SWR-indicator here which uses a blue LED as its null indicator, and it works just fine with half a Watt of RF.

[lesmw0sec]

You can either have a short or an open circuit at the end of the line. What you are trying to achieve is a high SWR. An rf indicator of some sort (a bulb if the power is high enough) will reveal peaks and nulls of energy at wavelenth intervals along the line. While this is practical for VHF and above, you can visualise the length of feeder required to measure 40 M plus!

[trh01uk]

I was also told by a WWII veteran engineer that it was a routine way to measure frequency in the days when frequency counters were yet to be invented. Just note the position of two peaks (or troughs) then take a ruler and measure the distance to get the wavelength. For open wire lines the velocity factor is near 1, so you can use the normal formula to then get frequency. He told me his lab had a pair of lines all set up ready to go with a bulb pickup - for him it worked well because he was using 50MHz or thereabouts so the wavelength fitted in the room!

Richard

[dave walsh]

An elderly Amateur, whose home I used to visit in the sixties, had a large Bungalow with Aerial's terminating on the roof and he used mainly open wire feeders moving a bulb along to "improve" the SWR re his [much modified] and prone to flashovers Heathkit DX100 Transmitter. He explained this was a pre-coax technique but I can't claim to be more knowledgeable than that. I recall that the wire he used wasn't that new or sophisticated at all-maybe fencing gauge [He lived next door to the Corporation Depot and Storage Yard so there could have been a connection]

Dave

[G6Tanuki]

Originally Posted by trh01uk

I was also told by a WWII veteran engineer that it was a routine way to measure frequency in the days when frequency counters were yet to be invented.
Richard

Yes, a "Lecher Line" https://en.wikipedia.org/wiki/Lecher_line

Works well up to UHF, as Randall & Boot discovered when they wanted to measure the frequency of their first Cavity Magnetron during WWII.

[Gi4CZW Cliff]

Well done Lawrence ms660. I put some twin-feeder, loop, led's, and a dummy-load together for fig2B and yes it works. I did find that my solid state Tx doesn't like high swr and folded back its power-out but I got just enough, I'm now ready for tomorrow's QSO. Many thanks. Cliff.

[Gi4CZW Cliff]

Oops, I should have made clear that my TX folded back its power when I simulated high SWR to light the reverse power LED. Cliff.

[LeakyGrid]

In the 1930s and 1940s, before and after the war, my uncle (G3QQ) had an aerial strung between a flag pole and a chimney. It was fed by an open-wire feeder made from wooden dowels and bare copper wire. I now believe this to have been a Zepp, which runs with a high SWR.

From time to time he would hang mains neon bulbs along the feeder, to check the standing wave pattern. I was about 6 or 7 at the time, and could only marvel without understanding.

Roger

[G8HQP Dave]

The distance between peaks (or troughs) is half a wavelength. You also need to take account of velocity factor, but for an open wire feeder this will be near 1 anyway.