RE: Which 50 ohm Loads are any good? Finding out with the FPC1500 Vector Network Analyzer

jw0752 — Sun, 13 May 2018 22:51:26 GMT

Hi Shabaz,

The only experience that I had with dummy loads was when I used to repair CB radios. I used to use a #47 bulb wired to a PL259. I'm sure it wasn't matched very good but at the time it was what I had and it at least allowed me to see if the output had power. Thank you for the nice job you did explaining how things work.

John

RE: Which 50 ohm Loads are any good? Finding out with the FPC1500 Vector Network Analyzer

three-phase — Sun, 13 May 2018 19:34:50 GMT

Very useful review of dummy loads and good to see the road test scope getting some more use with the tests.

Kind regards

RE: Which 50 ohm Loads are any good? Finding out with the FPC1500 Vector Network Analyzer

mcb1 — Sat, 12 May 2018 20:10:56 GMT

Interesting results.

There is a very large difference between terminations and dummy loads for transmitters.

If you intend to use a dummy load when testing transmitters, it needs to absorb the RF and radiate as little as possible.

Your tests show the VSWR but don't show the radiation from the load, which may be an additional deciding factor in what to buy.

My excuse is that I built this as a teenager

Now you know how bad it is, I suggest that you rebuild it and NOT use the case as part of the 50ohm transmission line [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-2c78d5c7-6c2d-4974-8328-7fa51c8097fc/contentimage_5F00_938.png:16:16]

I'll bet this one radiated like an antenna.

Many years ago we built some 'RF Snooping' devices for VHF and UHF.

They were a 50 ohm pass thru with the third connection having a 50 ohm termination near the inner connection and coupled across the air gap to give a 30dB loss at VHF.

These were a solid brass bar machined to the right dimension to suit the inner conductor and maintain the 50 ohm transmission characteristic.They were nickel ?? plated after being tested.

In those days getting a spectrum analyser above 1GHz was an major undertaking but the pass thru held up, while the 'snoop port' loss varied with the rise in frequency as we expected.

Those were the days, and I really don't play with proper RF since then.

Mark