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Monday, 27 May 2013

Well? You must have played a little by now?

Well,

Last time I mentioned that I had bought myself a Rigol DSA815 Spectrum Analyser with the built in Tracking Generator. I'm just about to head back to MAN for a flight to A71 land but I have had a bit of a fiddle this morning. First off, I made myself a very simple SWR bridge using some bits and bobs lying around, it's similar to the one I made back here for the power meter:

http://g0mgx.blogspot.co.uk/2012/10/well-since-progress-i-discussed-last.html

but smaller and not designed for the power, it looks like this:


What we have here at the top of the photo is a LH connector for the RF source, the top RH connector for the load (antenna). On the bottom the 2 parallel 100R resistors form a 50R termination for the forward power sample port and the reflected power sample port is connected to the bottom connector.

So, if I now connect the output of the Spectrum Analyser tracking generator to the top left (RF source), an antenna under test to the top right (RF Load) and the input to the Spectrum Analyser to the Reflected power sample port, we have a very expensive antenna analyser thingamabob!

With my recently constructed 40/80M dipole from here:

http://g0mgx.blogspot.co.uk/2013/04/well-further-to-my-musings-last-time.html

coupled to the RF Load port and the instrument appropriately configured, here's what we see:


Now, I've used the built in marker capability within the Spectrum Analyser to do a few things here:


What does this all tell me?


  • The 80M part of the antenna is nicely tuned within the band at 3.533MHz
  • The 40M part of the antenna is a mile out at about 7.765MHz
  • The 5dB width of the 40M resonant point is about 165KHz
I haven't done the maths yet so I don't know if my selection of a +5dB measurement is appropriate to see the bandwidth of decent SWR - I will do that later. But a summary of the summary?

It's too damn short!

Interesting though, egh?

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