Aide memoire

My 2-element quad's feed broke today, just as I was about to do some other work in the 32 degree heat.  It has always had a very cheap, aluminium outer conductor, 75Ohm 1/4 wave stub, and has now reached 6 years of age.

The 6m quad, battered by weather for 6 years, rises again!

 
Turning the quad towards North America, the 75Ohm coax broke at the feed point again.  The aluminium outer very rapidly corrodes to powder, especially near the sea as we are.

I decided to finally replace the stub, though I have had recent problems determining the proper electrical length using an antenna analyser.  It's worked perfectly for other bands, but for some reason, doesn't on 6m.

So I decided to cut some spare, all-copper low loss 75Ohm coax to the same length as the old cheap coax.  On this occasion, I was lucky, as it returned a near-perfect 50Ohm transformation - actually just over 48Ohms - the SWR being about 1.03:1 at 50.00MHz (the antenna proves to be lightly and inconsequentially too long). It remains well below 1.3:1 way out to above 50.4MHz.

Pretty pleased with that result!  For the record, the correct 75Ohm stub length is 115cm, with the coax characteristics apparently not being that important between types.  The length is also not super-critical, at least for this quad type antenna.