Hy-Gain 12AVQ mystery

Due to the miserably failed 10m/15m vertical HWEF experiments, I have put the old 12AVQ on my chimney. It’s the American version of the widely known (in Europe) Fritzel GP30 from Hy-Gain. It could well be that the Fritzel GP30 actually is a copy of a similar American antenna, I don’t know to be honest. Mine is old, probably 80’s when MFJ did not yet own Hy-Gain. It’s a sturdy antenna and even though the clamps have been replace and the nuts are an exotic mix of everything you can imagine, it still is a nice antenna.

At first I put it on my chimney on a 4 ft aluminium pole but I move it to the flat roof of my dormer so I have easier access to it. I’ve recently slipped on the wet roof tiles (not dangerous, the chimney was in front of me) breaking four of them and got a huge dent in my ego HI. I used two 3/4″ aluminium rods that used to be a portable 10m dipole as radials in N-S direction at 90 degree angle and 5m of wire (1/4 wave for 20m) right down the chimney (no longer in use) so making an angle of 180 degrees. 10m was OK, but 15m and 20m were terrible. To improve things on 15m, I added two MFJ 1615T-ish 15m mobile rods each around 7ft long in E-W direction but that didn’t improve things at all so I removed them. Then I added a single radial for 15m down the chimney but no luck. While I was – litterally – on a slippery slope, I’ve put the vertical on my flat (safe!) roof today. I only installed the 2 x 9ft radials for 10m and Guess what:

The darn thing shows very good VSWR and (!) resonance (X=0) on all three bands. I’m was flabbergasted. Really happy of course, but how this is possible, I just don’t have a clue, maybe you have?

Some important details:

  • There’s a choke right under the feeding point; 15 turns of RG58 on an FT240-52 toroid, that should be around 6kOhms on 20m and better on 15m and 10m. I therefore assume the coax’s braid does not act as counterpoise.
  • The antenne is on a galvanised iron tile stand with four 12″x12″ tiles. The mast is around 2.5 ft (75cm), the whole construction is 2 x 2 ft (60x60cm)
  • The tile stand is on a bitume roof, there’s no metal foil isolation or other metals inside the roof (just wood and rock wool), the flat roof was dry.
  • I’m wondering how the graph will look when the roof is wet. I put this line here as a reminder to add that later. I have a suspicion the 10m radials are “coupling” to the roof (in parallel, about 8″ above).

So if you have any ideas, just let me know!

73 de PA3HHO