Mod Bob – Part 7

I’m still intrigued by the idea of the “Mod Bob”, but this time as a multi-low band broadside array. Having two 60′ verticals opens up some new possibilities. I did model the feed being at the bottom – or elevated as shown here – and the elevated feed above the ground 10′ wins by quite a margin. My radial field is not connected, but according to Rudy, N6LF, in a paper he wrote about the Half Square – (an “Inverted Half Square” with a low phasing wire requires a really good ground underneath – or the phasing line high enough off the ground) – because you don’t want the current too close to the ground.

But this is not a half square or a half square feed. All bands from 160 – 30M model well. One thing for sure – I can do A – B tests between the two DX Engineering DV-80-P fed verticals and this Mod Bob like feed – with a balanced tuner in the center.

The only reason I am “musing” about this antenna again is that my ground radial field for 80M is really disjointed. For one vertical its awesome – the other – its “struggling”. The near field really wants to see ground near the base of the antenna – having it on the deck and elevated 5′ above a creek bed is lacking. However, if I elevate the feed – and just use the radial feed as a ground screen, things seem to get a lot better – and it even models better than the mono band DV-80-P, and it becomes a multi-low band array.

I still need to give the DV-80-P some time to tune. I might be able to fake the second vertical in thinking its as good as the first – electrically – and using a variable capacitor or even L-C circuit.

I’ve used the fantastic Palstar BT1500A before – but it has the two roller inductors ganged on one shaft, so they are tuned in a balanced way. They are always in phase. I’m starting to think maybe I want to be able to tune one vertical out of phase – and do it in a variable way. The BT1500A lets you switch between having the variable capacitor at the input or output of the circuit -and that is needed – especially because of 160 and 80 – where one is Low Z and the other High Z.

I’ll have to read up on variable phasing – it would be really cool to be able to have variable phasing on several bands, but the only system I have ever seen this on is a system that Array Solutions sells – and it switches in coax delay lines. I vaguely remember seeing other ways of doing this in the ON4UN Lowband DX-ing book. I’ll check this out.

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