The “Mod Bob” Antenna

Its somewhat rare that I keep an antenna up more than 2 years, but this one will no doubt be an exception. This antenna “sort of” looks like a Bobtail curtain, and at element spacing of 35′, it could be a 20M Bobtail. The only problem is that its not fed against ground, but fed as a dipole like feed at the bottom of the center. Think “tuned feeders” I guess (?) There is a ground screen underneath – so I have that in the model. ON4UN in his book “Lowband DX-ing” recommends a ground screen under any vertical type antenna. The verticals are not 1/4 wl on 20M, but would actually be 5/8 wl elements on 20M. That might even be OK, except my 2 element yagi up only 33′ is a better antenna. So, what the heck is this “Mod – Bob”?

Its easily the best “compromise”, non-resonant low band antenna I have ever built. It works quite well on 160M – 30M, and that really is a feat to achieve. Here is the crux of why its so good – rather than look like a low dipole on any of the low bands – it instead looks like phased verticals on all of the low bands. The horizontal wires look a lot more like phasing lines – like in a half square, bobtail or bruce array instead of like a dipole.  Sure – a single vertical will give you a decent omni directional fairly low angle pattern – but with a lot of loss in the radial field – and this Mod-Bob antenna is at worst – unity on 160M – and gain on all of the higher bands. On 160M, its better than an Inverted L – or shortened vertical because rather than have loss – it actually has a tiny bit of gain – with a nice low takeoff angle and less horizontal radiation than an Inverted L. On 30M it really shines:

In fact – that pattern, with the tell tale “ears” looks like a 30M Bobtail. Its has the same pattern and gain, so while it doesn’t walk like a Duck, its a Duck (er, Bobtail). Its as good as a 1/4 wl dipole up > 50′, so that’s pretty good. A dipole up at least 1/2 wl would normally model at 8 dBi as well – so I’m actually quite happy about this with this antenna.

This antenna came about because I had a 60′ Top Band vertical, and in a windstorm this past winter, the antenna bent over rendering usable but scary looking. I decided to just take materials I had – or that I could purchase from the VK0EK DX-pedition (think Spiderbeam aluminum push up masts), and I just put the biggest antenna up that I could.

Very surprisingly, its SWR is exactly where I want it to be on 160M – in or around 1.822.5 mhz – so it is actually a resonant antenna. It does not model well SWR wise in EZNec – and I have no idea why. It works very well on 40 and 30 with a tuner, but it has some problems with 80M and the KPA-500 folds back to only allow a couple hundred watts.

One thing I’ve just ordered is a balanced Palstar tuner – the BT-1500A. It has two switches that activate relays and switch the circuits to be either high or low impedance. This should help make the antenna on all bands – but specifically 80M, which I think will prove to be an important band during the upcoming Bouvet DX-pedition. If I were feeding this antenna against ground – then I am sure 160 and 80M would be very low impedance around 12 ohms – but this is a different beast. 80M must be either higher or lower than the other bands impedance wise – and far enough away from what the KAT-500 can match. There are no matching issues on 160, 40 and 30M.

Its a double L tuner for balanced line – and so it seems to be perfect for my needs – even if it only gets me on 80M. I will house it in a weatherproof box out back and manually change settings using my AA-30 antenna analyzer. Maybe in the future I will remote it and use motors to turn the shafts. Maybe an Arduino board can be hooked up so it becomes an automatic remote antenna tuner? C’est possible.

I’ve really committed myself to the low bands – besides this antenna, I have the fantastic DX Engineering PV-40-P 2 element phased vertical array on 40M, and I also have a 2 element yagi on 20M – but doubt it will get that much use.

So – that’s why my new blog is very seriously oriented toward the Low Bands – its a place where a lot of experimentation can be done and on 160M especially – there is a lot we do not even understand yet – propagation wise.

 

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