[VHFcontesting] Multiband VHF Antennas

[James Duffey]

Tom - It sounds like you are on the right path. Your gain numbers are similar to what DK7ZB gets on his designs; you may want to look at those designs if you have not already. He has a 4 element 2M beam with 5 elements on 432 MHz, and a 5 element 2M beam with 8 elements on 432 MHz. The 2M performance of these beams is essentially unchanged from the versions without the 432 MHz elements, but the performance at 432 MHz is down about 2 dB on the 432 over a similar length 432 MHz monobanders. 

My playing around with NEC and the open sleeve technique showed that starting with a 2 element beam on the low band, adding a closely coupled resonator for the higher band, then a reflector and finally a director or two on the higher band, yields a beam that is close in performance to monoband antennas with a similar number of elements. 

The problem comes with adding more elements on the lower band. I have done this modeling with 6M/2M and 2M/0.7M combos. With these combinations the higher band and the lower band are related by a factor of 3 so coupling is very strong. So when you add a parasitic element on the lower band, it will have significant current on it in the higher band. This will be in the 3/2 mode and skew the pattern. To bring this skew under control, one must closely couple a 432 element to it, which more or less brings the radiation back in line, but this closely coupled 432 MHz director element is not optimally spaced. So to get some gain back, one adds another parasitic element for hte higher band, between the driven element and the parasitic. This too will not be optimaly placed either, in fact it is hard to optimize this element, and so you will not have optimum gain for the boom length or number of elements you use. And this problem repeats itself and gets worse as you add more parasitic elements. 

But for modest lengths or modest number of elements it is a promising technique. The 2 element low band/3 or 4 element high band combinations model as nice antennas and I will get around to building one this spring. 

I am sure that you have found that there are a lot of variables to play with, perhaps even more than one cares to have, and they are not independent. The impedance of the higher frequency depends on the spacing to the lower frequency driver and the length of the element as well as the spacing to the first director and the reflector. With the close spacing of the first director in an OWA design, I am fairly certain that you have found this out. It is easy to start chasing one's tail and get to a design that is not viable. 

Keep us informed on your progress. - Duffey 
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KK6MC
James Duffey
Cedar Crest NM