At 06:25 AM 2007-02-12, Jim Miller KG0KP wrote:
>I have two Butternut vertical antennas (HF6V and HF9V) now that I
>have rebuilt the parts one I acquired. I would like to consider
>phasing them and expect it would work for "any" particular band but
>wonder what the effect would be on the other bands. Would other
>bands be degraded or could both antennas be driven with the proper
>phasing (by band) and be effective? What physical separation
>distance would be recommended? Same physical distance must serve all bands.
>I expect this could be modeled separately as monoband verticals but
>I am new at modeling and do not know how to model phasing.
I'm not too familiar with the specifics of these antennas, but as
long as they are fairly close to the same size (in wavelengths) and
similar feedpoint impedance on each band, you shouldn't have too much
trouble phasing them.
You can obtain either of two bidirectional patterns (90 degrees
apart) on the band where the antennas are 1/2 wl apart. You can get
cardioid (unidirectional, endfire)) patterns on the band where they
are 1/4 wl apart. It is possible to get endfire patterns on a band
where they are 1/8 wl apart, but the feed impedances can get really
low due to the mutual coupling and make it hard to get good
efficiency (unless you like laying lots of radials).
On higher bands where the spacing is 3/4 wl or more, there will be
several lobes, regardless of the phasing. However, you can steer the
lobes (and nulls) by adjusting the phasing.
Modeling phasing is easy. Just put a current "source" at the base of
each antenna and adjust the phasing to get the pattern you
like. (EZNEC comes with a sample 2-element phased array with 1/4 wl
spacing and 90 degree phasing in a file called CARDIOID.EZ , plus a
similar one with a transmission line feed network called CARDTL.EZ).
Getting equal currents and the desired phase in the real world is a
heck of lot harder than modeling them!
73, Terry N6RY
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