On 2/24/11 11:21 PM, Dick Green WC1M wrote:
> Yeah, but there's another variable besides forward gain, bandwidth and
> feedpoint impedance: front-to-back ratio. The 3-el and 4-el SteppIRs have
> only about 11 dB F/B on 10m, which is less than half the F/B of a 3-el or
> 4-el 10m monobander with optimized element spacing. I'm no antenna design
> expert, but I have to believe this is due to the fixed element spacing. In
> that sense, the SteppIR is more like a tribander than a monobander (c.f.,
> the Force12 C3E, which has similar poor F/B on 10m).
>
I haven't actually tried a model recently, but I would think that you
could get arbitrarily good front/back ratios (as in a very good null at
180 degrees), but not a generalized front/everything "behind". But
that's driven more by number of elements than length of boom. to a
first order, boom length sets maximum gain, number of elements sets
number of nulls/lobes, again to a first order you get 1 less than the
number of elements for antennas smaller than a wavelength (that is a 3
element antenna gets you one lobe and one null or two lobes or two
nulls).. the usual scheme is one lobe (front) and one null (back).
you get superdirectivity when you have extra elements to suppress lobes
in the "wrong" directions.
So, if the SteppIR gets worse F/B than some other 3 element antenna of
comparable size (i.e. no fair comparing a 20 foot long boom to a 40 foot
long one), it's because the SteppIR was setup that way deliberately..
maybe to narrow the forward beam a bit? maybe to eke out a few tenths of
a dB forward gain?
One of the big advantages of a SteppIR (or a three element antenna with
a variable impedance network at the middle of each element) is that you
don't have to try to also get "broadband match" so combinations of
element lengths that have narrow bandwidth are viable.
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