Towertalk
[Top] [All Lists]

Re: [TowerTalk] book Tribander comparison test

To: towertalk@contesting.com
Subject: Re: [TowerTalk] book Tribander comparison test
From: jimlux <jimlux@earthlink.net>
Date: Thu, 24 Feb 2011 23:39:02 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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.
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>