[TowerTalk] C31XR versus SteppIR

[Jim Lux]

At 03:08 PM 5/13/2007, john at kk9a.com wrote:
>F/B,Gain and SWR are easily measurable.  Obviously, since F/B and Gain don't
>both occur with the same element lengths and spacing, there is always a
>compromise, however there is an even bigger compromise when trying to use
>the same element spacing over multiple bands.   The most noticeable effect
>will be reduced F/B as the reflector is electrically further away from the
>driven element as you go higher in frequency.

Without going through an analysis to be sure, but I would think that 
you can probably get almost infinite F/B ratio with almost any 
spacing (other parameters would certainly suffer.. especially loss on 
lower frequencies), until the spacing is >> 1/4 wavelength (2.5m, 
about 10ft, on 10m)


>  Gain is less effected and
>apparently the 22 ohm impedance is maintained by the element length
>settings.


Gain, for a 3 or 4 element design, is going to be strongly affected 
by boom length, less so by other factors.



>   The SteppIR is an excellent multiband antenna, however I am sure
>a properly designed monobander with the same boom length that would have
>better F/B and Gain on at least some of the bands that this antenna covers.

I sort of doubt that, but can't prove it trivially.  The SteppIR has 
to only work at ONE frequency at a time, while any fixed design has 
to by necessity cover a band of frequencies.  The basic 
Chu-Harrington limits say that for a given physical size, there's a 
tradeoff between gain and bandwidth (where bandwidth can be measured 
in a variety of ways), with maximum gain at narrowest bandwidth.


Jim, W6RMK