On Fri, 20 Jun 1997 force12@interserv.com wrote:
> Yes, we have tried this in several combinations. From the practical side, a
> second reflector does not significantly add to the F/B to warrant the extra
> mechanical and cost factors. At UHF, maybe it would be worth it. At HF,
> it is surely not to be considered in a horizontal Yagi! With the modeling
> software, this can be achieved by re-tuning the design, with a focus on F/B.
> One other item is that most folks are looking for forward gain and adding an
> element to possibly improve F/B is not very attractive. Oh -- if the second
> reflector is located on the boom, behind the first reflector, it has very
> little value, as the first reflector contains the majority of the current
> and the second one has little to work with.
As you suggest, the region behind the reflector may not be a fertile area
for F/B-improving hardware. However, if one was inclined to experiment in
this area, the following is submitted for completenenss:
Along the lines of improving yagi F/B via less-than-traditional design
techniques, there was an interesting article in a HamRadio Mag back around
1980. Think the author was a G or ZL.
The concept was ingenious in its approach: What the guy figured was, hey,
here's all this unwanted RF coming in from the back...let's slurp it up
and dissipate it before the reflector has a chance to even deal with
it, heaven-forbid should any get by and wind up in the driven element
where it can flow down to the receiver.
He added an additional element resonant at the design frequency and
located at a suitable spacing *behind* the reflector. In the center of
this new element was a *resistor* whose function was to "use-up" any
energy acquired off the back (or going out the back during transmit).
Of course, all this was before the desktop-PC and modeling-software-in
every-shack days, and although performance claims were made, I never heard
anymore about the approach. Sounded like it was worth a try though....
Big Don
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|