[ARDF] HB9CV dimensions? in PVC?

[Dale Hunt, WB6BYU]

Matthew -

   You know it is tuned right by the pattern.  I end up using a lot
of W4RNL's designs by scaling them to other frequencies.  Even 
though my scaling method isn't perfect, they are good enough to start
with that a bit of rounding error usually won't hurt.  Besides, 
LB has a lot more time and experience for optimizing antenna designs
than I do, so I'll let him do the work.  Fortunately he is very good
about sharing his experiments with us.

   I tried the Moxon because Kuon doesn't like a big antenna.
She was using the VK4BRG version of the HB9CV but it appears to
be tuned for maximum gain and the F/B measured around 6dB (making
it very easy to go the wrong direction by mistake.)  The Moxon
is narrower than a conventional yagi and has a very clean
pattern.  For her "travelling version" I made it so the elements
come off and pull apart at the end insulators, so the parts pack
quite well.  But the mechanical construction is not optimized.

   How did I tune my ELT antenna?  Actually, I didn't.  I've  never
built one.  But after getting the question I spent some time playing
with the online Java Yagi Modeller at:

http://fermi.la.asu.edu/w9cf/yagipub/index.html

Since I was wasn't designing for max gain, but rather for a
reasonable mix of gain and pattern, the boom length wasn't
particularly critical.  It doesn't change a lot if you change
the spacing by half an inch or more.  I ran the model for a
1/2" diameter element, which should correspond to a 1" wide
tape measure.  Some day I'll build one and try it out, but in
this case just cutting the elements to the specified length
seems to work.  I didn't bother trying to match it for low
SWR, but you could calculate the hairpin length for 20 ohms
and then trim the driven element length for low SWR if you
wanted to.



The dimensions for 121.5 MHz with 1/2" elements are:

DE: length 48"
DIR: length 44.25" spacing 7"

Input impedance around 20 ohms, gain over 6.5 dBi,
F/B better than 20dB.  If you put this into the modelling
program you can see what the pattern does as you vary
the different dimensions: the pattern on the 2m version
remains useful over much of the band, so minor variations
in construction shouldn't be a problem.  (When I have to
change the element lengths in steps smaller than a quarter
inch to optimize the pattern, I figure it is too fussy
to build reliably.  These dimensions are more forgiving
than that.)  Actually they are probably pretty close to
the spacing that Cebik is looking at, but since I'm willing
to put up with a F/B ratio worse than 20dB at the edges of
the 145 - 147 band on 2m (compared to a proportionally much 
wider bandwidth he is trying to cover on 10m) then it
isn't nearly the problem that he makes it appear.

   Try it and see!

   Good luck,
         - Dale WB6BYU


Matthew Robbins wrote:
> 
> Dale,
> 
>     Thanks for the ideas.  I found the Cebik article about 10 minutes
> after I hit "send". It's here: http://www.cebik.com/phase/hb.html
> 
>     The question I came up with is:  "How do you know it's tuned right?"
> 
.....
> 
>     I've looked a little at Moxons, but the tape measure elements are
> so good and are easy enough to pack that I haven't felt the need to try a Moxon.
> 
>     What are the dimensions for your CAP antenna?  Is the boom length
> touchy?  Cebik says, "More efficient at producing raw gain are
> 2-element Yagis using a director closely spaced to the driven element.
> ... However, such beams have very narrow
> operating bandwidths."  How did you tune yours?
> 
> Matthew
> ,
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