Hi Matt,
The gain is free space.
I am 100% on board with the merits of hairpin matching. My other
homebrew yagi, a 5el 15m, is hairpin matched.
I have another optimization set of results for my 20m yagi that uses the
hairpin match. Overall, it is slightly better in gain and F/R. It
looks like an OWA, too, with D1 close to DE. The vswr bandwidth is
excellent, covering the whole 20m band less than 1.3 to 1.
I have played around with YO enough to observe that no matter what
parameters I adjust, or the weights that I apply to them, that gain
seems to always occur. One of my runs that gave very good results, I
set the gain weight to 5%, the F/B at 50%, and SWR at 45%.
Regarding your 20m stack - nothing wrong with that setup! Paul, W0AIH,
uses a 4 stack of 204BA's as his main 20m antenna. They work very well.
I found a technical paper on the internet written by a university
professor that describes very well the math behind the hairpin (beta)
match, and how to use a modelling program to estimate the parameters
needed. I wrote a spreadsheet that uses these equations and methods,
and I can quickly zero in on a hairpin match using the "Source Data"
feature of EZNEC. I would be happy to share it if there is any interest.
73,
Charlie N9CO
On 7/9/2014 10:38 PM, Matt wrote:
FB Charlie - understand you want the 50 ohm match. I have a pair of
stacked 4 element 20m beams on 26ft booms (yea - I know it's too short but
it's what I have...) I optimize my 3-el or larger Yagi's for -25J ohm
reactance and then back off when the impedance starts to go south of about
20 ohms. This seems to work very well for beta match, which is my
preferred method of feeding. I like the fact that the beta grounds both
sides of the DE (I have some element tips with holes burned right through
them from lightning strikes - no other consequence). I feed everything with
baluns so beta is not an issue.
With respect to optimization, I generally optimize for F/B over bandwidth
and forward gain usually falls right into place. You don't mention if your
forward dbi is over real ground or free space ???
Matt
KM5VI
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