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
-----Original Message-----
From: Antennaware [mailto:antennaware-bounces@contesting.com] On Behalf Of
Charlie Ocker via Antennaware
Sent: Wednesday, July 09, 2014 9:42 PM
To: antennaware@contesting.com
Subject: Re: [Antennaware] FW: Fwd: Modelling yagi split DE
Hi Matt -
Thank you for the useful information! I really do appreciate it.
I believe that I have convinced myself that the YO generated design is close
enough to begin construction. EZNEC 3.0 agrees very well with the YO model
in all areas except F/R - EZNEC reports a better F/R than does YO.
A bit of background - the yagi is a 4el 20m on a 26' boom. I designed it in
Y0 7.65 in 2005 with a hairpin DE. I put it up that summer, and it has
performed very well. High winds this summer caused a failure in the DE -
one element half came down - so I decided to redesign the DE to be more
mechanically robust.
In the process, I went to a different taper schedule, which meant that I
went back to YO for optimization. Along the way, I played around with the
settings, and discovered a way to drive YO for a direct 50 feed. It looks
like an OWA, with D1 being close to the DE. For whatever reason, I've not
been able to get YO to drive this way before.
It's an interesting design - it gives up a few tenths dB forward gain, and
the F/R is not as flat as the lower impedance version - but I like the idea
of less components (hairpin) in the air that could fail.
Both YO and EZNEC predict a VSWR that varies from 1.27 at 14.0 MHz, dipping
to below 1.1 at 14.175 MHz, and rising to 1.35 at 14.350 MHz.
Gain is 8.4 dBi at 14.0 MHz and rising slowly to 8.6 dBi at 14.350 MHz, but
the F/R is better lower in the band. Good for me, as 99% of my operating is
on CW.
73,
Charlie N9CO
On 7/9/2014 7:29 PM, Matt wrote:
> Hi Charlie,
>
> Just wanted to throw in a little more info that might be of benefit...
>
> The real life feed point actually starts where the wires first split
> off of the core inside the balun - one can observe this in measuring
> the performance of a balun from where the load side connection is made
> - as soon as the test points are moved outward from the core, the
> reactance begins to show up in the measurements. So for practical
> purposes, modeling the DE as a single, center fed element under NEC 4
> produces reasonable results. In reality, the model should be able to
> get you close to the right length and trim it off in the field from
there...
>
> To reinforce the point that Guy brought up, I have read that NEC 4
> does not seem to handle abrupt changes in geometry (such as aggressive
> tapering and/or abrupt changes in direction) within finely segmented areas
very well.
> I have found this to be true in my own experience. You might want to
> revert back to NEC 2 for these cases which seems to produce more
> realistic results, albeit with the limitations of that engine.
>
> Hope this is of some value to you.
>
> Matt
> KM5VI
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