Charlie,
If you're concerned about the balun leads, it's probably better to
model them as a short (4 to 6" long) section of ~400 Ohm transmission
line. I do not remember if YO will handle a transmission line but you
can always take the YO design and move it to EZNEC or 4NEC for final
tweaking.
In practice if you can keep the balun leads to 3 or 4 inches, at most
you may need to shorten the DE by a couple inches to put the minimum
SWR where you want it. However, with an OWA type design it's probably
broad enough that you will never notice. Pattern/gain are nearly
independent of DE length - the only thing it effects is resonant/match
point.
73,
... Joe, W4TV
On 2014-07-09 10:42 PM, Charlie Ocker via Antennaware wrote:
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
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
|