[Antennaware] FW: Fwd: Modelling yagi split DE

[Joe Subich, W4TV]

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
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