Could you post the dimensions for those of us without 4nec2?
Here is a similar wideband 75/80m dipole with direct 50 ohm feed and a
little more mechanical complexity than your design.
It sims on EZNEC to about the same performance as your SWR graph.
http://home.comcast.net/~ross_anderson/80Wire.htm
I'm dumping my bazooka and ladder line fed 80m "all banders" for a pair
80/40/20 fan dipoles at right angles. They didn't work well in NAQP,
the 20m nulls line up from the clover leaf with the antennas at 90
degrees. Any other orientation messes up 80 and 40 coverage.
A broad banded 80 over 40 and 20 seems like a much better way to go,
particularly with coax feed where I know a choke balun will work also.
Grant KZ1W
On 1/18/2011 10:13 PM, Kevin Normoyle wrote:
> Here's a 80m dipole I'm going to put up. Thought I'd see if I overlooked
> something someone might comment on.https://sites.google.com/site/knormoyle/
>
> It uses a close-spaced open-sleeve parasitic to broaden the bandwidth. Sims
> say
> I've got 1.5:1 SWR from 3.5 to 3.975Mhz. (at the end of the ideal 20 meter
> match
> section)
>
> (2:1 from 3.45 to 4)
>
> I want to do 1500W across the full band without tuning or relays or other
> adjustment.
>
> The background that got me thinking about this: I had put up a folded dipole
> with a open-sleeve parasitic, with a target impedance of 450 ohms, per N6LF
> (google) and that worked well.
> http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf
>
> This new one will be at a right angle to that one.
>
> However, it seemed easy to get the same effect with two wires instead of the 3
> N6LF used. Yeah I know about wide cage dipoles, and cutting two dipoles and
> keeping them spread to avoid interaction.
>
> This same-band open-sleeve design wants interaction. Spacing is kept at
> constant 6" with ladder-line like spacers.
>
> The use of parasitic open-sleeve is talked about everywhere (since 1946?) to
> broaden bandwidth. But most of the cases I find on the web are about adding
> different bands (like Force12 20m-15m-10m open-sleeve feeds)
>
> A variety of journal articles make the same-band case seem obvious. But I had
> to
> play around to find the best lengths. What I found, is that it doesn't work
> well
> to try to aim for 50 or 75 ohm impedance at the antenna.
>
> The antenna impedance sweet spot I ended on was about 125 ohms. I then use
> about
> 1/4 wavelength of 75 ohm to get a feed match to 50 ohms.
>
> In the nec file, I have an ideal 20M length of 75ohm transmission line for
> this,
> so it can be simulated with 50 ohm drive. The target height is 100 feet. The
> nec is setup so the parallel wires have the same number of segments, for nec-2
> accuracy.
>
> I know the N6LF antenna works as advertised. And I see that people put up wire
> pairs for the same reason (to get full 80M band). I guess I'm throwing this
> out
> for comment in case I've missed something. I think it's going to work well.
> It's
> no problem keeping the wires spaced at 6" with spacers every 8' or so.
>
> Anyone curious enough to replicate my sim: I put the nec and some graphs up at
> the url above. I run with 4nec2.. the default 50 ohm drive impedance works
> because it drives at the end of the matching line.
>
> The swr on the 75 ohm line is less than 2<1 so it doesn't look like there's
> much
> loss there.
>
> I had simmed it without the match line, but if you remove it and drive at the
> antenna, you want to use a 125 ohm impedance.
>
> -kevin
> ad6z
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