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On 3 May,
wrote:

wrote:
On 3 May,
wrote:

I was refering to the case where there is nothing to adjust the match
and getting something under 3:1 at the end of the feed line.

It was well within 1.5:1 within (part of) 10 and 12m bands.

After spending several hours with EZNEC trying to get that combination
under 5:1 with no success, I find that hard to believe unless something in
the system is really lossy (like the feedline) and masking the SWR or the
length of the feedline just happens to be at a length to help the match.

Well I modelled it and matched it fine on the two bands The ends of the
feeder are about 4" apart and effectively the inner ends of the longer dipole
are an 8" length of 600 ohm line, so that length comes off the 12m dipole
elements. I couldn't get it to match with it as a genuine fan (inner ends
paralelled) with any small spacing at the ends of the dipoles, so I made them
about 8" apart.

Then they matched up nicely. The feeder is short, and low loss, as seen by
the out of band (mis)match.

The trick is to make the dipoles far enough seperated so as to be seen by the
feeder as seperate elements. This lost me much time.



--
BD
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On Thu, 3 May 2012 00:37:47 -0000, wrote:

wrote:
On 3 May,
wrote:

I was refering to the case where there is nothing to adjust the match and
getting something under 3:1 at the end of the feed line.

It was well within 1.5:1 within (part of) 10 and 12m bands.

After spending several hours with EZNEC trying to get that combination under
5:1 with no success, I find that hard to believe unless something in the
system is really lossy (like the feedline) and masking the SWR or the length
of the feedline just happens to be at a length to help the match.

EZNEC lacks an optimiser. Try 4NEC2 and let the optimiser tweak the
wire lengths for best VWSR in the various bands.
http://www.youtube.com/watch?v=kYy6Yur127A 25min
The bad news is that since the fan dipole works on several bands, the
optimizer has to be run individually for each band. Fortunately, it
is easy to limit the parameters that it changes to just the wire
length for that band in question. I'm tempted to build an NEC2 model
and see what happens. Maybe this weekend.

I blundered across this article on fan dipole construction:
http://www.hamuniverse.com/multidipole.html
http://sstowers.com/aa4cv/
If I can find a long straight location, that doesn't come too close to
a tree, I might try a fan dipole again.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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On 3 May,
wrote:

The next modification will likely be finding a 10 M trap at a swap meet
and installing it at about 5/8 wavelengths to push the 10 M lobe down.

Why not put a quarter wave stub for 10metres about half wave down from the
top to make it radiate as two half waves in phase on 10? That would also
guive a little loading on the more LF bands increasing efficency slightly.
I'm not sure without checking what the effect on the bands below 20 would be.

--
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Sal wrote:

wrote in message
...
Jeff Liebermann wrote:

snip


FYI I spent an afternoon with EZNEC exploring fan dipoles.

What I found was the farther apart the wires, the less the interaction,
which is what one would expect from common sense.

From my limited runs it appears that somewhere around 20 degrees is about
where the minimum separation needs to be for practical length adjustments.

At angles less than that be prepared to spend a lot of time pruning.

It doesn't matter if the wires are separated vertically or horizontally
other than horizontal separatation means you need lots of support points.

If the bands are too close together, i.e. 20-17, 17-15, 12-10, you can

I had a 10-20 fan dipole laying on a roof for a while. Matched OK after a
bit of pruning but I abandoned it when I found both bands were better with
antennas that were clear of the roof.

I wonder something (and I suppose I'll have to try it, now) : Say I had a
20m horizontal dipole up at 30 feet, could I feed it through a coax that had
a Tee-connector located 22 feet up and simultaneously feed a 15m antenna
from that Tee-connector?

Just as they would be in parallel as a true fan dipole with a common feed
point, so also would they be in parallel -- just not sharing a common feed
point.

Normally I avoid Tee-connectors because they introduce the Evil Mismatch but
this time ...

I can see one problem already; some of the 15m energy that divides at the
Tee-connector would go up to the 20m antenna and be partially radiated and
partially reflected. Standing waves. The coax length would alter the
effect.

If I were going to do that, I would use ladder line between the elements
and connect the coax to the bottom.

It would be easy to model this with EZNEC, even the free demo version, so
I would do that first.

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