First off, thanks to LB Cebik & Steve Best for help with feeding stacks of
antennas.
It looks like there are two ways to do it, and from what I can see they give
equivalent results.
1) Use two tranmission lines (TL) coming to a single short wire then just feed
the wire.
To adjust the phasing, just adjust the length of the TL's.
2) Excite the two antennas independently with two EX cards. To adjust the
phasing,
just alter the real & imaginary parts of the input voltage.
Next, I started playing with 15m beams and stacks. I've already got a Force-12
EF-515 at about 85 ft, so I was looking to add another 15m beam at about 40 ft.
I noticed that the Hygain 155CA is quite a bit cheaper than the Force-12 so I
started
looking at combos of those antennas. I created models from the data at
Hygain's web
site and the Force-12 manual, then applied the taper correction algorithim that
is in
the ARRL hand book. (I don't think it is as acurate as Leeson's but it appears
to be
close - and since raw NEC2 won't handle tapered elements I had to convert back
to an
untapered design). Here's what I found:
Upper Lower Gain (in-phase dBi) Gain (out of phase dBi)
EF-515 155CA 16.30 (10 deg) 15.43 (21 deg)
EF-515 EF-515 16.75 (9 deg) 15.62 (22 deg)
155CA 155CA 17.00 (9 deg) 15.69 (22 deg)
Then I noticed that the 155CA doesn't have enough spacing between the elements
to swing around my tower on a side mount. So I started playing with the model,
and the results look interesting. (All data in freespace)
Freq Gain (dBi) SWR F/B
21.0 9.88 1.18 (45-j6) 10.09
21.1 9.90 1.14 (49-j6) 11.34
21.2 9.89 1.13 (51-j6) 12.79
21.3 9.87 1.06 (52-j2) 14.66
21.4 9.79 1.13 (54+j5) 17.41
21.5 9.58 1.49 (64+j18) 20.63
So it looks pretty good, except for the FB (not so hot there .. if I'm reading
the data out of NEC2 correctly). But it is direct 50 ohm feed, and has a
little better forward gain than a stock 155CA (at least by my models). When I
moved the 1st & 2nd director away from the center a little more, it appears that
the beam started acting like an OWA design. The 1st director seems to control
the
feedpoint impedance more than the gain.
Here are the element dimensions (inches):
Dist from refl Half length Diam
Refl 0 137.0 .875
Driv 67 132.0 .875
Dir 1 90 126.0 .875
Dir 2 203 129.0 .875
Dir 3 309 125.0 .875
So now the problem is how to convert this untapered model back to the real
world.
I'm thinking about buying a 155CA and modifying it to this design, I'm just not
sure what the new element lengths will look like. Anyone have a good way to
measure
the resonant freq of an element in place with a MFJ-259B (without splitting the
element)?
If anyone plays around with this model and comes up with something better, let
me know. Comments and suggestions are appreciated.
73
--Lee - KT4ZX
leewells@lexmark.com
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