N4ZR wrote: (full text on bottom)
I run the type of array that Jim describes on 80 (with the bottom halves of
the dipoles brought back to the bottom of the tower), using the dimensions
in K3LR's August 1994 QST article, and am quite pleased with it. I have
built a number of models, both with the antenna alone and with my tower in
the middle. There is a good deal of current on the tower in the models,
which suggests to me that it contributes usefully to F/B ratio.
My reply:
Pete (TT)
I only tried this once, on 75, and had good results.
Didn't model it. Didn't fold the ends back, just
let the dipoles stretch out, and sag a bit, as they
snaked through the forest. A tall pine tree was the
vertical element, so no tower impact.
If they're 133', and at 45 degrees, then .707 of that,
or 94' is the radius of the subtended circle. So, you're
right, it's NOT a small antenna for 75. For 40, it'd
be half of that, or just under a 100' radius.
I seem to recall having spoken with someone about models
with the legs folded back vs. not, and it didn't make
much difference.
The unanswered question, though, is CAN THIS BE DONE USING
MONOPOLES FED AGAINST GROUND, USING THE SAME FEEDLINE SCHEME?
I think it's a worthy experiment, but needs to be modelled
for spacing. I suspect a 1/4 wave element spacing with a
3/8 wave feedline might work ok, based on the typical cardoid
pattern which derives from 90 degree spacing fed 90 degrees
out.
N2EA
jimjarvis@ieee.org
-----Original Message-----
From: Pete Smith [mailto:n4zr@contesting.com]
Sent: Tuesday, February 22, 2005 07:30
To: jimjarvis@ieee.org
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] 4square/spitfire alternative
I run the type of array that Jim describes on 80 (with the bottom halves of
the dipoles brought back to the bottom of the tower), using the dimensions
in K3LR's August 1994 QST article, and am quite pleased with it. I have
built a number of models, both with the antenna alone and with my tower in
the middle. There is a good deal of current on the tower in the models,
which suggests to me that it contributes usefully to F/B ratio. Also,
there is a certain amount of current on the longest set of guy wires, which
may detract a bit. In practice, I find that F/B can be as high as 35 dB on
high-angle signals, and on DX, it is quite common for the DX to be
inaudible when the antenna is pointed 180 degrees away. The front lobe is
quite broad, so that often switching just 90 degrees away will have
relatively little effect.
Re modeling, if you use transmission lines in NEC-2, the result is pretty
high gain but indifferent F/B ratio. However, NEC-2 transmission lines are
lossless, and if you substitute loads with the characteristics of the same
length of RG-8X, the calculated gain drops about a dB but the F/B ratio
improves substantially.
The models indicate, and experience seems to prove, that this array works
quite well for several hundred KHz above the resonant frequency of the
dipoles. I use it to good effect in the 75m DX window, even though my
dipoles are cut for 3525. However, 100 KHz below the resonant frequency,
the pattern is essentially omnidirectional.
Just one final comment on the limited space issue. If you hang these
dipoles from rope, you will discover that the array requires quite a large
circle for the outer attachment points of the ropes -- I would estimate
about a 125-foot-radius circle for 80m. This is because the required
tension on the ropes goes up very rapidly as you try to pull the top halves
of the dipoles straight at the required angle, due to the weight of the
feedlines. If you can incorporate the top halves of the dipoles into your
top guys (as W9LT/WA7LT, who first showed me this array, does), then you
can get around this problem.
73, Pete N4ZR
At 05:58 AM 2/22/2005, you wrote:
>kk9k wrote:
>
> >I want to try this antenna as my marginal cost will be zero. I have 4
33'
> >verticals, an Ameritron RSC 8-V switch and plenty of radial wire and coax
> >on hand.
> >
> >Any info on design and tuning will be appreciated. I believe the design
> >has 3 "reflectors" that use the inductive reactance of the open feedline
to
> >make them "reflectors," right. How long should the feedlines be? I can
> >tune with the MFJ 259B.
> >
> >Any models out there to tell me what to expect?
>
>You didn't mention what you have for vertical support, and this may be
>the dominant deciding factor. If it were me, however, I would look at
>an array of 4 sloping dipoles on 40...switched at the center, and fed
>in the center. If I recall the design the ARRL antenna book has, they
>use 3/8 wave feedlines to a center switch. The result is one DE in
>the desired direction, and 3 or 4 passive reflectors elsewhere.
>
>I've seen both 4 and 5 dipole variants of this. The feedline shields
>need to be isolated from the switch at RF, for optimal f/b & f/s.
>The dipoles can slope straight, or the lower halves can fold back to
>the base of the center support, in a kind of diamond. They don't require
>a tower as a center radiator, so you can hoist 'em in a big tree.
>
>This would be my 'small space' 80m choice. Possibly even 40, if I didn't
>have clearance for a yagi.
>
>N2EA
>jimjarvis@ieee.org
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|