[TowerTalk] 4square/spitfire alternative

[Pete Smith]

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 at ieee.org
>
>
>_______________________________________________
>
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>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
>any questions and ask for Sherman, W2FLA.
>
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