[TowerTalk] RE: 80 meter 4 square array notes from N0AH

[Dick Green]

Paul,

Thanks for reporting your results in detail. They will be invaluable to
anyone putting up a 4-square, especially on 80M. Just a few
questions/comments:

> I used black nylon cord to guy each vertical.  They are guyed at 20 feet,
35
> feet, and at the 39.5 foot level using the tophats pulled out with nylon
> rope.

I had some trouble visualizing this. At first I thought the tophats were
pulled out horizontally, but it sounds like they slope down from the 39.5
foot level and are attached to nylon cord at the bottom. If so, about what
angle did you use?

> - Lots
> of data on rasied radials but none of seem conclusive that a 1-4 wire
raised
> radial system would be better than a 60 ground radials per vertical system
> used for an array-

I think I read here on TT that the element and radial system would have to
be raised a minimum of 1/4 wavelength (or is it 1/2 wavelength?) to be as
efficient as a ground-mounted system with 60-120 radials. Anybody know if
that's true?

> The antenna array takes up about 1 acre of land, apprx 46,000 sq ft, and I
> use another 40,000 sq feet to keep everything at least a full wave length
> away from the tips of the array's radials-  Thus a total of 76,000 sq feet
> were used to accomodate the system.

I've never heard that it's important to keep the radial tips separated from
other objects. K4SQR did tell me that the elements themselves should be at
least 1/2 wavelength from any metal structure, especially a tower. I can see
needing to maintain separation for the tips of raised radials because they
must be carefully tuned to resonance. But is it really necessary to give
ground-mounted radial tips that much of a buffer?

> Now this is a touchy subject because mutual coupling does not always
produce
> a 100KHz jump.  You might get only a 50KHz jump or have one as big as
200KHz-
> such as the case with mine-  But 100KHz tends to be the average and the
> standard suggested by many 4 square operators-

Now that you mention it, I've always wondered if the jump depends on the
band for which the array is designed. My 40M 4-square jumped about 100KHz.
Should an 80M array jump 50 KHz, 100 KHz or 200 KHz?

> Ok, so what were some of my results-  Well, first of all, my resonant
point
> jumped almost 200KHz......so while the feedlines and spacing between the
> verticals are cut and measured for the phone DX window-  I need to
lenghten
> my verticals to bring them down a bit-  Right now, my minimum power dump
is
> 5% at 3.900MHz and around 8-10% in the DX ssb window-  it is a whomping
20%
> in the CW band but I can fix this by reducing the current array resonant
> point-

Although 4-square owners tend to be very exacting on the feedline length and
element spacing (I know I was), I really don't think that they have as much
impact on the frequency at which dumped power is minimized as element tuning
does.  I say this because my 40M 1/4-wave phasing lines had incredibly broad
nulls, and I suspect that 80M phasing lines would be even broader. I do
think that changes in feedline length and element spacing have more effect
on maximum gain and F/B.

However, individual element tuning has a *massive* effect on the point of
resonance. Small changes there can make a big difference.

(snipped and moved)

> As for tuning, I tested each of the individual verticals and found them
all
> to bottom out around 3.900 MHz...so they are are acting like clones
although
> the bandwidth of the SWR bell curves vary by as much as 40KHz.............

This doesn't make sense. If the individual elements are resonant at 3.900
MHz, the array's resonant frequency should be higher than that. Did you
disconnect all of the verticals before you tested the individual resonant
point of each one? If not, the measurements are invalid. You must test each
element at the feedpoint (not through the phasing line), with the other
three disconnected. That way, you eliminate the effects of mutual coupling.
As you say above, for resonance at 3775, you would probably shoot for 3675.
However, I wouldn't rely on just measuring one vertical and building the
rest of the elements to the same dimensions. Instead, you should tune each
element separately to 3675, after it is mounted over its set of radials
(again, with the other three in place and disconnected.) I found that there
were differences between the physical dimensions required for each element,
and the impedance at resonance was not exactly the same either (it was very
close on three, but one is a little higher -- probably a minor defect in the
balun or coupler relays.)

FWIW, I designed my array for resonance at 7.025 MHz (I'm a CW op and I
wanted to hedge in case some WARC pushes the band edge down a hundred KHz.)
I used a SWR analyzer to set the four elements to exactly the same resonant
frequency, which was something like 6.950 MHz (I think.) The point of
minumum dumped power ended up at 7.050 MHz, and I decided to leave it there
because it was only 2%, and the highest it got in the CW portion was 2.5%.
It was only 8% at the top edge of the band and less than 6% below 7.300 MHz.
I guess it's easier to build a 40M 4-square than an 80M 4-square!

> Front gain looks to be 6-8dBs based again, on testing the array with DX
and
> stateside contacts and using a Butternut HF2V for comparisons-

That would be gain over a vertical, not a dipole. I believe a full-sized
ground-mounted vertical is something like -3dB gain compared to a dipole
(but, of course, the angle of radiation is lower.) I was able to run many
comparison tests between a GAP Titan multiband vertical and a single
ground-mounted element with 60 radials during the winter before I built the
rest of my array. By comparing the array to the GAP, I deduced a gain of
about 5-6 dB over the single full-sized element. That would make the array
about 2-3 dB better than a dipole, which is about what you would expect from
a 2-element shorty forty.

>
total.......................................................................
..
> ....$1,623

I spent about $1,000 on my 4-square, but I got the coupler in a non-cash
trade. Clearing the land of trees cost a lot more, however...

One way to save a little money, and get more accurate dumped power
measurements, is to use a wattmeter with a remote directional coupler. I
used a Radio Shack digital SWR/Power meter that had been discontinued and
was very cheap. I needed three wires for the coupler anyway, so I ran a
six-wire cable and used the other three for the directional coupler. It
sends DC signals back to the wattmeter in the shack. I tested it with 500
feet of #24 wire and the voltage drop did not have a significant effect on
the wattmeter reading. The cable I'm using has about 350 feet of #22 (#18
for the ground returns.) 400 feet of RG/213 would have about 1.5 dB of loss
at 3.7 MHz, so wouldn't the reading through your inexpensive coax be about
25% lower than the actual dumped power?

73, Dick WC1M





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