[TowerTalk] 4 Squares and Coils from N0AH
Dinsterdog@aol.com
Dinsterdog@aol.com
Mon, 6 Sep 1999 00:12:26 EDT
Greetings all-
When I last left you- I was trying to figure out some tuning issues with my
recently completed four square for 80 meters. As a reminder, I was using
ground mounted shortened verticals, made from aluminum radiators, 44 feet
long, with top hats of 20 feet connected at the 39.5 foot level- I used a
Comtek hybred coupler to phase the system- 60 1/4 ground radials per antenna-
Alrighteee- I had an interesting time experimenting with coils in trying
reduce my 4 sqr's resonant point from 3.900MHz to 3.800MHz. Added to my task
was trying to improve upon the array's over-all efficiency-
In the beginning, as I've mentioned in a previous posting- I made the first
vertical resonant at 3.700MHz and made the other three verticals exactly like
the first.
I expected the array's resonant point, , once the four verticals were up, to
jump up 100KHz so that the array would most efficiently resonate at
3.800MHz. (Comtek's manual states to expect this jump and you can find
reference to why this happens in ON4UN's 3rd edition on Low Band DX'ing- and
many other references on array's )
But the array jumped up 200KHz and I was stuck at 3.900MHz. Now what? I
really wanted to be perfect inside the DX ssb window-
So, I decided to explore ways to bring the array's resonant point down
100KHz. The best method would be to take the verticals down, and replace the
20 foot top hat wires with longer ones....say, 21 1/2 feet- but what a mess
and drawn out affair in high winds and rain-- -
So the next option was to increase the angle of the existing top hats from
45% downward slope to around 30% downward slope, thus, in effect, making the
top hat wires more horizontal. (See March 98 QST artical on shortened
verticals by AC7A, awsome!) But this would take a lot of work, more space,
and a lot more rope- so that idea was vetoe-d-
The third option was to use coils- and try to reduce the resonant point with
a bit of inductance- blah.........(K0RF kept suggesting I do this, but did I
listen?) - that mean't I had to disconnect my siliconed feedpoints
etc.....but it was a lot easier than the other options I ruled out-
Per a lot of opinions, I would not need much of a coil, and besides, I knew I
would need to try coils eventually for working CVW contests etc........
So this week, when N2IC wanted to come up and run CW sweepstakes from WY, as
a member of the Wyoming Split Rock Club (K7KU), I decided to get the array to
play on 80 meter CW. I planned to try even bigger coils than I had made for
lowering the array's resonant point into the ssb window--
Recall, that a four square resonance, using a hybred coupler, is measured by
the amount of power not used by the array, that is dumped into a dummy
load off the coupler- The higher the power dumping into the dummy load, the
less effective the array.
Most 4 sqr operators, using hybred couplers, shoot for less than 10% of the
power going into the dummy load with 5% percent being considered good, and
less than 5% considered excellent- Over 20% is considered bad as the F/B gain
of the array begins to go south-
Ok, with all this said, and another claim that I am not an engineer, and only
pass on these results as an amateur, and I mean amateur, - here are the
results of power dumped using no coil, using a short coil, and then using a
long coil.
I used 500 watts out and measured power levels using rat shack power meters-
(they are not perfect, but they did give enough movement for an educated
guess on power dissapaited by the dummy load)
All coils were constructed from 6 AWG copper-
The short coil, that I decided to use for the ssb DX window, has a 2 1/4
inch diameter, with 3/4 inch spacing between coils, and four turns. Each coil
is about 3 1/2 inches long.
The longer coil, with the same diameter, that I decided to use for the CW
window, has 11 turns, at 1/2 inch spacing, and is around 5 /12 inches long.
Each vertical was tested and coils slightly adjusted so that the minimal SWR
point was within 1-5 KHz of one another before the array was put back
together via the hybred coupler-
Power dumped measurement results-
Freq No Coil Short Coil Long Coil
3.500 27% 20% 11%
3.525 25 19 10
3.550 24 18 11
3.575 22 18 11
3.600 18 16 11
3.625 18 15 11
3.650 18 12 11
3.675 16 12 11
3.700 16 9 14
3.725 13 7 18
3.750 11 7 18
3.800 10 7 19
3.825 9 8 20
3.850 8 8 22
3.875 7 9 23
3.900 6 10 26
3.925 7 11 26
3.950 8 12 28
3.975 9 15 30
4.000 9 18 30
What I can conclude from all this testing is that adding coils did not allow
me to obtain a significantly lower minumum power dump point vs leaving the
coils off.
This was disappointing as I was hoping to get the array down to less than 3%
of power dumped but being able to play with the coils a bit still did not
allow me to do this- oh well-
But the coils did allow me to tune the array enough to play well inside both
the ssb and cw DX windows-
However, I was not able to get the array to be as resonant inside the cw
window as I got it in the ssb dx window- In fact, I could not get the array
to down to less than 10% using the longer coils inside the cw portion of the
band vs the 7% inside the ssb dx window and 6% at the array's natural
resonant point without coils- Pwrhaps just too far away from the natural
resonant point of the antenna- or perhaps my coils stunk- I'm not sure why-
(Inside the cw window, I did see a dip at around 3.525MHz to 10% but it went
back up to 11% down around 3.500MHz-)
Since I seldom go above 3.850MHz, I have decided to leave the short coils on
for now- and now that I have constructed the longer coils, I have an easy way
to convert the array for CW contests/operations-
I dont think I will ever take the verticals down to add longer top hat wires.
I'll still need coils to play in the CW dx window.......-
So there you have it- if you ever plan to put up a shortened vertical 80
meter array, and you want to use coils, this might help for reference. So why
all the trouble????-
Well, there can very very small differences between good and excellent- just
look at the downhill times between olympic skiers... a few micro seconds can
separate good from excellent even though the differences are not really
measurable in the real world-
I dont see any DX station seeing my signal being any different at 92%
efficiency vs. 97% efficiency...but it is fun trying!
I don't know if I can do anything else to get the array to put out less than
5% into the dummy load- but trying to has been an overall positive
experience-
EFFECTS OF MUTUAL COUPLING-
Of note, I did make a mistake last week in measurements regarding the effects
of mutual coupling in a email to towertalk 8/31/99.
I reported that the mutual coupling effects of having all four verticals
raised in the proximity of the four square raised each individual vertical's
resonant point. Well, it turns out the original single vertical I measured
to be resonant at 3.700MHz, in free space, without the other verticals up,
stayed at 3.700MHz in the array-
The bounce from the individual antenna's resonant point at 3.700MHz to the
array's resonant point at 3.900MHz seems to be caused when the antennas are
joined together using the Comtek coupler- Perhaps another 4 sqr owner using
a comtek hybred coupler can verify this???
I though that the mutual coupling of the verticals, independent of the
coupler, caused this jump. But not by today's testing- Each vertical,
completely disconnected from the feedlines leading to the coupler, tested to
be resonant at around 3.700MHz-the same as the original vertical- (I may
have gotten false results on the original tests because I left the feedlines
connected at the coupler, although I had the center conducters disconnected
at the feedpoints-)
That said- Stick a fork in it- I'm done and ready to DX-
73 Paul N0AH
Carpenter, WY
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