[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 

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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