[TowerTalk] K8UR wire 4-SQR characteristics vs 1/4 vertical array design from N0AH
Dinsterdog@aol.com
Dinsterdog@aol.com
Tue, 8 Aug 2000 14:12:48 EDT
After having a lot of good results with a 80 meter 1/4 wave vertical 4-SQR
array, I decided to try a 4-SQR for 40 meters and contacted Jim at Comtek
System for some ideas. He sent a diagram showing a K8UR style 1/2 wave
dipole array so I thought I'd give it a go-
You can find out more on this design in ON4UN's Low Band DXing Book, third
edition, page 11-79 (c) for a diagram. I know Jim is doing a lot right now
testing out various ideas with the design, so he is a great resource to
contact as well-
After talking to Jim, I finished a Rohn 25 tower I started 2 1/2 years ago
(one of those projects) and topped it off at 65 feet with a 10 foot mast.
Each end of the dipoles were connected at the top and bottom of the tower
with the feedpoints pulled out 25 feet from the tower.
I used 50 foot ropes tied to the W2DU current baluns to pull out the feed
points and tied them to T-posts. You really need to use the current baluns
for this design. The Hybrid coupler was placed in the center of the tower-
When all the dipoles were in, they formed a diamond shape pattern. Each
feedpoint was spaced at 1/4 wave distances from one another, (about 33 feet)
forming a square pattern. Diagonal spacing between the feedpoints was around
48 feet. I used 26 1/2 foot long feedlines using 75 ohm coax with a 78%
velocity factor. They sag a bit between the feedpoints and the tower-
Per Comtek instructions, I hung the top of the antennas (positive side (+)
of the dipole towards the top of the tower) about 5 feet of the tower mast
while to bottom leges were pulled out only around 18 inches. This was to
minimize the antenna(s) coupling with the tower. I also used insulated guy
wires to minimize coupling intereaction.
It was suggested that I get the top of the antennas as far away as possible
from the top of the tower, but I compromised a bit on the lower legs, which
represented the ground side (-) of the dipoles. While the top wires were
about 5 feet away form the tower, the lower legs were only around 18 inches
from the tower in order to keep the dipoles from blowing in the wind. I used
parachute cord to tie the ropes from the dipole insulators to the tower legs.
The array's performance, front to back, was a lot like the 80 meter vertical
array, around 20-25db's. I was really surprised, almost shocked, at this
high level of performance considering I spent a fraction of time and $$$ on
this array vs the 80 meter vertical design.
Tests on the dipole array, thus far, have been limited but promising- Most
testing done has been by listening to stateside stuff last night in heavy QRN
and JA's and VK's in the last couple of mornings. I've heard enough to see
how this array might play and it is very responsive. It is going to take a
full season to really give this antenna a good test run but initial results
have been great!
The most significant test measurement between my 40 meter K8UR dipole array
and my 80 meter 1/4 vertical array, is the minimum power dissipation into the
dummy load.
For those of you not up on 4-SQR's, if you use a Comtek hybrid coupler to
properly phase the array, using a Collins type feed system, you test the
efficiency of the antenna system by measuring how much of your power is not
being radiated. The less, the better. This is done by reading the amount of
power going into a dummy load port off the comtek coupler. It represents
power that is not being radiated out of the array due to resonant points.
Normally, power not radiated out due to resonant points is measured by SWR
readings. But the hybrid coupler dumps this unused power into a dummy load
port for reasons Jim at Comtek can better answer then I can...it's a neat
part of the science of array design.
Since the hybrid coupler hides true SWR readings, you need to measure the
power dumped into the dummy load to see whats up.
The 1/2 wave dipole array's minimum power dissipation into the dummy load was
at or close to 1%. I about fell out of my chair as It hardly moved the
meter. The 4-Sqr 1/4 vertical array design's minimum power dissipation was
at around 3%. It may not sound like a big difference, but trust me, when you
get into building arrays, it seems huge on bandwidth performance. For the
lower your minimum power dissipation, the more potential you have for greater
bandwidth.
For example, when my 80 meter vertical array was dumping a minimum of 7% at
it's most resonant point, I only had 150KHz of operating room on bandwidth
before the power dissipation readings exceeded 20%. (When you go past 20%,
you start to lose F/B performance and your dummy load can really get hot- so
20% is a good rule of thumb I've found to follow for measuring effective
bandwidth of an array).
However, by improving the minimum power dissipation by using more full sized
radiators, I went from 7% down to 3% which gave me around 400KHz of band
width between the 20% power dump edges. Not bad considering I only
lengthened the original top loaded verticals by 8 feet.
The moral here is that using full size 1/4 wave and/or full size 1/2 wave
radiators in an array is really a lot better than using compromised shortened
antenna designs. But this is only from what I've done here- I'm sure others
will have their opinions on the subject as far as what can effect minimum
power dissipation levels. Types of feedlines, styles of 4-SQR's, etc......
all have an impact-
While the 80 meter 1/4 wave vertical array's resonant point jump 180KHz from
where the individual radiators were tuned due to the effects of mutual
coupling, the K8UR array's resonant point stayed exactly where the dipoles
were tuned.
Jim at Comtek Systems told he not expect a jump with the dipole array and he
was right on the money. W0ETC told me his wire array jumped around 5%. So I
thought I'd end up somewhere in between- hihi
On the other hand, my 80 meter vertical array jump was expected to be around
100KHz, but it jumped 180 KHz. So go figure- The effects of mutual coupling
seems to be a case by case issue. But definitely, there is less potential
for a big jump in a K8UR array vs. a 1/4 wave vertical design.
If you have a tall enough tower, I really think you will get your money's
worth by utilizing a K8UR 1/2 wave dipole design vs. the time, $$$ space
needed for a 1/4 wave vertical array of some sort.
Because I was using these arrays for different bands, a sided by side
comparision was not possible. But from a performance standpoint, they sure
seem a lot alike.
How a 1/4 wave full size vertical 40 meter array with a decent ground system
would play against a K8UR design, I'm not as persuaded as I used to be that
there would be a huge difference in performance. But I'm sure the 1/4
vertical design would have the edge- but for the $$ and time spent, the K8UR
approach seems a decent alternative!
And I can't say enough about how nice it is to have these high quality Comtek
System Hybrid coupler units to use for my arrays.
If you are building and/or considering an array, and you need any assistance,
please email me at dinsterdog@aol.com and I will try and help where I can.
While there are experts out there on array's, I still have a tough time
finding all the information I'd like to on their performance and design. A
lot of home brewing will go into your array projects- But they sure are a
neat way to go!
73 Paul N0AH
Carpenter, Wy
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