After building three different 1/4 wave top loaded vertical arrays for 80
meters, I'd like to pass on some quick trivia from my experiences to others
considering such a project:
First, for quick reference..........The antennas used are made from Texas
Towers aluminum with a bottom diameter of 2 1/8 inches and a top diameter of
1.625 inches. 12 Gauge insulated copper braid wire was used in the top hats
that were attached at around a 45 degree angle just below the top of the
verticals. Three top hats per antenna were used. I use a Comtek Hybrid
Coupler to phase the antennas and 75ohm coax from the Comtek box to the
antennas.
Trivia Facts Discovered:
The shorter the verticals, the longer the top hats, the narrower the
bandwidth of the array. The 44 footers with 20 foot top hats only had about
200KHz of usable bandwidth while the 52 footers with 16 foot top hats
provided enough usable bandwidth, around 400KHz, to center the array at
3.650MHz. Thus effectively covering both the CW and SSB DX windows with
acceptable power loss into the array's dummy load. The 44 footers should be
designed for either the CW OR SSB window. I'm now using the 45.8 footers
that favor the SSB window but are very usable in the CW window as well.
Longer verticals with shorter top hats provide a lower SWR minimum, and
greater bandwidth, when individual antennas are tested prior to being put
into the array. My minimum SWR reading at resonance on the 44 footers was
2.3:1, on the 45.8 footers 2.2:1 and on the 52 footers 1.7:1.
Longer verticals with shorter top hats provide a lower "minimum" power
dissipation into the dummy load from the array. My minimum power dumped
using the 44 footers at the array's resonant point was 7%...it went down to
3% on the 52 footers.
Utilizing 44 foot, 45.8 foot, and 52 foot verticals, the length of the top
hat loading wires seemed to change the resonant point of the array by approx.
50Khz for every foot added in length. For example, on the 44.8 foot
verticals, 15 foot 6 inch top hats made the antenna resonant at 3.775 while
using 19.5 foot top hats dropped the resonant point down to 3.650MHz......
While it has been stated that dumping more than 20% of your power into the
dummy load effects F/B, I never noticed this effect.......F/B was, and is, a
killer on a 1/4 wave 4SQR vertical, anywhere in the band I have listened.
Just an opinion...hi
In all three designs, the resonant point of the array jump about 170Khz from
where the individual antennas were cut.
After 2 winters, the army surplus parachute cord used for guying the
verticals has held up with no problems-
Of note, I'd suggest never to mess around with one of these array's without
using a static bleed off resistor..............or ground the antenna with an
alligator clip to ground when working on it if it is up in the air-
The array seemed to hear weak CW signals better long path vs the beverages.
Short path was a toss up but typically went to the beverages due to higher
QRN levels. If you go to the trouble of installing a 4SQR, you may want to
consider going with a couple of NE and SE beverages.
Building three different designs of this array has been a learning
experience. The first one I built, I wanted to make better. The second one
was the best but came down in an ice storm. The third was a result of what I
learned from the first two.
Some things, like the usable bandwidth and minimum power dissipated into the
dummy load, had dramatic differences between the versions. While other
characteristics, like resonant jumps, or length of the top hat loading wires
effecting resonance, did not change much no matter which verticals I was
using.
Hope these notes help any future array builders. If you own one, please
email me any notes you may have on your 4SQR as I am putting together as much
information as possible for future reference. For more detailed information
on my array, please look in CQ Contest magazine Sept 2000 pages 27-33.
73 Paul N0AH
Carpenter, Wyoming
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