My goal is to build an 80/40 vertical antenna this summer. I've been
modeling the antenna and have some questions for those "in-the-know" on
Towertalk...
There is an article on the design on pp 7-16 of the 19th Ed of the Antenna
Book (ARRL) by Roger Cox-WB0DGF. The article describes the function of the
"Open Sleeve" antenna but no real design dimensions. Modeling the antenna
was the only reasonable approach.
I have access to substantial quantities of 20' lengths of used well casing.
It is 2-3/8" galvanized (thick wall) steel pipe. The main element ( 60'
with some top loading) is insulated from ground. Two grounded side elements
of ~ 1/4 wavelength (on 40 m)form the transmission line for the 40 meter
element. On 80 meters, they are almost transparent. For 80 meter
operation, the whole mess loads together.
After screwing around with AO and NEC2 for some time, I have the dimensions
for the sleeves. Using 1.315" chain link fence top rail material for the
sleeves dictates a modeled dimension of 32.86'. The spacing on the sleeves
is 2.45' on each side of the main element. The feedpoint impedance dictates
a series capacitor of about 1000 pf to resonate at close to 50 ohms on both
80 (3.550 mHz) and 40 (7.150 mHz). Both AO and NEC2 confirm similar
results. Using 2, 3 or 4 sleeves changes all the dimensions slightly to
obtain resonance. I see no benefit to using more than two sleeves.
Although I didn't model it, one sleeve may be sufficient.
The hat in my model is a pair of perpendicular extensions extending 3.88'
off the main element and the tips are connected with 14 awg wire. This
forms a square hat roughly 5.5' on a side with the corners supported by the
cross members attached to the top of the main element.
The model shows a good omni pattern (expected) and fair bandwidth. < 2:1 up
to 3.85 mHz or so and the whole 40 meter band. I can't wait to get my hands
on it this winter when the bands are quiet!
I have questions though. Perhaps some have had experience with this type of
antenna and can shed some light.
1) The model shows that the sleeve spacing is quite critical. Do I need to
be able to adjust this distance in place or can I simply fabricate a fixed
element bracket--slam it into the air and forget it? More to the point: Are
open sleeve designs a bear to resonate?
2) The main structure will be 60' with some top hat (see above). The well
casing has quite a thick wall but the couplings are threaded. Should I weld
these couplings solid first before hoisting it up?
3) I am hoping to get by with a two tiered guy system. Perhaps 50' and 25',
spaced 120 degrees. Adequate?
4) What distance should I place the guys from the tower? 35' OK?
5) I want to hoist the antenna using a derrick arrangement (well casing
again). Using a 6" x 6" post for center support (cemented in of course). I
will drill a large hole in the main element and run a large diameter bolt
(perhaps 1/2" solid rod threaded on the ends) through the main element and
the post. With the assembled antenna laying on the ground, a 35' length of
well casing will be temporarily attached to the post using a similar solid
rod arrangement. The guys will be attached all the way around. The derrick
will be vertical with the two guys attached to the end (top). Using the
pickup, I'll attach to the derrick and pull. The top of the derrick will
have ropes attached that run to either side of the tower to prevent it from
tipping over during the lift. Does this sound reasonable? The well casing
must weigh in at 60 - 80 lb per 20' length. Add the top load and the
sleeves -- it will be quite heavy. I want to succeed in building this
antenna. I don't want to kill anybody in the process!
Any and all comments, including "you're f....ing nuts!" are greatly
appreciated. I'll let you guys know my progress went there is some
progress.
Thanks for the bandwidth...
Ford-N0FP
ford@cmgate.com
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