In a message dated 8/29/01 4:44:53 AM Pacific Daylight Time, w5kp@swbell.net
writes:
<<
In all good spirit and courtesy, as a retired naval officer who in younger
days spent many hours in "turits", please allow me to correct the spelling
to "turret", which is a term generally reserved for gun mounts which are
completely enclosed and have humans running around inside, like the big guns
on battleships and those on tanks. The ones on battleships can throw a 16"
diameter 2,000 pound high explosive projectile over 20 miles with extreme
accuracy. They were a source of enemy terror on many an occasion right up to
the end of the Viet Nam war.
It is possible that older WW2 20mm or 40mm gun mounts might be available via
one of the large shipbuilding and scrap iron firms that dismantle and sell
the steel from retired navy ships. Keep in mind that the smaller the gun,
the faster it would train (move in azimuth). A 40mm gun mount would just
about give the crew whiplash if the guy on the remote controller got carried
away. It would take some serious machine/gearing work to make it feasible.
And even a relatively small 40mm mount base would weigh thousands of pounds
and be about 12' across, as I recall. An interesting but not very practical
idea. I'd think a base mount from a scrapped small crane might be a better
source for a large finely made ring gear and drive setup. It would make a
heck of a tower rotator, though.
73's and Anchors Aweigh!
Jerry W5KP
>>
You are right about the spelling. I copied what the original poster used.
When I read your correction I said "how did I miss that?" I've never been a
good speller.
I heard the Russian Hams used the gun turret from tanks somehow and that
wouldn't be too heavy. I also heard they used the motor that was greared to
and rotated the turret for rotators --which gave them the "Booming Signal."
Another rotating platform hams have used is the one car dealers used to
rotate cars in the show room. The U of Wash had one on the antenna range to
run patterns on VHF/UHF antennas on cars. I worked with Dr Donald Reynolds
W7DBA running patterns comparing several of the various antennas on his
station wagon and have a lot of the data.
One test that was startling was this. 1/4 wave verticals had to be mounted
dead center on his station wagon to give a reasonably decent balanced pattern
in all directions. It also had a fair lift off of the vertical pattern as
the car body was hot with RF. The pattern really got unbalanced when mounted
to the side of the car body or too far forward or to the rear. The same is
true for a 5/8's wave to a lessor extent.
However a 1/2 wave on 2M "was within 3 dB of being totally independent of the
car mounting at 4 measured angles of 0,10,20&30 degrees." I have these
patterns of a 1/2 wave vertical I made mounted at the rear top of his station
wagon showing no lift off of the pattern at these angles and no more then 3
dB variation all the way around for all 4 angles. It worked great also. The
1/2 wave is independently resonant and induces little current on the car body
by the L Network and the high current section is 1/4 wave above the car body.
I built an L network into a PL 259 connector in a sneaky sort of a way.
There was an inductor from the whip to the connector body and I made a
tubular or coaxial capacitor. I threaded the inside of the center pin for a
5/32 slotted screw that went into a sleeve the whip slipped into. I took 2
right angle connectors and added them in between the PL-259 connector and the
SO-239 mounted on the car for the initial tune up. I cut away one corner of
the right angle connector on one side so I could slip a screw driver blade up
to turn the slotted 5/32 screw part of the coaxial capacitor. Once tuned on
a car or HT it's removed.
Every once in a while one lucks out in guessing at what initial values to use
for L networks if you tune up enough of them. In this case not only did I
guess the exact values of XL and range of the variable Xc but the setting of
the Xc was exactly right for a 50 ohm match at 146 MHz. I though something
was wrong with the SWR bridge reflected power circuit. I had to detune it to
prove it was correctly set. The 1/2 wave often had S-Units advantage over
1/4 wave due to fewer nulls and far less lift off of the vertical patterns.
Low body currents is why the window antennas work well also. They need a
body connection at the box that houses the tank circuit also or the coax
shield is hot all the way to the rig
I've compared 1/2 to 1/4 waves on HF and the differences can be big for
somewhat the same reasons. I'll be running these tests again with the
antennas side by side a reasonable distance.
Then a local Marine Antenna stole the design and when mounted on a pole or
fiberglass cabin it's doesn't work well and they don't know why? There is
considerable RF Spill Over down the coax they are unaware of and don't know
how to stop it if they did. I'm going to have a chat with the new owners on
how to fix the problem. They have another Marine vertical with a similar
problem that has an easy fix I'll discuss with them also. If they don't
agree to pay for the solutions I hope I get out of their alive as I will
demonstrate it to them and they won't be able to see the simple solution.
They have a multiband Marine vertical similar to ham trap verticals with
close spacing between the traps using a 2" diameter all covered with shrink
on tubing for a cover that works great. Another company tried to copy it,
made a bunch of them and it didn't work right and didn't know why. There
would be a 2" coil part of the trap and 2'' aluminum tubing over the
insulated part on the inside as the connection to the next trap. The other
company copied everything exactly except for one thing. This 2" tubing had a
slot in it full length. The end of the 2" tubing was fairly close to the end
of the tank coil and without the slot it was a "shorted turn" on both sides
of the tank coil which created losses and detuned the tank coil circuit with
the fixed XC on the inside.
The Hy Gain traps had as similar problem but not as severe. The coil was
about 1 1/5" in diameter and the tubing was 7/8" I think it was and at least
3/4" away. I slotted this tubing for 3/4" and the resonant frequency went up
220 KHz on a 20M trap by moving the 2 shorted turns of the tubing end further
away from the tank circuit. This showed it was having its affect.
I also rewound the coils with larger copper wire than the aluminum wire used
and made double connections to the tubing. The traps were typically
resonated about 1 MHz lower than the band to lower the circulating currents
in the wire to reduce excess heating if resonant in the band. They would
over heat otherwise. The spacing needed to be reduced in-between the traps
which shortened the over all antenna length even more, reduced the bandwidth
even more as did the Xc shield over the coils. This lowers the Rr but the
Rloss increased with aluminum wire to maintain a fair SWR. The 40M trap was
a choke coil of about 100 uhy. The overall length was about 28'. I compared
it on 75M to a Rubber Duck on 2M and the performance was about the same.
Rubber Ducks are wound with resistance wire to get a reasonable SWR. It did
work better with traps wound with fewer copper wire turns resonant in the
band that required wider spacing between them. Fewer turns also increased
the spacing to the "shorted turn tubing ends". "Now you know the rest of the
story." k7gco
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