Side Mount Limits - Summation of replies

[Fred Cady]

Hello Tower Folks,

Here is a summation of the responses I got to the following question:

>When you mount a beam on a side mount, do you use some sort of limit switch
>to limit to rotation of the rotor (or rotator, as the case may be) or just
>let the boom crash into the tower leg?
>
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Good question!  I'm designing mine with limit switches on the 'swinging
gate' part.  Going to wire it like a 3-way switch so it stops rotation in
the limited direction but still allows going the other way.
73 Gary K7FR
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We let it crash, Fred...no extra stuff to break down like the limit-stops you
talk of.
Once a year or so it is necessary to realign the mast in the rotor, it isn't
a big deal for us because the dead zone chosen was between South and
Southwest. By making the dead zone an area where we NEVER have need to point
the beam, the number of incidences of colision of the sidemount with the
tower are very few, and thus, need to realign the beam is infrequent. The
ends of the rotation zone are nowhere near EU JA or W/VE, i.e. S-SW is a
"contest free zone" for our beams.
Jim k1zx                   
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Hi Fred.  At N3RS's station, we have been "letting it crash" for 
years, and have had not a lick of trouble.  We have always used Ham 
4s or Tailtwisters, and the motor merely stalls when the sidearm 
encounters the tower.  As for getting the T2X stuck at end of 
rotation due to inablility to "rock it in the other direction," 
perhaps we are lucky, but no trouble there either.
73 - Dave N3RD
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I've had a sidemount for 5 years. I just watch the dial and try to cut
power about the time the boom hits the tower.  Haven't had any trouble.
Just don't get greedy for that last few degrees of rotation...
73, don
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Hi Fred....I taped some thick rubber hose around the tour legs where the
side-mount would hit, just for a bit of cushioning. But I also taped over
the "blank"
part of the compass on the Tailtwister control box so I know when to stop
turning it
...73/Jon AA1K
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  You mark the rotator indicator dial with some marking that shows the
maximum point that it can be rotated in each direction.  (Unless, of course,
you like the sound of metal hitting metal and grinding gears...)
Sid K5XI
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Interesting question. Doug of IIX says to just let the tower limit the
rotation. Jack, W1WEF, tried this approach and had a problem when he
changed to a T2X from a Ham-4. Seems the brake engaged on the T2X and
the rotor jammed and wouldn't budge until he climbed the tower and
took the tension off.

In contemplation of putting up a side mount myself, and in convresations
with Jack, I came up with a solution:

Usually, the dead zone of the sidemount is arranged to conicide with
the rotation stop of the rotator (dead South, usually). If this is
so, the Rotator Pal circuit, described in Feb 1996 QST could be 
modified to provide a zone into which rotation is prevented. Add an
adjustable trim pot on one or either side of R39, the preset pot.
The trim pot(s) should be 250-500 ohms. This will move the ends
of automated rotation away from due south by an adjustable amount.

Never been tried; soon will be; hope this helps...

-Tony, K1KP
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Fred, if you don't mind a little electronics construction, another
possibility is to use the feedback pot signal to make "soft" limit switches
(assuming you have the rotator calibrated and it doesn't slip on the mast).
You can use a couple of comparators to sense the allowable range.  An even
better way is to use a single-chip micro with an A/D input to make the
"soft" limit switches.  This is preferrable to having to climb the tower
(for me, anyway.. hi).  Good luck.
73, Stan N3KK
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There is another alternative: let up on the rotator control switch BEFORE
the boom hits the tower leg.  You can mark your control box indicator so you
will know when to let up.  On the sidemounts that I built and use on my
towers, I decided not to complicate the design by trying to use limit
switches.  I rotate rather small antennas and use Ham-M or Ham IV type
rotators on Rohn 25 or 45 size towers.  As an experiment, I ran the boom
into the tower leg at full speed and it only stalled the motor.  I am sure
this is not a good idea to do all the time, but I wanted to know what would
happen.  I would not want to try this with a heavy boom and a prop pitch.

A good limit switch design would be a good idea, but switches, too, are
prone to failure.  If you count on a limit switch to save your beam from
destruction and it fails, goodby beam and maybe tower, too.  This is a
hazard of sidemounts.
w7ni
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Many, many thanks to all who responded.  

73, see the NW folks at the NW DX Convention.

Fred, KE7X

Fred Cady (ieefc@msu.oscs.montana.edu)
Department of Electrical Engineering
Montana State University
Bozeman, MT  59717
(406)-994-5976