Michael Black wrote:
On Sat, 30 Jan 2016, Allodoxaphobia wrote:

In rec.radio.amateur.antenna, you wrote:
gareth wrote:
One of the problems with beams is the cable winding around
the mast if you try to turn too far in one way or another.

These difficulties have long been solved in the world of radar,
so, has anyone conceived of an infinitely rotatable 50 ohm
coupling for our beam antennae?

Sounds like a $200 solution to a $0.75 problem, i.e. an extra foot or
two of coax to allow slack for turning.

With 4 different yagis on the mast, a large, bundled loop of
coax(es) *HAS* to be The Way To Go.

Yes, radar needed such couplings since the antenna went around 360
degrees, since you wanted to see in all directions.

And I'd like to think they've now moved to "electronically steerable"
arrays for radar, multiple fixed antennas that are switched to the
receiver via diodes or whatever. You see some of that in amateur radio
DF'ing.

Michael

They are called phased arrays; here are two I worked on:

https://en.wikipedia.org/wiki/AN/TPQ...refinder_radar
https://en.wikipedia.org/wiki/AN/TPQ...refinder_radar

These use active electronically scanned arrays.

A discussion of how it works:

https://en.wikipedia.org/wiki/Phased_array

https://en.wikipedia.org/wiki/Active..._scanned_array

Phased arrays are extremely complex and expensive compared to simple
rotating search radars.



--
Jim Pennino

On Sat, 30 Jan 2016 18:24:02 -0000, wrote:

Sounds like a $200 solution to a $0.75 problem, i.e. an extra foot or
two of coax to allow slack for turning.

Long ago, I had what I thought was a good idea. I was working for a
marine radio manufacturer. Getting decent range with 25 watts into an
omni antenna was problematic in some areas. What worked was a
directional yagi with lots of gain. The problem was how and where to
aim it. Since I was working on an ADF (automatic direction finder) at
the time, that displayed the direction as 0 to 359 degrees, I thought
that coupling the display to a rotator would be a good idea. The
problem was the conventional TV or ham antenna rotator didn't turn
fast enough to switch direction among multiple stations. So, I
installed a rotary joint which allowed swinging the yagi quite
rapidly. The rest was a tangle of junk made from a servo, shaft
encoder, discrete logic, and the usual spaghetti wiring. I switching
to 2 meters for testing.

The results were ummm... interesting. The direction finder output was
not very stable, so the antenna liked to vibrate and jitter over about
a 15 degree swing. If the station it was tracking crossed over 0
degrees, the antenna would swing all the way around to get to the
other side (logic error). A fast back and forth QSO would result in
the antenna doing a "wind vane" simulation. Management was thoroughly
irritated by the noise it made on the roof, which could be heard
everywhere in the building. The consensus among the other engineers
was that this was a truly stupid idea. I wanted to continue, but
testing was terminated when the mast bent at the rotary joint and
someone sabotaged the interface breadboard.

Anyway, an extra long service loop would not have worked for this
project.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 30 Jan 2016 12:04:49 -0000, "gareth"
wrote:

One of the problems with beams is the cable winding around
the mast if you try to turn too far in one way or another.

These difficulties have long been solved in the world of radar,
so, has anyone conceived of an infinitely rotatable 50 ohm
coupling for our beam antennae?

Clearly there would have to be some mechanical support other than
just the co-axial coupling to withstand the wind storms that we are all
now experiencing?

"Slip rings".
https://www.google.com/search?q=slip+ring&tbm=isch
https://www.google.com/search?q=coaxial+slip+ring&tbm=isch

For coax cables, it's a "rotary joint".
https://www.google.com/search?tbm=isch&q=rf+rotary+joint
I like to use a balun to go from coax to a parallel transmission line,
then to slip rings, and then to a dipole antenna feed. I once built a
direction finder that used a rotating yagi antenna, using that method.

As amdx mentioned, RF power levels are going to be a problem. My
interest is in direction finders, which normally do not transmit. If
you're going to run RF through the rotary joint while rotating the
antenna, the RF is going to arc and burn a nice neat groove in the
contact area. If the rotary joint is locked in place, it might
survive at low power levels.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558