Maybe somebody can shed some light on the following matter:

Recently I have installed an 18 GHz Microwave link, consisting of NEC-
Pasolink gear and Andrews Microwave Dishes.
Because it was a first for me, I had thought over the process of lining
up the dishes. I had heard of and read about the side-lobe vs the main-
lobe.
So I figured out the my coworker on the other end of the link should move
his dish and I would tell him the reading on my voltmeter. The higher the
better and at the peak reading he would fix the dish. Then I would do the
same on my end. And then his end a second time and then my end a second
time
We would repeat this procedure for the vertical line-up as well.
I had calculated an expected attenuation which could be converted in a
voltage reading.

Assume the reading should say 3,5 V. Well I never got anything better
than 2.8 / 2.9 volt.

COnsequently I asked NEC for advice and they said that 'you should line
up you own end using the voltmeter, and not the opposite end'. With this
advice we repeated everything and reached the expected reading of 3.5
volt. The 2.8 volt was a sidelobe of the antenna-beam

What I don't understand is that when I rotate my dish (either
horizontally or vertically) I can go from sidelobe to mainlobe to
sidelobe. I don't move the dish further left or right, I only rotate it
around a vertical or horizontal axis. When I would move the dish further
left or right or up or down, than I can visualize going from lobe to
lobe. Not by rotating the dish
That is why I used the opposite end to move the beam.

Anybody can explain where I go wrong

Regards, Ad

On Fri, 11 Jul 2003 21:35:16 +0200, wrote:

Anybody can explain where I go wrong

Regards, Ad

Hi Ad,

Looks like cross-polarization issue. As you rotate, they come into
conflict (the polarizations) and then re-emerge from conflict.
Actually, it sounds fairly bullet proof with what you got (plenty of
signal, the system probably doesn't need more than a volt, as you
would measure it).

73's
Richard Clark, KB7QHC

On Fri, 11 Jul 2003 21:35:16 +0200, wrote:

Maybe somebody can shed some light on the following matter:

Recently I have installed an 18 GHz Microwave link, consisting of NEC-
Pasolink gear and Andrews Microwave Dishes.
Because it was a first for me, I had thought over the process of lining
up the dishes. I had heard of and read about the side-lobe vs the main-
lobe.
So I figured out the my coworker on the other end of the link should move
his dish and I would tell him the reading on my voltmeter. The higher the
better and at the peak reading he would fix the dish. Then I would do the
same on my end. And then his end a second time and then my end a second
time
We would repeat this procedure for the vertical line-up as well.
I had calculated an expected attenuation which could be converted in a
voltage reading.

Assume the reading should say 3,5 V. Well I never got anything better
than 2.8 / 2.9 volt.

COnsequently I asked NEC for advice and they said that 'you should line
up you own end using the voltmeter, and not the opposite end'. With this
advice we repeated everything and reached the expected reading of 3.5
volt. The 2.8 volt was a sidelobe of the antenna-beam

What I don't understand is that when I rotate my dish (either
horizontally or vertically) I can go from sidelobe to mainlobe to
sidelobe. I don't move the dish further left or right, I only rotate it
around a vertical or horizontal axis. When I would move the dish further
left or right or up or down, than I can visualize going from lobe to
lobe. Not by rotating the dish
That is why I used the opposite end to move the beam.

Anybody can explain where I go wrong

Regards, Ad

When aligning the antennas you must swing each all the way left and
all the way right to where you are well past the beam width and any
lobes. Of course you note the signal strength as you do this. That is
the only way to be sure that you are not on a side lobe. Just moving
it a little each side of a peak or even moving it until you loose the
signal is not sufficient. You could hit a null between the main and a
side lobe and think that you have gone far enough but you still could
peak on a side lobe.
By swinging way past the lobes, each way, and noting the signal
strength as you go will assure that you find the main lobe.
Do the same in the vertical plane also.

73
Gary K4FMX

Gary, K4FMK wrote:
"Do the same in the vertical plane also."

Never had a minor lobe identification problem.

Never improved over the bubble-level set of vertical elevation angle on
long paths either, but I always tried.

I had always calculated my path gains and losses, and my best received
carrier power was very nearly always within a db of my calculations. If
not, I had a problem. Fortunately, that was very rare.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote:
Gary, K4FMK wrote:
"Do the same in the vertical plane also."

Never had a minor lobe identification problem.

Never improved over the bubble-level set of vertical elevation angle on
long paths either, but I always tried.

I had always calculated my path gains and losses, and my best received
carrier power was very nearly always within a db of my calculations. If
not, I had a problem. Fortunately, that was very rare.

Best regards, Richard Harrison, KB5WZI

Did you actually do very many?

The reason I ask is that there are other factors, and you
*can't* calculate them, and in some cases you can't even
adjust for them.

The best example that I know of was a TV microwave repeater that
was put in north of Tucson for a link between Tucson and Phoenix
back in the 1960's. This thing was located 50 miles from town,
at 8500 foot on a mountain top. They had a 50 foot tower
installed on a concrete base that cost $50,000 (think about that
in today's money!). And when all was said and done, they turned
it on, and it worked great!

But somebody had the smart idea to see what happens if they
slide the dish down the tower to see if the signal would
improve. It did!

So they positioned it vertically on the tower for best signal
strength. About 7 feet off the ground... :-)

(And darned if a few years later a build up of rhime ice on the
43 feet of tower above the dish didn't fall down and bend that
dish into a pile of ruble too.)

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Floyd Davidson wrote:
"Did you actually do very many?"

I`ve done single-hops, several-hop systems, and transcontinental
systems. I`ve done them on-shore, off-shore, and in a multi-hop loop
system on-shore and off-shore. I`ve done several systems with paths
sandwiched between tall buildings. I`ve done 960 radio, 2-GHz, and 6-GHz
systems. I`ve done space-diversity systems, hot-standby, and unprotected
systems. I`ve done solid-state systems, vacuum-tube systems, etc., etc.

I have no reason to say anything which is untrue. Floyd knows of an
anomalous hop in the desert. The path suffers reflections, else it would
not have great variation of signal with height. I know of many anomalous
systems, but I never built one. All of mine worked as designed.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote:
Floyd Davidson wrote:
"Did you actually do very many?"

I`ve done single-hops, several-hop systems, and transcontinental
systems. I`ve done them on-shore, off-shore, and in a multi-hop loop
system on-shore and off-shore. I`ve done several systems with paths
sandwiched between tall buildings. I`ve done 960 radio, 2-GHz, and 6-GHz
systems. I`ve done space-diversity systems, hot-standby, and unprotected
systems. I`ve done solid-state systems, vacuum-tube systems, etc., etc.

I have no reason to say anything which is untrue. Floyd knows of an
anomalous hop in the desert. The path suffers reflections, else it would
not have great variation of signal with height. I know of many anomalous
systems, but I never built one. All of mine worked as designed.

As I pointed out, that was the most _interesting_ example that I
know of.

However, you've just stated something that I can't quite get my
arms around. "All of mine worked as designed." is stated as if
the "anomalous systems" that have a path which "suffers
reflections" are somehow not common, or not well designed, or
not normal.

Yet you mentioned "on shore" and "off shore" each twice above,
and I'm having a real difficult time thinking you've ever
designed a microwave shot across tidal waters without having
"reflections" which could not specifically be calculated. And
there is simply no way that it "worked as designed" unless you
mean you just allowed for a large enough fudge factor to account
for signal swings from day to day. The original claim that they
*all* came in within 1 dB is just hilarious.

My bet is that you have hung around and do know how these paths
function over time, and I'll bet you just exaggerated a little,
that's all.

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Floyd Davidson wrote:
"This thing (TV microwave relay station) was located 50 miles from town
at 8500 foot up on a mountain top. They had a 50 foot tower (at the TV
studios I suppose)----they turned it on and it worked great."

So, if it wasn`t broke, why did they fix it?

Floyd also wrote:
"But somebody had the smart idea to see what happens if they slide the
dish down the tower to see if the signal would improve. It did!."

I wasn`t there, so I can only speculate, but I might have not been
surprised by those results.

On a 50-mile path with plenty of mid-path clearance, propagation is
similar to communications with a satellite. One difference at the
Arizona latitude is the vertical angle the dish path makes with the
Earth. The low angle the dish on the terrestrial path makes with the
Earth, makes it vulnerable to reflections from the Earth. The higher the
dish is placed, the more vulnerable it becomes. That`s a reason to go
high / low on a reflective path, and not high/high. (Low/low won`t make
the trip on most long paths due to Earth curvature).

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote:
Floyd Davidson wrote:
"This thing (TV microwave relay station) was located 50 miles from town
at 8500 foot up on a mountain top. They had a 50 foot tower (at the TV
studios I suppose)----they turned it on and it worked great."

So, if it wasn`t broke, why did they fix it?

Floyd also wrote:
"But somebody had the smart idea to see what happens if they slide the
dish down the tower to see if the signal would improve. It did!."

I wasn`t there, so I can only speculate, but I might have not been
surprised by those results.

On a 50-mile path with plenty of mid-path clearance, propagation is
similar to communications with a satellite. One difference at the
Arizona latitude is the vertical angle the dish path makes with the
Earth. The low angle the dish on the terrestrial path makes with the
Earth, makes it vulnerable to reflections from the Earth. The higher the
dish is placed, the more vulnerable it becomes. That`s a reason to go
high / low on a reflective path, and not high/high. (Low/low won`t make
the trip on most long paths due to Earth curvature).

A 50 foot tower on top of an 8400' mountain. What are you
talking about high/low etc etc. I wasn't there when the
decision was made to put it on a 50 foot tower, and neither you
nor I have any idea why that was done. Perhaps the topo maps
were wrong, and some obstacle they assumed was there didn't
actually exist. Perhaps the engineer made a mistake. I don't
know and you don't know. But, you don't suppose the engineer
knew exactly what he was doing, eh???

A 40 foot change in elevation suggests there simply were no
obstacles, so one has to wonder what the 50 foot tower was
supposed to accomplish... other than allow a range of adjustment
to find the best point for signal strength. Because this was
not a 50 mile shot, it was much closer to 100 miles and no doubt
they were very interested in optimizing the signal strength.
With no obstacles, not even earth bulge, calculating reflections
isn't so easy... unless you move the antenna vertically to find
the right position.

But, it doesn't emulate "a satellite" shot even in the slightest.
Or, not for real satellite shots at least.

E.g., the look angle for a geosynchronous satellite here is only
10-12 degrees or less depending on where the satellite is. I
don't agree at all that a 50 mile microwave path with no
obstacles is approaching similarity to a satellite shot! Some
satellite links have *less* clearance.

Of course, your exposure to satellites might not be that great.

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Floyd Davidson wrote:
"And darned if a few years later a build up of rhime ice on the 43 feet
of tower above the dish didn`t fall down and bend that dish into a pile
of rubble too,"

By Floyd`s logic the lesson must be that a dish must always top the
tower, out of harms way.

The antenna does not have to receive zero interference, though that
would be nice. We are always susceptible to some interference from
somewhere at some time. Multiphop systems often must reuse just a few
frequency pairs over and over. It`s all the regulators will allow.
Situations arise when anomalous propagation provides strong signals at
extraordinary distances.

Planning includes avoiding azimuths which would repeat to present
interference at a great distance along with a repetition of a frequency
which might interfere. A solution to interference is coordination.
Another is often high performance dishes which do a better job of
rejection. I`ve used the shielded variety from Antennas For
Communications (AFC) with good success.

Ice may be falling, but the sky isn`t in the case of microwave
interference.

Best regards, Richard Harrison, KB5WZI