[MAMS] April 2011 Microwave Activity Day

[Dr. Gerald N. Johnson]

On 4/5/2011 7:28 AM, Zack Widup wrote:
> On 4/5/11, Dr. Gerald N. Johnson<geraldj at weather.net>  wrote:
>>
>>
>> On 4/4/2011 4:39 PM, Zack Widup wrote:
>>> Surprisingly, my dish wasn't damaged, even after it literally flew
>>> over my car WITH the tripod at one point. I did get a scratch in the
>>> car.
>>
>> At CSVHF last summer Bill Clark didn't seem to think that the stock
>> Direct TV dish was made very well anyway, that its precision was poor.
>
> I wasn't using a DirectTV dish. I was using a two-foot diameter
> prime-focus dish that is very thick. It's been beaten around a lot and
> hardly shows any signs of wear at all. I had it at CSVHF in the Dish
> Row last year.

I didn't remember seeing it, but I spent that time period mounting and 
unmounting antennas on the VHF antenna range seeing that mine were 
tested thoroughly.
>
>>>
>>> My IF rig was damaged when the table blew over. I don't think a radome
>>> would've kept either the radio or the dish from blowing over.
>>
>> Actually I was thinking of a RADOME big enough to hold dish, gear, and
>> operator. It would practically have to be mounted on a truck with
>> stabilizing jacks like a crane or bucket truck and heavy enough to hold
>> it down in the anticipated wind.
>
> That would probably do it if I had anything like that. I figured a
> couple sandbags, one to hold down the dish tripod and one to hold down
> the table, probably would've done the job. I don't know how to keep
> the 902 looper from spinning. I had the bolts on the mast clamped as
> tight as I could get them and it still spun around.

The time I've been out with tripod and Direct TV dish on Buck Hill, I 
took a concrete block along for weight. The second day with the wind 
running 10 to 20 mph, I tied the tripod to the block with strands of 
baling twine (which I have by the mile having baled and sold hay on my 
farm years ago). Several of the more experienced stations there mounted 
the large deep cycle battery in that position to hold the tripod down. 
Scott (N0EDV) uses a surveyor's tripod with the legs spread nearly flat. 
Hurts the height above ground but will stand some wind. When the wind 
picks up the dish and blows it over the car, mere weight or widespread 
tripod stability may not be nearly enough. It may take something like 
9SNR's roof mounted array, or a large foot on the tripod that you park a 
car front wheel on for ballast. E.g. 500 pounds of ballast minimum not 
50 pounds of concrete or sand.

A saddle on the mast helps a great deal. Its big for loopers but HyGain 
makes a good one for their HF beams. Its a casting with serated V groves 
in both blocks held together with straight bolts. I've found that iron 
channel makes a good start to channel if its the right size. I want the 
channel to contact the mast mostly with the edges of the lips, not the 
bottom until I've squeezed the mast in tight. Probably 1-1/4" x 5/8" or 
1/2" for the RS masting is the right size. It was just right if I have 
to use a pry to release the mast. I just happen to carry a handy long 
flat blade screwdriver in my truck for tightening the trailer mirror 
mount and a claw hammer for popping hub caps off that provides adequate 
pry tools for that purpose.

Some TV rotor and antenna U bolts had serations inside the U to get a 
better grop. Many used a saddle of some for with notches for a better 
grip. The saddle of a muffler clamp provides a lot more friction area, 
though it doesn't look good after a week in the weather. A 5/16" or 3/8" 
U bolt grips a little tighter than a 1/4" and doesn't snap off when 
tightened as easily.
>
> There are a lot of trees and big limbs down around here after the wind
> we had over the last few days.

I picked up a few around here in addition to the ones I climbed up 
(during a near calm) and cut.
>
>>>
>>> Next time I won't bother to try to set up if the wind is that
>>> ferocious. I suppose I could've carried some sand bags for weight but
>>> there's only so much you can fit into a Toyota Corolla!
>>
>> We probably need to do some checks of path losses vs wind speeds for
>> various times of the year. I suspect that under the wind conditions you
>> had, signals are down seriously, like 20 to 40 dB on a 40 mile path,
>> making outings and exercise in futility. I've seen more variation than
>> that to Des Moines repeaters and packet nodes over the years, but I've
>> not bothered to compare to wind speeds. The 4th edition of Kraus
>> published for Asian consumption has many chapters on propagation and
>> includes computations for refraction and obstacle gains at microwave.
>> I've not read it all yet, but it may supply some research references if
>> not some insight to these wind effects.
>>> :-)
>
> It would be interesting to note that. My path to Wally is about 100
> miles. We've tried several times on 3456 and haven't made it yet but
> we've worked before on 902. If I'd gotten started earlier I'd have
> taken 144 MHz with me too, but given the wind problems I'm glad I
> didn't. It would've been another antenna to blow over and patch up
> later!

Overnight, I've designed the experiment but I don't have everything 
needed to make it work. I'm about 25 miles from three airports with AWOS 
automated weather observation stations that update their voice message 
at about 30 second intervals. The data includes time, temperature, winds 
(speed and direction), clouds, dew point, and visibility. The cloud 
observation is a little faulty, allowing high clouds to pass as clear. 
The dew point sensor shows a diurnal variation following the normal 
diurnal temperature even with no changes in local air, e.g. during a 
calm. Some ham gear CAT includes an S-meter indication. In my 857, there 
are only 16 steps, not too useful, but the R7100 has 4 binary coded 
decimal digits. I've not yet worked up a program to check the 
calibration of those.

Hourly AWOS reports are generally available from NWS
(http://weather.noaa.gov/weather/current/KAMW.html for the one I hear 
the best) updated a few minutes after the hour from observations made 56 
minutes after the hour. But they also broadcast on VHF AM with a new 
report about every 30 seconds. Its synthesized voice so voice 
recognition software should be reliable at gathering much data to be 
parsed and stored. The dual band antenna I listened with last night, I 
hard AMW best probably because its highest in the band (132 MHz) and 
others at a similar distance but below 120 I didn't detect. With the 
meter available to the CAT port, a computer program could gather 
instantaneous weather data as well as signal strength. And since the CAT 
port also allows setting the receive frequency it would be practical to 
listen to the pilot carrier of the HD TV stations about twice as far 
away as AMW on the same heading at channels 5 through 40 or 50 to get a 
cross section of VHF and UHF frequencies. As a practical matter, I'd 
probably neglect a couple weather observations to get several S-meter 
readings for statistical analysis on each frequency. The signal from 
KAMW is weak enough on the vertical that it would require a gain antenna 
though the TV stations probably wouldn't. 10s of KW ERP do make a 
difference along with 1500 to 2000 feet of antenna height. And for near 
surface paths the elevation of the TV transmit antennas might not be the 
most representative signal sources, but they are there. Some FM 
broadcast and TV translater or low power TV stations might be better 
signal sources from being on lower towers, though I'd expect to see some 
effects of frequency as well as wind and temperature.

I don't know of any archive of AWOS data by the minute. Finding that 
would simplify, but probably delay the data analysis of the experiment.

I scanned through Kraus 4th ed that I mentioned yesterday. It shows a 
formula for signal strength based on temperature and humidity gradients 
plus a bit of pressure gradient but doesn't quantify the effects of 
winds or inversions though it mentions inversions and ducts that tend to 
require light or no winds.
>
> 73, Zack
>
73, Jerry, K0CQ