[MAMS] 24/47 GHz

[Dr. Gerald N. Johnson]

One way to use a 10 GHz dish on a higher band without the complications 
of beamwidth too narrow for aiming (and for atmospheric variations) is 
to underfeed the dish. Say for 24 GHz illluminate just under half the 
width of the dish using a higher gain feed than it would take for the 
full dish. The focal point won't move. So if the offset fed 10 GHz dish 
has F/D = 0.7, then design the 24 GHz feed for F/D = about 1.6. Or the 
47 GHz feed for F/D = 3.4. Means the feed won't be small for the higher 
bands.

Riding an airplane in bumpy air, I muttered something about meteorology 
forecast programs that believe there's enough detail at 40 km 
computation points at 1 hour intervals. Over eastern Oregon or southern 
Idaho, I noticed that water sources on the ground had their own low 
clouds, whether lakes, ponds, or snow on hilltops. There is much 
micrometeorology that can affect refraction of the sorterwavelength 
signals. And its not going to be the same on any give path unless its 
dead calm along the whole path. Even then dead calm leads to more 
effective radiant cooling from low elevations (lower than a couple 
meters), sometimes there can be a 15 degree F gradient from the standard 
2m thermometer height and the ground in the same location. That's 
probably enough to enhance refraction where that applies. Radiant 
cooling is most effective in valleys where winds aren't circulating.

73, Jerry, K0CQ

On 5/10/2011 2:14 PM, Lloyd Ellsworth wrote:
> Dave, Zack,
>
> Don WW8M and I ran a pile of contacts, QSO's on 47 GHz. Various
> distances, and paths. Wind effects on signals, were very noticable.
> Especially on paths of say>10 miles. Many times, acting as a variable
> attenuator to signals, as the wind changed. To a lesser extent, we saw
> this on 24 GHz. Set up, was set up a fixed beacon, for a few hours,
> and let it run. Watch, record the changes. Kind of fun to watch. More
> watchable than most TV.
>
> So yes. Have seen it, and would agree that it is more prominent on 47
> GHz than 24 GHz. I would add, 47 GHz the signals seem to find more
> things to reflect off of. To bounce off of than 24 GHz. 5/10 GHz I
> would characteurize as being more forgiving with paths. Fringing,
> refracting, vegatation and so forth. As well as finding all sorts of
> things to reflect off of. 24 GHz, is not forgiving. Any vegetation,
> forget it. 47 GHz, in between. We found sort of a what I would
> describe as a wall at 45 miles, that took quite a bit to get past.
> More power was the answer.
>
> The SBMS mixer diode set up. About the best DX we have seen with
> stations using that. About 2 KM. PS, that is a really good, quick way
> to get on. Or make a beacon, for testing.
>
> We noted a pile of observations, that we are still mulling over, and
> trying to figure out.
>
> In general, I would agree with the theory that if conditions, signals
> are really good on 10 GHz, 24 GHz stands a good chance. Then the same
> between 24 GHz and 47 GHz. However, not always. Don and I found times
> and conditions, where 24 GHz was better than 10 GHz. And 47 GHz better
> than 24 or 10 GHz. One of those situations, involved fog. Took a pile
> of understanding and working with all this, to make our 2 VUCC's on 47
> GHz happen.
>
> Currently, working with Bob K2YAZ, to get him on 47 GHz. Currently has
> a SBMS mixer diode.
>
> On LO stability. What Don and I played around with, suggests that 47
> GHz, you can still use a basic xtal with heater oscillator technology.
> Yes, it will drift for 20 minutes or something, but will settle down
> enough to be useable. Our best guess is that 78 GHz +, you will want
> something better. More stable. 24 GHz. Max DX, has been about 120
> miles. 47 GHz, Max DX has been about 70 miles. Over water. And yes it
> was beyond LOS. On Lake Michigan.
>
> Another observation. 24 GHz, using a 2 ft dish, is about the maximum
> size dish you can practically aim. Bigger becomes just too difficult.
> EG 3 ft. On 47 GHz, about 1 ft is it. Bigger is just too difficult to
> aim. Even with a good solid telescope mechanical (Az El) mount.
>
> 73, Lloyd NE8I/r
> EN74 etc