[amsat-bb] Re: 2400 MHz
kd0ar at sbcglobal.net
Fri Mar 20 19:36:26 PDT 2009
Hi Gene, and thank you for the kind words.
Yes, I'm using fixed elevation, about 30 degrees or so. It works quite well for me if you consider that the satellites are rarely above about 45 degrees, and when they are, theyre not at a high elevation for very long.
3 elements on 2 meters is VERY broad. 6 elements on 437 is narrower, but I lose the satellite once it goes above about 60 degrees. So an overhead pass, I lose the bird for a little bit. I lose the satellite (AO-51) to the point I can no longer copy about 5 degrees above the horizon.
The gain on S band is greater, so the beamwidth is narrower, however, the signal strength on 2400 is so strong, I actually hear the bird BETTER on S band than I do on 70 cm. I get it a LOT better, except when its toward the south, where I have trees that are fairly close (less than 100 feet) away from the antennas. When the 2400 MHz antenna is looking RIGHT at the satellite, the signal strength meter on my FT-817 is full scale, and totally full quieting. When the satellite is between the trees and my northern horizon, I get it at least S-9 and full quieting until it gets to about 5-6 degrees above the northern horizon, but I can still copy it at 3 degrees above the horizon. I think a lot of the success I'm having is because the front end is so sensitive. I looked at it with a calibrated signal generator, radiating into a halfwave dipole radiator on the generator about 5 feet or so away from the converter, and although how much signal I was feeding
the generator, I dont remember exactly, it was VERY low, on the order of -50 or -60 dbm. Remember, this was radiated from the generator, as the converter does not have a spigot to actually physically connect the generator to actually measure its sensitivity.
The antennas are elevated and all of the support structure above the rotator is 1 inch ID PVC pipe. I raised it as far as I have above the rotator to keep the rotator out of the 2 meter field. I HAD the 2 meter elements vertical when I first put it up, but swapped polarization because I wanted to work some terrestrial 2 meter SSB. I probably could shorten the vertical support some, but it seems to be working ok the way it is. I may replace the rotator with a larger ham rotator and I also have a U-110 that I can use as an elevation rotator, but thats a lot of weight to put up, and would no longer be a one man job putting it up and taking it down.
The antennas are quite small, and the wind load is so little, I dont think that little bit of height above the rotator will not have any ill effect. The crossboom is about 3 feet above the rotator.
One thing I did not say in the article, because it was mainly about S band, but I've found that using an insulated support structure makes a difference, no matter what plane the antenna is in. Even with the 70 cm elements perpendicular to the support arm, I gained a couple S-units on the downlink changing from metal to the PVC. I recommend plastic ANYWHERE near the antenna proper.
----- Original Message ----
From: WB2LLP <wb2llp at optonline.net>
To: Michael Heim <kd0ar at sbcglobal.net>
Sent: Friday, March 20, 2009 4:05:46 PM
Subject: Re: [amsat-bb] 2400 MHz
I enjoyed your article in the new AMSAT journal and have been pouring over the picture on the front cover. It would appear that you have the "Bruninga fixed elevation" and I will be installing one shortly. However I do have two questions: Bob's argument was based on having a rather wide beam width on the antennas which I suspect you have on the 144 and 432 antennas. I was surprised that the S band antenna had a wide enough beam width considering the number of directors.
You appear to have elevated the antenna a couple of feet above the rotator which puts more of a strain on the rotator bearings and mast. Are you trying to get away from all the metal at the top of the pipe mast or is there something else I do not understand?
TNX es 73 de WB2LLP Gene
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