[ao16aprs] AO16 Experiment and Antenna Comments

[Jon Haskell <kb9cml@att.net>]

Hi all,

This weekend, myself and fellow satellite affieciendo AB9V , will hopefully start a week of Mobile AO16 APRS with a digi to our local digipeter here in Indianapolis.

Mike and I have worked the 1200 and 9600 baud LEO's over the years. Mike has retained his station, while mine has been in hibernation since I moved three years ago. Frankly, after the crash of AO13 I have had no interest in getting my sat station back up and running. But with this experiment, and the forthcoming launch of P3D, I think I'll get things fired up again and get the satellite tower up before cold weather.

Here is what we are going to try:

I will use an ICOM 28H, that has I think about 50Watt transmitter and start with a 1/4 wave vertical antenna mounted atop the roof rack on the Rover. I will probably use the KAMPlus TNC with a Magellan GPS unit. I am not sure how much power or ERPS is going to be needed to compete with a stationary station with some power and gain antennas.

Mike, will set his station on autopilot, and use WISP, a PSK  TNC for downlink,  and another TNC for the link to AFSK FM on 144.39. He does not have any stationary satellite antennas, so his az/el yagi array will auto-track the bird.

So look for W8JON mobile in Indiana next week. I usually monitor 7.258 or 14.300 so give me a call.

With respect to antennas for a ground station, Mike and I chatted about this and came up with some thoughts. Firstly, as I remember, over 60% of LEO orbits are 30 degrees or less to your location. (I gave a speech on this some years ago, but have forgotten the exact percentage.) With this in mind, antennas favoring this orbital characteristic would be best.

I used a homebrew J-pole design w/preamp quite successfully. It was not as effective as my az/el yagi system, but the design worked quite well and was easy to build. There was some fading as a result of high orbits and polarization issues. A preamp is almost a necessity to help you through the nulls.

Other designs that have been used in this application, include the Lindenblad and the Quadrifiler antenna design. Both antennas are circular polarized and omni-directional. These in conjunction with a low-noise preamp would work very well. I believe I am correct in saying,  because of the circular polarization, these designs would minimize fades from a linear source better than the vertical.

If memory serves me correctly, I saw a posting suggesting the use of a short, slightly elevated stationary yagi at stations located East to West across the country. At first blush, we are not sure if this is a good situation. Assuming the antennas are pointed North, as the foot print of the satellite moves south of the antenna location, the directivity of the antenna would not favor that area south of the antennas location. Would for instance,  a beaconing sailing vessel in the southern area of the Gulf of Mexico be received when the LEO is nearing the equator? The farther north the antenna location, the more pronounced the problem. Of course, the reverse scenario would apply, if the antennas were directed to the south.   Perhaps if two yagis were used, one facing north and one south, would solve that issue. Also, is there a polarization issue with the linear signal from the satellite using a non-circular polarized antenna?  

The bottom line is that if you are just receiving the downlink from LEO's,  we believe a gain antenna is not necessary; and perhaps not as effective as a omni-directional antenna.

Well, I throw this out for discussion. We would appreciate any comments from your experiences with these birds.

Jon W8JON




---
You are currently subscribed to ao16aprs as: lyris.ao16aprs@tapr.org
To unsubscribe send a blank email to leave-ao16aprs-833L@lists.tapr.org