OBSERVATIONS FROM SCOTLAND                           24th September 2009                              GM1SXX

Working linear birds.

After the article on operating FM satellites, I thought I'd write another short article on Linear satellites and throw in another video.

Linear satellites take a segment of the radio spectrum, typically 50Khz wide and transpose it onto a different band. They are *only* intended to handle narrowband modes such as SW or SSB and should never be used for FM operation.

Typical operating modes would be V/U VHF up and UHF down or mode  U/V ... UHF up / VHF down.  So they are cross-band space based repeaters designed to handle multiple concurrent users.  Unlike the FM birds where only one QSO at a time can be supported, Linear birds can support multiple QSO's. This nicely brings me to the biggest mistake made by newcomers.... running excessive uplink power.  On an FM bird, the uplink power doesn't matter. If you run too much, the transponder will go into limiting (fully quieting) so more power than is needed won't have any effect on the bird.

Linear transponders are totally different. In a linear transponder, the power available on the DOWNLINK is *shared* between all the signals in the satellite's 'pass-band'. That's to say that the downlink power available is shared between the users. This is why linear satellites have beacons. The beacon is your power reference. When you transmit through the satellite, your received signal should never be stronger than the beacon.

Why is this important?

It's important because you are using a shared resource. If you run excessive power, you rob others of  their share of the available power.   Not only that, you also risk damaging the satellite by drawing excessive power from the battery.

A QSO via a linear bird is full-duplex. You transmit and receive simultaneously, and in doing so, you can (and should) therefore monitor the doppler shift and the signal strength.  An S9 signal report is NOT what you want. keep the power as low as is feasible to make the QSO.  You'll find that you need more power near the horizon, but be aware and back off the power as the satellite approaches. Once it's flying away from you, you nay want to increase the power in small increments to maintain the QSO.  Because the QSO is full-diplex, it's a very good idea to use headphones to avoid feedback.  You may also notice a slight echo on the signal because of the round-trip time for the radio signal. This is normal. You'll get used to it quickly.

I have yet another short movie from Simon 2E0HTS in which he makes a contact using the Indian built HMSAT bird, also known as VO-52.  I chose this one to denonstrate that its possible to work satellites with modest antenna systems.  If you operate portable, it's possible to set the antenna(s) on a pole pitched up at 30 degrees and rotate the pole by hand.  For base station use, a simple cheap TV type rotator can be employed.   Anyone intending to try linear satellite operation is advised to read How much power is enough (1)  and How much power is enough (2)

Click on the button to watch the movie. 

Operating linear birds is VERY different from operating FM ones, where FM capture effect predominates.  Running excessive uplink power is greatly frowned upon and is very bad practice.  Most linear satellites have some sort of AGC (Automatic Gain Control) which when triggered, reduces the available power for *all* the users.   Play fair.

It's  a fact that if all the users lowered their uplink power to say only a few watts, all of them could have better satellite contacts. Human nature, being what it is, rarely allows this happy state of affairs.  There's always a plonker, waiting to spoil things for the rest of us.  Most of them live in Europe.

In Simon's video, he's using a dedicated satellite transceiver (the Yaesu FT-897 Earth-Station).  While nice to have and use, a couple of 70's vintage EBay multimode radios for 2 & 70Cms can do just as good a job.  Expensive kit is not essential and neither are large circularly polarised aerials. They help of course, but I find greater enjoyment in simplicity.  Simon uses some sort of very small dual-band beam employing a pair of horizontally polarised aerials on a common boom.  Large aerials and complex equipment are not necessary.  Common-sense is a far more useful commodity :-)

73 AL.
GM1SXX