FM vs AM birds.
Since AMSAT launched their first FM ‘EE-ZEE’ satellites, there has been a marked decline in the standard of operating practice. This is being carried across to new satellites such as HAMSAT and is causing real problems for operators of linear satellites.
Since linear transponders first flew on amateur satellites, the use of excessive uplink power by certain operators has been a perennial problem. Make no mistake, amateur satellites have VERY sensitive receivers, the problems are generally to be found on the ground. The most important part of any amateur satellite station is the receive antenna and its feed-line. No matter whether you are working LEO birds or high orbit birds, you should make your receive system your highest priority. The old adage applies even more to satellite ops than terrestrial, if you can’t hear ’em you can’t work ‘em. To this end, you should take extra care when setting up your receive system.
When working via an FM ‘ee-zee’ sat, there is a strong incentive to wind up the power to capture the transponder since the bird can only handle one QSO at a time. This positively reinforces bad operating practice.
By contrast, when working via a linear transponder bird, you signal should never be stronger than the satellite’s beacon. The beacon is your power reference, and you should aim to be no stronger than the beacon and preferably use as little power as is necessary to conduct the QSO.
Running excessive power on analogue birds has the following effects.
*It reduces the sensitivity of the receiver on board the bird. By activating the onboard AGC.
*This makes ALL the signals in the passband weaker.
*It flattens the satellite’s batteries.
*It makes you unpopular with good operators.
When AMSAT’s P3-D (AKA AMSAT Oscar 40) was launched, it carried a revolutionary ’alligator detector * system that could notch out stations running excessive power and warn then with an alarm on the downlink.
While this worked, it is sad that some people are such poor operators that it was even necessary to fly it. Unfortunately, poor operating practice on linear birds is all to common and the proliferation of FM ‘EE-ZEE’ sats has contributed to worsen the situation.
Antennas for LEO satellites.
For LEO satellites, a fairly modest system can be adequate. With Hamsat for example, a small yagi set at 15 degrees elevation and rotated by hand will work as a receive antenna. Set at 15 degrees elevation, the typical small yagi will cover 0-30 degrees elevation where you need the most receive gain and as the bird flies towards you the loss in gain is compensated to a large degree by the decreasing range to the bird.
Because antennas are reciprocal in nature, the same sort of system can be used to ensure that you don’t overload the satellites receiver front end as it flies toward and past you when transmitting to it.
Many beginners make the mistake of thinking that satellite working requires big yagi antennas and a lot of transmit power on the uplink. This is completely wrong.
For sure, larger antennas have their place with Molniya type orbits where the apogee is far out into space and the resulting path-loss is rather high but for the most common types of LEO bird, large antennas are not only not necessary, they are not desirable.
The extra work of having to continually re-point a long yagi as the bird shifts position makes them an unattractive proposition.
It IS possible to work the LEO’s from a small balcony or a small garden. K5OE does it all the time, and what’s more he does it with small home made antennas made from nothing more than ordinary wire and plastic tubes..
Don’t believe me? Well, ask him yourself or better still, check out his website.
Definition.. Alligator. A creature with a large mouth and small ears.