Some time ago I did some path loss calculations to see if a 10mW 434MHz licence exempt transmitter was viable for balloon to ground communications. The maths indicated that narrow-band communication should be feasible over that distance using a modest sized yagi as a receive antenna. To prove my calculations I conducted some tests using a 10Km line of sight path – the results were very good - the signal as respectible stength. I added a 20dB attenuator – thus simulating a 100Km path – the signal was still of respectable strength. Finally I tried a further 20dB of attenuation (simulating a 1000Km path) – the signal was still audible. So much for the theory – when the opportunity of Pegasus III came up I offered to build and track a transmitter.
The tracker was built using a standard licence exempt narrow-band 10mW 434MHz transmitter, a PIC microprocessor (to provide a modulating signal and power management) and a Photo-Lithium Battery (which should have been sufficient for almost a weeks operation).
The antenna is a simple ¼ wave vertical over a 4 radial ground plane.
The antenna was mounted upside down (i.e the radiating element pointing down) underneath the payload. This type of antenna was chosen because of its radiation pattern - as you can see (theoretically) no power is directed toward space - and most of the power is directed at low angles maximising the range when in flight.
The receive antenna was a 70cms 7 element ZL Special from moonraker antennas
Pegasus III was released about 7:35am – climbing relatively slowly – heading off about 10 degrees south of west – the maximum signal strength seemed to be inclined at about 30 degrees.
Over the course of the next hour or so the point of maximum signal strength steadily swung round toward the south and the inclination of the signal seemed to climb slightly. By 9:10 the signal was being received due south – inclined by about 40 degrees. During all this time the signals was of good strength - even on just on a whip antenna.
At 9:30 I got in my car to head off south. The signal was still very good – enough to hear it weakly on the whip antenna inside the car.
I headed south on the M11 – I had intended to pull off at Junction 9 – but found that you can't do this heading south – so I had to carry on. As I drove on the signal got stronger until it reached a maximum and then got weaker again – I got off the motorway at the next junction (J8 – Bishops Stortford) and parked up at the nearby Motorway services.
10:12 (M11 J8 Motorway Services) – I found a corner of the car park with a large patch of grass – laid the compass on the grass and took a bearing with Yagi – more or less due north and inclined by about 50 degrees . By my reckoning this put the balloon somewhere between Saffron Waldon and myself. The signal was a very good strength (definitely good enough to carry data).
I headed north on the M11 again – this was a mistake as after only a few miles the signal was completely lost. I pulled of the M11 at J9 and found a place to park about 10:45 – but despite a sweep of the sky nothing was heard. I headed south on the B1383 – stopping off here and there to take a sweep – nothing. After a while the main balloon team gave me a call to say that it was down and had reported its position just south of Stocking Pellham village.
As I got close to the village I started to pick up the signal again and at one point It became very strong (this would have certainly be strong enough to carry data) so I parked up and started tracking with the yagi. As I moved about I started to get some quite confusing direction readings – I tried attenuating the signal – but with little effect. After about 20 mins of walking about I got a strong and clear reading and within a minute or so I found PIII in the corner of a Oilseed-Rape field almost directly under some power lines. I suspect the power lines had been carrying the signal away from PIII that had given me the false readings. Fortunately PIII had landed upside down – which meant that the antenna was on the top side – had PIII landed right way up the signal would have been a lot weaker.
Frequency shift – most of the flight the frequency kept relatively stable – shifting a couple of hundred Hz. On landing the frequency jumped about 1KHz higher.