50km 146.5MHz DATV Contact
On Saturday 14th Feb i went to my favourite portable site which is 50km away from Terry G1LPS to do another 146.5MHz DATV test.
It was very cold setting up and total white-out with snow when i was taking the mast down but it was a productive test :-)
DATV was exchanged in both directions, I received Terry with about 13-14dB MER and the reverse path was not quite as good at about 8-9dB MER.
The system at either end was very similar.
G1LPS 50km Reception
G1LPS IO94EQ:
TX: 1W from the RD06/15 amplifier and DATV Express.
Rx: Mutek LNA - PYE Westminster filter - HSU-10 Upconverter (1GHz LO) - Satellite Line amp - Technotrend TT-1600 PCI card
Antenna: 7el yagi vertical polarity.
M0DTS/P IO94MJ:
TX: 1W from the RD06/15 amplifier and DATV Express.
Rx: Microwave Modules LNA - PYE Westminster filter - BFG135 LNA - Gelncom Upconverter (1.95GHz LO) - Technotrend TT-3200 PCI card
Antenna: 7el yagi slant polarity, 45deg - compromise when /p for H/V stations!
There could be a few reasons why the result was not reciprocal, the main points being:
1. The Rx system at G1LPS is lacking in gain so the PCI card was only seeing very low input, i found it was best at higher than -60dBm input.
2. G1LPS is surrounded by other houses and will see a much higher noise floor than i did at the portable site (even when i was running a 24-240V sine inverter!)
3. Something was probably faulty with our systems ;-)
The result of this test gives us hope that the next test over 100km will be successful....
Signal Margin/Improvements:
We can gain about 4dB by going down to 106Ks, this works well now with the TT-1600 card after F6DZP's software improvements.
We can increase the Tx power by 3dB with still very low spectral regrowth (-54dBc shoulders)
Using the same polarity Antenna at both ends will give another 3dB(?) or so.
Using a bigger antenna to get to the Full ERP of 25W will also give some more margin.
We could also use lower Noise Figure LNA's (G4DDK PGA103's probably) when /p to take advantage of the lower noise floor...
If we take it that 5dB MER is the minimum to decode, the current margin at 50km is 13-5=8dB.
Add the 4dB bandwidth gain, 3dB power gain, 3dB polarity gain(?) and 2dB or so extra anetnna gain then the margin would be 12+8 = 20dB.
This should be adequate to go out to >100km near LOS path, watch this space...!
