To find the line of sight P5 distance, go across your Total gain dB to your your AM ATV transmitters peak envelope power diagonal line and then go down from that point to find the snow free (40 dB video to noise ratio or P5) line of sight distance.

Total gain in dB = sum of Transmitter antenna gain in dBd (gain over a dipole) plus Receiver antenna dBd gain minus transmitter and receiver coax losses. Figure 1 dB coax loss per 20 feet for Belden 9913 or Times LMR400. If 4 MHz deviation FM ATV add 12 dB; if your downconverter or preamp noise figure is more than 2 dB, subtract each dB above 2. For power levels inbetween the diagonal lines, add or subtract dB's - ie. add 3 dB each time you double the power.

The power level diagonal lines were chosen to reflect the output of our 50-100 mW RTX23-.1 Transmitter or added Downeast Microwave linear amplifiers. The 3 watt RTX23-3 AM ATV transmitter would add 4.7 dB to the total gain when using the 1 Watt line. Check eBay and QRZ.com On Line Swapmeet for our older models.

Example #1: AM ATV repeater transmitter running a RTX23-3 driving a 2318PA Downeast linear amp, 100 ft LMR400 coax, Diamond U200 omni being received by a Directive Systems 2424LYRM Loop Yagi, Downeast 23LNAWPQ antenna mounted preamp and TVC-12S downconverter.
Total dB is 9 dB tx antenna - 6 dB coax + 16 dB rx antenna - 0 dB coax = 19 dB.
Going across to the 18 Watt diagonal, the P5 line of sight DX is 16 miles.

Example #2: A FM ATV point to point link using a Videolynx Z23B driving a Downeast 2318PA and OAL OAL TCR-23cm corner reflector to transmit, receive with a 2424LYRM Loop Yagi, antenna mounted Downeast 23LNAWPQ preamp and a 23/33FMR-3 receiver.
Adding up 8 dB TX antenna + 16 dB receive antenna - 6 dB 100 ft coax + 12 dB FM advantage = 30 dB on the total gain line. Going down from the 18 Watt line would give up to 54 snow free line of sight DX.

Tom O'Hara, W6ORG
(c) 2011 P. C. Electronics

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