The FM SATs decode discreet frequencies. The PL Tone goes up as 67 Hz and is detected only at that particular freq.
The Linear SATs receive a wide bandpass and the incoming signals are beat against the SAT local oscillator and the resultant freq is the differential. Therefore, the resultant freqs are different based upon that differential. With multiple signals coming up to the SAT on their different freqs within the bandpass, which one would only have exactly 67 Hz? Tune across any HF Band with CW/SSB signals and you'll hear the result of multiple signals/freqs.
Bob K8BL
On 2/3/2018 09:45, Philip Jenkins wrote:
I understand why FM satellites need an PL tone on uplink (to keep out
land-mobile services, like taxi-cabs, etc), but why is a PL not required
for the linear sats utilizing the same frequencies? A linear transponder is
certainly capable of receiving/transmitting an FM signal (albeit not
successfully demodulating that signal).
This question came up at a satellite presentation I did, and I had never
thought about it. ( Moreover, will radios even transmit a PL on SSB/CW
transmissions?)
Philip N4HF