Topband: J28MD, conditions

[Jim Brown]

On 11/5/2022 4:26 PM, Wes wrote:
> I'm far from an FT8 expert* but on F/H mode the Fox does move the 
> selected hound to his frequency.

Nor am I, but I learned a lot from studying the doc, and asking K1JT a 
question. And I've used F/H several times with good ops on the Fox end, 
guys like W1SRD and AA7V). What they have devised is a VERY slick 
system. It divides the AF spectrum (and thus the RF "channel") into two 
parts. The Fox owns everything below 1,000 Hz, Hounds can call til we're 
blue in the face above that. When the Fox chooses to work us, he moves 
us (via the software) down into his spectrum, which is free of random 
callers if everyone is behaving, and we respond on clear frequencies of 
his choosing.

The system breaks down two ways; 1) if random callers are QRMing that 
protected spectrum, or 2) if his TX power is divided between too many 
streams when he sends RR73 to callers, and his signal falls into their 
noise. As Joe has observed, each stream is reduced by 6dB per doubling 
of the number of streams to minimize IMD.

The logic for that reduction goes like this. The streams are constant 
amplitude, with frequency shift at zero crossings, which makes for very 
low IMD. But when transmitting multiple streams, the signal is no longer 
constant amplitude, so can excite IMD.

73, Jim K9YC