On 2/17/11 9:30 AM, Bob Camp wrote:
Hi
System design is always about compromise.
If you hop slowly, you "stomp" on each channel pretty hard. You are likely
to get noticed when you do. The idea is to stomp so rarely and for so short
a time that you aren't noticed.
If you hop fast, you need to adjust the sequence to match. If you have a max
distance of 50 miles / 270 us and a 270 us chip rate things are still pretty
easy. You only need to be able to adjust over a +/- 1 chip range. More or
less like a fine tuning knob.
If you are trying this with a conventional radio, there are some other
issues. The PLL lock times are way longer than the 270 us you are looking at
for distance. Based on using radios as scanners, anything dimensioned in
hops/second is likely sound bad.
My practical experience has been that the typical ham 2m or 440 rig (or
a land mobile rig, which is essentially the same) has a PLL lock time in
the 1-few millisecond range, and a hop length of 100 ms works pretty
well. The noise blanker helps deal with the "popping"..
I set up an experiment 20 or so years ago using a pair of FT-757GXs in
the 40m band, driving the computer control port with frequency change
commands, and it worked fairly well. Didn't radiate (transmit into a
dummy load, receive from leakage... the radios were sitting on the same
table)
Any VHF/UHF rig already flips the PLL frequency when going from tx to Rx
to accomodate the IF and/or repeater offset, so you can look at the
"Rx/Tx turnaround times" for packet radio to get a feel for it.
If you use digitized voice (say, CVSD at 16 kbps, which is easy and
doesn't sound too bad and doesn't have licensing issues) you can buffer
it up and transmit at a higher rate (e.g. if you have a "dead time" of
10 ms out of every 100ms, you transmit at 1.1 times the bit rate you
digitized at)
One caution...
Spread Spectrum communications is something that is desirable by various
"bad people" (because it's moderately secure and also low probability of
intercept), and so you might attract attention of people who worry about
things like that. You might have a car with small hubcaps and lots of
antennas sitting outside your abode one morning, with someone who "just
has a few questions".
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