Hi Jim, I largely agree with your good points raised here.
In respect of times when there's a crammed ham band chock full of brain dead
contesters, I simply turn the rig off and come back to the operating desk
when the mindless cacophony is over and the bands have returned back to
"normal".... :-)
My main point Jim was that the amateur / radio experimenter ham bands are
unlikely to ever have mandated Type-approved minimum IM performance specs
rigs or more stringent Tx requirements imposed on station licence holders
like the channelized commercial / military radio folk. No matter how good
the ham rig might be there will always be clueless / careless operators who
will mal-adjust things and cause objectionable havoc :-(
Joe nailed it when he stated "Broadcast transmitter designers have known
this truth for 30 years - their solid state transmitters have probably a 4:1
ratio of saturated power output to rated power output so that the devices
are kept well out of saturation where linear operation is required."
This system headroom is the key to attaining Tx chain spectral cleanliness
and godliness...albeit comes at a considerable additional cost.
Cheers,
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Saturday, 23 May 2015 2:50 AM
To: amps@contesting.com
Subject: [Amps] SPE 2K RTTY Duty Cycle / spectral cleanliness
Date: Fri, 22 May 2015 22:40:53 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: <amps@contesting.com>
Subject: Re: [Amps] SPE 2K RTTY Duty Cycle / spectral cleanliness
All good valid points that you make here Kevin.
There is also an equal fascination / obsession in some ham radio quarters
with IM3 and levels! We are talking here of HF amateur / ham band
transmitters NOT professional transmitters where spectral cleanliness is
more important.
It's the higher order IM stuff / crud that has to be curtailed...
The modern trend towards Adaptive pre-distortion techniques works well to
clean up mediocre PA transistors and sort of turn a sow's ear into a silk
purse.
Leigh
VK5KLT
## Nonsense Leigh. Where in the commercial world, do you have 10,000
brain dead contesters screaming their lungs out on crowded HF bands...for 48
hrs non stop ?
## it would be different if we were all on channels, spaced every 4 khz
apart. 4 khz spacing is what all the telcos here in NA used, back in the
analog cxr days..b4 the conversion to digital. It was all SSB, stacked nose
to tail, every 4 khz, then the entire mess, called baseband..was stuffed
into the microwave TX. IMD was important in that case, since one klystron
typ had anywhere from 240-1200 channels. If the tube went bad, and imd
degraded, the TX imd would affect all 1200 channels.
## marine, aircraft, broadcast etc, are all assigned channels. In the ham
radio world, we have vfos. One night a group of us were on 75m, with a rag
chew session going on..during a major contest. Some bozo starts up with the
cq contest stuff..... 2 khz above us. A half hr later, another bozo starts
up exactly 500 hz below us! We were all running 1.5- 2.5 kw output into typ
75m ants. We ask the bozos if they can hear us. Sure, I can hear
you...you are all S9++. WTF, then why not stay at least 3 khz away.
## half the problem is stations way too close to each other to begin with.
The rest of the problem is junk SS xcvrs, with sky high, higher order
imd...fed into linear amplifiers. The amplifier just amplifies a high imd
xcvr = total disaster. IMD3 and IMD5 is a meaningless spec. Whats
important is IMD 7-9-11-13-15. A static two tone imd test is equally
useless. Everything is in a static condition, not a dynamic condition.
The various voltages, including bias, alc, collector, B+ etc, dont budge.
## two tone testing isnt even allowed nor used in the commercial world.
By juggling the tone spacing, you can easily reduce IMD3 and IMD5 by quite
a bit. We don't tx 2 tones....we use ssb.
## as noted in my previous posts, marine and aircraft SSB IMD specs are
WAY more stringent than typ amateur radio junk. The marine stuff all runs
on 12 vdc too. Somebody mentioned the FT-990. Take a look at the yaesu
767GX from years ago. It had superb imd. But it used 200 watt
devices...run at 100w out.
## that doesnt cost much more. The power supply, LP filters, cooling, auto
ant tuner, etc, etc, are all rated for 100w.
The finals are rated for 200-300 watts.... but run at 100w. Use loads of
feedback, and you too can meet the FCC marine spec for SSB TX imd. No
pre-distortion nor class A blast furnace tricks required.
## we have regs for harmonic suppression etc, etc, but nothing for TX imd.
Too bad, cuz until we do, we will continue to get crap TX imd. The kenwood
870, when run at 100w out...has bad imd..like -30db pep. Back it down
to just 85w out, and imd cleans up a whole bunch, huge improvement. Reduce
it down to just 50 watts, and its superb. I gave up on the 100w xcvrs
years ago. Buy a 200w xcvr...then run it at 100w, or whatever is required
to drive your amp. Remember all the xcvrs with key clix ? Same deal, no
regulations in place.
later.... Jim VE7RF
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