[Amps] Duty cycle for processed SSB in contest conditions?

[Karl-Arne Markström]

Another "voice in the wilderness" comes here.
 
I would rather say that 75 - 80 dB of receiver adjacent channel suppression, IM or phase noise limited, 
can handle more than 95% of today's SSB signals. This was typical of the best early 1970's professional HF receivers. 
 
For the last 20 years, I have been involved as a consultant in trying to mitigate co-location problems in MF/HF/VHF systems for civilian and military use. The equipment standards in these systems are good, but not dramatically better than these found in amateur radio.
 
I have compared the SM5BSZ numbers with those that are found in typical "MIL-spec" equipment, and the major differences are
that the MIL-equipment  usually controls transmitter broadband noise better and does not have the "mic-gain control" or the aggressive ALC that are signs of amateur grade equipment.  Nevertheless it is seldom found that the first adjacent channels are suppressed by more than 55 dB or the second more than 65 dB, using "CCITT phosphometer shaped noise" as excitation.
 
Looking at the amateur bands at times of good propagation and high activity (contest weekends) using an high-performance SDR connected to a large antenna is revealing interesting things.
 
Heavily overmodulated and overprocessed SSB signals often have first adjacent channel IMD sidebands which are only 25 - 30 dB down from the center frequencies, and extend several SSB bandwidths at each side before falling off to an acceptable level. 
The same can be said for many CW signals that are found, they have too often keyclick sidebands extending many kHz each side.
It is quite obvious that those generating these signals either haven't read or understood the provisions in the ITU Radio Regulations:
 
"25.8   § 5.    1) All pertinent Articles and provisions of the Constitution, the Convention and of these Regulations shall apply to amateur stations. (WRC-03)"
 
 
Although the administrations seem to have lost their interest in both the technical and operational qualities of amateur radio, it still does not give us a carte blanche for emitting lousy signals. We have an unique exemption for building and modifying equipment on our own, as the authorities assume that we still have the knowledge and skill levels that are necessary to keep the technical characteristics of our equipment within internationally agreed limits.  
 
I personally believe that if and when the administrations finally catch up, the results will be lower power limits and type accepted equipment, as in other radio services.
 
If  a "backwards calculation" from what power levels other radio services use, and what might be reasonable in the
European view of public exposure for electromagnetic fields is made, we end up in the order of 100 W ERP on the HF bands.    
 
To return to the original question, this is composed of both the steady-state and transient thermal properties of the transistor die and its mounting. A very high thermal power density exists at the die mounting that needs to be removed very rapidly. 
 
Looking at the thermal design of professional equipment with MOSFETs I find it reasonable to dimension the steady-state heat-sink when used for highly-processed SSB for at least half the key-down CW dissipation at the highest ambient temperature that may be encountered.
 
My own forced-air cooled SRT SSA1020 1 kW amplifiers (8 BLF177's) were tested at the manufacturer using both a two-tone test and a key-down CW test of 30 min duration at 55C ambient. The permitted heatsink temperature rise for two-tone was 20C and for CW 35C.
This gave a margin of about 5C before the overtemperature alarms went off and reduced drive by 3 dB.
I have a similar recollection at the FAT:s of the Rockwell-Collins PA-2250 amplifier.
 
73/
Karl-Arne
SM0AOM
 

----Ursprungligt meddelande----
Från: g8on at fsmail.net
Datum: 2014-02-23 08:00
Till: <k8ri at rogerhalstead.com>, <amps at contesting.com>
Ärende: Re: [Amps] Duty cycle for processed SSB in contest conditions?

Roger said:


>Only recently have SS rigs began to approach the IM figures of the old 
Collins S-Line.  Even some of the mighty expensive rigs have pretty 
crappy IM figures.<  

SM5BSZ has done some work on this, and let me use some of his data for my presentation at the RSGB Convention last year - which I called 'Spreading the Sewage' as 'Slinging the s**t' wouldn't have been acceptable! In it, I used an analysis of published equipment reviews back to 1972 (123 transmitters) to look at how the IMD performance of transmitters has degraded with the move to solid state. What is especially bad is the big increase in high order IMD, in some cases up to the 11th order. The rigs that were really clean were those Yaesu rigs where the PA could operate in a heat producing, lower power, very low IMD, Class A!

>They have receivers that have passed the point of 
practical numbers for intercept points, dynamic range, selectivity, and 
have a sensitivity that is far, far below the band noise, yet they seem 
to be ignoring the transmitted signal which is one of my pet peeves.<

I am somewhat amazed that the manufacturers appear not to realise that the intermodulation limited dynamic range and the phase noise limited dynamic range need to be comparable, although the phase noise limited one can arguably be required to be better by about 10dB. This is because the phase noise moves linearly with signal level, dB for dB, while IMD drops faster with reduction in signal level - often not the theoretical 3dB/dB  but usually more than 2dB/dB. The analysis I did that were published in QEX and NCJ some years apart (sunspot cycle peak and trough) showed that in the UK at least, in a situation with noise meeting ITU 'rural' levels, about 95 to 100dB of DR (phase noise or IMD) was all that was needed. I did a paper on this subject at RF Expo in Anaheim in 1986 (Phase Noise Intermodulation and Dynamic Range in Receivers), so it's hardly new. There was a ham radio reception at that RF Expo with door prizes and I was a lucky guy - a nice new Bird 43 with 1kW HF slug
  came my way!

So, Roger, you aren't a lone voice crying in the wilderness, but the number of voices is very low.....

73

Peter G3RZP
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