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@fsmail.net
Datum: 2014-02-23 08:00
Till: <k8ri@rogerhalstead.com>, <amps@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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