Rick.
As others have said, your suspicions re the Chines "Auction site" PA's are largely correct. But
also, many "Expensive instrumentation" amps, are by their broadband nature, not that clean re
harmonics either. -20dBc is a common figure, if the harmonics are "in band", and somewhat better
(but still not communications grade) if they are out of band. Regardless of the actual technology, Bipolar,
FET, Vacuum Tube, or TWT...
Some of the cheap PA's can be very good linearity wise, if you add filters,
very good heat sinks, and a very capable (very good dynamic regulation) DC
power source. (Or, run them in Class A, and use a SMPS to power them, with
more filtering on the DC lines!)
As in all things amplifiers, the power supply, and cooling are critical. Skimp
on either of them, and you'll have trouble, period!
But a note too, regarding "if there has been a noticeable change to the bands over
the years."
Oh yes! There are now hoards of poorly suppressed digital gadgets that
populate the place, and between them produce a noticeable level of QRM from
near DC to way up the spectrum. MF/HF being particularly badly hit.
Domestic LED lighting (not the LED's themselves, but some types of switched
mode constant current drivers.)
"In House" Power line Networking (data over the mains) devices.
Plasma TV's (the older they get, the worse they get.)
And now VDSL. (High speed broadband over phone lines.)
Plus the gazillions of SMPS based wall warts/chargers etc, that can be
amazingly noisy at RF when they have no load! (Not that they are particularly
quite when working into a load.)
Some older games console PSU's, the Wii "grey brick" types are particularly
bad... Don't ask me how I know..
Remember, that the EMC regulations regarding radiated noise *from* a "Product"
are designed to protect Broadcast Reception, where the RX's are relatively deaf, and
wanted signal levels are high. Sadly, our interest in weak signal working using
sensitive RX's with good antennas, result in us being unduly affected by the urban EM
Fugg we now have to endure.
Coupled with the sad EMC lore, that "nothing radiates below 30MHz anyway", so there are few if any
radiated emissions checks below 30MHz for "consumer" products, it's just conducted EMI that's
tested for, and under artificial conditions too. So, even a product that passes such a test, in practical
use can still cause a huge problem when it has a "better antenna" (the mains wiring, or other hook
up leads) to feed it's QRM into and get radiated!
If you do have unduly high levels of noise on the bands, run the RX from a
battery (A KX3 here is a natural of course for this sort of test) make sure you
have the RX connected directly to the antenna, note the noise levels, power
down your house with the main breaker, and see what changes.
If it did markedly change for the better, then at least you have control of
whatever is causing the trouble. Finding it though, can take a while, unless
you can power up each part of the house separately. You might be surprised at
what you may find!
If there was little to no difference, then any of your neighbours, or local
infrastructure can be causing it, then you have a near vertical uphill struggle
to even identify the source, let alone getting a fix.
If you want help, make a recording of the noise, with a RX in AM mode, and as wide a filter as
possible, and make that file available to others via (for example) dropbox or similar. If it's
high levels of "white noise" that is definitely coming from outside your property, that
could indicate badly balanced phone lines carrying VDSL signals, possibly linking fibre fed telco
cabinets to users homes (common here in the UK now) but there are clues you can hunt for (narrow
gaps or "guard bands" between different levels of noise.)
Lastly, even with high levels of local QRM, some of the new Digital modes, can
work very well indeed, but again, the learning curve is steep, if you've only
ever used RTTY or PSK31 in the past.
Welcome Back Rick, and 73.
Dave G0WBX.
~~ Original Message ~~
From: rick jones<[email protected]>
Subject: [Elecraft] [OT] Off Shore RF amplifiers
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
First of all, I'm NOT entertaining purchasing one, I'm happy with my 100W K3. I've been
away from the hobby for a while and I'm catching up on a few things. I'm noticing quite a
few add on amplifiers targeting the portable crowd on the auction site. Most are
originating from China. Many show an internal picture and I do not see any obvious forms
of LP filtering. Is it legal to run one of these amps without FCC acceptance? If the
answer is "no" then is an effort being made to educate people why these amps
may be causing harm to the airwaves (in the test question pool, for instance)? Hams,
being a notoriously thrifty lot, may be attracted to the low price but may not be aware
of, or have the ability to monitor their own signals. I'm not trying to stir up any wars,
I'm simply curious about the proliferation of cheap equipment of questionable quality
control and if there has been a noticeable change to the bands over the years.
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