[Elecraft] [OT] Off Shore RF amplifiers

[Dave B]

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<n3ikq at yahoo.com>
Subject: [Elecraft] [OT] Off Shore RF amplifiers
Message-ID:
	<1112441812.6599075.1469643690164.JavaMail.yahoo at mail.yahoo.com>
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.