(Ham radio’s dirtiest issue, which few dare discuss)
By Matt Erickson, KK5DR, with commentary by Adam Farson, VA7OJ/AB4OJ
Currently, the way things are in ham radio is a mess! The HF ham bands are far more filled with interference than any other frequency range within the HF spectrum; this is due to mostly ham-generated noise. We are our own worst enemies in this aspect. Many hams have demanded of the radio manufacturers that close-in selectivity performance be emphasized above all else. No one dared say that the reason for this demand was the fact that there are a huge number of very “dirty” transmitters operating within the ham bands. In this article, I am daring to say it! My own estimates of the numbers of ham transmitters that are what we can call “dirty” run about thirty percent of the currently active HF hams. In the contesting and DX’ing segment of HF ham operations I would say that as many as seventy percent of the transmitters are splatter-generating noise machines. Many hams have come to accept this as “normal”. I’m saying that this isn’t the way it has to be!
It is pretty certain that anyone that has operated on HF for any amount of time has heard the God-awful noise of an OT running his station for all it is worth, “fire-walled”, “all knobs to the right”, “maximum smoke”, etc. This is NOT the way you should run your gear! It is a rather touchy matter to tell a fellow-ham that his transmitted signal is too wide, splattering, or has so much background noise in it as to generate a “virtual” carrier on SSB.
As my friend Adam VA7OJ/AB4OJ says: “Then there is the ‘elephant
in the bathroom’ which none of the contesters or ‘big-gun’ DX'ers wish to see;
the limiting factor in close-in amateur HF operation is not so much receiver
dynamics as the poor spectral purity of most transmitted signals. Transmitted
IMD3 products at -15 to -20dBc, and CW keying sidebands at -30 or -40 dBc, are
by no means uncommon. Concerted regulatory intervention (nationally, or via the
ITU) may ultimately be the only way to correct this problem. In addition, one
of the worst aspects of the dirty-transmitter phenomenon is the practice of
‘tweaking out’ the RF output control for those few extra watts. We should not
forget that IMD3 increases by 3 dB, and IMD5 by 5 db, for every dB of
increase in power output above the transmitter's design rating.”
Aside from the technical aspect of this issue, there is the greater ethical issue of knowingly operating a station that causes harmful interference to your fellow- hams. It used to be a matter of pride to run a “clean” station and cause little to no interference to others on the band. The decline in operating standards and ethics seems to be a part of the ever-decreasing knowledge of how radio works, and the increased emphasis on features and “plug-and-play” operation, with little to no concern that the equipment is operating properly. It is a classic case of the “dumbing-down” of ham radio, and of society in general. It is also a matter of competition; we compete for space on the bands, we compete for contacts, and we compete to have the “biggest” signal on the bands. It is all a vicious cycle generated by us. One ham increases his power output to the point that it becomes very “dirty”, another ham has difficulty receiving the desired signal on a nearby frequency, so he generates his own splatter in retaliation, and so on. The amateur service is the only radio service outside CB that tolerates this type of operational interference.
Few hams own even the minimally required test equipment to insure that their transmitted signal is as clean as can be. An RF power meter is NOT the only test instrument required for proper adjustment of a station; an oscilloscope would be a basic station test unit to own and learn to use. In this way, the operator could ensure a “clean” transmitted signal on our ham bands.
You might ask; “I don’t know how to read an
oscilloscope trace?” See below what to look for.
Fig. 1
Fig.2
Fig.3
Figure 1, shows a two-tone test of a transmitter that is properly adjusted. Figure 2, shows a transmitter that is being over-driven (notice the flat-tops). Figure 3, shows a transmitter that has idle current that is too low, and or poor carrier suppression. (Notice, the cross-over points are not well defined, hence cross-over distortion.)
Coupling of the oscilloscope can be done by coiling a short length of wire around the outside of the transmitters coaxial cable, then adjust the scope voltage for a display size such as you see in figure 1., next adjust the sweep speed for a stable display that does not move horizontally by very much.
It is generally accepted that a two-tone test is good enough for most stations.
Then there is the HF operator who deliberately transmits a broad, spectrally-impure signal to establish a “clear-channel” by brute force. This seems to occur more frequently than many would like to admit. Just because it works, does not make it right!
Transmitter distortion can occur regardless of output level. This does not mean that a ham can simply reduce his RF output and become clean; even a QRP station can have a very bad signal with distortion and key-clicks due to poor power supply regulation or ripple, RF feedback or other sources.
It would be nice if all hams made the effort to learn as much about their gear as possible, and the theory behind it, so as to be able to know how to diagnose or fix a problem and take corrective action. But sadly, I do not think that is likely to happen anytime soon.
The technical literature
states clearly that the load impedance presented to the transmitter must be as
close to resistive as possible. In a practical amateur station configuration,
this means that even if the antenna is close to resonance (VSWR < 1.5:1), an
antenna tuner must be placed between the transceiver (or amplifier) and the
antenna. The tuner must be adjusted for the best possible match to 50W resistive.
The statement: “I don’t need a tuner, because my SWR is only 1.5:1” simply doesn’t wash!
The universal acceptance of the typical solid-state HF transceiver by the HF amateur community has actually simplified the task of generating a clean signal. As a solid-state RF power amplifier is peak-power-limited, it is quite easy to set it up so as to deliver a clean signal to the antenna or high-power amplifier. For the typical “100W”-class HF rig, powered from a well-regulated power supply (another must!) all we need to remember is to set the RF power output control for 100W CW into a 50W dummy load, as measured on an accurate power meter. The next step is to hook up an oscilloscope as explained here, and adjust Mic Gain for 100W PEP on voice peaks, using no more than 6 dB of compression. If you don’t have an oscilloscope in the shack, I strongly encourage you to beg, borrow or buy one. These scopes are cheap and plentiful at nearly every ham-fest; there is little or no excuse not to have one and use it.
Tube-type HF amplifier set-up:
Proper adjustment of a tube-type
amp is required for a good clean signal. Please read this article for the proper tuning
method.
ALC is absolutely
required for proper operation of an HF
SSB station. It does not matter what you have heard about it; ALC is
required! It is not an optional connection! Tune your system for whatever the
maximum rated output of the amp is, and set the ALC for a slight reduction in
output, and the setup is complete. It
is foolish not to use ALC! It might
just save you a lot of money on replacement tubes too.
If your radio is not equipped with
an ALC input jack, you’re on your own.
Solid-state amplifier set-up:
Connect the RF drive, PTT, ALC,
band-selection and other required cables between the transceiver (exciter) and
the amplifier, according to the respective user manuals. In some cases, all
required interconnections except for the RF drive jumper and ground wire are
incorporated in a single control cable. Connect an RF power meter and a
suitably-rated 50W load to the amplifier output. Always disengage the
exciter’s internal autotuner.
Next, set the exciter’s RF output
control just above the level required to drive the amplifier to full output (typically
75W for a 1 kW amplifier). Verify that
the exciter and amplifier are set to the same band, then calibrate the ALC
according to the amplifier manufacturer’s instructions. Set the amplifier’s ALC
control to just level the RF power at the amplifier’s rated output. Now reduce
the exciter output until the amplifier output just drops, then turn it up until
leveling occurs. The station is now ready for operation. (Further reading.)
Never
operate the amplifier above its maximum rated output. Let’s do the math: Consider
a 1 kW amplifier “pushed” to 1.25 kW. That is 1 dB, or all of 1/6 S-unit! For that, are you really
willing to radiate 3 dB more IMD3 and 5 dB more IMD5, and risk destroying $800
worth of RF power devices?
If the amplifier is fitted with an
internal autotuner, set up the tuner per the amplifier user manual. (Warning: Always disengage
the amp’s internal tuner when using an
external tuner!) The autotuner will also function as a
tracking preselector on
receive.
Warning: Correct ALC connection and calibration is absolutely mandatory when driving a solid-state amplifier. The ALC line is the amplifier’s first line of defense. Do not install a solid-state amplifier if your transceiver does not have an ALC input jack!