...As far as the health of my transceiver is concerned, if my transceiver is putting out 100 watts and 10 watts are reflected back that's much worse than putting out 5 watts and having 1/2 watt reflected back. Same SWR but ... I want to protect my transceiver first and put out the most signal second.
No, it's not any different if we assume that the final in the QRP rig is designed for 5W output.
It still is subjected to the same potential over-voltage and/or over current compared to the
design values. That is, the percentage change in voltage or current as seen by the output
device will be the same in both cases, so you have the same percentage increase in heat
Now, if you are using a 100W rig turned down to 5W instead of a rig designed for 5W output,
then your comments start to make more sense. But that isn't clear in your post.
In the end it comes down to the design margin
of the specific design: how close are
the final devices being run to their physical limits? If you are running a 100W radio at 5W,
certainly it can tolerate a higher reflected power than at 100W. But a radio designed for 5W
in a small package may already be pushing the safe power dissipation due to poor heat sinking,
and thus will be more susceptible to changes.
If I can keep the absolute level of reflected power below a certain threshold my transceiver should be fine although my antenna may be radiating poorly.
Now my next concern is where to establish that threshold. What is a safe amount of power to reflect back at my transceiver?
Being only concerned about SWR seems to me to miss the mark. My SWR when running 100 watts has to be much lower than when I'm running 5 watts to keep the reflected power below the threshold so I'm really not concerned about SWR, within reason.
Something else that may help is that I'm not only talking about QRP transcievers and power levels but also talking about portable operation with an antenna tuner so working to minimize SWR (other than the tuner) may not be much of an option.
Am I making my point or am I thinking all wrong?
First, SWR doesn't damage transistors
. Transmitter finals may fail due to several factors,
the most common of which are:
2) maximum peak voltage
3) maximum peak current
4) loss of cathode emission (for tubes)
(Tubes and transistors have different responses to the latter two, as tubes can often handle
up to twice the rated plate voltage under momentary conditions, and cathode emission limits
the maximum tube current.)
The effect on the transmitter depends not really on the SWR or the amount of reflected power,
but on the load impedance
that the rig is trying to deliver power to. In fact, when we look
at it that way, we can ignore the meter reading for reflected power and just look at what happens
in the transmitter for a particular load impedance: that is really what determines the conditions
for the final.
(As an example, I remember one of the old Johnson tube rigs was rated to match antenna
impedances from 40 to 600 ohms. If it saw a 500 load at the end of a piece of coax it would
be perfectly happy delivering power into it, even though the SWR on that coax would be 10 : 1
and the reflected power meter would be jumping all over the place. But a 25 ohm load, with
an SWR of only 2 : 1, was more of a problem, even though the number of reflected watts was
Of the common causes of failure, overheating is by far the most common in my experience, other
than loss of emission in tube equipment (especially those using sweep tubes). This can be due to
extended transmissions at high duty cycles, poor circulation of air to the heat sink, poor tuning of
the output that increases heat dissipation, misadjusted bias currents, etc. It isn't immediately fatal,
in that heat has to build up over time until the temperature of the transistor junctions gets too hot
(though if the thermal resistance between the transistor case and the heat sink is too high, it can
happen pretty quickly.) A small fan aimed at the heat sink on the back of the radio makes a big
difference in providing cooling: damage may occur due to the failure of a built-in cooling fan or a
clogged dust filter.
If you run high duty-cycle modes such as AM, RTTY or SSTV (or even SSB with large amounts of
speech processing) then the output devices have to dissipate more heat. With unprocessed SSB on
a normal voice, dissipation is less, so the rig will be tolerant of a higher SWR before overheating
becomes a problem.
Fortunately the heat sink temperature isn't difficult to monitor, even if it just means sticking
a "digital temperature sensor" on it - if you burn your finger, it clearly is too hot. If you can
leave your finger on the heat sink it probably isn't a problem.
Transistors can be quickly destroyed by excessive voltage and current, but actual damage due
to that is not that common (except, perhaps, for transients from lightning strikes or power surges.)
Most commercial rigs use transistors that are capable of nearly double the rated output power
(at least for a very short time) if linearity is not a concern. That would mean that there probably
is at least a 40% safety factor in normal use, and the self-protective circuitry in most commercial
rigs will kick in before that to protect the finals.
So there isn't an exact answer to what I think your question is, because the answer really
depends on the specific design, both electrical (choice of transistors, impedance matching,
fuses, protection circuits) and thermal (heat sink area and effectiveness, thermal resistance
of connection between transistor and heat sink, provision for fans, air filters, over-temperature
sensors, etc.) Generally for most commercial 100W solid state transmitters, at 10W output
there is virtually nothing you can do to damage the rig as you crank an antenna tuner. The
same might be true at 25 watts output. If your rig has a protection circuit you should be OK
with whatever it will permit. As long as you have sufficient cooling on the finals, you might
even get by at 25W reflected power, at least for short periods of time. All my rigs have
survived intermittent antenna connections at full drive more than once, so they really aren't
as fragile as some might make out to be. The only times I've blown a final in a rig were
(1) an FM rig mounted under the dash in my car, right where the heater was blowing on it
in the winter (and that only when I used it for an extended period at full power); and (2) the
final of a QRP rig I was building when I turned up the power too high: the transistor was
only rated for 150mA, and it blew somewhere under 200mA when I tried to get 2W from it.