I am sure most of you know all about SWR (i.e. Standing Wave Ratio), antenna
tuners, and reflected power. However, it might be worth our discussing it
anyway for the benefit of those who might not be as familiar with these
relationships as others. Many of us use antenna tuners, both manual and
automatic, for obtaining the best possible impedance match at the output
point of our transceivers with the objective of getting the SWR as close to
1.0 to 1 as we can. A perfect match for most rigs is when the transceiver
sees a 50 ohm load at its output. Anything above a 1:1 SWR represents some
level of mismatch. Most of our transceivers can tolerate some mismatch
before it begins to reduce the power output as a means of protecting the
circuitry of the transceiver, and if the SWR or mismatch gets high enough,
it will cause the transceiver to reduce its power output to zero. Many of us
have transceivers, like the Kenwood TS-590 and similar rigs, that have
built-in automatic antenna tuners. These tuners work extremely well for
matching the impedance presented to the transceiver at its output, providing
this mismatch isn't too high. How much is too high varies to some extent
from one make and model of transceiver to another. If the mismatch is so
large that the rigs internal automatic tuner cannot bring the SWR down
within an acceptable level, you are faced with either adjusting your antenna
to achieve a better match at the point where the feedline connects to the
antenna, or you will need to obtain an external tuner to do the job.
External tuners, whether manual or automatic, tend to have much wider ranges
for matching the impedance of the feedline to the 50 ohms the transceiver
wants to see at its output. A match at the transceiver's output does not
mean there is a match at the point where the feedline actually connects to
the antenna. Even if the impedance at the output of the transceiver is
matched perfectly with an antenna tuner, there can be a very substantial
mismatch at the other end of the feedline where it connects to the antenna.
The larger this latter mismatch, the less actual power will be radiated by
the antenna and the more power from the transceiver will be reflected back
or remain in the feedline. What happens to the reflected power depends
largely on the type of feedline used. The difference between the power
radiated by the antenna and the output power of the transceiver is the
reflected power and may be totally lost power, depending upon the loss
rating of the feedline. If coax is used for the feedline, this reflected
power ends up dissipating as heat in the coax and is pretty much totally
lost power. If this heat is more than the coax can handle, it can burn up
the coax. In contrast, open wire feedline is considerably less "lossy", and
much of the reflected power ends up being radiated by the open wire feedline
as opposed to ending up being dissipated in heat. 

 

I am building up to two very important points that, in my opinion, do not
received nearly as much attention as they should. The first point is that
reflected power is not good for your transceiver, or if you are using a
linear amplifier, it is not good for your amp. If you are using a tube-type
linear, it can handle or tolerate more reflected power than the solid-state
type amps, but regardless, reflected power is bad for your rig and it is bad
for your amp. Here is    my second point: just because your antenna tuner is
able to get the SWR down to 1.0 to 1 or an acceptable level at the output of
your transceiver or amp, that does not mean the reflected power is zero or
reduced to an acceptable level. Most rigs and amps today have built-in
protection circuits that detect high SWR and high reflected power, and they
will shut completely down if either the SWR or reflected power is too high.
For example, I have a Kenwood TS-590 driving an Ameritron ALS-600, and my
antenna is an 80 meter, full-wave loop. I feed the loop antenna with 50 ohm
coax and use a 2.5 to 1 matching ballun at the feedpoint of the antenna. The
loop works extremely well on all bands 80 through 10 meters, including the
WARP bands. However, there is a sizable mismatch on some bands at the
feedpoint of the antenna. Without an antenna tuner, this mismatch would
present such a high SWR at the output of the transceiver or amplifier that
the protection circuits would reduce the power to zero or, in the case of
the amp, it would kick it off. The mismatch is so high that the internal
tuner of the TS-590 cannot handle it. So, I use a LDG AT1000PRO II external,
automatic antenna tuner, which has a much wider matching range, and it can
handle the mismatch and the power of the ALS-600 just fine. However, even
though I am able to get my SWR down to 1.0 to 1 on some bands and 1.5 to 1
or better on the others, I cannot run the amp at full power because if I do,
the reflected power is so high, over 50 watts, that it causes my ALS-600 to
kick off. Therefore, I am forced to reduce my output power down to the point
where the reflected power is low enough that it won't shut the amp down.
Consequently, instead of putting out 600 watts from the amp, I have to
reduce the power to where I am putting out only about 300 watts. This keeps
my reflected power level below 50 watts, which is the critical level for the
ALS-600 amp, keeping it from completely shutting down.   

 

Many years ago, when I was a young, foolish teenager, I put up an antenna
that had such a high SWR that I could get RF burns from the Microphone of my
transmitter. The transmitter was a Heathkit DX-40, which had a PI-output
circuit that would allow me to load-up about anything for an antenna, but
back in those days, there were no protection circuits to automatically shut
the rig down if the SWR or reflected power was excessive. As a result, the
high SWR and reflected power from trying to load and extremely mismatched
antenna caused my tank coil to get so hot that it melted the plastic
insulators that separated the windings of the tank coil; this caused the
tank coil to short, which, in turn, created a chain reaction, ending up in
wiping out the entire power supply. My point is to simply illustrate what
high SWR and reflected power could do to your transceiver or amp if it
weren't for the protection circuits that are built into the modern gear.
However, even with the protection circuits, it is important to strive to
achieve the best possible resonant frequency match with your antenna and
then, between the feedline and your antenna and again between the feedline
and the output of your equipment.

 

I've had three people write to me off list asking me questions that related
to this whole topic, and so, I thought I would simply write this up and post
it to the list. I'm not trying to show-off what I know; rather, I might be
showing-off what I don't know because there is a lot more about this stuff
that I know very little about, and I am absolutely certain that many of you
know tons more about some of this stuff than I do. I'm just trying to be
helpful to some of the people that might be new that do not understand some
of these things. 

 

  

 

 

 

Dr. Ronald E. Milliman

Retired Professor of Marketing

 

President: Millitronics, Inc. (millitronics.biz)

President: South Central Kentucky Council of the Blind (SCKCB.ORG)