Why an antenna tuner
may be a waste of money
August 12, 1997
I wrote the following explanation about why not to assume
that an antenna tuner is necessary, or even useful for good reception.
Some of the discussion, that followed the posting of my article
on rec.radio.shortwave, follows the article here too.
The fact is that an antenna tuner won't help you much, unless
the level of the noise brought from your antenna to your receiver
is significantly lower than the level of the noise that your receiver
generates, internally. IMHO, that won't be true almost all of
the time. If you wish, you can perform a simple, practical test
(see the end of this article) and decide for yourself.
In my opinion, an antenna tuner might provide some marginal
help in the reduction or elimination of interference from signals
well off the desired signal's frequency, but it will not
increase the signal to noise ratio on the frequency to which it
is tuned. Therefore, unless, for example, you're having trouble
with interference from MW stations screwing-up your reception
on 6 MHz; or you are lucky enough to have an antenna in a very
electrically quiet location; an antenna tuner won't help you
Here I quote from page 582 of the 1974 Edition (the old one I
have readily at hand) of The Radio Amateur's Handbook:
"Most hf receivers are sensitive enough that exact matching is
Whether you use an antenna tuner or not, some of the signal will
get to the receiver. Any signal or for that matter any noise voltage,
that is present at the input to the receiver will be amplified,
so long as the signal or noise frequency is within the passband
of the amplifier, and the combined signals and noises present
do not exceed the amplifier's dynamic range (i.e. they do not
overload the amplifier). Modern receiver front-ends are sensitive
and quiet, and if a signal has a good enough signal to noise ratio
it will be heard. Using an antenna tuner doesn't help because
when you "peak" the tuner to increase the signal strength of a
signal by a few db, you also "peak" the noise level at and near
that frequency by the same number of dB. The signal to noise ratio
in dB for a signal strength of S dB and a noise level of N dB
is given by:
[Signal to Noise Ratio] = S - N
If you increase both S and N by the same number of dB, S - N doesn't
While it is true that an antenna tuner will help a little
to reduce or eliminate off-frequency signals, there is a better
way to do that. If, for example, you're having trouble with interference
from MW stations screwing-up your reception on 6 MHz, an antenna
tuner might help to reduce the interference A LITTLE. But an antenna
tuner is designed as an impedance matching device for transmitting,
and only incidentally would be a (relatively poor) off-frequency
signal receiving filter. A preselector (preferably an un-amplified
one, or one that can be operated with its amplification switched
off) is a device specifically designed as an off-frequency receiving
filter, and does a MUCH better job of it.
Bottom line: IMHO, unless you have convinced yourself that your
location is VERY QUIET (RF-wise), don't use an antenna tuner.
If and ONLY IF you're having a problem with out-of-band signals,
consider getting a preselector. Otherwise, if you want more signal,
consider improving the antenna.
Simple practical test
1. With your antenna connected, tune your receiver to an unused
frequency, near a frequency that you might want to receive.
2. Disconnect your antenna from the receiver.
3. Using as short a wire as you can, reliably short
circuit your receiver's antenna connection point to your receiver's
ground connection point.
4. Turn up the audio gain, and the radio-frequency gain (if your
receiver has one). Set your receiver's local/DX switch to DX (if
your receiver has one). Verify that the receiver noise
you hear is nothing but noise (or possibly no noise at all).
5. Remove the short circuit and reconnect the antenna to the receiver.
Verify that the total noise you hear is nothing but noise
(or possibly no noise at all).
6. If you judge that the total noise heard with the antenna is
NOT audibly louder than the receiver noise heard with the receiver's
antenna connection short circuited to ground, then PERHAPS an
antenna tuner MIGHT make a difference, and you MIGHT want to try
Don't wast your money on matching
(June 19, 1997)
A. King writes: A passive series-resonant tuner (MFJ-956,
Grove Mini-Tuner) may improve matching and signal transfer from
about 5MHz and below.
As has been pointed out on this Newsgroup before, although the
better matching and will increase the signal transfer, the matching
of naturally occurring noise at and near the tuned frequency
also will be improved by the same factor that the signal is
improved by. The result is that there will be no improvement
in signal to noise ratio, and therefore no increase in signal
readability. Don't waste your money merely for the sake of matching
the antenna to the receiver, it just ain't useful.
I suppose that we could get into a real nitpicky argument about
how the front end of the receiver has a signal-to-noise ratio
of its own, and that by providing a stronger signal at the antenna
input you are improving matters at this point irregardless of
whether or not you're improving the relationship between signal
and noise as it exists on the antenna. But since it's clear that
you already have you're mind made up on the issue, I won't bother.
A. King writes:
Yeah, Dan. What about the a.v.c. threshold? If you are below
that threshold you have: signal + noise from the antenna and
circuit noise from the i.f. (and r.f., if applicable) amplifiers|
running full gain.
So what you're saying here (correct me if I'm wrong) is that the
antenna tuner will increase the very-weak signal + noise from
the antenna to lower the gain of the receiver so you can hear
the not-quite-so-very-weak signal+noise from the antenna. That
probably would make sense in a low-ambient-noise laboratory environment.
On SW frequencies, where the naturally-occurring, ambient noise
level on and near the frequency to which your tuner is tuned is
not low compared to the internal noise of the receiver, the ambient
noise is what's keeping you from hearing the signal in the first
place, with your "amplifiers running at full gain." How does the
tuner alter the situation?
Try this. Remove your tuner, and connect your antenna directly
to your receiver. Find an unused SW frequency, and note (or better
yet measure) the noise level you receive. Put the tuner back,
and match the antenna to your receiver. Note (or better yet measure)
the noise level you receive. Wasn't there more noise with the
tuner? If the tuner were to multiply the naturally-occurring,
ambient noise level on and near the frequency to which your tuner
is tuned, by the same factor that it also multiplied a signal
on the frequency to which your tuner is tuned, how would the tuner
have improved anything?
A tuner which corrects mismatched impedances will increase
the signal. A passive 50 Ohm in/out preselector (e.g. MFJ-956)
sure what you're saying here. Surely you don't mean to say
that the tuner, made up only of coils and capacitors, is an active
device. Surely you don't mean to say that a preselector, which
I think (correct me if I'm wrong) has a built in amplifier, is
a passive device.
Assuming that the tuner or preselector is used, as a tunable band-pass
filter to attenuate interference that is well off the tuned frequency,
and assuming that the preselector is run with it's amplifiers
(if it has any) switched off, then the choice of a tuner or a
preselector might be based on price and performance. An antenna
tuner may be cheaper or more readily available than a preselector.
OTOH, a preselector, because it probably has a narrower passband
than antenna tuner, might work if the antenna tuner didn't.
If the cheaper solution works, go with it. If not, try the more
Random wire and antenna tuner
(August 28, 1996)
Let's say a random wire is connected to a balun (like the MLB),
and then the coax from the balun connects to an antenna uner
|then to the radio. What is tuned when the ATU knobs are adjusted?
The wire or the coax? In other words, does the ATU match the
coax impedance to the receiver input impedance, or is the entire
system, from wire to the ATU, tuned to match the receiver? I've
looked through many books and can't find an answer. Maybe experimental
evidence will answer the question.
A receiver is matched to an antenna, at a given frequency,
when the receiver's input circuit, the tuner, the two transmission
lines, and the antenna are in resonance at that frequency,
and the antenna looks down the line to the receiver and "sees"
a resistance that is equal to the resistive component of the receiver's
antenna input impedance. This is called complex conjugate matching.
For the purposes of the following discussion, frequency means
any or all radio frequencies below 30 MHz, that are within the
tuning ranges of both the receiver and the antenna tuner. For
the purposes of the following discussion, antennas are fed with
A SW receiver's designer probably has tried to present a known,
fixed, almost-purely-resistive load at the receiver's antenna
connector. In a modern SW receiver, usually the antenna input
circuit is unbalanced (i.e. one side is grounded). It is likely
that the designer of a modern SW receiver has tried to make the
impedance at the antenna connector equal to either 52 or 72 Ohms.
(52 Ohms or 72 Ohms are the characteristic impedances of several
commonly-available, coaxial cables.)
If an antenna tuner is used (and I don't believe its use generally
is justified), typically the tuner will be located quite close
to the SW receiver, in order to facilitate tuning. If the cable
run between the SW receiver and the tuner is short (perhaps a
meter or shorter), and the characteristic impedance of the coax
used corresponds to the rated input impedance of the receiver,
mismatches between the tuner and the receiver can be neglected.
Therefore, practically speaking, the purpose of the tuner is to
match the impedance of the antenna, as it is transformed by
the transmission line between the antenna and the tuner, to
the impedance of the receiver.
Transmission lines act as impedance transformation devices, but
not necessarily as simple transformers). For a given frequency,
if the impedance of the antenna is purely resistive and it equals
the impedance of the transmission line, then regardless of the
length of the transmission line between the antenna and the tuner,
the impedance presented to the tuner will be the impedance of
the antenna. On the other hand, for a given frequency, regardless
of the antenna's impedance, if the transmission line is an integer
multiple (0, 1, 2, ... ) of half-waves long, the impedance presented
to the tuner will be the impedance of the antenna regardless of
its amplitude and or whether or not it is purely resistive. Since
we wish to receive signals at many frequencies, we also must consider
the much more common classes of cases, where neither is the antenna
impedance equal to the characteristic impedance of the line, nor
is the line length an integer multiple of half-waves.
As the frequency varies, a fixed-length transmission line, whose
length beyond an integral multiple of half waves varies as a function
of frequency, will transform the impedance of an antenna whose
impedance also varies as a function of frequency. As the frequency
varies, the amplitude of the impedance presented to the tuner
by the transmission line may be large or small. As the frequency
varies, the impedance presented to the tuner by the transmission
line may become resistive, highly reactive (either inductive or
capacitive), or some combination of resistive and reactive.
The simple answer to your question is that a receiving antenna
tuner must tune the combination of the antenna and the transmission
line from the antenna, in a way that varies complicatedly with