Why Antenna Tuners Aren't Necessarily Useful for Shortwave Listening
         by Daniel A. Grunberg   --   Kensington, Maryland U.S.A


    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.  

IMHO, 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 at all.  

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 
     not necessary." 

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 change.   

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 one.  



++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



Subject: Re: NEW TO SHORTWAVE
From: ce369@FreeNet.Carleton.CA (Daniel Grunberg)
Date: 1997/06/19

Dave Rickmers (=rickets@earthlink.net) writes:
| On Tue, 17 Jun 1997 12:48:33 -0400, "Christopher A. King" 
| wrote:

||Daniel Grunberg wrote:

||| "Christopher 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.

| 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 (correct me if I'm wrong) what you're saying here 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?  


| Chris:  A TUNER which corrects mismatched impedances will
| increase the signal.  A passive 50 Ohm in/out preselector (e.g.
| MFJ-956) will not.

I'm not 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
expensive solution.  



+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++



in a posting to rec.radio.shortwave
        Subject: Random wire/balun/coax/ATU question
           Date: Wed Aug 28 11:37:18 1996

"Bruce Wilson" <bew4568@acs.tamu.edu> said: 

|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 tuner
|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 coax.

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
frequency.  







This article was last updated on 12 August 1997.

If you have any questions, feel free to Email me ce369@freenet.carleton.ca . I'll do my best to confuse you completely (:-). (Comments or corrections also are welcome.)



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