Receiver Impedance Matching Transformer
(Balun or Unun or tuner - they are all transformers.)
When your received signal is lower than, or close to, your receiver's noise floor, it's time to think about
a more efficient means of transferring the energy from the antenna to the radio. If you chase DX or
would enjoy listening to a quieter weak signal, you may benefit from better impedance matching.
The use of a tuner (transmatch, coupler) or transformer can improve reception in the following ways:
1. Greater energy (signal) transfer from antenna to receiver due to correct impedance match. This may be a moot
point if the incoming signal is much above your receiver's noise floor, unless any of the following apply:
2. Out of band noise can be reduced with improved matching, since the whole antenna circuit is more efficient
when matched at the desired frequency, and less energy transfer takes place at other (offending)
frequencies (if in fact, there is strong out of band noise present). Of the three categories of external noise;
cosmic, atmospheric (both QRNatural) and man made (QRMan), I have observed the reduction of some of the
characteristic noise that shows up on the scope about an octave or so removed from the received signal. These
are tests done with wide and varied types of antennas that are mismatched by at least 6:1 or 10:1 and when out
of band noise is available - particularly static crashes. This seems to be less true of cosmic noise and mostly
noticeable with certain atmospheric and QRM (neighborhood) radiation.
3. Utilizing a transformer (not a "Tee" circuit tuner or Pi network, see below) adds the protection of maintaining a
constant static electricity (DC) bleed to ground. This is a very important consideration with transistor radios,
with their delicate front ends.
4. The use of a tuner or matching transformer is usually indicated ahead of any passive preselector in order
to control the impedance-selective filters. This would be less true when the antenna is being brought to the
receiver via 50 ohm coax, and more the case when feeding from an end-fed wire antenna, a loop, a single-wire
(true Zepp or Marconi), inverted "L", ribbon, or ladder line.
Here's why: The Q, or skirt selectivity, of the passive filtering in a preselector, head-end filter, or any front end
preselection within the radio, is going to be impedance selective (unless preceded by some kind of impedance
controlling active circuitry, which should be avoided as that tends to add noise to these microvolt stages of the
receiving chain). Passive preselectors are generally optimal at between 6 and 12 ohms (within the filter loop), with
lesser impedances being somewhat impractical due to ohmic losses. So, with an impedance transform of 4:1 or
8:1, the filtering Q can be very effective. I have settled on 18 to 24 dB (skirt depth) with a 10 KHz bandpass, which
is about as effective as we can get with passive components. It's important to note that impedance mismatches
ahead of passive signal preconditioners can all but destroy the filter's Q-factor. I believe that some of the
complaints concerning the effectivity of signal preconditioning can be traced back to that very situation.
Preconditioning signals in this way can help the receiver process the signal in its front end and IF filtering stages -
you are as healthy as your diet!
This is a small (about 3/4") core wound with
24 gauge wire. The core is type 43 ferrite, but
I have used 61 and 75 with good results. The
loss within the transformer is a fraction of a
dB. Bench testing was conducted from 150
KHz to about 10 or 12 MHz. Extensive A/B
testing has be ongoing out to 15 MHz with no
When should you think about receiving antenna matching?
A handy Tee/Pi network
switch for a very versatile,
broadband tuner. Note that
none of these will provide
a DC static bleed to
ground. For a basic DC
and impulse protection
circuit, see the input part of
the schematic, here.