Impedance matching

From wikipost

Most RF transmitters nowadays are designed to work with a 50 Ohms load impedance. (Read here: Why 50 Ohms? ) Many antennas however do not have a 50 Ohms input impedance, so the need for Matching arises.


For related information on the subject, see also our articles on common mode interference and baluns.


Why do we need/want impedance matching?

  • Maximum power transfer happens when the source (transmitter) impedance and the load (antenna) impedances are matched. If there is a feedline in between then the feedline needs to be matched as well.
  • Impedance mismatches will cause equal-and-opposite (differential-mode) currents going back and forth in the feedline. This may damage the radio.
  • Built-in protection circuits in radios prevent delivering maximum output power if a severe enough mismatch is detected (e.g. higher than SWR > 2:1).




Voltage Standing Wave Ratio (VSWR, or just SWR)

SWR is be caused by an impedance mismatch between source and load.

Common areas where the impedance might not match the source or the feedline:

  • antenna feed point (e.g. dipole is 73 Ohms, quarter-wave vertical on ground is 36 Ohms)
  • type of feedline (coax is usually 50 or 70 Ohms, twin-lead can be 300 or 450 Ohms)
  • routing tracks over a circuit board (vias, corners, thick-to-thin transitions)



SWR is an indication of how much of the transmitted power to the antenna is reflected due to impedance mismatch and comes back to the radio. SWR is entirely contained inside coax or on the conductors or twinlead.


Coax close up
Twinlead close up


To give you an idea what the various SWR numbers actually mean in terms of reflected power.

VSWR and Reflected Power
VSWR Power Out Power Reflected
1:1 100W 0W
1.5:1 100W 3.9W
2:1 100W 11.1W
2.5:1 100W 18.4W
3:1 100W 25W
3.5:1 100W 31W
4:1 100W 36W


Why we don't want high SWR

  • cable losses could add up significantly and cause output signal attenuation
  • high voltage at radio can cause 'RF in the shack', 'hot' microphones
  • RFI to nearby devices due to over-stressing radio final power amplifiers and protection circuits



So in short:


High (Voltage) Standing Wave Ratio
Cause Effect Symptoms
Impedance mismatch due to:

inherent antenna design

feedline not matched to antenna or radio

antenna not matched to feedline

SWR cable losses due to attenuation

rf in the shack

stressing power amplifier and protection circuits and cause RFI

less than maximum power transfer to the antenna




Various ways of matching feedline to the antenna

Gamma Match

More info: http://myweb.tiscali.co.uk/g8hqp/radio/gammamatch.pdf

T-Match (basically a Gamma match on either side of the driven element)


Delta Match


Hairpin Match


Stub Match


Omega Match



The device that matches the radio to the feedline is often called an Antenna Tuner or ATU (Antenna Tuning Unit), but this name is slightly incorrect as it doesn't actually tune the antenna; it adds capacitance or inductance to whatever is connected to the Antenna port of the Tuner to cancel out any reactance.

If matching the feedline to the antenna is not done at the antenna feed point it is still possible to match the radio to the feedline. This is often done for practical reasons in that a tuning network can be situated close to the radio (sometimes even on the same circuit board) and is away from the harsher outside environment where the antenna is. The biggest drawback of course is that any mismatch between the feedline and the antenna is propagated all the way down to the radio until it reaches the antenna tuner. This is less of a problem with lower frequencies (e.g. < 100MHz), but at higher frequencies there may be significant attenuation of the signal simply due to the length of the feedline.


Transmatch (Antenna Tuner) designs

T-Match


Z-Match


L-Match


Pi Match


S-Match




Some myths dispelled:

  • The length of feedline does not alter the SWR (apart from making it less due to losses in longer feedlines). Feedline length DOES affect impedance.
  • Antenna Tuners do not need to be encased in metal enclosures as there are no harmonics or non-linear products generated
  • SWR does not radiate off the feedline
  • Common mode currents on the feedline can still occur in a 1:1 SWR antenna system