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.
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.
To give you an idea what the various SWR numbers actually mean in terms of reflected power.
|VSWR||Power Out||Power Reflected|
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:
|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
T-Match (basically a Gamma match on either side of the driven element)
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
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