Filters And Couplers
An antenna coupler is a device which makes it possible for one or two antennas to do the electrical work of many, as opposed to using a separate antenna for each individual wavelength. Ideally, every signal would have an individual antenna perfectly measured to resonate to its frequency.
Antenna tuner, matching network, matchbox, transmatch, antenna tuning unit, antenna coupler, and feedline coupler are all equivalent names for a device connected between a radio transmitter and its antenna, to improve power transfer between them by matching the specified load impedance of the radio to the combined input impedance of the feedline and the antenna
Antenna tuner, matching network, matchbox, transmatch, antenna tuning unit (ATU), antenna coupler, and feedline coupler are all equivalent names for a device connected between a radio transmitter and its antenna, to improve power transfer between them by matching the load impedance of the radio to the combined input impedance of the feedline and the antenna.
What antenna tuners do & don’t, can and can’t (ref. 1, 2):
Antenna tuners do not tune the antenna, in the sense of shifting the resonance frequency of an antenna to the operating frequency of the transmitter.
Antenna tuners cannot change the feedpoint impedance of an antenna.
Antenna tuners cannot change the impedance along the feedline between the tuner and the antenna.
Hence, an antenna tuner cannot change the SWR along that feedline between the tuner and the antenna.
An antenna tuner can, however, change the load impedance that appears at its connector to the transmitter. Hence, it changes the matching at the transmitter.
Conversely, an antenna tuner can and does change the source impedance that appears at its connector to the feedline to the antenna. Hence, it changes the matching at the feedpoint of the antenna.
The function of a “tuner” is to provide impedance matching for the purpose of maximizing power transfer between the transmitter/transceiver and the antenna system ( = antenna + feedline).
A tuner/coupler is only a “local impedance transformer”. It provides (or improves) impedance matching only at the point where it is inserted into the feedline. All other impedance mismatches elsewhere along the feedline (and associated losses) to the antenna are not corrected!
More precisely: maximum power transfer requires conjugate impedance matching between the source and the load:
Simply put: cancel out the non-resistive (i.e., inductive, capacitive) part of its load impedance, and transform the resulting purely resistive impedance to 50 ohm ( = “matching”). So, it would be more appropriate to call a tuner/coupler an “impedance matching unit”.
In principle, a “tuner” can be inserted anywhere along the feedline between the transmitter and the antenna.
If it is placed directly at the antenna, it is often referred to as an “antenna coupler”. Here, the “tuner” matches the impedance of the antenna to the characteristic impedance of the coax-feedline between the tuner and the transmitter. This minimizes feed-line losses. Obviously, if the tuner is located at the antenna, it must have remote control. Typically such remote tuners/couplers are automatic: the transmitter outputs a low-power carrier, and the tuner does its thing.
When the tuner is located closer to, or at the transmitter, it may very well be able to provide perfect impedance matching between the transmitter, and the “feed-line + antenna” system. The feed-line may be balanced (ladder line, twin-lead), or unbalanced (coax). Whereas the transmitter may be very happy in this configuration and see SWR 1:1, feed-line losses may actually be (very) high: a tuner that is not inserted at the antenne feedpoint does not correct the mismatch between the feedline and the antenna.
All elements placed between the transmitter and the antenna cause power loss: feedline, tuner, balun,… Power loss in a lossy item means dissipation, i.e., heat generation. Note that power loss in tuners is not limited imperfect components: in compact tuners, a coil that is placed close to the metal housing will cause induction heating (and also reduces the “Q” of the tuner). In general:
The larger the impedance mismatch that the tuner/coupler has to correct, the larger the tuner/coupler losses. These losses may be a very significant part of the transmitter output power!