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Antenna Tuners

posted Feb 17, 2014, 9:40 PM by Charles Boling   [ updated Feb 17, 2014, 9:40 PM ]
(a/k/a "Antennas as electronic circuit stages")

Some highlights from last night's lecture:
graph of antenna impedance vs. frequency
To a radio, an antenna looks no different from any other collection of electronic components, and the input characteristics of any antenna can be duplicated with the right combination of resistors, capacitors and inductors.

At a given frequency, an antenna system presents a specific impedance (resistance + reactance (inductance + capacitance)).  Ideally, it will present 50 ohms resistance + 0 reactance -- which is what the outputs on modern radios are designed to match.

The graph to the right (taken from the ARRL Antenna Handbook) shows how the impedance of an antenna (in this case, a 100' dipole made of .1" wire) changes with respect to frequency.  In classic "Smith Chart" style, resistance is shown on the X axis, with reactance on the Y (Inductance up, capacitance down), and the frequency in MHz is labeled along the trace.  This antenna is a half-wave dipole at a little below 5MHz, presenting a theoretical 72 ohm purely resistive load.

If your antenna is a bit too long, it gains inductance, increasing the overall impedance; too short, and it loses inductance, becoming capacitive. Note that the same antenna can also be used at 3x its design frequency, as well as 5x, etc.; also, there it is resonant (i.e. no reactance) but with a much higher resistance at multiples of the full wavelength.

Any circuit stage (e.g. your final amplifier) can transfer power most efficiently to another circuit stage (e.g. your antenna) when their impedances are the same.  Any time there is an impedance mismatch anywhere in the line (e.g. coax with a different characteristic impedance than your antenna or radio, maybe a kink somewhere in it, a bad connector, etc.) some power will be reflected back from that junction.  The more power is reflected, the more trips it makes down the coax, causing greater signal loss but perhaps making your coax cozy warm. One web page that does a pretty good job of explaining reflected power, SWR losses, etc. is http://www.wc7i.com/reflection%20section.htm.

The load connected to the output of a tuned circuit such as the RF amplifier on your radio can affect the output circuit.  If you put an antenna on it that's e.g. too short and looks like a massive capacitor, it's going to load your output circuit in such a way that it detunes it, makes behave horribly, and unchecked, not only will you get rotten radiated power, the circuit components (usually those nice expensive power transistors) may overheat until they melt.  (Fortunately, most modern radios have "fold-back" circuits that automatically reduce output power in this situation and can save your radio from such costly mistakes!)

An antenna tuner, whether the good ol' fashioned kind with cranks and huge components, or a computerized one with 20 different relays that go clickity-clack when you hit the TUNE button, uses capacitors and inductors to adjust the reactance of the rest of the antenna system to match the impedance of what's connected to it.  While it's true that losses in coax can be significant in the presence of extremely high SWR, the most important thing is making the radio happy, and it's a lot easier/cheaper to operate a tuner inside your comfortable shack instead of up your tower, so most tuners go between your radio and your feedline.  Antenna too short?  Add some inductance in series with it.  Too long?  Make it more capacitive.  There are different arrangements used for the components in different tuners, and some work better w/ high-impedance antennas, while some can handle lower-impedances fine, etc., but the basic principle is the same.

In addition to L-C (inductance-capacitance) circuits, impedance can also be changed by a transformer, and even a particular length of mismatched coax!  (Yes, a 1/4-wavelength of e.g. 75-ohm coax can do nice things to the impedance the radio sees, in the right situation.  Baluns and coax sections are often used with or without tuners to make a particular antenna a better match to the radio.  A 4:1 transforming balun will often allow you to match a specific antenna that would be otherwise beyond the reach of a particular tuner. 

Tuners are particularly convenient, because they allow you to use a single antenna at frequencies other than what it was designed for, allowing you to e.g. work low bands with a smaller antenna, and multiple bands with a single antenna.  No, using a tuner is never going to be as efficient as a full-size antenna designed for the frequency you're operating on, but done right, it can be close!


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