Something has bugged me for a long time — the way some radio manufacturers market and sell their transceivers with built-in automatic antenna tuners. These are almost always intended to be used with antennas that are sold by the manufacturer (for instance, a specific mobile antenna) but this is seldom clear in large print in the adverts.
An unsuspecting ham may think the auto-tuner in his shiny new HF rig is going to work with the new dipole he hung up between the trees, only to discover it’s a hot mess.
Due to their size, most internal tuners (there are some exceptions) can manage only a slight mismatch in impedance and cannot begin to cover a wide range like that presented by ladder line and wire antennas. The explanation for this is fairly straight forward.The antenna and feed line present a load to the output of your transceiver. Most modern equipment has been designed to work with a load impedance of 50 ohms. Get close enough to that, and the transmitter is a happy camper. But if the load impedance is something other than 50 ohms, you have a mismatch. A mismatch causes a certain amount of the power that you’re trying to get out to be reflected back down the line — where it encounters forward power from the transmitter resulting in standing waves on the feed line.
You’ve doubtless heard of this in discussions about SWR or standing wave ratio.
A high SWR can cause considerable RF voltages in the feed line — and at the output of your shiny new radio. This isn’t healthy for modern transceivers (tubes are a little more forgiving) so manufacturers have taken to protecting equipment from this condition. When it’s sensed, your rig may fold back its output power, or shutdown altogether in order to prevent damage.
And this is why hams use antenna tuners. Most of us would like to be able to operate over many frequencies with a single antenna. Since the impedance of that single antenna will change depending on the frequency of the transmission, a tuner becomes a useful tool.
Basically, a tuner is a combination of inductors and capacitors that act to balance the load reactance at the transceiver end of the feed line. With the reactance effectively canceled, the load impedance at the transceiver is 50 ohms and all is right with the world.
Well, maybe not everything. Your antenna might still be a highly inefficient radiator — but with a 50 ohm load, the transceiver will happily pump all of its juice up into that inefficient aerial system as though it were perfect.
What’s really taking place here is that the load mismatch has been moved down the feedline from the output of the transceiver to the output of the antenna tuner. The reflected energy and standing waves still exist, though a well-designed tuner should be able to handle it better than your transceiver. The tuner protects the transceiver and permits it to generate full-power output.
Higher power and wider ranging tuners are physically larger than can be made to fit inside most tiny, whiz-bang transceivers. Physics is a tough master that demands attention at some point.
The moral of the story is that wire antennas, ladder line and open feed lines are great systems for launching RF into the aether, so long as you have a proper antenna tuner in the circuit. But internal tuners offered with most modern transceivers simply won’t cut it with these kinds of antenna systems.
There are notable exceptions. The low-power internal tuners offered by Elecraft and the slightly larger internal tuners offered by TenTec are downright amazing. There may be others that I’m not aware of so do your homework but as always, the buyer should beware…
Filed under: Ham Radio Tagged: antennas