Ken Hough's Website
|This section includes information about antenna tuning units, more correctly referred to as antenna matching units.|
An ATU is most commonly used to transform the output impedance of a transmitter to match impedance presented at the input to a transmission line, or by an end fed antenna. An ATU does NOT tune or otherwise affect the resonant frequency of an antenna and is more correctly referred to as an AMU or Antenna Matching Unit. However, I shall use the commonly accepted term of ATU.|
End fed (long wire) and other antennas that present impedances which vary considerably with frequency should always be used with some form of ATU.
Resonant antennas such as dipoles, Yagis, and 1/4 wave ground planes types can be designed to present impedances close to 50 Ohms and with the use of matching feeder cables, can often be connected directly to a transmitter with out the need for an ATU.
Dipoles, etc will typically exhibit quite sharp resonances that do not fully cover the frequency band for which they have been designed and resonant impedances are unlikely to be exactly 50 Ohms. ATUs can be used to "tweek" these antenna systems to provide accurate matching to a transmitter.
Much has been written about the various possible configurations for ATUs. For further information refer to the ARRL Handbook or the RSGB Communications Handbook. Most modern ATUs are of the "T" type and can provide very wide impedance matching ranges.
Some tranceivers include automatically tunable ATUs.
I have a stand-alone manually tunable ATU that can operate over the entire range of HF bands from 160m to over 10m. It includes a "ground matching" circuit that can help in optimising RF ground connections or the use of ground counterpoise elements. A balun is provided which allows the use of balanced feeder on the antenna side. As with most modern ATUs, there is also an SWR/RF power meter.
All of my HF antennas are resonant types that are fed via coax cables and where appropriate, I always include some form of choke balun so as to prevent RF from travelling back down the outside of these cables. Consequently, I do not presently need to rely heavily on RF earthing from within the shack. I find the ATU with it's built-in SWR meter useful to check on antenna operation and to "tweek" matching on the HF bands.
My VHF and UHF antennas all have fairly broad operating bands and match very well into my VHF/UHF transceiver without the need for an ATU. For normal operation I rely on the SWR meter that is built into the transceiver.
"T" type ATUs versus "Pi" type ATUs
"T" and "Pi" circuits are shown on the left. The "T" circuit behaves as a high pass filter, whereas the "Pi" circuit has low pass characteristics.
Modern PA stages usually operate in class "B" (or possibly class "AB") linear mode and generate relatively low levels of harmonic energy. Additional low pass filtration is not usually needed, so that "T" type ATUs are quite satisfactory.
The PA stages of most older valve based transmitters were run in class "C", thus generating significant amounts of harmonic energy. Low pass filtration was needed to reduce harmonic content to sensibly low levels. This was typically achieved via a "Pi" type ATU.
A "T" type ATU will provide a greater matching range than a comparable "Pi" type ATU.