From LF and up through the HF bands an ATU is required to match the nominal 50 Ohms output from a transmitter to the impedance presented by the antenna. If the antenna already is a reasonably close match then the ATU doesn’t have to do much work. However, if the antenna has to be an all-band antenna (really a misnomer as it may or may not be resonant on any of the amateur bands), such as a Long Wire, the ATU has to be capable of converting the transmitter 50 ohms output to a wide variety of impedances on the different bands. This is where things can get more difficult.
However, one of the attractive things about ATU’s from a constructor’s viewpoint, in particular, is that they are relatively straightforward, typically comprising just three components – two variable capacitors and one inductor - and there are plenty of designs out there with construction articles to guide you. Providing you follow the basics about the types, quality and RF power-handling capabilities of components you shouldn’t have too much trouble putting one together.
For the intrepid homebrewer there are three or four basic ATU designs to choose from, the pi-network, t-network, and SPC around which you can build various switching facilities, such as bypass, internal or external dummy load, and metering (VSWR and power meter) for example. There is also the classic Z-match, which is a little different, and several other designs using balanced circuits to match the transceiver to a balanced antenna.
Over the years there have been some classic commercial designs built on these basic circuits, including the Johnson Matchbox, the Collins 180-S, a pi coupler with variations, the Drake MN2700, and not forgetting the popular made-in-Britain KW Z-match. In recent years manufacturers such as MFJ and Vectronics (now part of MFJ) have produced a multiplicity of designs but all are variants of the above basic circuits.
What has now changed however, is the growing number of auto-tune ATU’s appearing on the commercial market, with LDG, MFJ and others producing some nice units. However we again have to remember that these are still variants of the same basic circuits: the electronic relay selection of the tapped L and C components directed by a PIC or microprocessor taking the place of your hand carrying out the tuning manually. A big advantage of the auto-ATU of course is that it can be positioned remotely from the shack and does not therefore have to take up space on the operating desk.
Fig 1. The W3EDP antenna tuner by Ted G4TTY
The selection brought along by members to the ATU Evening was broadly representative of the above. We had a couple of commercial units by Vectronic and MFJ, together with a venerable but clean KW Easy Match, and some very nicely constructed units built by Peter G4LEG, Ted G4TTY and dare I say it, myself G3VLH.
Fig 2. Peter G4LEG’s very neat QRP Z-Match
Fig 3. G3GRO demonstrating his Variometer-tuned VLF ATU
The most different of all was that built by Derek G3GRO, which he put together to support his VLF operation. This design uses a variometer (a rotating coil within a fixed outer coil, the axis of the inner rotating relative to the fixed axis of the outer), and thereby varying the total inductance.
All in all this was another enjoyable and well supported Members Evening, and it was an added pleasure to welcome Mike G0KAD, home on leave from distant parts, and son Matthew.
Fig 4. Mike G0KAD and Matt eyeing John G3VLH’s ATU
FOOTNOTE
For those who have an interest in digging deeper into the design characteristics of ATU’s you are referred an excellent 2-part article “How to Evaluate your Antenna Tuner” by Frank Witt AI1H in the April and May 1995 issues of QST, which describes, analyses and compares a number of the classic designs. For anyone interested I will be happy to provide photocopies.
John Longhurst G3VLH
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