The Elecraft T1 Miniature ATU

The Elecraft T1 is a compact automatic antenna tuner covering 160 to 6m, suitable for use with radios of 0.5 to 20W output. About the size of a pack of playing cards, weighing next to nothing (well, less than 150 grams) and powered by an internal PP3 battery, it's the perfect companion for anyone who likes to take small, low-powered radios out into the country or up into the hills.

The T1 is the first Elecraft product that is available both ready-built and as a kit. There's only a small difference in price ($24 US) between the two versions. However, for me, building is at least half the fun. As with my K2, I ordered the kit direct from Elecraft's website.

Buying

Some folk are leery of buying direct from overseas, but it's no different from buying from a UK dealer's website: the product just takes a bit longer to arrive. The kit shipped on the day Elecraft promised, and a fortnight later a man from ParcelForce was knocking on the door asking for money. There is no customs duty on radio equipment, but you still have to pay the postman VAT plus an £8 collection fee on top of the advertised price (to which must be added a $28.60 shipping charge.) Even so, with the current favourable pound/dollar exchange rate, it's a very good buy.

One of the things about Elecraft products is the enthusiastic customer base and the company's responsiveness to criticisms and suggestions for improvements. A couple of early builders had apparently found that the components in the tuner's SWR bridge warmed up a bit too much under certain conditions, so Elecraft produced a modification, substituting some higher wattage resistors and adding an extra toroid core to beef up one of the transformers. An envelope containing the revised instructions and new components plopped through the door the same morning as the kit arrived, saving me the task of having to modify the kit once built. Impatient builders on the other side of the Atlantic weren't so lucky!

Building

Building and assembling the kit should present no problems at all to anyone who has mastered the art of soldering. The two boards - the main board and a smaller control board containing the switches and LEDs - are quite tightly packed, and some of the solder pads are closely spaced, so you need a steady hand and a clean bit to avoid creating solder bridges. To keep the size down, the main board uses a number of surface mount (SMD) capacitors, but these are preinstalled, so there is no need for anyone unfamiliar with this type of component to worry about them.

The boards are clearly marked with component locations, and the instructions are exemplary. It's hard to see where you could go wrong. Elecraft has anticipated every possible pitfall, such as warning you to check you haven't installed a component in the wrong hole, where two holes are so close that a mistake is possible. My only complaint is that some obvious construction steps are explained at such length that I ended up re-reading the text several times in the belief that I must have missed something.

There are two toroidal transformers and seven inductors to wind. They aren't difficult to make, and I find this one of the more enjoyable aspects of kit-building. The instructions include excellent photos of each of the coils, so you if you're worried that you may not have done it right you can compare yours with one made by an expert. Still, some people don't like toroid winding. It's possible, for a small charge, to buy a set of toroids pre-wound, but as this makes the kit more expensive than the ready-built version it probably won't be a popular option.

Squeezing the beefed-up transformer T2 into the slot designed for the original, slimmer component was a bit of a challenge. The only other building issue worth mentioning is the small control board. This is installed as a daughter board directly above the T1's microprocessor control unit (MCU), and there's little more than a millimetre of clearance between the bottom of the board and the top of the processor.

Elecraft recommends flush cutters to be used during construction. Like most constructors, I only have an ordinary pair of miniature side cutters. I worked around the problem by trimming the leads of the control board components before soldering. Not a recommended construction technique, perhaps, but it worked, although I needed to make fine adjustments to a couple of component leads before I could move a slip of paper freely between the MCU and the board to confirm that there was no contact between them.

Circuit Description

The T1 has seven fixed inductors and seven capacitors, which are switched in and out of circuit using relays to tune the matching L network. The capacitance can be switched to the transmitter input or the antenna output, giving a total of 32,768 different combinations, with a resolution of 0.05uH and 10pF. (The T1's designer, Wayne Burdick, N6KR, tells me that a constructor whose interests lean towards LF could double the value of each inductor to improve the matching range on 160m, at the expense of 6m.) Latching relays are used, so the tuner draws no power from the battery, except when tuning.

A Stockton bridge circuit is used to detect forward and reflected power, from which the SWR is calculated. A modulated SSB transmission can be used for tuning, with almost as good accuracy as a constant carrier. The microprocessor tries a coarse tuning algorithm to roughly determine the antenna impedance. This is followed by fine and very fine algorithms to seek the best possible match (unlike many auto-ATUs which stop searching once they have achieved an SWR below a certain level.) The settings and band are stored, allowing the T1 to return to this setting instantly the next time that band is used.

Unlike many other auto-ATUs the T1 is switched off once it has found a match so it does not constantly monitor the SWR in order to automatically re-tune if the SWR changes. The user must manually initiate a re-tune if required. The T1 can also be turned on using the external control interface, which can provide information about the band selected on the transceiver, allowing the T1 to automatically tune the antenna using the previously stored settings. Currently, this is only possible with the Yaesu FT-817 transceiver and optional adapter, but Elecraft has provided information about the serial data protocol used, to allow interfaces for other low-power transceivers to be designed.

Moment of truth

Several people have reported taking six hours to build the kit. I didn't time myself, but in two sessions over a weekend I probably took a bit longer. However, eventually it was time to install the battery and try the T1 out. It's always an anxious moment, applying power to a new project for the first time, but thanks to the excellent instructions (and my carefully following them!) everything worked as expected. If it hadn't, help would have been just an email away, on Elecraft's Internet reflector.

The T1 is controlled through two push-buttons on the top panel, and gives you information through three LEDs. There are several options, depending on whether you tap or hold the buttons, but don't worry if you have a memory like a sieve like me: there's a crib sheet printed right above them.

To initiate a tune, the power button is held until the green LED comes on; you must then transmit within three seconds. Elecraft recommends that a power of 2 to 5 watts is used for tuning, but the T1 will tune using as little as half a watt. One user reported that it survived an accidental tune-up using more than 30 watts! This probably didn't do the relay contacts much good, though, so it's not advisable to make a habit of it.

Elecraft cautions FT-817 users against tuning using the full 5 watts, as the early FT-817s (not the FT-817ND model) seem prone to blown finals, which might be caused by the extreme mismatch conditions encountered using certain tuning combinations. This is not the fault of the T1, of course, but a problem that could be encountered using any automatic tuner.

Tuning

The initial tune-up on a band can take several seconds. After that, the band settings can be recalled from memory. Re-tunes, such as when you change frequency within a band using a sharply resonant antenna, take a fraction of a second. The SWR achieved is displayed using the front panel LEDs. Using the T1's Info mode (another button tap combination) you can find out even more information, such as the internal battery voltage, inductance and capacitance selected, network configuration and band in use. This data is sent in slow Morse using one of the LEDs.

This tiny tuner can match a surprisingly wide range of loads for such a small unit. It matched my small non-resonant attic loop, which has an SWR of more than 10:1 on some frequencies, on all bands from 3.5MHz to 50MHz. My 5W signal received good reports, suggesting that the power is being radiated by the antenna, not dissipated in the tuner.

I also tried a 5m length of wire draped over a curtain rail, using the central heating system for a ground. This was matched with a 1:1 SWR on 30m and up, and worked better than one of the short tunable whips like the ATX Walkabout or Miracle Whip.

The insertion loss of the tuner, measured into a 50-ohm dummy load, ranged from 4% at 3.5MHz to 7% at 28MHz, and 8% at 50MHz. This is a good figure - many ATUs have an insertion loss of 10% or more - but as with any ATU, the loss is likely to increase when the tuner encounters a severe mismatch. Even 7% may sound like a lot, but it won't be noticeable at the receiving end, and it's still less than the loss you would get trying to use a mismatched antenna.

FT-817 adapter

As I own an FT-817, I also ordered the optional T1-FT817 Remote Control Adapter. This an 18in long screened cable fitted with a mini-DIN plug for the 817's ACC socket, and a jack plug that plugs into the side of the T1. Inside the mini-DIN plug, is a tiny circuit board made with SMD components, including a tiny microprocessor control unit. It's powered by the FT-817 itself, drawing a tiny current. The MCU monitors the band data signal from the FT-817. When the band changes, it turns on the T1 and sends it band information, allowing the antenna to be automatically tuned to the new band.

The adapter is a great boon, allowing you to switch from band to band without tempting you to send a burst of spectrum-polluting RF to re-tune the antenna. But at a third of the price of the ready-built ATU it's a luxury some FT-817 owners may decide to forego. It might be best left at home when you take the 817 out in the field, too, as it would be easy to lose it.

It's also worth bearing in mind that, because it uses the FT-817's CAT port, you can't use this adapter and connect the radio to a computer at the same time. Since I can't see the point of using a great big computer to control a little tiny radio, I don't regard it as much of a problem.

Conclusion

The T1 is a desirable accessory for all low-power radio enthusiasts, but especially owners of the Yaesu FT-817. The T1 is small enough and light enough to take everywhere the FT-817 goes without destroying the radio's unique advantage: it's small size, and it's capable enough to turn almost any piece of wire into an effective antenna. Yaesu should have built something like this into the FT-817 in the first place.