FRI-Match ATU, a single-coil Z-Match type:

(Freematch line matching unit etc. published in RSGB's RadCom 1989 july and a recent version of Rothammel's Antenna Book.



This ATU, I call my design FRI-Match, has been devised in 1972 as an unbalanced tuner for improving the SWR at the transmitter end of coaxial feeders to resonant antennas (eg verticals, dipoles, trapped dipoles, G5RVs, Yagis, loopquads etc.).

PAT Hawker wrote in Radio Communication: "surprisingly, it seems to have attracted relatively little attention when it featured in TT July 1989. In view of the current interest in this approach it seems worth repeating this pioneering 1989 item". 


It is a modified version of the well-known Z-match and is designed as the result of experiments in reducing the SWR on the five HF bands between 3.5 and 28 MHz (incl. the WARC bands) without the necessity for switching coils and with a minimum of knobs.

Tom Seed, ZL3QQ, has published a basically similar approach with a detailed explanation of the theory of operation in BREAK-IN March 1992. Bill Orr, W6SAI, has featured this ATU in the August and September 1993 issues of CQ.

After experimentation with various antennas and complex loads, one 5 - 8 µH coil tuns out to be the best tuning system for 10 to 80 m. The efficiency of the tuner is good, because it can be considered as an auto transformer with inductive and capacitive taps on the coil. Originally the design was a three knobs tuner (the left model). The results of further experiments were the middle and right model (with two knobs). The twin varicap has almost the same effect as the switched capacitor at the input of the left model. So a switch could be saved.

One of my first constructions was with two tiny plastic varicaps as used in portable AM radios.



Although intended for QRP, they were often suitable for even 100 W if the loaded impedance was relative low, e.g. very close to 50 Ohms. My QRP ATU no longer exist, but accidentally PA4M build a device (fig ») almost similar to my former unit.




Wire diameter of at least 1 to 2 mm.


4.5 cm

5.5 cm


5 cm

4 cm





3, 6, 9

3, 6, 9




The coil is made with: 15 turns of 2.5 mm diameter enamelled copper wire or 6 mm² tinned copper wire, 4.5 cm long with 5 cm ID or 5.5 cm long with 4 cm ID. Taps on 3, 6, and 9 turns from earthy end.  


If random sized 5 – 8 µH coils with n turns (see PE1ADY's ATU) are used the taps for equal efficiency on all bands should be at: 


Random size but: ± 5 – 8 µH



Tap 1

n ÷ 5 turns from earthy end

Tap 2

2n ÷ 5 turns from earthy end

Tap 3

3n ÷ 5 turns from earthy end



It is essentially a 'kiss' approach cheaper and almost faster than an automatic ATU provided that the calibrated settings on each band for minimum SWR are known so that the capacitors can be quickly reset.

In practice the ATU has proved more flexible than expected and in many cases permits matching to non-resonant antennas.

With an extra 3 – 6-turn bifilar winding over the earthy end of the coil (fig») a "balanced" output for 50/75 ? twin wire or 300 ? ribbon feeder is feasible.

However, it should be noted that this design couldn't satisfy all possible conditions such as random length wires and antennas. This can sometimes be overcome by increasing or decreasing the length of the coaxial feeder or ladder line and/or reversing the input/output thermals of the ATU. To meet all possible matching conditions a more complex arrangement should be necessary.




With this FRI-Match I do not recommend the use of a coupling coil but have the preference for a suitable balun at the output of the ATU.


The coiled ribbon performs similar as a choke balun.

If limited space in the back garden occurs, roll up the ribbon.

Roll up if there is too little space.






Taps on

5, 10, 15

A toroid, self-shielding because oh it's low external field, facilitates compact construction. For ± 400 W power, a T200-2 toroid can be used with 25 turns on 75% of the circumference

Taps 5, 10 and 15 turns from earthy end. A 5 to 8 µH coil seems to be the best as the result of experiments.

Before winding, several layers of Teflon plumbing tape must be applied to the core, to insulate it from the coil-windings. Another method of insulation is to cement two flat isolating washers (e.g. made from bare glass fibre board, see figure) on each side of the bare core. Apply a small quantity of super glue, possibly only a few drops, around the sides of the core. Work swiftly; the glue hardens quickly. The glue prevents the washers from moving out of alignment while the core is being prepared for winding. For a T200-2 core, the inner diameter should be 28 mm and the outer diameter 55 mm. With this last construction it might be even possible to use bare copper wire for the windings.





Home-made ATU (w × h × d), 17.5 × 6 × 12 cm with a T200-2 toroid.


PE1ADY's home-made FRI-Match ATU's.


Twin-gang from vintage AM radios

Variable capacitors can be receiver-type twin-gang 10 – 490 pF per section, for power up to 400 W. For QRP to 100W operation a T200-2 toroid and two air-dielectric variable capacitors salvaged from vintage AM radios can be used. If 2 × 350 pF capacitors are used the coil should be increased to about 8 µH. If 3 × 350 or 3 × 490 pF are used in some cases the FRI-match permits limited matching on the 160 m band. It will then still work on 10 m.




HA8LUA:"I looked for a really little portable ATU which is very universal (can be used with all my field day antennas: doublet with ladderline, G5RV, FD3, verticals, etc.). A few years ago I built a Z-match QRP ATU for my FT-7 and it worked fine! I have a FT-897D now, that’s why I needed a 100W model. The Z-match principle was given for me and I read PA0FRI’s very-very nice article on his website. The result":




In Karl Rothammel's famous Antennenbuch a lot of information is published about antenna systems, tuners, coaxial cables, etc. It is a good informative reference book. My original 1978 book was printed in the former GDR on a cheap type of newsprint. An owner of a much newer version discovered that my design was also published in the Antenna book and that is nice to know.


Review and experience of professor R. Jayaraman, VU2JN.

To make a long story short, my Fri-match ATU was completed in March 2011, nearly half a century after I first thought of building an ATU! Fig. 4 shows a photo (fig») of this ATU. It has just 2 controls, and no rotary inductor. It outperforms the conventional Z-match with regard to ease of tuning and tuning range, and is almost as good as the SPC Transmatch. And interestingly, so long as the Fri-match ATU is able to match an antenna within its tuning range, it is able to bring down the SWR to exactly 1.0. This is something that I had not expected from a 2-knob ATU that is free of the burden of a variable inductor! No reduction drives are used in this ATU. Though the tuning of the condensers is very sharp, it is manageable, even for a person aged 75 years! An analog SWR bridge is needed for tune-up. A point to be kept in mind is that, if one of the condensers is very much off-tune, tuning the other condenser would not produce any dip in the reflected power. Therefore, in the absence of calibrated dials, visual monitoring of the condensers is necessary. The body of one of the condensers has a RF potential but, since it is tied to the transmitter output, there is no hand-capacitance effect.

The Fri-match ATU sits to my right near the front edge of the operating table, not far away from the FT-840 transceiver. From the antenna switch, a 70-ft. length of RG-223 coax feeds a 40-metredipole antenna, and a 50-ft. length of RG-213 coax feeds a HY-GAIN 12AVQ 3-band ground-plane antenna. The ATU enables me to use the 40-metre dipole on 20, 40 and 80 metres, and the 12AVQ ground-plane on 10, 15, 20 and 40 metres --all with a SWR of 1.0 as seen by the transceiver. So much so, the ATU is useful even when a resonant antenna is used for the band of operation. On 20 metres and the higher bands, I normally use the 12AVQ ground-plane. The only time I operate with a non-resonant antenna is when I use my 40-metre dipole on 80 metres. Signal reports then indicate that I am roughly 1 S-point weaker than similar stations using a 80-metre dipole. That's not bad, and I am quite happy with the performance of the ATU.

I recommend this ATU to all hams. When an ATU is available, we can fabricate a dipole, ground-plane or any other antenna simply to the dimensions suggested by theory, and dispense with the trimming of the antenna. In many situations, trimming of the antenna to lower the SWR is unscientific, because the problem is not in the antenna, but elsewhere! It is better to rely on the ATU to take care of the fine tuning of a resonant antenna.

-- VU2JN.

See original topic of professor R. Jayaraman, VU2JN: Frimatch-ATU .