2m-atu

An Antenna tuner for 2 Meters
An Antenna tuner for 6,4,3 Meters

by VK3ZZC August 2016

There is not much point having an antenna tuner for 2M. If you have significant VSWR on your line you will have significant line loss anyway, so merely disguising this with an ATU will ,at least, make your transmitter a little happier.

There are very few options for ATUs for VHF. Roller inductors have way too much inductance up here and also too many spurious parasitic resonances to be usefull. You cannot really switch a fixed tapped inductor for the same reason. A real ATU still requires some sort of variable inductance. (susceptance for all you Smith charters !)

This design is a basic high pass T match tuner, nothing new to see here. However what is new is my variable inductor. It is nothing more than a variable short circuit, but at VHF has a usefully small amount of reactance to make a VHF tuner a practical reality.

This "inductor" was purchased at a recent hamfest. It must have sat at the bottom of the prior SK owner's junk box since world war two. This is probably the actual truth. It originally would have been the output tank of a WW2 vintage radar oscillator or output amplifier ground station for VHF air band. Its solid construction and heavy silver plating says world war2 surplus. I dithered for a while over this relic at the hamfest, it came with a handfull of assorted junk and I really do not need more boat anchors. I felt that if I did not bite, I would regret it, as this item is made from unobtainium. The variable capacitors came from my junkbox, which I have been hoarding for over 40 years ! It was time they left home a got a job . They also came from a piece of WW2 surplus that I had found on a hard rubbish collection all those years ago. The choice of Olive Drab vinyl finish on the case material was just coincidence, I just happened to have scavenged a large sheet of this material.

The case was hand made by myself so the sole reason that the total path delay through the tuner had to be minimized. The RF ports must have the minimum practical path to the capacitors so this required this custom box to be beaten into shape. There is just barely enough clearance for the short circuit slider to clear the capacitors and base. The box is lined with brass flashing so as to provide a good low resistance path for the coaxial connectors. The capacitor shafts must be insulating, I have used 6MM wooden dowel that has been shaved to 1/4inch for the legacy hardware. The shaft couplers were a necessity due to the lack of precision in my box construction.

The reason for the holes in the top of the case is to permit visual inspection of where the inductor is up to. I could engineer some sort of turns counter, but visual inspection is best. I still will add some sort of ruler at the bottom to calibrate the slider. The handle on the knob of the variable inductor really is needed. Turning a knob twenty times is hard work without the handle.

I have found that the most satisfactory method of tuning this device is to use a return loss bridge, a wide band noise source and the 2M transceiver as a sensitive detector for the bridge. Successfull tuning for my 2M J-pole occurs at about 1/4 minimum inductance and then to fiddle with the capacitors for maximum return loss . Perhaps in future years, to facilitate field day operation I should build in a little noise source for tuning purposes with external return loss bridge. I now have more faith in the operation of the RLB then any SWR meter I have yet encountered. The true null setting is hard because all three controls are interactive with the T Match. Start with the inductance for the first null and fiddle with the capacitors for a perfect null, seems to work for me.

I believe that this could well be the first practical antenna tuner for the 2 meter band . I have seen nothing significant in numerous ARRL , RSGB, et al handbooks. The limiting factor was always the lack of a practical variable inductance at this frequency. This unit does work and I would urge its reproduction to those lucky enough to be able to source or make the sliding short circuit. It still remains to be said that at VHF your antenna should be designed and matched at the antenna end of the feedline. An ATU should not be required. Its only purpose is to make your commercial transceiver a little happier. It may have automatic high SWR power reduction protection. An ATU like this can make an appallingly bad antenna into a merely bad one !

The 2 Meter ATU, christened The "hurdy-gerdy" by my fellow grumpy old codgers

insulating shafts are using 1/4 inch wooden dowling stock

low impedance conductors and positive RF grounding are required

A proper crank handle is required for the variable inductor

Here it is, construction is complete.

The variable inductor in my tuner came from a set of 3 tuning stubs from the hamfest. Very glad I had the presence of mind to snap these up. These longer stubs will form the basis of a 6-meter/4-meter ATU when I get my next set of high voltage tuning capacitors. Got them ! I will begin this project of a 6-meter T match after completion of my valve HF linear. See Below !

An Antenna tuner for 6,4 and 3 meters
As promised, using the longer tuning stubs, another unit was constructed with larger variable capacitors. As above, this is a common inductor T match, a configuration that can tune and match an extremely large set of impedances without physical reconfiguration than could be obtained with a simple L match. The only thing worthy of note here is the use of brass shim stock for forming an internal grounding plane that is certifiably good for RF. You cannot reliably ground RF to aluminium due to the inability of making a gas tight metalurgical pressure bond to aluminium. All aluminium connections are metal-oxide-metal sandwiches with variying degrees of effectiveness. Do not muck about, just use brass or copper. The box IS aluminium, a junked project box that was repurposed. The unwanted second hand holes on the front panel are hidden behind a new plate which has only MY holes. The big transmitter capacitors came from an under the table hamfest special. They were heavily encrusted with 60 years of baked on decomposed grease and heavy congealed dust and spider web between the plates. They were made from silver plated mild steel. This permitted the capacitors to be boiled in caustic soda solution in a stainless steel pan which completely dissolved the gunge. Do not try this with alumium plate capacitors, that will end in tears ! The capacitor shafts are live, they connect with the front panel controls via a couplet and 1/4 inch wooden dowelling rod for effective and cheap, insulating shafting. How effective is this unit ? Sadly, it just misses out at 6 meters which is what I had really intended. It matches my groundplane at 54Mhz but not at 50Mhz, due to the lack of length in the tuneable inductor. Oh well, it really was an experiment. It does work at 70Mhz and there is a strong possibility that Oz hams will get a 4 meter band in my lifetime. (Although I wonder why . I think that a 25M ham band in the old and now mostly unused SW broadcasting segment would be the bee's knees) I have also fitted 2KV gas arrestor tubes at both outlets and 5M6 resistors for draining static. You should consider doing same for all your external antenna equipment .


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Mon Sep 26 18:10:06 EST 2016 Thu
Feb 9 18:28:54 EST 2017 added 6M atu