Today I had some free time and I decided to build something.But what would it be that could fit in my 2 hours break?An idea came at once!I always wanted to have a tuning accessory like the famous S.E.M. Ezitune, why not build one?
The Ezitune device is nothing more than a noise bridge which can be permanently connected between the transceiver and antenna tuner. There is a switch to bypass it during normal transceiver operation and switch it in for antenna tuner adjustment.I have seen them on ebay from time to time (they are really rare) at prices around 50 GPB and one German distributor (WIMO) sells a newer version of it by the same name.However, I find them too expensive for what they do.
The main concept of a noise bridge is to measure an unknown impedance using a receiver as a null indicator. The unknown load is connected to the one port of the bridge and the receiver on the other. The bridge has two controls , one for resistance and one for reactance (Xc or Xl) and generates a wideband noise at a level of around S9. In order to find the unknown impedance at a specific frequency we tune the receiver to that frequency and using the bridge’s controls we try to achieve a null at the receiver (that is, the lowest indication on the S-meter). When the null is achieved, the unknown impedance value is equal to the readings of the bridge controls (resistance + reactance).This is based on the bridge’s property that “a null is achieved when the known impedance equals the unknown”.
The idea behind Ezitune (as I understand it at least… I never managed to find its schematic) is to use this feature to make its magic. Instead of providing a measurement range using a variable resistance/reactance it sets the “known” impedance to pure 50 Ohm (no reactance) which means that when the null is achieved the “unknown” impedance will be pure 50 Ohm (or very very close to that).Isn’t that exactly what we are trying to achieve when we adjust our antenna tuner? Trying to make our whatever-we-have-as-antenna to look like a 50 Ohm load to the transceiver.Ezitune makes it extremely easy.Let’s build one!
All we need is a simple noise bridge, with components which are already lying around in some bin.I have one in mind from previous google searches: the New England QRP club noise bridge (original article here and the construction details can be found here ) . (Note: The bridge is not my design and I do not pretend it to be, it is a Dave Benson’s design.I am posting the schematic here just in case the original article is removed)
In order to convert the above bridge to an Ezitune device, all we have to do is to remove R1 and C1 and replace them with a 51 Ohm resistor.During my initial tests I also omitted C6 and the bridge worked fine. The reason I omitted this capacitor is because I think that it will introduce unnecessary reactance to the antenna port. Of course, it may help to prevent any DC voltage that (may) appear at the antenna port to reach our receiver.
The finished device is shown below
On the top right, just on the left of the toroid core, you can see the “known” impedance of 52 Ohm (a 30 Ohm + 22 Ohm resistor in series, as I ran out of 51 Ohm ones). The transformer is made of 4 turns of the flat ribbon cable on an FT50-43 core.The right coaxial cable, leaving out of the green ribbon is connected to the receiver, while the coaxial leaving out of the yellow ribbon is connected to the antenna tuner.
The next step is to see whether it is working as intended and whether is it really easy to adjust the tuner with this. My antenna is a 15m long doublet fed with 450 Ohm line and I can get a pretty good match from 40m – 10m using the MFJ-901B versa tuner. I cannot get 1:1 SWR but it is always less than 1.7:1 . On some bands (30m and 12m) the best I can get is somewhat 2.5:1 SWR but at the moment I am not really active on those bands.
For my first attempt I chose 20m where the tuning is very sharp and every time I change bands it needs very delicate touches to adjust it.I played a bit with the controls into positions I know for sure do not work for 20m (I know by heart the rough settings for 20m). The noise was a constant S9 on the receiver. As the controls were approaching their “good position” I noticed a slight dip which suddenly became deeper for a short time and then back to S9. I went back a bit and played with the other control. The null was getting deeper. I adjusted the controls alternatively and bang!The S-meter was now at S2!Wow!The adjustment of the tuner was really fast! The original noise of S9 was reduced to S2 when the tuner controls were where they should be. The interesting part is that using the bridge is easier to see and hear the dips and adjust the tuner for better match.
My method of tuning the antenna is to use my Autek RF-1 antenna analyst to find the tuning settings per band and note them on my notebook. When I change bands I preset them, connect the RF-1 and adjust the tuner knobs for the lowest SWR. On 20m the best I can get is 1.4 – 1.5. This does not have to do with the tuner/antenna but the RF-1 takes 1 or 2 measurements per seconds and it is easy to miss the optimal point.Otherwise you have to go really slow (this means time!). I am happy when I get 1.5- 1.6 and I usually stop there.
After I adjusted the tuner with the bridge I unplugged it and connected the RF-1 to see the SWR. It was 1.3 !Much better and much faster.
I tried again on 40m and 17m. There I noticed something interesting. Even though I could not achieve a perfect null (the S-meter indicated S5 during null) the result was good as it was 1.5:1. So, that means that the lowest the S-meter indication the better the match (the lowest the SWR) but even if it is not S1, the match is probably good. I will have to test if the same SWR value results on the same (or almost the same) S-meter indication across bands.
What is left is to put it in a box and add a bypass switch so that I can have it permanently connected.
Not bad for a 2 hours project, right?