How to build a SWR meter

01/10/2009

In case of an imperfect match between the transmitter’s output impedance and the antenna’s impedance (both of them wich should be 50 ohm), some of the power transmitted will not be radiated and will return in the transmitter’s final amplifier stage to overload it, first by overheating and in serious cases even by damaging it.

To check the correct adaptation of the antenna, a device called SWR meter needs to be used; the achronim comes from Standing Wave Ratio, and it will determine the ratio between the transmitted power and the radiated power. In ideal cases this ratio should be 1:1, but in usual cases values of up to 1.5:1 are OK.

A SWR meter is very simple to build, you just need a type of transformer that will pick up the waves circulating in both ways, rectify the picked up voltage and measure it with a voltmeter. The schematic is below:

The transformer is made of 3 copper lines on a PCB, each of them 65mm long. The center line, that carries the power to the antenna, is 8mm wide, and the “secondaries” that pick up the signal for measurement are 4mm wide. The 68 ohm resistors are used to adapt the secondary lines to the 50 ohm impedance of the line and for large power measurements should be able to hold at least 0.5W of thermal dissipation. D1 and D2 are high-frequency rectifing diodes, for large power 1N4148 is OK but for precision and lower powers germanium-junction diodes like EFD 109 are a must.

Here is how this looks, mounted in the case of my old President TOS-01 SWR meter, that had burned-out PCB:

P1110557

I combined more resistors to get a value as close to 68ohm as possible, this is important in getting the correct reflected power; replacing these with 100 ohm will make the SWR meter compatible with 75 ohm feed lines.

“DIR” will be a voltage proportional to the output power’s voltage, and “REF” will be a voltage proportional to the returning power’s voltage ( the voltage is proportional with the square of the power = you double the voltage, the power grows 4 times). Generally you want “REF” to be as close to zero as possible in the frequency band you are usually working, or at least in the middle of it. Mine is almost zero from 27.1Mhz up to 27.7Mhz, but this depends alot on the type of the antenna you are using.

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The simple way to a high power 12V PSU

28/09/2009

Most CB transcievers are actually built for use in a car, therefore the supply voltage of +13.8V available in an automobile has become a standard for CB radio units also. If you do not have a better solution at hand, when you use your CB radio from your home a quick fix is to modify a computer ATX power supply. For the standard legal CB radios a cheap ATX power supply is just fine, provides good regulation and plenty of current, all you need to do are the quick hook-ups described below and you’re ready to go.

First of all, take a good look at the large ATX connector and try to identify it with the picture:

atx_pinoutThis is actually the pinout for the older 20-pin models, if you power supply has 24 pins do not worry, everything is the same, there are just 4 extra pins that continue to the right of this picture for extra current support required by newer PC’s.

The first step is to connect the green wire (pin #4) to one of the black wires (#3 for example), so the PSU powers on by itself whenever you connect it to the mains (~220V or ~110V for you other folks out there). Very simple:

atx_1

But because it usually provides many voltage outputs, the ATX power supply will become unstable if you just draw current from the +12V line. Therefore, some kind of consumer will be needed on the +5V line too: a car lightbulb (12V / 21W) or a power resistor will do just fine; i used a 2.2ohm / 20W resistor with great succes but beware, it will get HOT. Just hook it up like this:

atx_2Now you’re pretty much ready to go. Just connect the yellow wire from the power supply to the +13.8V red wire of the CB radio and the black wire to the black wire, power up and enjoy. The output power will be just slightly lower that with a dedicated true +13.8V power supply, but it’s entirely unnoticeable.

This solution is actually so good and simple that i’m using it right now to power the 150W SSB radio. A 50Euro PSU, these mods, some extra RF filtering and some other easy adjustments to the PSU internals and it’s good for 30Amps at +12V.