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Posted - 6th May 2017
A SWR meter is used to measure Standing Wave Ratio, the power reflected back to transmitter by unmatched antenna impedance. There are many variants of these devices regarding SWR detection method, like directional coupling, wheatstone bridge, etc. Of course, each method has its own advantages and disadvantages.
I decided to build one based on wheatstone bridge. This will not measure the actual VSWR, but it will indicate when antenna is matched to 50 ohms for given frequency. This type have some pros and cons:
It can't be left into antenna circuit during regular transmissions because it will attenuate received/transmitted signal about 4 times or -6dB, when antenna is at resonance.
Click to enlarge.
The input is the left BNC connector and the output is the right BNC connector. As it can be seen, the wheatstone bridge consists from R1, R2, R4 and antenna, a diode detector formed by D1 and C1, a low pass filter formed by R3, L1, C2, and the indicator formed by RV1 and LED D2.
When the antenna does not represent a 50 ohms resistive value for input frequency, then the bridge is unbalanced and a voltage apppears between its legs, across the detector. Further, the current flowing through detector will be smoothed out by the low pass filter and applied to the LED indicator. The LED sensitivity is set by potentiometer RV1.
So when LED is turned off, means the followings:
I used SO-239 connectors, a small alu box and an LED as indicator in order to keep things small and simple. Also, for portable operations, BNC connectors can replace that big SO-239 ones, a SPDT switch can be useful in order to speed things up when you want to disconnect the SWR bridge from the circuit, because it is very annoying to screw/unscrew connector caps every time you want to adjust the antenna.
All 50 ohms resistors must be metallic film 1% precision resistors with power rating bigger than 1W. The LED is super bright green LED and I used no PCB.
It's very easy to use:
This video demonstrate the power transfer between a transceiver and an antenna using some tools like SWR Wheatstone Bridge and EMF Strength Meter. Basically, when SWR is at minimum the maximum power is transferred to dummy load or antenna.
After some time using the LED version of this SWR bridge I wanted a more refined indication of the null point. Therefore, the LED was replaced with an analog indicator and other internal changes were done too.
Another big problem of this design is that you need to insert the bridge in and off from antenna system. This can be easily accomplished by a DPDT switch, but the problem resides in fact that you can forget to switch the bridge off from the antenna circuit. If you forgot to switch off the bridge and work QRP your correspondent might not hear you, but if you work in QRO mode and forgot to switch this off, you will burn the resistors in a few seconds. It happened to me many times to forgot to switch off the bridge, and two times were fatal.
Those 1W resistors were replaced by 2x5W resistors, so the power handling was increased from 4W to 40W maximum, but practically ... see thermal images.
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