Poor-mans portable antenna analyzer
To build and tune short-wave wire antennas, I needed a cheap and easy to use antenna analyzer. Of cause, I could use my scalar network analyzer for that task, but it requires a PC to operate it. A stand-alone solution would be much easier to use. Commercial products, even those addressing HAM radio operators and hobbyists, are way to expensive for such a simple task: All I need is a RF generator with an SWR bridge (three resistors). I am generally not interested in the complex impedance of my antenna, I usually only want to know how close it is to real 50Ohms or at which frequencies the antenna has real 50Ohms. Hence a simple resistor SWR bridge is sufficient.
If you are looking for some more bells and whistles (SNA, Power meter, etc.), check out the Antuino by Ashhar Farhan, VU2ESE. If a simple SWR bridge + RF generator is, what you are looking for, consider my poor-mans portable antenna analyzer.
The circuit consists of a RC-Oszillator (VCO, 74HC4046), a buffer amplifier ( 74HC00), a resistive SWR Bridge (R7, R8 & R9) as well as a logarithmic LED bar-graph driver ( LM3915). The Opamps (TL072) are just used as buffer to provide the reference voltages for the bar-graph and SWR bridge. They could easily be replaced by two emitter-follower transistors.
Please note that the generator produces a square-wave signal which is rich in harmonically related frequencies. Although the low-pass (C19, C20 & L2) will reduce the harmonics above 30MHz, some harmonics will still pass through, particularly at low fundamental frequencies. This has an effect on the SWR reading: Although an antenna may have exactly 50Ohms at the fundamental frequency, it will likely not have 50Ohms at higher harmonic related frequencies. The SWR bridge, however, will also measure the (absolute value of the) impedance of the antenna for all harmonics passing through the low-pass filter (although with some way smaller weight). To this end, the SWR bridge will likely never show a perfect 1:1 match at lower frequencies, just tune for minimum SWR at lower frequencies.
The VCO is tuned by a ten-turn 10kOhms potentiometer. To cover the complete short-wave range, the resistor of the RC oscillator can be switched from 22k to about 5.2k splitting the frequency range into a low-band (1-16Mhz) and a high-band (10-30Mhz).
Being a RC oscillator, the frequency will drift a lot. Hence, a buffered output of the RC-VCO is available directly at a separate output (X2), this allows to measure the exact frequency while measuring the SWR. For tuning simple wire antennas, however, the frequency stability and accuracy of the VCO is sufficient.
The circuit runs with a 9-12V supply and can therefore be powered with a 9V block for portable operation.
Best & 73,