Poor man's SWR Analyser
The circuit below serves as an SWR analyser for frequency range 2.9 - 16.0 MHz (Lo) and 12.0 - 39.0 MHz (Hi). A CMOS VCO excites an RF bridge. In the bridge, the antenna under test is compared to a 50Ω resistor. The bridge unbalance is displayed on a meter with SWR scale.
A square wave signal is used to excite the bridge, so the SWR can only be considered a rough indication. Due to the harmonics of the square wave, the indicated SWR is probably above the 'real' SWR when measuring antennas - especially at the low end of the band (80 meter).
The analyser has an output for a Frequency counter. The VCO potentiometer can be equipped with a scale to indicate frequency.
Power source is a 9 Volt alkaline battery. Voltage is stabilized at 5 Volt for the logic circuit using an LM78L05
A VCO from the 74HCT4046 PLL integrated circuit outputs a square wave. This output is routed through a buffer (74HC00) to an output for a Frequency Counter (BNC) and a 50 Ohm RF bridge.
The bridge (DC) output is buffered using a FET input OPAMP. A second OPAMP is used to generate a 'virtual ground' for the analog circuit.
The bridge is balanced if the attached antenna has a resistance of 50 ohm. The analog meter will not deflect. Full scale deflection is reached when the bridge is in full unbalance (ie: An open circuit or short circuit is connected to the antenne port). The SET potentiometer must be used to set the meter at FSD with an open or short circuit on the antenna connector (BNC).
The fully assembled meter.
Bottom cover removed.
The PCB taken from the top cover
Manufacture a scale for the meter as follows (P equals meter deflection, 1 = FSD):
Scale |
|
|
VSWR |
P |
|
1 |
0,000 |
|
1,1 |
0,048 |
|
1,2 |
0,091 |
|
1,3 |
0,130 |
|
1,4 |
0,167 |
|
1,5 |
0,200 |
|
1,6 |
0,231 |
|
1,7 |
0,259 |
|
1,8 |
0,286 |
|
1,9 |
0,310 |
|
2 |
0,333 |
|
2,1 |
0,355 |
|
2,2 |
0,375 |
|
2,3 |
0,394 |
|
2,4 |
0,412 |
|
2,5 |
0,429 |
|
2,6 |
0,444 |
|
2,7 |
0,459 |
|
2,8 |
0,474 |
|
2,9 |
0,487 |
|
3 |
0,500 |
|
4 |
0,600 |
|
5 |
0,667 |
|
6 |
0,714 |
|
7 |
0,750 |
|
8 |
0,778 |
|
9 |
0,800 |
|
10 |
0,818 |
|
Set |
1,000 |
|
Disclaimer I will not be responsible for damage to equipment, your ego, personal injury or worse that may result from the use of this material and material found on any links on my pages. You are responsible to make sure that your use any of my designs is legal in your country. |
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| Copyright: Erwin Gijzen |
| Version: Aug-2007 |
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