Making an SWR meter with a Tandem Match

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We made a Tandem Match (Stockton Bridge) for the antenna tuner project but we still had parts leftover and other odd and ends we routinely pick up at flea markets and swap meets so we decided to make a stand alone SWR meter.
The same question arises, why make one when we can just buy a commercial grade meter, more stylish, compact, and of proven performance?
The same answer is the pride and satisfaction of crafting something that works and is unique.
This is the second attempt at making an SWR meter.
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As with the previous project, first state the goal:

An RF meter useful for nulling out SWR when used with an antenna tuner.
Capable of measuring power in two ranges, 20 and 200 Watts, 1.8 to 30 MHz.
The RF directional coupler and the metering parts may be easily disconnected for experimentation in other projects.

  1. We had a spare coaxial static surge protector and decided to put it to better use.


  2. Removed the existing circuitry and drilled holes to secure an additional enclosure.


  3. Normally we would put all the assemblies in the same case (with proper shielding).


  4. But we had to use what was available at hand, and our two enclosures were not identical.


  5. At some point we have to unite these two.


  6. The choice was to use FT50-43 toroids. There is much discussion as to use ferrite mix (FT) or iron powder (T) ones. My first one years ago was done with T50-2 and it worked well, but we wanted to experiment.


  7. So how many turns? The calculation called for 26 turns of 26 AWG enameled copper wire for 30 dB coupling. If we have 100 Watts of incident power, 10 dB down would be 10 Watts, 20 dB down 1 Watt, and 30 dB would give us 0.1 Watt (100 mW). This power level can also be expressed as +20 dBm and is the maximum power some measuring instruments can take without damaging the input. This is also the equivalent of 3.16 Vpeak or 2.24 Vrms (sinewave into 50 Ohms). These are safe levels for simple experimentation without invoking a visit from the Magic Smoke.


  8. There is a large amount of information, both practical and theoretical, readily available online. I wanted a simple design. Make sure details, notes, diagrams, and calculations are well documented. All this will serve in the final evaluation.


  9. Just about any type of 50 Ohm type of coax can be used but needs to pass through the toroids with the winding. The piece shown in the picture below was enough for the two coupling legs.


  10. This is one of the two coupling legs. I wanted to insure a common ground bus between all the ground points as the aluminum cast case itself may not be up to solid RF conduction. The use of a wire as a ground bus may not seem proper for RF work because of the possible added inductance, but the top frequency goal was 30 MHz. Have read that Teflon tape used for sealing pipe joints is recommended to make the toroid fit snugly over the coax.


  11. Again, things are easier if following a drawing. In this case we are looking at the smaller enclosure.


  12. The two completed RF coupling legs or "tandem".


  13. Now we proceed to "marry" them together.




  14. Note the ground bus connecting the two assemblies.




  15. The complete Tandem Match (Stockton Bridge) directional coupler. A number of tests were made with other transceivers, SWR meters from other tuners, RF rectifying diodes, RF loads, and preliminary results indicated a sampling ratio of 29 dB below incident power. This was later revised to 28 dB. After more reading, calculations, and measurements, we finally settled for 29.6 dB. The calculated ratio was 30 dB. Am always skeptical when results come close to theoretical calculations for we know that Physics (of which Electricity and Electronics are part of) is not an exact science.


  16. After searching E-bay (and the garage) for a suitable meter movement, we found this one for a reasonable price. This particular model was intended for mobile use, so it originally had a remote RF coupling "head", possibly similar to ours. It came without it, but we did not care, as we only wanted the meter movement. The switches and other hardware were a nice bonus.


  17. This meter dated from the 80's and there were no schematics available online anymore so we had to "reverse engineer" it to find where all the input wires connected to. This is part of the fun of "cannibalizing" and rebuilding.


  18. After some live tests, we had to replace a 10K Ohm resistor to 15K Ohm to get one of the adjusting trimmer pots to be centered.


  19. Now it was time to finally connect the Tandem Match to the indicator head.


  20. The RF detector circuits were hastily tack soldered "ugly bug" style to calibrate and continue the characterization of this new SWR meter. Only had to adjust the 20 and 200 Watt trimmer pots.


  21. At this "ugly bug" stage, we fed 100 Watts at 14.2 MHz through the bridge and measured 3 VDC at the FWD detector when the meter needle was also indicating 100 Watts on the 200 Watt range. According to theory, one has to add the voltage drop across the 1N34A germanium diode, typically 0.3 VDC, to obtain the peak voltage of this rectifying circuit.
    Using available online calculators, this corresponds to a sampler output of 109 mW or 20.7 dBm.
    In turn, knowing that the incident power is 100 Watts, we can calculate the coupling ratio to be 29.6 dB. The design called for 30 dB. Close enough for amateur work.
    Did the same at the 20 Watt level and measured 1.15 VDC on the 20 Watt range and the needle reading full scale. The rest of the math is still magic to me, so I stop here.


  22. We had to order two BNC male panel connectors to house the Forward and Reflected RF detector circuits, and in the meantime also made a holding base with leftover metal from a computer case and pieces from a mobile mounting bracket.


  23. Both the FWD and REFL RF detecting circuits were soldered to the respective BNC connectors then protected with shrink tubing. Both circuits are identical and, as shown in a previous diagram, consist of a 47 or 51 Ohm terminating resistor, a 1N34A diode, and a 0.01 to 0.02 uF disc capacitor.

  24. Finished Tandem Match (Stockton Bridge) SWR meter.


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As with any project, at the end, state the conclusion:

In the "Power" mode, once internally calibrated, meter consistently reads both 20 and 100 Watts over the 1.8 to 30 MHz range.

Both FWD and REFL indications are sufficiently good for HF amateur work. The "Calibrate" control is a tad "noisy" in some spots but I considered it not bothersome enough to warrant replacement. This may be addressed at a later date.

Unit was tested on 2M and surprisingly works well as an SWR meter. VHF power measurement is not accurate. With a known 25 Watt source, it reads 15 Watts at 144-146 MHz, and improves to almost 20 Watts from 146-148 MHz without pegging the meter (even works up to 152 MHz). This means we could use this SWR meter to tune 2M antennas, but not to measure VHF power. Still a bonus as we did not intend to use this meter at VHF frequencies.

Testing at 440 MHz shows excessive coupling between components as POWER, FWD and REFL readings just peg the meter full scale without rhyme or reason. Clearly there is a limit on the useable range.

Parts cost:
$19 Meter movement
$2 FT50-43 toroids
$8 BNC connectors
$1 1N34A diodes
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$30 Total

Lesson learned: In the process, got to read and review all about toroid cores, rectifying diodes, RF load resistors, RF probes, Ohms' Law, power calculations using logarithms, decibels, dBm, as well as other types of RF directional couplers.
The price for commercial grade SWR meters, available online and at swap meets, ranges from $25 to $150, give or take a few, but the experience, expectation, hope, and pride of crafting one is, for some of us, priceless.
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