Making an antenna tuner with SWR meter

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We now have air variable capacitors and a variable inductor, so we can make a tuned circuit to present a 50 Ohm transceiver and transmission line with a suitable impedance from an antenna over a range of frequencies.
Of course, why make one when we can just buy a commercial grade tuner, more stylish, compact, and of proven performance?
There is always the pride and satisfaction of making something that did not quite exist 200 years ago.
This is also my first attempt at making an antenna tuner!
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As with the previous project, first state the goal:

Capable of matching complex impedances to 50 Ohms, 1.8 to 30 MHz, and handling 300 Watts RF.

  1. We now have two air variable capacitors and a roller inductor.


  2. Located a suitable enclosure for the tuner. We had an old computer from the 1990s, with floppy drive too.


  3. After removing the computer contents we ended up with lots of space and some spare aluminum.


  4. We tried to position the caps and inductor on several places. Rear, side.




  5. Finally decided using the top of the computer case as front panel.


  6. Reduced the computer case to a more suitable size.




  7. Installed the caps and inductor in a T-Match configuration.
    This could change depending on future levels of experimentation.


  8. For now, BNC connectors should suffice. Using SO-239 connectors remains an open choice for future upgrade.


  9. We can "tune" by listening to the input noise or received signal level but it is better to null the SWR to a minimum for transmitting. Now we need to make an SWR meter. The computer power supply and the old motherboard had a few ferrite cores.


  10. We did not try to make a meter movement as this was a delicate part and we had no tools or materials to make one. This meter is sufficiently good for our project, and it cost $20 USD.


  11. Now we need to place the meter on the front panel. We had no easy way to make a square hole.


  12. But we did finish with a square hole. Note the chrome tubular cabinet handles. They serve to protect the front panel and add a classic touch too.




  13. To avoid making more holes on the front panel. We used epoxy glue to secure the meter to the front panel. In the process, we figured we could make have two power ranges in case we went QRP, so we decided for 10 and 100 Watts.


  14. Same procedure to install the power range switch.










  15. Now we proceed to cut aluminum for an RF directional sampler enclosure.


  16. The excess aluminum from the computer came in handy.



  17. Also had to find a suitable location for the input to the tuner and the sampler.


  18. After much reading and examining the design of various existing commercially tuners, decided to use a "Tandem match" for the RF directional bridge. The more common alternative, "Bruene bridge", uses a small value compensating trimmer capacitor (a few pF), and I could not get such a part at a reasonable cost. The "Tandem match" seemed simpler and I also had two identical toroids.




  19. Final wiring of SWR meter, calibration controls, and cabinet.


  20. Completed Tuner, inside view.












  21. External view. Note the custom made knobs with Lucite acrylic material to insulate user from the metal parts.
    Still deciding how to make the artwork for the front panel.


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

Lesson learned: As of this writing, April 2020, we have not been able to produce RF in sufficient levels to test and calibrate the SWR meter. We did test the tuning network with a helical antenna cut for 20 meters and it works as expected with an SDR receiver. More to come.
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