Bench Testing an HF Voltage Balun

2024-01-13

I decided to experiment on this principle with some tools that I have, and with my open BALUN, that I showed as a photograph earlier.

I simply attached a 50 ohm resistor (wattage doesn't matter) to the output terminals of the BALUN, and attached the BALUN to an Antenna Analyzer. Since many folks have something like the MFJ-269, or such, and did a basic SWR check at about 3.6 MHz, and higher.

Suspecting that my BALUN was a 1:1 Voltage-wound BALUN, the SWR would be very low.

  1. At 3.5 MHz the SWR measured 1.1, Rs=50 ohms, and Xs=8 ohms
  2. At 4.0 MHz the SWR measured 1.1, Rs=51 ohms, and Xs=7 ohms
  3. At 7.0 MHz the SWR measured 1.0, Rs=52 ohms, and Xs=2 ohms
  4. At 7.35 MHz the SWR measured 1.0, Rs=53 ohms, and Xs=1 ohms

Now, with a 75 ohm resistor attached to the output terminals, instead of the 50 ohm, as expected, the SWR jumped to 1.5. (see photo)

  1. At 3.5 MHz the SWR measured 1.5, Rs=75 ohms, and Xs=10 ohms
  2. At 4.0 MHz the SWR measured 1.5, Rs=75 ohms, and Xs=9 ohms
  3. At 7.0 MHz the SWR measured 1.5, Rs=70 ohms, and Xs=15 ohms
  4. At 7.35 MHz the SWR measured 1.5, Rs=69 ohms, and Xs=16 ohms

Bear in mind that a carbon resistor would have been best, but this thin metal resistor still proved the point.

As I decreased the frequency down to below 2 MHz, the SWR changed to 1.7, with an apparent Rs=66 ohms and Xs=27 ohms.

This would be because of the small number of windings around the core, with much lower Mutual Coupling between windings at these lower frequencies.

Now repeating this with a RigExpert AA-600 Antenna Analyzer, shows an expected flat line across the band with an SWR=1.5, except at frequencies below 2 MHz.

I'm sure that any of my AEA Analyzers would show nearly identical results.

Now, with this information, and if you do not have an analyzer, then you could repeat this process, by using a high wattage resistor (preferably carbon), with your transceiver on very low power and simply check the SWR Meter of the transceiver during transmission.

Dave - WA7RSO