Efficiency Test using the MFJ 941B Purpose: The purpose of this test was to see what instruments I have that makes measurement of a tuner possible. In particular I wanted to use my Vectronics (MFJ) Antenna Analyser (Vec) as the source of a signal and read the power (as voltage) on a 50 ohm load. Test Setup: The MFJ tuner was hooked to the Vec and a 2 watt 50 ohm resistor was placed across the coax output. The voltage across the 50 ohm resistor was measured with a Fluke Model 81RF probe into a Tectronics Type 422 Oscilloscope. The scope measured the dc voltage that came out of the Fluke probe. Calibration: I was amazed! The Vectronics was hooked to the 50 ohm resistor direct and the voltage measured at the scope was 0.2 volts dc almost exactly from 1.8 to 30 MHz! This made it easy to run the measurements since each was just one reading. So all measurements were run with a power of P=E*2/R so the input was 0.8 milliwatts. This must be what MFJ sets there devices to. Measurement: The Vectronics was placed on a frequency in a Amateur Radio band. Then the tuner was tuned to the best SWR on the Vec. Then the tuner settings were recorded and the voltage was read across the 50 ohm load, and recorded. Data: Frequency SWR Power Out Loss 2.0 1.1 6.85 14% 3.5 1.1 6.85 14% 7.0 1.1 7.22 10% 10.0 1.0 7.22 10% 14.0 1.0 7.22 10% 18.0 1.1 7.22 10% 21.0 1.1 7.22 10% 28.0 1.0 8.0 none Conclusions: No tuner is required to match a 50 ohm load to your rig. But it is possible the tuner might not work as well as a piece of coax did. In fact it turns out there is about a 10% loss in power through the tuner to a 50 ohm Unbalanced load. My earlier measurement done a different way confirms these numbers. I can read a change of .02 volts very well on the scope. A 5% drop in power could not be seen in this test. The taps on the tuner coil were made when there was only 160, 80. 40, 20, 15 and 10 meters. It was not possible to get a 1:1 SWR on all frequencies in a short time. So when the output was seen to be peaked it was reported. On an easy load this tuner works without serious loss on all the bands. But it's possible to miss-tune the tuner so that SWR looks good but a 50% loss occurs as it did at 3.0 MHz. This needs further investigation. September 5, 2001 Unbalanced Output Load Tests Test Conditions: This series has a 200 ohm resistor (1/2 watt) as a load on the Coax Out which we should call the Unbalanced or Unbal connector. Power into the tuner was 8 x 10*-4 watts. All the powers listed are X x 10*-4 watts. Data: Freq. SWR Power Loss (%) 2.0 1.1 8.0 0 3.5 1.1 8.0 0 7.0 1.0 7.6 5% 10.0 1.0 8.0 0 14.0 1.2 7.6 5% 18.0 1.1 7.6 5% 21.0 1.0 7.6 5% Test Conditions: This series used a 1000 ohm load which is near the high limit of this tuner. No low SWR could be found on 160 meters. The load was on the Unbal input and the power into the tuner was 8 x 10*4 Watts. Data: Freq. SWR Power Loss (%) 2.0 Not able to tune 3.5 1.0 8.0 0 7.0 1.0 7.74 3 10.0 1.1 8.0 0 14.0 1.2 7.06 12 18.0 1.0 7.06 12 21.0 1.3 6.4 20 28.0 1.0 6.1 24 Test Conditions: This series used an Unbal 20 ohm load which is near the low level limit of this tuner. All bands got a low SWR and the power into the tuner was the usual 8 X 10*-4. Data: Freq. SWR Power Loss (%) 2.0 1.1 5.2 35 3.5 1.5 6.9 14 7.0 1.1 7.7 4 14.0 1.5 7.7 4 18.0 1.1 8.0 0 21.0 1.0 8.0 0 28.0 1.0 8.0 0 Conclusions to this point: The MFJ-945B appears to have a working range of impedance from about 20 to 1000 ohms. If the impedance is much below 20 ohms the tuner has high loss in the 50-80% range. If the impedance is above 1000 ohms the tuner may not be able to tune it to 50 ohms output. The tuner should do well with a G5RV type of antenna that has Coax to the tuner's Unbal input. What data I have seen on the impedance of this antenna puts it within this tuners efficient range. 160 meter operation with this tuner is going to be poor. The C-L-C or Tee tuner configuration with a toroid as the L has more loss than all the other bands covered. Test Conditions: The load was 5 ohms and the tuner was difficult to adjust. Some bands it was not possible to get a good 1:1 SWR. This data was collected just to complete this section. Data: Freq. SWR Power Loss (%) 2.0 1.2 1.6 80 3.5 1.0 1.8 76 7.0 1.1 2.5 69 10.0 1.1 1.0 88 14.0 1.8 2.5 69 18.0 1.4 3.0 63 21.0 1.1 3.5 56 28.0 1.2 6.9 14 Conclusions: The tuner loss with a 5 ohm load is too much to stand. If your antenna feed-line has this low an impedance at the tuner, it will not work. I don't think there is a tuner that will work well at this low impedance. This does prove that the MFJ 945B tuner will not work with this load.