Tuner & counterpoise efficiency tests using an RF Current sensor

The objective of these tests was to try to determine:

 1) The most efficient combination of tuner and counterpoise that will produce greatest RF current in a half wave, end-fed, field deployed, 40m, antenna. 

2) Determine the best combination based on received signal reports by ZS4SF based in Welkom. 

3) Determine the difference in signal strength between the best and worse case.

PROCEDURE: refer to the attached detailed schematic and pictures showing:

1) Test configuration.

2) Local conditions at the test site

3) Receiving station ZS4SF details

4) The 3 tuner details

5) The test results

GENERAL CONCLUSIONS

1) The combination of the 'Granadilla' tuner and 17ft counterpoise produced the best 'stable' result showing a marginal single S point improvement of 569 over the average signal reports of 559 using 2 Watts transmitter output.

2) The combination in 1) above also showed good consistent output on the RF current meter although the combination of the Granadilla tuner and 8.5Ft counterpoise showed a higher RF current, however the setup was unstable and variable.

3) The Z Match showed the worst performance of the 3 tuners with all 3 counterpoises. The DC meter sensitivity control had to be increased in order to obtain an RF current report. However 559 and 549 signal reports were still received.

  

KEY CONCLUSION

1) Based on the signal reports from ZS4SF there was a marginal difference of  less than 1 'S' point (estimate 4dB) between the best and average cases. Indicating that all combinations are acceptable for real field deployments.

OBSERVATIONS

1) It was difficult in a real field deployment to get a consistent RF current measurement with the output varying depending on proximity effects. 

2) None of the combinations were completely stable indicating that tweeking is required to reduce the ground currents and to obtain a resonant antenna with minimum reactance present. (difficult to achieve in the field).

PROXIMITY AFFECTS

1) Upon terminating the Granadilla tuner on the bench with a 4.7K resistor after the field tests, it was determined that the resonant frequency was at 7.487MHz. This is 7.487 - 7.020 =  467KHz difference. Assuming the tank circuit inductor is fixed at 4uH, this reveals a capacitive contribution of 16pF from the antenna/counterpoise system contribution. If this contribution came entirely from the antenna wire then the effective antenna length can be calculated as 71.4ft (as opposed to 66.66ft actual length).  This seems to be a big difference in length and may explain why the system was unstable during the tests.

GRANADILLA TUNER ADVANTAGE

The Granadilla tuner has two key differences when compared to the Altoids tuner. It uses an Air variable panel mounted, screw driver adjustable, 30pF capacitor, in parallel with a 10pF silver mica capacitor with a 500V rating. The Altoids tuner uses a 150pF polyvaricon capacitor. Both tuners use the same T-50-2 inductor. The Z Match utilizes two polyvaricon capacitors. Could it be the polyvaricon capacitors that exhibit losses in this environment. 

Assuming a radiation resistance of 3000Ohms and 2 watts, the peak-to-peak voltage across the tank circuit = SQUAREROOT( 8 X P X R)  = SQUAREROOT(8X2X3000) = 219Volts. Does this voltage stress the polyvaricon capacitors?

This was a fun project and very instructive! My thanks to my friend OM Monk in Welkom ZS4SF for hanging in there and providing 12 signal reports. Thanks Monk! 73.

Z Match, Granadilla Tuner, Altoids Tuner

View of the RF current sensor connected to a sensitive DC analog meter (0-1mA)

View of Antenna, Cedar Lakes upper picnic table, Fourways, Gauteng, South Africa 

Granadilla in South Africa is known as passion fruit in the USA. Yummy!

The 7020Khz capacitor pure resistive position terminated in a 4.7K resistor. Antenna reaction is tuned out in the field.

Understanding the RF Current sensor (2)

Following from tests conducted and recorded in the previous blog I had to now compare the Z Match output with my Tank Circuit coupler that I have used for many years to feed a 40m end-fed. This was done using additional bench tests in the same manner as before.

I could not match the 3000 Ohms to the tank coupler with 1:1 SWR. The best obtained was a 1.7:1 match. I then substituted the tank coupler with the Z Match. I was able to tweek this to an SWR of 1:1.

Results:   Tank Coupler   DVM reading = 315mV  SWR 1.7:1
               Z Match          DVM reading = 340mV   SWR 1.1:1
               Z Match          DVM readings = 277mV, 331mV, 330mV, 277mV various settings SWR 1.7:1

Some interesting observations.  At the 1:1 SWR on the Z Match the greatest output was obtained. However if the input power is adjusted for SWR 1.7:1 = 93.3% efficiency, thus input power = 1.2*0.933 = 1.11Watts.  If I reduce the power input to the Z Match by 6.7% I derive a DVM = 315mV. This being the same as the Tank Coupler. Looking at the Z Match DVM, SWR 1.7:1  readings they are highly dependant on the settings of the capacitors. However they are all in a similar 'ball park'.

Thus I conclude that the couplers have a similar efficiency at least at 7020KHz.

When comparing to the theory. The DVM readings are significantly lower that the calculated value of 463mV (see previous blog). Is this an indication that the loss is attributed to the RF current transducer?

Understanding the RF Current sensor

I thought it might be instructive to set up a bench experiment to try to understand the performance of the RF Current sensor. To keep it simple I measured the sampled RF current at just 1 frequency 3579KHz.

In order to simulate an end-fed halfwave input impedance, I terminated my Z Match in a 3000 Ohm resistor. I then linked the termination through the RF current sensor. After measuring the K2 output at 1.2 Watts I then connected the K2 to the Z match and tuned the Z match for a 1:1 SWR. Best I could get was in fact a 1.1:1 SWR. I measured the RF current sensor output at the RF point across the 270 Ohm resistor using my scope with a 10X probe. I measured the DC output using the Keithley DC DVM.

I then compared the results with some theoretical calcs which assumed that the system was 100% efficient. ie all power from the K2 is transferred to the 3K load AND all power from the current sensor is transferred to the DVM and Scope readings. Here is a summary of the results obtained.

DC voltage output (calculated) from the sensor = 463mV. Measured DVM DC voltage = 255mV
Voltage output (calculated) across 270 Ohm resistor = 763mV peak. Vpeak measured = 490mV.

In the end the only observation would be that the measurements were 'in the ballpark' of the expected results.

There could be many reasons why the discrepency. Listed as follows.

1) K2 Power output measurement error on the Power Meter
2) 1.1:1 SWR is not a perfect match.
3) Losses in the Z Match.
4) Load not exactly 3000 Ohms at 3579 KHz (did not measure the actual resistor values)
5) Core losses in the current transformer
6) Incorrect theoretical calculations
7) DVM and Scope measurement errors
8) Loading effect on the circuit from the DVM/Scope
9) Assumed forward voltage drop of 300mV across diode is incorrect

Looking at item 6 it is not clear how accurate the assumption is on current transformer core action. In practice this could be widely different to the theoretical 10:1 turns ratio. I assumed that all available current would be extracted across the resistor? This is probably not correct. I don't know how to calculate this value. More education required on transformers. Without knowing what the actual efficiency is of the Z Match it is not possible to narrow down the discrepancy.

The most practical next step would be to perform the test with another 2 transmatches. This way it would be possible to get some idea of the difference in efficiency of the transmatch section of the test.

Test Setup showing the Z Match terminated in 3000 Ohms and with the RF Current sensor inline. 

DVM = 248mV and Scope showing peak to peak RF across the 270 Ohm resistor of 980mV

RF Current sensor with scope probe in place

RF Current measurements on a Long Wire W3EDP antenna

Having seen that it is difficult to obtain accurate RF radiation measurements using a field strength meter, I next resolved to measure the RF current on the 84ft inverted L W3EDP antenna deployed in my garden.

I built an RF Current measurement transducer according to the schematic developed by Dave ZS6AZP. I built this transducer in an evening into an altoids tin. The build came together very easily.

Having previously built a DC analog meter for my Field Strength meter I simply connected that meter to the RF current transducer. This scheme worked very well.

I found it much easier to obtain consistent readings using the RF Current sensor as opposed to the Field Strength meter.

Terminating the Current meter in a 51 Ohm load and driving the system with my K2 TRX, I established that the minimum power detectable with this meter into a 50 Ohm load is approximately 100mW across the HF band. No doubt by using a germanium diode lower currents could be detected.

The results were revealing to me. On both 80m and 40m the peak RF current was definitely not at the 1:1 SWR load point. Refer to attached for more information of the test. Why is this?

I also noted that at the test power level of 300mW that the output RF current dropped (amount not quantified) as the test proceeded. Was this the effect of heating in the Z Match?

By maintaining the same output power from the K2 TRX and keeping the DC Meter sensitivity the same I was able to compare the RF Current on 80m and 40m when connecting the system to a 17ft and 33ft counterpoise respectivily. I then loaded the system against the two different counterpoises by adjusting the Z Match. I noted that the 17ft counterpoise in both cases, resulted in more RF current than the 33ft counterpoise. In fact the meter did not even deflect with the 33ft wire attached. The 17ft counterpoise is over the grass lawn whereas the 33 ft counterpoise lies along the concrete path. More experiments with counterpoises warranted. Noted that the W3EDP design in fact calls for a 17ft counterpoise. Perhaps W3EDP knew what he was doing :).

In my morning sked at 6:30 am on 3579KHz with OM Barrie ZS6AJY, Barrie reported that I was a 579 but that my signal was 'the strongest he had heard'. hi. I was using the 17ft counterpoise.

NOTES: The shunt resistor across the inductor = 270 Ohms.
               The inductor is a FT50-43 with 10 Turns on the secondary and 1 turn on the primary (pass the                       antenna wire through the toroid).
               Calculated inductance = 44uH, Measured inductance = 45.8uH

 

RF Current sensor inline with an inverted L 84ft W3EDP antenna with 17ft counterpoise and Z Match tuner.