RF-Ampere Meter

I've been wanting to have a Powermeter for a while but didn't want to spend an awful lot of money for a commercial unit. I'm not talking about units like the LP-100 or the BIRD series of Powermeters, I'm talking about Yae-Ken-Ico's etc. . As usual you get what you pay for. But it doesn't have to be that way. An easy way to measure power is by using an RF-Ampere Meter, huh measuring current doesn't give you power! Oops, yes it does! Remember Ohms Law! Check out the below formula.

Yep with a little math we'll be able to measure POWER. I'm going to use a current transformer to be more precise, a 40:1 current transformer to measure FORWARD power.

Say we'd like to measure 400 W, what is the expected current reading. Using the above formula (F.2) we get approx 2.8 A.

So to get a suitable voltage from the secondary, our load resistor should be reasonably high, but not to high as we want to avoid reflecting a low impedance back into the primary. So a 10:1 ratio, which would make the load resistor 500Ω, would be a good compromise. Let's have a quick look at the impedance reflected back into the primary. Impedance transformation is proportional to the square of the turns ratio i.e. 1600 (40²). So the impedance presented to the primary will be 1/1600^th of the load resistor (500/1600), approximately 0.3Ω. In practice it would be very unlikely that this would cause disturbances to the measurement.

Lets assume 50 W of RF is flowing through the transformer, which is a current of 1 A, this in turn equates to 25 mA in the secondary (1/40^th or 1000 mA/40). Now 25 mA flowing through the 500Ω load resistor will be 12.5 V_AC. To rectify this voltage we'll use a simple silicon diode and a filter capacitor. This combination, diode/capacitor, will establish a DC potential of 1.3 time the E_RMS i.e. 16.25 V (1.3 x 12.5).

As it happens, I had a few 50µA Analog Panel Meter lying around which are perfect for this little project. I'm going to choose two ranges, 0.5 A and 5 A, which should make the meter usable for QRP (<1 W - 12.5 W) and QRO (10 W->1000 W). Lets figure out the series multiplier resistors (R_s) we need. On the 5 A scale we want a 10 µA deflection for a 1 A reading, therefore R_s should be 1.625 MΩ (R=E/I) and so for the 0.5 A scale the multiplier is only 1/10^th of the 5 A scale and therefore R_s should be 162.5 kΩ. In the schematics below I used what I had at hand, 10% value resistors. Measure the value of the resistor, you find the ones that are close to the calculated values.

Pic 1 shows the dead bug style build of the RF-AmMeter. The next two pictures, Pic 2 and Pic 3 shows the through line loss and the return loss against a 50Ω resistor (dummy load).

I will have to change the dial to read power, rather then RF Ampere. Below are a some measurements, which are against an LP-100D.

From the above, it is clear scale and refraction errors are playing a big role. However, with the data at hand it is possible to construct a more accurate Powerscale, in either dBW (dBmW) or Watt (W), which should improve accuracy of the RF-Ampere Meter to around 10%.

Parts List:

• • • • 1x 470Ω 3W NON inductive (Load Resistor)

      • 1x 1.5MΩ (Series Resistor 5A scale)

      • 1x 150kΩ (Series Resistor 0.5A scale)

      • 2x 1N914 (Silicon Diode or similar small signal diode)

      • 2x 10nF (ceramic capacitor)

      • 1x LO-1230 (Ferrite Toroide or similar FT50-43)

      • 1x 50µA Analog Panel Meter

      • 1x Toggleswitch

      • 1x Diecast Aluminum Case to suit (approx.110x60x50)

      • 2x RF connectors (SO-239 bulkhead or chassis)

      • 1000mm 0.5mm CuL (#24)

      • Teflon tape (Plumbers tape)