Dr. FB

The Bird Model 43 RF Wattmeter (Bird 43) is a power meter that every amateur radio enthusiast wants, and every shack should have one. The RF output power displayed on this power meter is absolutely reliable. it is said that "Did you measure the power with a Bird 43?” in the end. So, I decided to test a Bird 43.

There are other power meters in the shack, and all power meters display very similar power values, but they are slightly different. To be honest, I'm not sure which is the actual power value. In terms of price, the Bird 43 is the most expensive. It may be the long-established name value of Bird. that makes you think that the power value measured by the expensive Bird 43 is always more accurate than that measured by an inexpensive power meter.

Figure 1 Connection example for power measurement using a Bird 43

Is the Bird 43 really accurate?

As a result of the experiment shown below, it is not established because it is not clear about what value is accurate and what value is inaccurate. In conclusion, the power value measured by a Bird 43 shows a difference of about 0.5 W at 10 W with respect to the variation of the length of the relay cable and the load. Although it is also called a CM type power meter depending on the passing type power meter or product, it is a general understanding that the power value measured by them changes with load fluctuations. What I felt in this experiment is that Dr. FB's candid impression is that the Bird 43 certainly changed the indicated value due to the change in load, but it was not as expected.

Experimental background

The pass through wattmeter is a very convenient measuring device for amateurs, as it can measure traveling wave power and reflected wave power while transmitting. As mentioned earlier, it is known that the indicated value will change with a change in load, but in that case, an expensive terminal type power meter using a thermocouple as shown in Figure 2(a) can be used. How many amateurs are you using? Most amateurs can see the power of the traveling wave and the reflected wave as shown in Fig. 2(b), and in many cases, they use a pass-through wattmeter that can measure SWR.

The Bird 43 we tested this time is a pass-through wattmeter, but it is not cheap. While many people think that it is a standard for electric power meters, I decided to measure its performance by changing the load and the length of the junction cable.

Figure 2 Examples of high-frequency power measurement

Test result

First, from the experimental results. The transmission power of the IC-9700 was measured with the connection configuration shown in Figure 1. As a result, although the conditions for the IC-9700 transmission output was not changed, the readings have certainly changed if the length of the junction cable connected to the input side of the Bird 43, or the impedance of the load connected to the output side is changed.

The change is not that big, but Dr. FB who has no accurate RF power meter as a reference did not really know which was the true power value. Although details are not described in the article in this experiment, a similar experiment was performed with a pass through RF power meter that can measure SWR often used by amateurs as shown in Figure 2(b).

As a result in the test in Figure 2(b), I can observed a power differences that is much more than the one tested with the Bird 43 when the length of the coaxial cable used to connect the IC-9700 and the RF power meter and the load impedance of pass through RF power meter. The power indication difference was about 2-3W to 10W. The difference may be in the structure of the detection part, but we have not examined it to that extent.

Preparation for experiment

In the experiment, I considered to connect the following three conditions of load. Under each condition, the length of the cable between the IC-9700 and the Bird 43 was increased by 10 cm, and the meter indication of the Bird 43 was observed. The frequency and mode were 145,000 MHz and RTTY respectively. Note that the PL-259 (UHF connector) used at both ends of each coaxial cable must be new and purchased from the same manufacturer at the same time so that the loss at the connector is the same for all coaxial cables.

At the time of the experiment, the APC (Automatic Power Control) function of the main unit operates due to the deterioration of the SWR of the load connected to the IC-9700. If the power decreases, accurate results cannot be obtained, so the deterioration of the SWR, I first checked to see if there was a power drop. As a result, I found that the APC of the IC-9700 does not work if a load with an SWR of approximately 2.0:1, so the experiment was proceeded with the maximum SWR of 2.0:1.

(1) Experiment 1: Directly connect a 50 Ω dummy load to the load side of the Bird 43.
(2) Experiment 2: Connect a dummy load with an SWR of 1.5:1 to the load side of the Bird 43 through coaxial cable (3.5m-LFV of 3m).
(3) Experiment 3: Connect a dummy load with an SWR of 2.0:1 to the load side of the Bird 43 through coaxial cable (3.5m-LFV of 3m).

1. Experiment 1_ Directly connect the 50Ω dummy load

Figure 3 Measurement set up for Experiment 1

1-1 Preparation
Connect the IC-9700 and the Bird 43 with a connector. Adjust the transmit power of the IC-9700 so that the Bird 43 reading is 10W. Based on 10W at this time, the power of IC-9700 will be constant thereafter.

1-2 Measurement results
Figure 4 shows photographs of the Bird 43 reading when the length of the cable between the IC-9700 and the Bird 43 was increased by 10 cm

The Bird 43 readings change slightly each time the junction cable is changed. As whole, the meter readings do not increase more than 10W and shows less than 10W.

1-3 Consideration
When the length of the cable was further increased by 10 cm from 10 cm to 30 cm, the power on one scale of the meter reading decreased. However, when it reaches 60 cm and 70 cm, the meter reading is slightly up again. The same measurements were repeated multiple times for reconfirmation, but the results were the same. I'm not sure what it is, but it seems that the decrease in the meter reading is not caused by the loss of the coaxial cable.

Figure 4 Photographs of observations from Experiment 1

Experiment 2_ Connect a dummy load of SWR=1.5:1

Figure 5 Measurement set up for Experiment 2

2-1 Preparation
Connect the IC-9700 and the Bird 43 with a double male UHF connector. Adjust the transmit power of the IC-9700 so that the Bird 43 reading at that time should be 10W. Based on 10W at this time, the power of IC-9700 was kept constant thereafter, and a dummy load of SWR=1.5:1 was connected to the load side of the Bird 43 in order to be close to an actual operation.

2-2 Measurement result
The length of the cable between the IC-9700 and the Bird 43 was lengthened by 10 cm and the reading of the Bird 43 was observed. Figure 6 is photographs of the meter readings.

2-3 Consideration
The meter reading of the Bird 43 changes slightly as in Experiment 1 each time the junction cable is changed. When the length of the cable between the IC-9700 and the Bird 43 is set to 30 cm, 40 cm and 50 cm, the meter reading is slightly over 10 W. Other lengths do not exceed 10W. It is also a mysterious point that the change in the meter readings do not change as much as the meter readings observed in Experiment 1.

Figure 6 Photographs of observations from Experiment 2

3. Experiment 3_SWR=2.0:1 dummy load connection

Figure 7 Measurement set up for Experiment 3

3-1 Preparation
It is almost the same as Experiment 2 above. The difference is the impedance of the load connected to the output side of the Bird 43. In Experiment 3, a dummy load with SWR=2.0:1 was connected.

3-2 Measurement result
The length of the cable between the IC-9700 and the Bird 43 was increased by 10 cm, and the meter reading of the Bird 43 was observed. Figure 8 is photographs of the meter readings.

3-3 Consideration
As with Experiment 1 and Experiment 2, the meter reading of the Bird 43 changed slightly each time the length of the cable was changed. As a result of Experiment 3, even if the length of the cable between the IC-9700 and the Bird 43 was changed from 10 cm to 300 cm, the meter reading did not exceed 10 W as in the result of Experiment 2. I was surprised that the power value would change greatly because it was a poor matching condition such as SWR=2.0:1, but it was not.

Figure 8 Photographs of the measured values of Experiment 3

Summarize

Even though it is the Bird 43, we found that the meter reading changed more or less depending on the load change and the length of the coaxial cable used for connections. This can be said to be the fate of a pass through wattmeter, but to use the pass through wattmeter as an accurate wattmeter, always use a fixed 50 Ω dummy load and always use a fixed length of coaxial cable for connections. And it is important to have them calibrated by a laboratory that calibrates test instruments.

FBDX