Measuring Radio Power Output

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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Measuring Radio Power Output

Postby jimh » Fri Dec 29, 2017 10:42 am

The VHF Marine Band fixed-mount radio I happen to own, like most others, is rated for a power output of 25-Watts. The manufacturer's specifications say:

INPUT VOLTAGE = 13.8 VDC ± 20 %
POWER OUTPUT = 25 Watts (and no tolerance is given)

To test the radio's power output, I first have to connect it to nominal 12-Volt power source. I use a conventional lead-acid battery at full charge with a terminal voltage of 12.65-Volts. The voltage specified by the manufacturer is 13.8-Volts but can be in a range from 11.04 to 15.56-Volts, so my power source is in the allowed range, but well below the target voltage.

We must also consider the FCC regulations for VHF Marine Band ship station transmitters. The applicable federal regulations appear at 47 CFR 80.215 (g) - Transmitter power:

The carrier power of ship station radiotelephone transmitters, except portable transmitters, operating in the 156-162 MHz band must be at least 8 but not more than 25 watts. Transmitters that use 12 volt lead acid storage batteries as a primary power source must be measured with a primary voltage between 12.2 and 13.7 volts DC.

To test the radio under the voltage specified by the FCC regulation, the power source cannot exceed 13.7-Volts DC. The middle of the specified range is 12.95-Volts, and the voltage for my test, 12.65-Volts, will be just below that midpoint. With the power source determined to be appropriate, I next connect a measurement device to determine the radio power output.

In reviewing the specifications from the manufacturer, I note that the preferred load impedance for the radio is not specified, nor is there any mention of a VSWR limit on the transmission line. The manufacturer does mention using transmission with a characteristic impedance of 50-Ohms, so I infer that the transmitter is designed to provide power into a 50-Ohm load. I connect a purely resistive 50-Ohm load, often called a dummy load, that has sufficient power dissipation to absorb the rated power output of the radio, at least 25-Watts. The dummy load is also designed to present purely resistive termination even at 156-MHz, is composed of a non-inductive resistor, and is shielded to prevent radiation of the signal.

Neither the manufacturer or the FCC specify what frequency ought to be used in testing; I presume any frequency in the VHF Marine Band can be used and the radio should meet specification. I will use 156.800-MHz for the test, as that is the primary channel (16) for the VHF Marine Band.

To measure the power output I will use an in-line directional wattmeter. This is a device that is inserted into the transmission line and can measure the forward-power and reverse-power on the transmission line. By measuring the forward and reverse power separately, the VSWR can be deduced; the power delivered to the load, called the absorbed power, can also be calculated: it is the difference between forward and reflected power. For my test I use a very good, industry standard directional wattmeter, a BIRD Electronics Model 43.

The BIRD 43 wattmeter has a very crafty design in which the frequency and power range of its measurements is determined by plug-in detectors (called elements). The element I will use is rated for a frequency range of 100 to 200-MHz and a full-scale reading of 100-Watts. The Model 43 has a rated accuracy of ± 5% of the full-scale reading.

The test configuration is as follows: the radio output connector is connected to the input port of the directional wattmeter by a short cable; the dummy load is connected to the output port. The length of the cable connecting the dummy load is not particularly significant. Any power lost in that cable occurs after we have measured the power. Any power lost in the length of the cable connecting the transmitter to the wattmeter is significant; power lost in that cable cannot be measured by the wattmeter.

The cable connecting the transmitter to the wattmeter is three-feet long and made with RG-58C/U cable. That cable has a rated attenuation at 150-MHz of -6.2 dB in 100-feet. We can calculate the loss in three feet as proportional.

-6.2 dB/100-feet × 3-feet = -0.186 dB

Since the cable also contains two additional UHF series connectors, we will assume they contribute a very small loss, and allot the entire connector and cable as having a loss of -0.2 dB. (We will use this figure later to calculate the actual power output at the radio.)

Now for the test. The radio is put into transmit on the HIGH power mode. The DC power to the radio under load sags slightly to 12.6-Volts, and the wattmeter indicates as follows:


The ABSORBED POWER to the load is thus 22-Watts.

Now we must calculate the actual power output at the radio, allowing for the loss of 0.2 dB in the short jumper cable. The relationship between any two power levels, P1 and P2 is described as

dB = 10 log (P2/P1)

Since we have measured P2 we substitute the measured value, 22, and then calculate the original power P1. We have already calculated (from the cable specifications) the power lost in the cable in terms of decibels: -0.2dB. This gives us the relationship:

-0.2 = 10 log (22/P1)
-0.02 = log (22/P1)
22/P1 = 10-0.02
22 =P1 ×10-0.02
P1 = 22/10-0.02
P1 = 22/0.955
P1 = 23

Now we assess the measured power results. The radio produces what we calculate to be 23-Watts. This power output occurred at a DC input voltage of 12.6-Volts. The manufacturer specified a nominal voltage of 13.8-Volts.

In the FCC regulations, the radio power output is specified to not exceed 25-Watts, but must be at least 8-Watts. The radio tested certainly meets those criteria. The test conditions of providing input DC power at 12.6-Volts also comply with the regulations.

It should be noted that the manufacturer's nominal DC power input voltage of 13.8 VDC exceeds the regulatory input voltage range which specifies 13.7 Volts as the maximum. I suspect that the power output from the radio will increase when the input voltage is increased. It is reasonable to expect that if the radio were tested with an input voltage of 13.8-Volts as the manufacturer suggests is the nominal input, then the power output would increase and be the specified 25-Watts.

Next we calculate the reduction in transmitter power that will occur if we have a 23-Watt output instead of the maximum-allowed 25-Watt. Again the relationship is

dB = 10 log (P2/P1)

We solve for

P2 = 23
P1 = 25

dB = 10 log (23/25)
dB = -0.36

This loss of power represents a very small change in signal level.

In any measurement the accuracy of the measurements must be considered. In these tests the DC voltage was measured with a FLUKE DVM, which is rated for accuracy of about three-percent. The power was measured with an in-line directional wattmeter, which is rated for an accuracy of about five-percent. With these tolerances, it is quite possible that the radio could produce the specified 25-Watt power output and the measured power provided from the directional wattmeter would indicate 22-Watts, due to an error in the wattmeter. The voltmeter error could also affect the measured input voltage, but the range of input voltage allowed is wide and, even allowing for error in the DVM, the input voltage was very likely within the allowed range.

Although the cited FCC regulations do not appear to specify any measurement method, I suspect that if one were diligent and investigated further into the regulations there may be some additional specifications referenced for test procedures. In any event, radios offered for sale in the USA must be certified as being compliant with the regulations, and no manufacturer can offer a radio for sale before submitted extensive test data to the FCC to substantiate that the radio meets all applicable regulations.

In a follow-up article, I describe how to measure antenna VSWR with a directional wattmeter.