1. Accurate Power Measurements
By Fred Francis, Owner Xenirad Broadcast Engineering
Power measurement in broadcasting has been a subject that too often leads engineers to make assumptions or leaves
them with questions not easily answered. As an engineering field service contractor I have had the opportunity to measure
power on many transmitters both radio and television and have found too many transmitters operating at a power level less than
is licensed.
Many stations have inline wattmeter’s external to the transmitter which can be very accurate or in many cases can be
very in-accurate. The elements in these units do go bad and can cause the meters to read significant errors. Calculating the
power output by efficiency is not an inherently accurate method as transmitter efficiency can vary over the life of the transmit-
ter or as a result of tube wear, tuning methodology, or antenna system condition. If you have significant VSWR your meter will
not read as accurately as with a properly functioning antenna system.
If however you have access to some test equipment described in this procedure you can calibrate your transmitters
metering and verify your external wattmeters readings. You will need either a spectrum analyzer with tracking generator or a
network analyzer and a power meter.
If you have an external wattmeter with a removable element you can get from the manufacturer of that meter a test
element that will provide a sample output on a BNC connector. The amount of sample varies with wattage and line size. This
element will provide a place to accurately measure your power if the following steps are taken. First you must determine what
the actual measured sample is. DO NOT simply go by the stated sample stamped on the element as the antenna condition will
cause this to vary. To do this you will need to disconnect your line from the transmitter and connect a test adapter to the line.
Next you will need either a network analyzer or a spectrum analyzer with a tracking generator. Calibrate your test equipment
according to manufacturer specifications. Generate a signal into the line as if it were the transmitter and measure the sampled
level at the test element. Write that number down in this example we will use –50db. Now to verify directionality of the test
coupler turn it around as if you were going to read reflected power and check it again. It should read >20db lower than the pre-
vious measurement, write that number down. Reconnect your transmitter back to your antenna and go back on the air.
Connect a power meter to that sample element and read the power when the element is facing toward your antenna.
As an example let’s say our transmitter is supposed to have a TPO of 20Kw which translates to 73.010dbm and we are reading
23.010dbm on the power meter and the sample measured –50dbm. 23.010+50=73.010 or 20KW. Now you can turn the element
around and measure the reflected power and calculate the VSWR based upon that reading. You now have accurate power meas-
urement data to calibrate your transmitters metering and you can verify that your external wattmeter is reading accurately. If the
reflected reads –7dbm+50=43.010dbm or a reflected power of 20 watts. An RF calculator that converts dbm to watts and vice
versa as well as VSWR calculations can be obtained free from Bird Electronics web page at
http://www.bird-technologies.com/en/Resources/TechnicalTools.aspx#.UsS0n_RDt8s
If you do not have an external inline wattmeter you will need to buy or borrow a line section with a directional sample
port to perform the previous measurements. In the picture below the measurements were taken using the directional input sam-
ple ports of a multi-station combiner.
Fred Francis
Owner, Xenirad Broadcast Engineering
304-416-3269
Fred@Xenirad.com
www.Xenirad.com