POWER MEASUREMENT AND A SIMPLE DUMMY LOAD / WATTMETER

Prior to the introduction of the Bird 43 RF power measurement and antenna tuning in the VHF/UHF range was difficult. Most methods were more suitable to the laboratory than field operations or wildly inaccurate. Most methods were based on heating of the RF load. This method is still the most accurate and is still used for power measurement in the milliwatt region and for microwaves. 

A BRIGHT LIGHT
The simplest method of power measurement (I still use it on occasion) of RF power indicator is the light bulb. Perhaps the best example of the light bulb dummy load is the WWII AN/URM-13 test set. This little box contains a device that looks like a small opened ended metal can with a "N" connector on the closed end. I paid $5.00 for mine at a Hamfest twenty five years ago. 

Inside the can are four small carbon filament lamps! Yes, carbon, just like Edison made. Carbon was used so with four lamps the resistance stayed close to 50 ohms regardless of how bright they lit. This little combination dummy load / tuning indicator has a frequency range of up to 400 MHZ and a power range of 5 to 20 watts. At twenty watts, the lamps are VERY bright! With AM modulation they flicker beautifully with the modulation. 

Over the years, my little mil-spec dummy load has been very useful for quick and dirty testing and tuning of low power radios. You can even make accurate power measurement with this type of device. Prior to WWII a "Oil Spot Photometric" VHF Wattmeter was manufactured. Truly a curious looking device, in reality it worked very well. It consisted of a two foot long black painted wooded box with a carbon filament light bulb at each end. 

One bulb was the RF dummy load. The other was connected to a simple regulated power supply and a large rheostat. The rheostats' dial was calibrated in RF Watts. In operation the operator turned on the radio transmitter (lighting the RF lamp). He then turned on the power supply lighting the DC lamp at the other end of the box. 

In the center of the box was a sheet of paper with a oil spot in the center. Using a mirror and eyepiece the operator watched the oil spot as he turned the rheostat knob increasing the power to the DC lamp. When both lamps were the same brilliance, the oil spot appeared to disappear (since it was lit to the same degree from both sides). At that point the operator logged the reading on the rheostat. The amount of DC power required to light the DC lamp is the same as the amount of RF power required to light the RF load lamp. Sounds complicated, but it worked! 

CALORIMETER
Obviously light bulbs make terrible dummy loads as they change impedance with filament temperature (RF power). Nor do they work well above 400Mhz due to stray reactance. To counter this the next common method of RF power measurement used by hams was the calorimeter. For many years the ARRL handbook gave details on building your own UHF/microwave calorimeter. The principle is simple. You measure the amount of heat energy dissipated by a dummy load over a given time period. You can then calculate the true RF energy required to produce that amount of heating. 

The apparatus is simple and gives you pause to think about losses in coaxial cable. The dummy load consisted of 100 foot of RG-174 solid dielectric 50 ohm coax. This 1/8" diameter coax is just about the lossiest ever made. At 1,296Mhz the losses are so high that almost no RF can ever reach the other end! For practical purposes 100' of RG-174 appears at 1.2GHZ as a perfect 50 ohm dummy load. 

The loose cable/dummy load is placed in a styrofoam cooler filled with water to absorb the heat generated by the RF losses of the cable. A thermometer placed in the water measures the heat rise after the transmitter is turned on. Knowing the time, beginning and ending temperature, the power output of the transmitter in watts can be calculated. As primitive as this system sounds, it is so technically sound that it is still used (although with more sophisticated apparatus) to measure the power of high power FM and TV transmitters. 

TODAY
The task of power and SWR measurement becomes simple if your the proud owner of a Bird 43 series wattmeter if not, life gets much more difficult. The Bird 43 was developed during (and has remained unchanged since) the second world war for the testing of the new VHF/UHF radios that emerged during that period. 

For amateur purposes the directional wattmeter (such as the Bird 43) has supplanted other methods. These meters are essentially a calibrated directional coupler. That is, they are little more than a simple RF pick up loop parallel to the center conductor of a section of coaxial line. This loop samples the current flowing in the center conductor of the coaxial line. If the sampling loop is correctly designed, the current flowing in it will be proportional to the current (power if the load is 50 ohms resistive!) flowing through the coaxial line. 

A simple diode detector converts the RF from the sampling loop to DC to drive a meter. The response of the diode RF detector is logarithmic, thus the scale of the meter is very compressed on the high end. Keep this in mind when you buy the range determining elements for a Bird wattmeter (called "slugs" in the trade). Study the scale on the meter before deciding what the range should be of your elements. I've found 5 and 100 watt slugs to be the most useful. 

If you do decide to buy a professional wattmeter I strongly recommend buying one with "N" series connectors on it. As you progress to the higher bands, you will have to go to N connectors. N to UHF adaptors won't cut it as the RF still has to go through the mismatch of the UHF connector. Its much better to use UHF adaptors on a meter with N connectors if you have to. 

The Bird 43 uses "plug in" connectors (actually four screws hold them on, just the center RF pin plugs in) so you can easily change between connector series. I have both UHF and N connectors for my meter. Rarely do I use the UHF connectors, its easier to simply use a set of UHF to N adaptors. 

SWR is measured indirectly by the Bird 43. The element can be rotated in the line section to make comparative readings of forward and reverse power. A chart supplied with the meter allows you to translate wattage into SWR. One advantage of this system is that the design of the Bird element allows it to make accurate readings anywhere in a feedline. 

Next Month: Using your Dummy Load and SWR Meter. 

Comments or submissions for publication can be sent to muldoon@seits.org.