Bird “43” Thruline Watt Meter Plug-in Elements determine the power and Frequency
The meter is at the top and has 3 scales. they are not linear but are expanded on the left and compressed to the right. the scales are 0-30 watts, 0-50 Watts and 0-100 watts. The element determines the multiplier of the scale. The base unit I purchased from a re-seller at ARNSW trash and treasure sale at Dural probably about 2010 and it cost me $150. IT was surplus from the RAAF according to calibration stickers.
Bird “43” was a standard in the Radio transmitter testing but it had +/_ 5 % error stipulated in the manual. Bird made many variants of the 43 and other models. They also made other RF test equipment. But back to the 43, The first thing you see is the meter uncluttered and below is a socket to put an element in to determine the power and frequency. The element is often referred to as a slug. The slight in this meter is 25 Watts at frequencies of 400 to 1000 Mhz. It also has a large arrow across th e middle, this tells the direction of the RF being tested. There are 2 connectors one on each side into which you connector of choice is held in with 4 screws. I have UHF,N and BNC connectors for the 43.
If the transmitter under test is connected to the the left input and the Arrow is pointing to the right connector where a dummy load or antenna is connected the it will indicate the forward power. The slug can be turned through 180 degrees so the Arrow is pointing to the left it will indicate the the reflected power level. It does not directly read SWR but it has instructions in the manual how to convert the reading to SWR. Obviously you want a very low reflected power to operate on an antenna, The slug is retained by a spring loaded catch to stop it falling out.
The slugs should be treated with care as I dropped one and it did not work afterwards. I did find how to repair and calibrate the reading. It has a small potentiometer and the jolt had caused the connection to the arm of the pot was open circuit. It was simple to put a small screw driver and adjust it backward and forwards a couple of times and it has worked since.