Q Kaehu Shaprio, WH6WW, asks, “I have about 130 feet of 3/4-inch 75-Ω CATV Hardline that I’d like to use for 2 meters and 70 cm. I looked in The ARRL Antenna Book and found a description of a broadband transformer, but it’s only for 3 to 30 MHz. I also saw an article in the September 1998 QST on how to make a matching transformer, but it seems to only work on one frequency or band. Is there another matching transformer I could build so that I could achieve a 50-Ω match to my transceiver on both of these bands at the same time?”
A I do not recommend that you use 75-to-50 Ω transformers in this application. At 2 meters the loss in your CATV Hardline, if perfectly matched, would be 0.8 dB/100 feet, or a total of 1.02 dB. If you operate this line at a 1.5:1 SWR, the additional loss caused by the SWR would be 0.07 dB. It is very unlikely that you could obtain less than 0.07 dB of total loss between two matching transformers—one on each end. Instead of building transformers, why not simply use the Hardline as it is? The SWR on the line will be approximately 1.5:1 and the loss, even at 70 cm, will be negligible. Most likely, your transmitter will be perfectly happy to deliver full power into that load.
From January 1999
