Yes, All My Coax Cables Are 75 Ohms (and everything works OK!)
C02KK exposes some myths about using 75-ohm feedline with 50-ohm rigs and shows us how to build a "cotangent transformer," a simple matching device to make it all work.
By Arnie Coro, C02KK"
: ow many times have you heard line, maybe your mind went to that tail If the 75-ohm cable has the same kind '-' that 50-ohm cable is a must? end of low-loss, 75-ohmCATV cable that of dielectric and the shield or braid cov- I 1 Equipment instruction manuals, the technician at the local cable compa- erage is equivalent to that of the 50-ohm advice from the experts, and antenna ny offered just for the asking not too long coax it replaces, signals from 100 kHz to handbooks all seem to agree that present- ago. Or perhaps you remembered what 150 MHz will show practically no mea- day amateur radio technology is all set for you paid for the latest run of 50-ohm coax surable difference. 50-ohm impedance devices only. for your new 2-meter antenna, and you I've conducted this experiment time Of course, you probably haven't had hurriedly compared it to what an equiva- and again to show my friends what actu- the opportunity to actually measure your lent run of 75-ohm TV-type cable would ally happens. My test bed is a typical transceiver's real output impedance, with have cost. And maybe, you're now think- receiving setup for SWLing (shortwave really professional instruments. But if ing how many newcomers to the hobby listening), using a broadband Tilted Ter- you ever do, be ready for quite surprising could get started at substantial savings by minated Folded Dipole (TTFD) antenna numbers coming out of those dials! I'll using lower cost 75-ohm TV cable for covering 6 to 30 MHz. The antenna is give you a hint-the reading probably their rigs. located about 100 feet (approximately 30 won't be 50 ohms. ~eeithinking about possible sources meters) from the receiver. of 75-ohm coaxial cable, and I'll happily Changing the 50-ohm coaxial cable for show you how to use it in many applica- an equal length of 75-ohm cable, of the tions, without the slightest chance of same average quality, produces no mea- But let's go back to the myth: "50-ohm damaging your transmitting equipment. surable difference on signals received. cable or nothing!" Now, reading that last For those really tough-to-convince !\" DDiffg~gn~gc.- "&~e guys, Iuse a very low power signal source *Arnie Coro, C02KK, is a professor at located about 10 wavelengths away from the University of Havana and broad- Thorough testing of 75-ohm coax used the antenna. The solid-state, crystal-con- casts two weekly radio shows on Radio in place of 50-ohm cables for receiving trolled 10-meter source is installed at my Havana's shortwave service-"DXers applications shows practically no detect- closest ham neighbor's shack (about a Unlimited," devoted to SWLs and other able difference, even at frequencies as city block away) so I can run the tests radio hobbyists; and "Breakthrough, " a high as 150 MHz. Try it yourself. Mea- around 28.5 MHz with the 10-meter band science and technology program. A sure a length of 75-ohm cable to replace closed and have a very stable signal at the licensed ham since age 17, Arnie 's main an equal length of 50-ohm coax, use the receiving antenna. Again, changing from interests are VHF/UHF, HF propaga- same type of connectors, arrange your the 50-ohm RG-213 to a 75-ohm RG- tion research and promoting amateur setup so that you may switch cables eas- 11/U makes no difference on any of the radio among young people. ily, and watch your results. receivers I've tried.
75~2coax to antenna
Figure I. The cotangent transformer. Two ,081-wavelengthpieces of coax, one 50-ohm and the other 75-ohm, connected as shown, allow you to use lower-cost 75-ohmfeedline with no loss in signal strength.
-.------. - , . ,i v-----r- ----Z------liil ------_I___- *I_II _ * - -ul - _ --_ _ ___ ------14 CQ VHF * July 1997 Ham Radio Above 50 MHz