On 9/17/2012 10:33 AM, Fred Townsend wrote: > Personally I don't want a 75 ohm cable in the shack where I might use it on > my Ant 3 (K3) connection where it will make a difference. Each to his own.
There are times when 75 ohm cable is preferred, regardless of the rated load impedance of the rig or power amp. A high dipole (as a fraction of a wavelength) is a closer match to 75 ohm cable than to 50 ohm cable. Excess loss in the coax due to mismatch is determined by the match between the antenna and the line, NOT between the rig and the coax, although some matching may be required at the rig to keep the output stage happy and transfer full power to the load. In addition, because the current in 75 ohm coax is one third lower for the same transmit power as in 50 ohm coax, 75 ohm coax of equivalent size and construction typically has a bit less loss than 50 ohm coax. The 80/75M band has a bandwidth of nearly 10%, so 75 ohm coax provides lower SWR on a high dipole over a greater bandwidth. Yes, it's wise to be aware of cable impedance so that we're not generating mismatches by inserting cable of the wrong impedance, but don't throw the baby out with the bathwater. :) I use mostly 50 ohm coax, but have at least 300 ft of Belden 8213 feeding my two high 80/40 dipoles, and several resonant lengths of CATV hard line feeding my 20M and 15M yagis. There are also some really neat broadband matching tricks that one can play with resonant lengths of 75 ohm cable. I'll be talking about that at Pacificon Antenna Forum next month. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
