Hi Peter, If you make a dipole (not folded) antenna for any frequency it will have a characteristic impedance of 72 Ohms in free space. It will also have, essentially, the same impedance until you get real close to the ground. A properly made electrical 1/4 wavelength vertical antenna will have a feed point impedance of 35 Ohms.
In the old tube equipment, the output stages were designed with variable components such that the adjustment range could compensate for a wide range of impedance's, both above and below the above values. This was easy to do because the tube plate impedance's were in the order of 2000 to 5000 Ohms. The transformation was easily handled by variable components. In the solid state world things are a bit more complicated. First the solid state devices were typically very low impedance. The low impedance made it possible to design very wide band circuits without the need to adjust anything when changing frequencies, unlike the tube counterparts. Unfortunately the wide band design requires selecting a fixed output impedance that cannot easily, if at all, be made adjustable. In an effort to standardize, the industry selected the mid point between the 35 Ohms and the 72 Ohms, that being 50 Ohms. This forced the antenna manufacturers to design their antennas for 50 Ohms or provide a matching network. The television world originally used 300 Ohms at the antenna, along with 300 Ohm "Twin Lead" and the 300 Ohm input to the television itself. The reason for the 300 Ohms was due to the use of a folded dipole which has a characteristic impedance of 300 Ohms. The folded dipole design could be tweaked to provide a wide frequency range needed to cover all the TV frequencies, especially with the UHF channels. The switch to coax for TV use came about in an effort to prevent or greatly reduce ghosting problems and for cable systems as a reliable means of transporting the signals to many locations. "Twin Lead" cannot tolerate being near metal objects and is unable to be buried. "Coax" contains the signal entirely within its own shielded structure and therefore can be buried and laid next to other metal objects without degrading the signal quality. The reason 75 Ohms was selected for the TV world was because a simple, easily constructed, 4:1 balun (transformer) would transform 300 Ohms to 75 Ohms. Trying to go from 300 Ohms to 50 Ohms would require a 6:1 ratio with increased I/R losses and greater difficulty in obtaining wide band operation in the early days of ferrite mixes. I am sure there were other considerations in the process, but at the moment I cannot think of any. At any rate, the above is the primary reason why things went the way they did. I see that a range of comments have been already made, but I think they are off base. The natural constants of the universe dictate the natural phenomena of antenna systems and that is what dictated the course of history. I hope this helps explain the history a bit. Bill....WB6BNQ Peter Vince wrote: > I came across some telecom equipment the other day which had > reference outputs marked as 75 ohms. I work in television, not > telecoms, and we use 75 ohm connections for video, but with most RF > stuff being (I believe) 50 ohms, and certainly all the HP and other > counters seem to have 50 ohm inputs, I rather assumed telecoms used > 50 ohms - obviously not! Can anyone tell me how and why the 50/75 > ohm distinction came about? Was it perhaps a VHS/Betamax type issue > with different manufacturers going their own ways, and then with so > much equipment out in the field, neither side was willing to change? > > Thanks, > > Peter (London) > > _______________________________________________ > time-nuts mailing list > [email protected] > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
