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#11
December 31st 08, 01:05 PM
posted to rec.radio.shortwave
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The Characteristically 50 Ohm Impedance Coax Cable is 'only'50 Ohms Nominal when . . .
RHF wrote:
On Dec 29, 12:37 pm, Telamon wrote: In article , RHF wrote: On Dec 28, 8:36 pm, Telamon wrote: In article , John Smith wrote: SNIP I never even commented on where the placement of the matchbox would be, and, as everyone knows, anywhere along the line you can place it. The best place would be between the coax (feedline) and the antenna- Right. That's because you are to stupid to understand a concept until someone rubes your nose in it. This would not even occur to you until someone else brought it up. -that is, taking for granted that the match from your rig to the feedline is perfect. SNIP You are really worried about the match of 50 ohm coax to your radios 50 ohm output? Now that's funny. IIRC - The Characteristically 50 Ohm Impedance Coax Cable is 'only' 50 Ohms nominal . . . Until you attach something to it. SNIP Nope. The cable itself has a characteristic impedance of some design value. The spacing and size of the conductors along with the dielectric constant of the insulator between them dictates the impedance of the coax. You are confusing the characteristic impedance of the coax with its ability to be an effective transmission line. The coax only behaves as an effective transmission line when both ends of it are terminated at its characteristic impedance. -- Telamon Ventura, California Telamon, OK -restatement- The "Measured" {by You} 50 Ohm Impedance Coax Cable is 'only' 50 Ohms nominal* * Until you attach something to it. -IF- You then attach a Transmitters Output that is a Nominal 50 Ohms to one end of the Coax Cable and then the 'other' end will still "Measure*" about 50 Ohms. * This is what the Antenna will see. However -if- You attach an Unknown "Z" Antenna and Ground to one end of the Coax Cable; then the 'other' end may "Measure*" near or far from 50 Ohms. * This is what the Transmitter will see. That depends on the length of the transmission line. |
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