[AMRadio] Antenna/Coax


Geoff/W5OMR ars.w5omr at gmail.com
Sun Dec 31 10:38:24 EST 2006


Jack Schmidling wrote:
> Ed Sieb wrote:
>> You're missimg nothing, Jack.
>>
>> There is no issue, really. Hooking up 50 ohm coax to a 75 ohm anteena 
>> only
>> provides an SWR  of 1.5:1, which is not excessive.
>
> From a student's point of view, most of the discussions on the subject 
> start out with titles such as the importance of matching antenna to 
> feed line.  A 50% error or 25% deviation does not seem trivial.
>
> But from a practical standpoint, as 75 ohm coax is readily available, 
> wouldn't it be better to use that?

Electronics is sometimes called "the exacting science of 
approximation".  Sometimes when you're building a circuit, your 
calculatoins require (arbitrarily) a 432,354 ohm resistor (for 
example).  Resistor makers don't -make- a 432,354 ohm resistor.  You 
could get by with a 427,000 ohm resistor, or even a 470k ohm resistor, 
and makde adjustments as necessary if it's *that* critical in your circuit.

The Exacting Science of Approximation.

We would -all- love to have things work in the REAL world, as well as 
they do, theoretically, or on paper.  Practical experience says that 
nothing is ever perfect, so we get as close as we can, and then don't 
worry about it.

If you're open-minded enough to listen to a suggestion, in order to feed 
an antenna, consider using 450 ohm ladder line, or some open-wire 
feedline, instead.  The Velocity factor of wire is immensely faster than 
coax, and therefore not nearly as lossy.

--
73 = Best Regard/*S*/
-Geoff/W5OMR
Happy New Year, ya'll!






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