Tom Rauch wrote:
>>Radiation resistance increases along the length of an
>
> antenna. Higher
>
>>radiation resistance means less current for a given amount
>
> of power.
>
>
> Not so Gary.
>
> Radiation resistance associated with any particular antenna
> is a single value based solely on the radiation from the
> antenna system and the net or effective current causing that
> radiation. That's the basis of the IRE definition of
> radiation resistance.
>
> We shouldn't confuse a useful term like radiation resistance
> with a nearly useless parameter that actually is just the
> real part of the feed impedance as the feedpoint is moved
> around to different parts of the antenna. Doing that will
> just confuse us.
>
> Radiation is caused by charge acceleration, and that is why
> lower current areas of an antenna contribute less to
> radiation than high current areas. It all comes down to
> ampere-feet. The greater the ampere-foot product the more
> radiation we have from that area.
>
> 73 Tom
Hi Tom,
I agree with you on the definition of radiation resistance as being the
equivalent resistance that a given amount of current passed through it
causes an amount of power to be radiated. I squared R.
As you know radiation resistance is a fictitious term used to represent
another but very useful. (the power radiated) We can find the
radiation resistance at any point on the antenna that we may choose just
as well as finding it at the "normal" feed point.
Radiation resistance is a function just as loss resistance is a linear
function along the length of an antenna.
If we go by the accepted definition of radiation resistance then it can
be anything with the proper amount of squared current through it that
represents the amount of power that is actually radiated.
We know that current is lower as we approach the high impedance end of
the antenna and radiation resistance is higher.
It is the field around the antenna that radiates not the movement of
electrons through a particular part of the antenna. The field is created
outside the antenna conductor.
The ampere-foot product is useful for comparing one power level to
another to determine the amount of radiation but it is not the lone
determining factor in how much radiation there is from a given part of
an antenna.
An antenna can be viewed as a transmission line. A quarter wave antenna
has the same properties as a quarter wave transmission line. Just
because one end of the line may be a high impedance compared to the
other doesn't mean that the high impedance end does not see any power
transferred to it.
If power didn't get transferred to the high impedance point then a half
wave length vertical antenna would never work.
Just like the transmission line, we can replace parts of the antenna
with lumped components, coils and capacitors, and they function in a
similar manor.
73
Gary K4FMX
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