[Antennas] Re: Hairpin monopoles

Wed, 7 May 2003 14:20:51 -0500

As you express it, "radiation resistance" can be considered an
"explanation."
Since all the power delivered to an antenna must be either radiated or
dissipated in heat (guess that is radiation also!), the ohmic resistance
loss in the antenna structure is used to explain/determine the power not
radiated but delivered.
The remainder of the delivered power that is actually radiated is modeled as
the power that a fictitious resistance - termed radiation resistance - would
radiate if fed the same amount of power as that which is radiated.
It is not a physical resistance, thus it cannot be identified, isolated, or
directly measured, in a sense. But, it is a useful construct for accounting
for radiated power.
A similar situation is found in the notion of characteristic impedance of a
transmission line, expressed in the form R+jX, where R is the resistive
component and X is the reactive component. Lines operated at HF generally
have reactance so small as to be negligible so we use Zo as a purely
resistive value. At audio frequencies, however, X can take on quite large
values and all the usual transmission line equations must deal with the
complex nature of Zo.
So, this is another instance wherein we use the notion of a resistance to
define a property although it is not a physical resistor that can be
identified and isolated. Neither does it dissipate any power! The resistive
component of Zo is separate and apart from the ohmic and dielectric losses
of the line which account for its dissipative losses.
Although the matter is less clear in the r-f amplifier area, arguments have
been presented that the "internal impedance" of such an amplifier is
essentially resistive and shares the property of being non-dissipative.
However artificial the radiation resistance construct may appear, it is of
critical importance in understanding and computing the efficiency of an
antenna.
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>
----- Original Message -----
From: "Chris BONDE" <[email protected]>
To: "Roy Koeppe" <[email protected]>
Cc: "Antennas reflector" <[email protected]>
Sent: Wednesday, May 07, 2003 1:27 PM
Subject: Re: [Antennas] Re: Hairpin monopoles
> At 08:02 AM 2003年05月07日 -0500, Roy Koeppe wrote:
> >Oh, oh, regarding,
> >
> >"It's not a matter of what I think - it's a simple fact. The resistive
> >component of the antenna's input impedance (the radiation resistance) is
> >generally higher than a conventional, non-folded radiator."
> >
> >. Actually, most folded monopoles have a slightly LOWER
> >radiation resistance than their single conductor counterparts. This
> >obeys the law about "the thinner the conductor, the higher the radiation
> >resistance, for the same self-resonant element." Fat elements are
> >physically shorter than thinner ones for the same resonant frequency,
> >hence have a lower radiation resistance.
> >
> >73, Roy K6XK/0
>> This seems to me to be a little out. To me the size of the conductor is
> not what changes the impedance, it is the number thereof, ie the
> transformer effect. The radiation resistance is just a method of
> explaination. Has anyone really measured radiation resistance? From what
> I understand it is used to explain what happens or whatever the missing
> part is in the equation.
>> Now I have not studied antennas systems that much lately, but when I was,
> radiation resistance seemed to offer the most resistance in understanding
(
> after Poynting vector)
>> Chris opr VE7HCB