No ... it is *not* the radiation resistance. It is the feedpoint
resistance. They are not the same thing and nowhere in the section of
the ARRL Handbook that discusses folded dipoles does it call that 300
ohm feedpoint impedance the radiation resistance. Go read the section
on radiation resistance (section 3-2 or thereabouts) and you will note
that discussion refers only to a horizontal half wave antenna being
roughly 70 ohms and does not distinguish between whether it is a single
wire or a folded one.
Let's try another way of looking at it. Let's say you have a single
half wave wire and you feed it in the middle. The radiation resistance
is 70-some ohms and the feedpoint resistance is equal to the radiation
resistance plus any ohmic losses. If you then decide to feed the thing
several feet off center the feedpoint resistance changes but you haven't
changed the radiation resistance at all ... it's still just a half wave
dipole.
The same thing happens if you make the dipole a folded dipole. It's
still just a horizontal half wave antenna, albeit with a built in
transmission line that multiplies the original feedpoint resistance by a
factor of four. It is fundamentally no different than any other
impedance transformation scheme in relationship to the radiation resistance.
Dave AB7E
On 10/1/2013 6:38 PM, dave wrote:
I don't understand the disagreement over the radiation resistance of a
folded dipole. ARRL Antenna Book, 19th edition, page 6-1, plainly
states that a folded dipole will have an impedance of approx 300 ohms.
This has been widely known for decades. This is for a folded dipole up
in the air, ran horizontally. This *is* the radiation resistance. It
is approx 4x the impedance (radiation resistance) of a regular,
unfolded, dipole.
73 de dave
ab9ca/4
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