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Old January 19th 08, 04:50 PM posted to rec.radio.amateur.antenna
Richard Harrison Richard Harrison is offline
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First recorded activity by RadioBanter: Jul 2006
Posts: 588
Default folding a dipole in half

Roy Lewallen wrote:
"Thus we see that the (quarter wavelength) folded dipole exhibits the
qualities of a shunt-tuned circuit and has a terminal resistance value
Ra of high magnitude."

Yes, my copy of Bailey says the same. I got the 6000 ohms from page 509
in the catalog section where large diameter rods were used for the 200
MHz antenna.

Looking at Bailey`s page 508, Fig. 8-14, it seems a 1/2-wavelength
perimeter can be opened into a loop instead of a folded dipole and iits
radiation resistannce drops to about 5 ohms. This is a mixed blessing
because as Roy says, "There`s no free lunch". A loop configuration does
not lower the loss resistance in the conductor and this will extract a
larger percentage of the energy fed to the conductor with a lower
radiation resistance.

A closed loop has an inductive reactance at frequencies below first
resonance (when the wire perimeter is almost 1/2-wavelength). This
reactance can be compensated with a simple low loss series capacitor. It
is unfortunate to not have room for a straight radiator.

Best regards, Richard Harrison, KB5WZI