[Elecraft] Random Wire???

[lhlousek]

>>Half wave wires are theoretical infinite impedances on their end, and are
the wire that is hard for some transmatches to tune.<<

This is only true if the theory neglects losses in the antenna.  Such a theory would also predict that a dipole has zero feedpoint impedance.  A resonant half-wave wire will have infinite impendence when end-fed, or zero impedance when center-fed (dipole), only if it has infinite Q.   Q is limited by losses in the antenna and radiation resistance is the dominant loss for simple wire antennas (although there can be other significant losses such as wire resistance and mutual impedance).  It produces a feedpoint impedance for resonant half-wave wires of around 70 ohms when center-fed  and limits it around 5000 ohms when end-fed.
 
Q is equal to the capacitive or inductive reactance in a resonant circuit  divided by the effective resistance. Because fatter wires have higher capacitance and lower inductance (both lower reactance), a fatter antenna will have a lower Q and hence broader tuning .  The lower Q reduces the feedpoint impedance of end-fed half-wave wires, so a fatter end-fed half-wave wire will have lower feedpoint impedance than a skinny one.  Figures 2 through 5 in chapter 2 of the ARRL Antenna Book depict this.

There is nothing too difficult about matching a resonant 1/2 wave end-fed antenna with 5000 ohm impedance. It's often done with a simple matching network at the feedpoint.  A double Zepp consists of back-to-back 1/2 wires usually fed with ladder line, the matching done by a transmatch, and maybe a 4:1 balun, in the shack.

Lou W7DZN






--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---