[Boatanchors] Good (And Bad) Lengths For End-Fed And Dipole Antennas

[Paul Christensen]

>Back in 2013, Jack, VE3EED (SK) published a scientific analysis of good and
bad lengths of antennas regarding ease of matching.  Basically, any length
of wire that was a multiple of 1/2 wavelength would present an extremely
high impedance (high voltage point) that will be very hard to match to any
transmitter OR trans-match."

The premise is that a half-wave of transmission line or 1/2 wave multiple
thereof repeats the load Z at the input to the transmission line.  So, in
the case of a full wave center-fed dipole or end-fed half-wave wire, the
load R and X values are high, complex, and presumably "difficult to match"
at the end of a line.  

I'm skeptical about generalizations like this.  While it may be more
difficult to match a transmitter at these lengths with a line of one
characteristic Z, it may be easy with another.  When using a 600-ohm Zo line
with voltage-fed systems, it's generally easy to match the line end to the
transmitter for all-band operation and have minimal loss (like Rob's EFJ
Matchbox). 

Rather than rely on generalizations, I force myself to model the antenna
feed-point Z on multiple bands using 4Nec2 software.  Then, using
transmission line tools in 4Nec2, SimSmith, ARRL's TLW, or TLDetails, we can
vary both the transmission line type and modeled length to get an idea as to
system loss and the complex Z presented at the end of a line.

Paul, W9AC