[TowerTalk] Mosley Antenna Question

[Joe Subich, W4TV]

On 2015-02-26 2:29 PM, Kelly Taylor wrote:
> It's also been fairly well established down through the ages that an
> optimized 2 el. monoband yagi equals or betters a three-element
> tribander, owing to the compromised element spacing and trap losses
> of the tribander.

Close but a bit wide ... while trap losses contribute to the lower gain
of the three element tribander the big problem is the need to tune the
antenna elements to provide a 50 ohm feedpoint or a feedpoint impedance
that can be matched by a single fixed value "hairpin".

The antenna design software will show that one can obtain the same gain
for a specific boom length no matter where (within reason) the driven
element is placed in a three element antenna.  However, if the director
and reflector are tuned for maximum gain, the feed point impedance will
vary widely based on the boom length and distance from reflector to DE
in wavelength.  It's that "detuning" that is responsible for a large
part of the tribander under performance.

73,

   ... Joe, W4TV