The Channel Master 4221
The CM 4221
is also known as the 3021.
This is a top
performing medium range antenna: Lots of
gain but not very directional. Typically
it is used in the 20-35 mile range. It
is good at rejecting multi-path from the rear.
But multi-path usually doesn’t come from the rear, so this is not
usually a good antenna for multi-path.
If you want to avoid the hassle of a rotor, consider this antenna.
A new version
of this antenna is being introduced and should be available late in 2008. The new version looks lighter.
I am amazed at how many people want to
build this antenna. Pictures of several
handmade versions can be found on the web.
In the process of simulating it I discovered it is mathematically a
rather complicated antenna. Adjusting it
experimentally is not practical. For
example if you adjust the screen-dipole spacing, it appears that the gain
changes a lot, but in fact it is mainly the terminal impedance that is
changing. It might be practical to
adjust it for your weakest station. But
unless you have modeling software and a good model, the best overall strategy
for you is to copy a known good design.
The 4221 dimensions are:
of dipole element wire = 7.8” x 2
vertical separation = 8.0”
separation = 4.5”
bus wire separation = 1.2”
bus wire diameter = 0.100”
The Feed bus is a transmission line
and its separation/diameter ratio
determines its characteristic impedance.
The ARRL Antenna Book is an excellent reference for all antenna
This page is part of “An HDTV Primer”, which
starts at www.hdtvprimer.com