2/14/09

The Channel Master 4221 4-Bay

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.

Antenna builders:

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:

Length of dipole element wire = 7.8” x 2

Dipole vertical separation = 8.0”

Screen-dipole separation = 4.5”

Feed bus wire separation = 1.2”

Feed 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 builders.

This page is part of “An HDTV Primer”, which starts at www.hdtvprimer.com