The Channel Master CM-4228HD Nist Harness
Ken Nist, KQ6QV, has published recommendations for redesigning the CM4228HD harness on his "HDTV Primer" website that have been widely discussed in a number of DTV forums.  The screenshots here provide a comparison of a stock CM4228HD to a CM4228HD fitted with a Nist Harness. The high level details of his recommendations are described here.

[Note: the notch at about 480MHz is the result of an imperfection in the Spectrum Analyzer's Tracking Generator and can be ignored.]

Wideband UHF Results

Bottom line: The emperical results substantially confrim Mr. Nist's assertions that the stock harness leaves some money on the table. The above capture is intentionaly wider than the 470-700MHz UHF to illustrate where the main effects of the Nist design occur; mainly at the high end.

UHF Band Results

This capture is taken for with the span reduced to just outside the standard (North America) UHF band (470-700MHz), and with the vertical resolution set to 2 dB per division.  The 300MHz span results in 30MHz per major horizontal division, and we see the positive effects becoming evident at about 540 MHz, in the neighborhood of UHF Channels 25, 26, with about a 1 dB improvement.  This increases to an approximately 5dB improvement at the highest UHF Channel (51) just below 700MHz.

Hi-VHF Results

Alas, the effects on the Hi-VHF band response were not positive across the band.  Althrough there is a modest 1dB improvement on the higher channels 11-13, there is a roughly 5dB decrease in response across the lower channels 7-9.

One speculation would be that the stock harness itself does apparently play a significant active role at the Hi-VHF frequencies, but the improved balance and impedance matching of the Nist Harness diminishes this role.  Another consideration is that the construction of this implementation moves the horizontal span of the harness several inches closer to the plane of the reflector.  The author's perspective is that there may be other ways to enhance the Hi-VHF response in a more predictable and controlled manner; experiments along this line wait for another day...

As Mr. Nist has claimed, the evidence is that the Channel Master CM-4228 could indeed benefit benefit from some additional harness engineering, and Mr. Nist has does done a substantial piece of that work for them!  

Construction Details
Photos and drawings related to the implemenation are here.