CM-4228 UHF Antenna Feed Modification




This page documents my experiments with a Channel Master 4228 UHF antenna.

Conventional wisdom around the net is that the feed that connects the two halves of the antenna is suboptimal, and can be improved. So I decided to investigate this possibility.

Each image is a link to a larger image, and some of them are linked to the original 8 Mpixel images.


Experimental Setup


Test Setup
Original Feed



 

The antenna is in the middle of my hardware lab, North of Austin, TX. It is pointed South at the main complex of TV transmission antennas about 12 miles away. The CM-4228 is mounted on a custom built stand made of 1 1/4" PVC pipe. Please excuse the clutter!

The original feed configuration uses the stock 300 to 75 Ohm balun. For test purposes, I have added a 75 to 50 Ohm impedance matching transformer in series to match the input of the spectrum analyzer.

The testing was performed using an HP 8558B spectrum analyzer in an HP 182C mainframe. The analyzer was set to 20 MHz/div horizontal, with 300 KHz resolution, and 10 dB/div. The reference level is -40 dB.
 

Test Setup
New Feed



 

The original feed & crossover network has been removed, and the new feed installed.

Each half of the CM 4228 has it's own 300 to 75 Ohm balun and a short RG59 feeder leading down the front support. Because the phase relationship between the input twin lead and the output F connector is unknown, a second test was run with one balun flipped.

The RG59 leaders meet at a wideband RF signal splitter used as a combiner.


Test results


 

 

Original

 

New Feed

 

New Feed / Bad Phasing


470 MHz -
670 MHz

 
 
 

610 MHz -
810 MHz

 
 
 


Update

  I completed another round of tests, this time I upgraded both the instrumentation and the test article. The antenna balun RG59 feeds were replaced with shorter RG6 quad shielded cable. On the test side, I replaced the 182C mainframe with an 853A digital spectrum analyzer mainframe, and the HP 10519A test cable was replaced with a 6 foot length of RG214 with N connectors. In these tests, I also looked at the VHF high band, as well as the FM band (there are no local VHF low band transmitters). For the VHF high band, the sweep was set to 10 MHz/div, while the FM band was 2 MHz/div.  


New test results


 

 

Original

 

New Feed

 

Both


VHF High
170 MHz -
270 MHz

 
 
 

470 MHz -
670 MHz

 
 
 

610 MHz -
810 MHz

 
 
 

FM
88 MHz -
108 MHz

 
 
 


The FM Band

  Due to the poor performance in the 88 MHz - 108 MHz band, I tested the received power of just the center feed section, disconnected from the rest of the antenna, and of a common folded dipole made from twin line.  


Original feed only

 

FM folded dipole


 


Conclusion


It appears that the performance of the original antenna feed works pretty well as designed. The variance observed in the received power spectrum with the new feed does not appear significant. The FM performance is abysmal, especially for the modified feed design. It appears the bulk of the FM signal is picked up by the stock feed itself. VHF high band performance is hit-or-miss. I'm open to suggestions for experiment redesign and retesting.