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Old 06-09-2013, 12:39 AM   #18  
Pete Higgins
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Join Date: Aug 2012
Location: California, 58 miles @112 degrees from Mt. Wilson
Posts: 265
Default Adding to the confusion?

IDRick,

I should have made myself clearer! On day one (6 June) I was focused on comparing the HDB8X to the CM-4228. Both 8-Bay antennas were wired as described and unamplified. The HDB8X didn’t show any High VHF response. Meteorological conditions were obviously good so the CM-4228 High VHF performance shined. Don’t forget, it was all the stations I occasionally got with that thing last summer that started this whole OTA mess.

The 91XG has never shown any High VHF response either. As I said in the paragraph above the first chart, “I also included measurements from my <1 year old Antennas Direct 91XG/Antennacraft Y10-7-13”. The High VHF SNR numbers in that column are from the Antennacraft Y10-7-13/PCT MA2-M feeding the VHF port on the UVSJ. Those two antennas along with my 10 element FM antenna are on the 40’ tower. It is a completely separate antenna system that doesn’t share anything with the HDB8X/CM-4228 including the cables that run through the walls.

In the 2nd day (7 June) table, the first column was copied over from the previous day’s testing. The single panel test with the combiner bypassed used the AP-2870 amp with the VHF input connected to an Antennas Direct FM band filter with its input open. While the VHF channels were checked, even with the amp, they didn’t show any signal.

For the next test I connected both panels through the combiner and that’s when I lost most of my LA channels. I went back up and on the roof and connected the Winegard YA-1713 to the FM trap and reran the complete test, turning the YA-1713 to record San Diego & LA. Its on the same rotor as the CM-4228. That’s where the High VHF numbers came from in that column. None of the UHF readings changed by even 1 dB on that second run.

The final column was just to compare the difference between using the AP-2870 and the PCT MA2-M. Since it only has one input, I just hooked-up the HDB8X.

wizwor,

I’ve had the CM-4228 for over 40 years. It has been stored outside on my patio roof since 1980 when I bought this house. Reception here has been so poor that everyone is on cable or now Satellite.

Last fall, I came across this forum, was intrigued, and decided to see what I could get with what I had. It wasn’t real reliable but I was fascinated that I got anything at all.

I originally had the CM-4228 on the tower at ~32’ AGL.

Going by Tigerbangs prescription, I eventually ordered a 91XG & Y10-7-13, added another 8’ to the tower and started to play.

Because of all the first-rate 91XG reports and the fact that my CM-4228 was 40 years old I was expecting a big improvement. VHF actually did improve quite a bit but UHF didn’t. Fortunately, the 91XG was mounted at the top of the mast.

While pretty widely separated from each other, all my San Diego channels and all my LA channels are within a couple of degrees of each other so once the antenna was aimed at a channel in one city it was good for the rest from that city.

I went through the 91XG channels and jotted down the SNR for each.

The tower raises & lowers with an electric winch so it only takes me 40-45 minutes to pull 2 guy wires, lower the tower, swap the antennas and raise the tower back up and start taking a new set of readings.

Instead of an F-71, I used a short piece of coax to connect the CM-4228 to the 91XG’s amplifier. Everything else in the system stayed the same. I then went through the same channels with the CM-4228. To my surprise, most of the SNR’s were within 1 dB and none were over 2.

The tenths digit typically changes by a few tenths every second so I have to interpolate an “eyeball average”. Consequently, I don’t think we should ascribe much significance to a 1 dB difference.

Bottom line I think I took around 30 readings and maybe 13 were higher for the CM-4228 17 were higher for the 91XG, but they only differed by a small amount (probably due as much to interpolation error as even a moderate performance difference). Had I left the CM-4228 in place I doubt I would have noticed a real world difference.

There was one difference between the two antennas that is worth mentioning. The CM-4228 has never received channel 43 (CBS 2.1 TV Fool -112.40) reliably, the 91XG usually does. The 91XG has never received channel 36 (NBC 4.1 TV Fool -107.40) reliably and the CM-4228 usually does. This anomaly followed both antennas when they were swapped between the tower and the push-up mast –go figure.
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