Hi-UHF on the CM 4228 - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 14 Old 08-22-2007, 10:18 PM - Thread Starter
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I'm still a bit puzzled by one aspect of how my CM 4228 receives signals. The 4228 does a gorgeous job with signals around channels 20-35. It does an awful job the closer you get to 50.

This applies to both digital and analog. I have several signals coming in from the same direction (Pittsburgh, PA) at 60mi.

KDKA-DT (UHF 25) is rock solid. Strong signal, very stable. I can pick it up across a wide range of angles. Even on a bad day we're talking about 45 degrees.

WPGH-DT (UHF 43) tends to come in well during the evenings. Higher channels, WTAE-DT (51) and WPXI-DT (48) depend heavily on the weather.

The same goes for analog channels. 22 is gorgeous, 53 is crap.

I've heard a few comments about modifying the CM 4228 to be better sensitive to the upper UHF channels. I'm curious how effective this has been, and how much it alters performance for the lower UHF channels.

Also, is it possible that some of this is just how the frequencies behave at this range? My understanding is that the lower the channel, the longer the range and the greater the ability to sneak into valleys and past trees and such.
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post #2 of 14 Old 08-23-2007, 01:35 AM
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Quote:
Originally Posted by MeowMeow View Post

Also, is it possible that some of this is just how the frequencies behave at this range? My understanding is that the lower the channel, the longer the range and the greater the ability to sneak into valleys and past trees and such.

Most likely this is the case. While the beamwidth will narrow slightly at the upper frequencies it will not be enough to prohibit good reception after proper aiming. The gain will actually increase slightly.

Several factors are at work. Most of all at 60 miles you are at the limit of the radio horizion and there will be very little propagation beyond line-of-sight at the upper frequencies. No antenna can mitigate that problem without a significant increase in antenna height over average terrain.

Trimming the length of the bowties can optimize the response of the antenna to the upper end of the band at the expense of the lower end. I don't believe this will help you at 60 miles.
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post #3 of 14 Old 08-23-2007, 03:33 AM
 
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Quote:
Originally Posted by MeowMeow View Post

I'm still a bit puzzled by one aspect of how my CM 4228 receives signals. The 4228 does a gorgeous job with signals around channels 20-35. It does an awful job the closer you get to 50.

Also, is it possible that some of this is just how the frequencies behave at this range? My understanding is that the lower the channel, the longer the range and the greater the ability to sneak into valleys and past trees and such.

The gain of the 4228 drops a few tenths of a db on channel 50.
http://www.hdtvprimer.com/ANTENNAS/comparing.html

Your second explanation is more likely. A pair of antennas will gain you about 2.5 db.
http://www.hdtvprimer.com/ANTENNAS/16bay.html
Depending on your terrain and obstructions you might get better results with more antenna height.
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post #4 of 14 Old 08-23-2007, 04:53 AM
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Quote:
Originally Posted by Tower Guy View Post

The gain of the 4228 drops a few tenths of a db on channel 50.
http://www.hdtvprimer.com/ANTENNAS/comparing.html

Maybe. Other published data suggests otherwise.

http://www.starkelectronic.com/cmg5.htm

Regardless, it's all derived from computer modeling and should be taken with a grain of salt. Gain is unlikely to be the issue.
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post #5 of 14 Old 08-23-2007, 07:12 AM - Thread Starter
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I was hoping the magical wishing method might help. Dammit.

Does anyone know how effective this method of trimming the bowties is?
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post #6 of 14 Old 08-23-2007, 07:54 AM
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It's very effective if it helps... sometimes it doesn't .
The thing to keep in mind is... if it makes matters worse, there's no going back.



If you can’t explain it simply, you don’t understand it well enough – Albert Einstein
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post #7 of 14 Old 08-23-2007, 08:57 AM
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have you checked on the transmitter power output for the stations in question ? It is possible they are not as powerful as the lower channel stations and nothing you do to the antenna (other than raise it's height) will cure that.

The actual published gain of the 4228 is higher at the higher UHF frequencies, so I would tend to think is might be a problem on the transmitted signal side of the equation.
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post #8 of 14 Old 08-23-2007, 09:19 AM - Thread Starter
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Almost everyone in western PA is at full power. Only two exceptions: PBS out of Pittsburgh is around 700 kw and CW is not up with a transmitter at all.
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post #9 of 14 Old 08-23-2007, 09:22 AM
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Quote:
Originally Posted by NightHawk View Post

Trimming the length of the bowties can optimize the response of the antenna to the upper end of the band at the expense of the lower end. I don't believe this will help you at 60 miles.

More specifically, it optimizes the antenna for frequencies that have been reassign for purposes other than TV. If you really need gain in the upper freqs, you should go with a yagi design ... of course, the low range will take a hit with the yagi.

The bottom line is that the 4228 *should* be nearly perfect wrt gain across the post DTV transition UHF range.
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post #10 of 14 Old 08-23-2007, 09:26 AM
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I do not generally suggest trimming the bowties on the cm4228. the trimming process falls back to the past when uhf was ch 14-83. the trimming was geared towards improving gain for reception of the repeater stations that typically were found at the upper reaches of uhf.

trimming the bow ties are too chancey, you may not see improvement as you expect at the higher channels, and the lower uhf channels may get worse.

you can take a chance and trim the bowties to the score lines and see what happens. if things do not workout as planned you can not fall back and make the bowties longer.

an alternative may be just replacing the cm4228 with a 91xg with the cm7777 preamp. if you need hi band vhf later you add a winegard ya1713 on the 5' mast and combine the vhf and uhf with the preamp...as I suggested several times in the past concerning your situation.

in the long run I have found that this combination (91xg and ya1713) is the better compromise.
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post #11 of 14 Old 08-23-2007, 09:32 AM
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Do you have an amplifier? The feedline loss is greater at upper frequencies.

Mike Glass
Indianapolis
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post #12 of 14 Old 08-23-2007, 09:32 AM - Thread Starter
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I hadn't thought about the old UHF range. So, trimming is pretty much useless unless I intend to listen to cell phone frequencies... Fair enough.
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post #13 of 14 Old 08-23-2007, 09:48 AM
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Quote:
Originally Posted by HDTVChallenged View Post

The bottom line is that the 4228 *should* be nearly perfect wrt gain across the post DTV transition UHF range.

agreed
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post #14 of 14 Old 08-23-2007, 10:23 AM - Thread Starter
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Quote:
Originally Posted by mglass1646 View Post

Do you have an amplifier? The feedline loss is greater at upper frequencies.

Mike Glass
Indianapolis

CM 7777, with rotator.
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