help -- ota hd issues

[erh1117]

This may or may not belong in this forum, so I've cross posted...

I'm getting my Denver local digital channels using via the DISH 921 and a rooftop antenna. I get all but my local ABC affiliate fine off a rooftop Yaggi. I got greedy though and wanted ABC, so I added a DB4 aimed at ABC and combined the signals with a combiner. Now I have severe but sporadic multipath (I guess).

The symptom is that I'm having issues with the signal strength jumping up and down between 70 and 0 and then no picture. This happens on a variety of channels broadcasting from a variety of locations at different times of day and different dates. Sometimes all channels come in fine. Other times, I will lose Channel 2, 9, 7, or 31 (their digital channels) in this same manner.

I've had DISH replace the 921. Same result.

My antenna is a rooftop yaggi type with a combiner to a DB4 so I can get channel 7. No preamp or amp. Don't know the gain.

I'm told this could be "multipathing," but what baffles me is that things work well for a few days, then crap out again. Random times. Random channels. Random weather conditions. Also, I've tried an inline attenuator to dumb down signal strength in case one OTA was overwhelming another.

Some people have suggested to me that it might be the fact that I'm connecting two antennas and that it is the combiner. Is that correct? They suggest using a separate cable run off the Db4 for one channel (my tough to reach ABC affiliate) and the Yaggi for all other channels, but an A-B switch , not the combiner.

An installer has suggested removing the combiner and Diplex the signal from the two antennas on my roof and then separate them again at the TV putting an A-B switch between them to toggle between the DB4 (channel 7) and the Yaggi (all other channels). Yet, someone has also opined that this still would be mixing signals at the same frequency - there would be no way to reseparate them.

I didn't install it or the DISH and don't plan to get on the roof. Unfortunately, the guy who installed the DISH system knows less about antennas than my eight year old niece. And, who knows if the problem is reproducible when he’s here anyhow.

So, does anyone out there: (1) have any idea about solving the problem or (2) any good antenna experts here in the Denver area?

Or am I wasting my time because it is really a 921 issue due to the known crappy stability of the 921's terrestrial tuner (see http://www.satelliteguys.us/showthread.php?t=3951). If so, will L213 fix it?


Please respond to my email address erh1117@eazy.net

 

[Tony N]

The only sure soloution would be to get another cable run so you can experiment with the A-B switch box. If things then are fine all the time on each of the stations, now you know the receiver is ok. With both the cables handy at your receiver you could conviently experiment with combiners, filters etc to try to eliminate the inconvience of the switch.

Tony N.

 

[erh1117]

Thanks Tony. That is a great idea.

 

[mchomicz]

I live in Charlotte, NC and have an identical problem with my setup. I have a completely different HD receiver (an ATI HDTV Wonder PCI card in my Windows Media Center PC) and I have tried using a signal combiner as well as connecting two antennas with flat cable and connecting thus combines signal to a matching transformer. No matter what I try or what type of antenna I use - I’m getting the same results: all channels come in perfectly fine for a day or two. Then things suddenly break (same symptoms as you – 80% for a few min; going down to 40% then down to 0% then back to 85% again).

So I guess it’s a phenomenon unrelated to your specific receiver. In my particular location one transmitter is much closer than the other (5 miles versus 25 miles - or so). To avoid multi-path from the antenna aimed at closer transformer I tried "shielding" it with some aluminum foil placed behind it and in the path of signal coming from the more distant transformer - to no avail.

I have just about given up. I really don't want to use an A/B switch because I wan to be able to automatically record from ANY channel without having to remember to set the switch properly. There is a guy on ebay selling "information" stating that this very setup is SUPPOUSED to work (combiner putting together signal from two or more transmitters). So I guess it works in some situations.

Here is my question: in your setup – is one of the transmitters much closer than the other one(s). I'm pretty sure you are familiar with www.antennaweb.org/aw/Address.aspx to find your distances from transmitters - but I'm including the link just in case.

I'm subscribing to this thread. If anybody with the same problem comes across as solution - I'm all ears!

Thanks,
Michael

 

[erh1117]

Update on the OTA digital local reception issues I've had...

Recall that I have suffered form a loss of all signal on different channels at different times and experienced the oscillating pattern of 0--70--0--70 which we came to conclude was multipathing due largely to the combining of two antennae into one cable.

Sunday, I (well actually my installer) uncombined the signals from my two antennae by removing the combiner. Then I ran cable from the 2nd antenna to the house and connected both antennae to the 921 via a Radio Shack remote controlled A/B switch located on top of the TV. So, now we have two antennae feeding the local digital OTA's off separate cables that can be toggled via the remote.

And the result......?

Well, so far so good. Two days into the A/b experiment, I am optimistic. Immediately before the uncombining surgery was performed, this patient was suffering from no signal on channels PBS 6 or NBC 9 (I mean the digital equivalent of these). Last night, I had no signal on NBC 9 in the evening and none on ABC 7 in the morning.

And, pre-op the signal strength on the channels I was getting looked like this:

WB 2 72
CBS 4 95
ABC 7 80
Fox 31 87

After the surgery, I am pleased to report that immediately post-op I received all of my OTA digital locals (all but Fox 31 on the DB4 antenna and all but ABC 7 on the Yaggi). And the signal strengths increased dramatically for each:

WB 2 105
CBS 4 103
PBS 6 111
ABC 7 97
NBC 9 123
Fox 31 95

As for Michael's question ("is one of the transmitters much closer than the other one?"), yes -- several transmitters ere in Denver are downtown, several are 20+ miles away, one is at the station itself near downtown, and all are low-powered.

Yeah, I'd rather have all channels on one cable and not have the A/B because recording could be affected if I forget and also because I can't watch ABC HD and record Fox HD (and vice-versa), but it beats the multipath issue.