Before you go looking for the 3rd harmonic, go look for a broken wire. Or more likely a bad coax shield connection. A corroded or broken foil shield connection or an improperly installed F fitting can affect lower frequency channels but sometimes not the higher ones.
John Willkie wrote:
Barry;To rule out intermodulation/image issues, you will need to do the appropriate engineering analysis. This can be a significant issue. I've never done it with digital, but with analog (and there are still analog stations around), the separation between an undesired station 7 channels below a desired one is almost the same distance as between first adjacent channels. That's because the IF frequency for analog tv sets is 45 mhz, which is between 7 and 8 channels and is almost as significant as first adjacent channels.The antenna preamp might also be interacting with a slightly out of tolerance component in the Dish receiver.If you've got a friend with the same receiver, it might be informative to trade receivers for a few days. If that receiver has the same issue, you might try to put successive 3dB pads in the antenna line and see what happens.You could also be getting more VSWR than is desirable due to mismatches between the impedance on the preamp output, the receiver input, the connectors on the cable, and the cable itself. You could take the receiver in question up into the attic along with a receiver, and hook it up to the antenna with and without the preamp.Lots of luck on this one.John Willkie------------------------------------------------------------------------From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On Behalf Of Barry BrownSent: Monday, January 15, 2007 8:18 PM To: opendtv@xxxxxxxxxxxxx Subject: [opendtv] Re: Analog v Digital TVI appreciate all the comments, however, front end overload doesn't seem to be a condition at my location. I'm about 35 miles from the closest transmitter. There are no stations in the area that are adjacent to the problem channel. Intermodulation, I guess, could be a possibility if it's a condition created by the problem transmitter. Also, I'm not the only one in this area experiencing the indicated random signal loss of this channel. The fact that I have to take the signal down 21 dB in a grade B area is somewhat a mystery. BTW, I have a preamp on my attic antenna, but that is required to receive my weaker channels.On Jan 15, 2007, at 6:36 PM, Allen Le Roy Limberg wrote:Front end overload, possibly by nearby channels. This is owing to the AGC design of the receiver, not something to do with the ATSC modulation scheme.Al ----- Original Message ----- From: Barry Brown <mailto:barrysb@xxxxxxxxxxx> To: opendtv@xxxxxxxxxxxxx <mailto:opendtv@xxxxxxxxxxxxx> Sent: Monday, January 15, 2007 11:53 AM Subject: [opendtv] Re: Analog v Digital TVOn Jan 15, 2007, at 8:57 AM, Craig Birkmaier wrote: Anything downstream in an ATSC transmission is error protection with the sole purpose of trying to get the data in the files I described above to the decoder in the receiver with the lowest number of errors possible. IF you can receive the ATSC transmitted bits perfectly, you can reconstruct the MPEG video stream at the same level of accuracy, as a decoder that is connected to the output of the encoder ( i.e. NO CHANNEL ERRORS).What ATSC signal conditions, other than multi-path errors, would cause some receivers to give an intermittent "Loss of Signal" indication (might be loss of sync) where inserting 21 dB of attenuation in the antenna connection corrects the problem? One channel in my area is such a case. It is not the strongest or weakest channel nor is there any indication of multi-path errors on analog channels (not adjacent) either side of the problem child . In fact one of the analogs is transmitted from the same tower as the DT channel.
