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Thursday, August 14, 2008

Thin-Air ATSC (And NTSC): The De-Boost (Or Over-Boost) Debacle

Aug 14 2008 9:25AM | Permalink |Comments (12) |

The latest in an ongoing series…

As I mentioned last Friday, I've been temporarily using Antennas Direct's CPA 19 amplifier in an attempt to boost the over-the-air broadcast signal strength that reaches my home office from Reno-area transmission sites. The CPA 19 explicitly disregards low-band VHF, but that's ok in my particular case; all of the region's ATSC signals are high-band VHF or UHF. And I can tell from visual inspection of the before-and-after KRXI (Fox affiliate on NTSC channel 11) analog picture that the CPA 19 is delivering some benefit…thought I'm still not able to snag KOLO's (ABC affiliate on ATSC channel 9) nearby-tower digital signal.

But since my agreement with Antennas Direct was for the CPA 19 to be loaner-only, I went ahead and ordered Motorola's model 484095-001-00 signal booster from Amazon last week. Judging from overall positive customer feedback on sites like Amazon and Buy.com, I hoped I'd have a successful end result. Alas, the converse was the case. Not only could I still not receive KOLO, I was no longer able to receive KRNV, either. KRNV = NBC. NBC = Olympics. This is a problem.

Disconnecting the 'booster' fortunately restored KRNV's signal. So what's going on here? Judging from the overwhelming positive feedback on the Motorola 484095-001-00, I don't think that the product is faulty in design (though I suppose I could have a defective unit). Here's my alternative theory: note that KRNV is one of the two ATSC transmissions that I've been able to receive from the very beginning of this experiment, even though my antenna was substantially off-axis from KRNV's tower at the time.

I later re-oriented my antenna directly at the Slide Mountain tower cluster that includes KRNV's equipment, thereby (presumably) further boosting the station's received signal strength at my location. And then, in striving to successfully tune in KRNV's next-door tower neighbor, KOLO, I inserted an active signal booster into the coax cable chain. I suspect that as a result, I actually ended up with an excessive-signal-strength situation on KRNV, which overloaded my tuner's pre-amp and thereby degraded reception.

Please note that this (or for that matter, anything I've discussed so far in this series) isn't an ATSC-only issue…in fact, I've frequently found that where I was able to receive both the NTSC and ATSC version of a signal (with KRXI, for example), the digital version was far preferable. With that said, I welcome your alternative theories as to the root cause of my Motorola 'booster' setback. And have I mentioned yet how much I detest RF stuff?

Sigh.

p.s...For another amusing-and-saddening tale of television reception woes and creative solutions, check out this CommVerge writeup from late 2002.

Reader Comments

at 8/14/2008 1:48:58 PM, stiggle said:
What, no tuner AGC?

at 8/14/2008 1:56:22 PM, alternety said:
You could try some attenuation in the input to the amp. If you don't happen to have a nice broadband variable attenuator, buy some decent and/or cheap splitters (places like Radio Shack have them). They should show you how much loss there is on each port (3 DB for a 1-2 splitter). Buy a bag of terminators for the unused posts. String them together until you see results.

at 8/14/2008 2:00:55 PM, Steve said:
Since your in the experimenting mode I'd forget the "noisy" gain path (read "active amplifiers") and pursue another direction - you've established that there's plenty of signal at your site - you just need to get it to your set.

Go get yourself some high quality quad-shield coax and high quality "F" connectors - and substitute same for your current in-home wiring. . . . . I'll wager you get significant improvements over your current wiring.

at 8/14/2008 2:19:34 PM, Brian Dipert said:
Dear stiggle and alternety, with all due respect, gang, if a booster doesn't enable me to get the added station I want, and conversely causes me to lose a station I had been receiving just fine pre-boost, why would I want to keep using it at all? Aside from satisfying my professional curiosity as to the root cause of the station loss, of course...;-)

at 8/14/2008 2:22:50 PM, Brian Dipert said:
Dear Steve, 'time' (ie personal bandwidth) is the key issue with your valid suggestion, unless you're aware of some human cloning breakthrough that I haven't yet heard of. Along with, of course, snakes and spiders (and bears? Coyotes? Mountain lions?) in the crawlspace under my rural mountain home...;-) Please note, too, that in an earlier writeup I'd pointed out that KOLO's signal was even unacceptably weak when measured from the parking pad outside my home using a high-quality UHF/VHF antenna, before it ever even touched my potentially sub-optimal coax

at 8/14/2008 2:36:20 PM, Steve said:
Brian -

My suggestion was not yet to make a new cable run - but simply throw the new coax up to the antenna, through your front door and conect to your set - to determine the relative improvement in signal - then you could decide if the "time" issue was of benefit.

When you say that you used a "high quality" antenna on your pad - I'm curious, at what height was this "high quality" antenna (received signal strength varies significantly relative to the heights of receiving and transmitting antennas) ALSO, do you have any idea of the relative "gain" of this "high quality" antenna ?

A multi-element yagi will provide you with far more "noiseless" gain than any active preamp you might buy . . . . combining same with minimal loss on your transmission line would be my starting place, albeit the most intensive from an installation standpoint.

at 8/14/2008 2:50:34 PM, Brian Dipert said:
Steve, please check out my previous posts in the series for all of that information in detail...including pictures! ;-) The engineering team from KTVN will be visiting me again tomorrow to run some more measurement tests (both on their signal and their competitors' ;-) ) so expect another writeup from me shortly thereafter

at 8/14/2008 6:28:04 PM, TV_Guy said:
I tend to agree with alternety because there seems to be at least 3 channels adjacent to each other: 7,8 and 9. The basic problem could be tuner front end overload from those channels (and possibly others). This could be severe enough cause excessive uncorrectable errors on KOLO. When the booster is added, it just makes the situation worse causing other channels to be distorted and lost. I think the idea of adding attenuation would be a good test.

at 8/15/2008 12:54:43 PM, Larry M said:
Just to state the obvious, the easiest way to insert some attenuation is to simply rotate the antenna off-axis again (as long as there are no other signal sources in that general sector). That's related to Brian's success in pointing the antenna so as to "split the difference."

at 8/27/2008 2:42:07 PM, gjd said:
The motorola amp is extremely broadband. Perhaps some of the LOW Band signals or even cell phone signals are overloading the amp.

at 8/28/2008 10:00:39 AM, Chicago South said:
Its been my experience that broadband amplifiers sometimes act as oscillators when the gain is high and there are strong signals nearby. Poorly shielded coax could theoretically cause the same thing.
Plasma monitors give off EMI so good coax is important. Keep the cable away from the set as much as possible. Also, the patch cables packaged with VCRs and video games are junk. Throw them away and use quad-shield.
Have you grounded the coax with static discharge block and antenna mast?

at 8/30/2008 12:29:51 PM, Mike M said:
Using a cable TV amplifier could be causing problems as cable TV has additional channels that are not used for broadcast TV. As it seems you are having significant overload problems with the cable TV amp, and since your TV Fool plot shows stronger signals than I would recommend for the best pre-amps (e.g. CM7777), I am going to recommend the Winegard HDP-269 pre-amp. It doesn''t have tons of gain (12dB), but it is the most overload resistant model that I know of. Note that tons of gain on an amp is only needed if you have very long cable runs and/or lots of splits. It can''t make the antenna any better than it is, though small gains are useful in overcoming noise figures even on short cable runs.

Still, I don''t see that you installed a VHF 7-13 antenna? I expect that would go a long way towards solving the KOLO problem. In Mystic, CT, I was not able to get WTNH on 10 at all with an antenna that supposedly had VHF capabilities (Winegard HD-1080), but the Winegard YA-1713 gets it just fine.

Note that KOLO will be turning off analog 8 in January, in advance of the 2/17/09-2/18/09 transition. I''m assuming they will have DTV 8 ready soon after that. So maybe that will help if you can''t get 9 to work until then.

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