Splitters, Attenuators, Filters, Diplexers, Other Signal Gear

[Jase88]

@majortom: I've seen that kind of interference before. At first, I too believed it was electrical interference. After some troubleshooting, I discovered that it was very strong FM noise. If you can somehow attentuate FM, or use an UVSJ to attentuate VHF, you may discover that the noise will disappear or diminish. I also found the issue to be intermittent--furthering my initial suspicion that it was electrical in nature.

When you're in close proxmity to an FM transmitter, the interference harmonics will roll on up to your UHF stations as well (if your equipment allows it). And this issue is exacerbated if you use a broadband pre-amp that amplifies the FM band as well.

[majortom]

I've seen that kind of interference before. At first, I too believed it was electrical interference. After some troubleshooting, I discovered that it was very strong FM noise. If you can somehow attentuate FM, or use an UVSJ to attentuate VHF

Thanks Jase,

Yeah could very well be pumped up broadband noise from the fm transmitter down the road (within a 1/2 mile). I know for sure if ya drive around the neighborhood of that thing with a crappy FM radio, u can hear that station at all ends of the FM Dial (That's gotta be front end overload & intermod, not harmonics). Used to happen at my parent's house which was like 1.5 miles from it, even 10 -20 years ago.

The "splitter" I indicated is so old, ya can't read it, actually came from an install I took down while some friends and I were re-roofin a friends house. It was a tear off so we took down their UHF Yagi & VHF antenna which they never used. It's all rusty & crusty so I can't read anything. Since it came from a UHF & VHF install, it's highly likely to be a UVSJ. Also just so happens to be the one I have presently as the first split of the antenna downlead.

All makes sense, assuming it's really a UVSJ and not just a splitter, what I don't get though is why the VHF output side of it would still be clean?
a 50/50 shot as to which side I'm feeding the Rat Shack Splitter (cause I can't read it). Later on, I'm gonna place the Rat Shack 5-600 Mhz splitter in-line first, to see if the noise comes back. Temporarily re-arrange to reproduce the interference and take some Pictures of what this interference looks like on the analog stations and post 'em, ya can have a look. May be of some benefit to others. Especially since in a little over a year, we won't have that benefit of using the analog stations to help isolate interference issues.

What also threw me for a loop is with Strong FM, I woulda expected the signature of herringbone patterns on the VHF Hi stations. Which isn't what I'm seein (FM Trap is IN on the Winegard). I guess you could say that assuming the Strong FM signal is "Clean" around it's skirts.

For now, it's definitely completely eliminated when I use the old UVSJ / Splitter first.
In fact I've even seen CTS 35 this morning, which I haven't seen in months...
I'll buy some new UVSJ online sometime, can't find those locally.

[PanaMark]

guys keep in mind, local police and EMS scanner frequencies. They too can cause interference as well as FM.

[Jase88]

Originally Posted by majortom

What also threw me for a loop is with Strong FM, I woulda expected the signature of herringbone patterns on the VHF Hi stations. Which isn't what I'm seein (FM Trap is IN on the Winegard). I guess you could say that assuming the Strong FM signal is "Clean" around it's skirts.

One thing I've noticed on my Sony Bravia--and I'm sure it's applicable to all modern TV units--is that the tuner cleans up analog video. When I tune to a weak or "messy" analog station, immediately the picture begins to clean up a bit; smoothing out snowy images, etc. So it's possible your tuner is cleaning up any herringbone artifacts.

[ralph_sinclair]

Back in post #802 I was complaining of poor results while using an HLSJ. It turns out the problem was simply a bad cable in my chain. I had some more time recently and tested each cable. After eliminating the bad cable, the HLSJ is working exactly as expected.

Since all of my local OTA's are VHF-HI and UHF, I can put the channel 3 output of my satellite STB into the LO side of the HLSJ, and my unamplified antenna input into the HI side. I then split this two two TV's in the house that don't have satellite STB's. Using the UHF satellite remote I can control what is on "channel 3" while my regular OTA channels are at their usual numbers.

The HLSJ doesn't seem to bother the OTA signals but the split has a bit of an effect on the weakest OTA.

[schoenbe]

What are the methods to determine/calculate adjacent channel interference and co-channel interference between two DTV stations, for a given location?

Case in point:

• CFCF and WVNY in Montreal
• CFJP and WFFF in Montreal

In Montreal, WVNY (ch 13) is currently suffering from CFCF's powerful signal. CFCF's (ch 12) will reduce ERP from 325 kW (analog signal) to 10.6 kW (digital signal). How can the impact of this reduction be determined - or better - calculated?

Similarly, WFFF (ch 43) may be affected by the signal of CFJP (ch 42).

[Jase88]

Engineers use a prescribed testing procedure/tool to model interference probability (can't recall the name of the test off the top of my head).

The reduction in ERP and introduction of digital modulation will certainly reduce the effects of adjacent channel interference--but not eliminate them entirely. The effects of adjacent channel interference don't appear to be as drastic with UHF channels, even on analog.

Nonetheless, I have a similar situation in my local market, where nearby channel 13 completely obliterates channel 12 (a distant station I've been trying to pull in).

A sharp-cut notch filter can be a solution. But due to the tight frequency tolerances, expect to pay premium $$$ for such a fine-tuned filter. Tin Lee of Toronto can provide you a quote (do a search on the board here for their website and contact info). Whether such a filter would be required post-transition is a good question that would depend on your proximity to the over-powering station, and other variables. My "guess" is that we might be okay...

[Jase88]

It's referred to as the Longley-Rice Methodology to determine interference probability.

[tvlurker]

Bear in mind that:

- stations are not protected from interference outside their protected contours
- American stations are not protected in Canada, and Candian stations are not protected in the US, even within what would otherwise be their protected contours.

Also, Industry Canada's BPR-10 does not explicitly account for FM second harmonics interfering with ATSC VHF-Hig stations. (BPR-3, the FM broadcast procedure, does mention this, but that is for new FM stations, not for new ATSC stations.) WVNY on channel 13 is subject to second harmonics, either -80dB harmonics generated by the transmitters, or preamp-induced harmonics generated at the receiving end. There are several high power stations between 105 and 108 MHz in central Montreal that could conceivably cause problems for channel 13.

I've heard that IC hasn't considered this an issue, and technically they may be right, since most legal FM harmonics will be a problem only outside a stations protected contour. However, I suspect this may be an issue for channel 10 Barrie.

[schoenbe]

Do FM second harmonics propagate like regular radio waves (in terms of reflection, diffraction, etc.)? If so, would a highly directional antenna pointing towards the weak DTV transmitter remedy the situation?

[JamesK]

Harmonics, from a transmitter, behave exactly like "regular radio waves" because that's what they are. Radio waves are radio waves and propogation depends of the usual characteristics, such as frequency etc. It does not matter how they were generated. So, a 2nd harmonic from a transmitter around 100 MHz will behave exactly like a signal from a 200 MHz transmitter.

[holl_ands]

For a given location and terrain, www.tvfool.com calculates received signal levels.
You should click on the call letters of each station of interest to compare the ERP power
levels assumed by TVFool (extracted from the FCC database) with what you think is
actually being transmitted (e.g. from the Canadian database)...adjust if necessary.

Impact of interference across an entire REGION can be calculated by various FREE programs,
including an FCC/NTIA program, Radio Mobile and SPLAT! Or by various commercial S/W.

Next-Adjacent, 2nd Adjacent (N+/-2), and other "Taboo" signals can be much stronger than
the desired signal, usually expressed as a NEGATIVE Desired to Undesired (D/U) power ratio. Co-channel (same channel) signals must be AT LEAST 15 dB weaker than Desired
station. However, in practice for all types of interference, it is advisable to add another
10+ dB of Fade Margin to ensure the Desired signal remains locked when subject to
multipath fading.

ATSC A/74 Receiver Performance Guidelines tells you what GOOD receivers SHOULD be
capable of in bench tests, although many older DTVs weren't quite that good and ONLY
the CECB (NTIA coupon eligible low-rez) DTV Receivers were REQUIRED to "meet spec":
http://www.atsc.org/cms/index.php/st...nded-practices

Tests REPORTS on actual DTV Receivers can be found here:
http://www.fcc.gov/oet/info/documents/

=================================================
Second harmonic interference from FM stations into the VHF TV band are nearly
always due to non-linearities ("overload") in the receiver system, rather than
being generated in the heavily filtered and monitored transmitter. Hence the
expected signal strengths can be calculated using www.fmfool.com.

Non-linear Interference generation explained:
http://www.odyseus.nildram.co.uk/Sys.../Linearity.pdf
http://www.fiber-span.com/Applicatio...ic%20Range.pdf
http://www.arl.army.mil/arlreports/2007/ARL-TR-4235.pdf

The effect of FM interference for DTV reception was discussed here:
http://www.tvfmtranslators.com/PastP...09FMixtoTV.pdf
http://tvtechnology.com/article/82716
http://www.tvtechnology.com/article/93086
http://www.tvtechnology.com/article/88282

Narrow-band Notch Filters targeted against Adjacent or Next-Adjacent interference will
degrade the desired signal due to excessive envelope delay & freq response non-linearity.
Steerable Nulls from a dual antenna system, or physically blocking the undesired station
are effective remedies....depending on how much "help" is needed....as usual...YMMV....

Narrow-band Notch Filters targeted against a strong FM station can be effective, BUT it
will also degrade Ch6 if tuned to the lower part of the FM band. A more expensive,
FM Full-Band Stop Filter would be needed if there are multiple strong FM stations.

[schoenbe]

Second harmonic interference from FM stations into the VHF TV band are nearly always due to non-linearities ("overload") in the receiver system

... and therefore special care needs to be taken when amplifying antenna signals, i.e., limiting amplification to just the amount required for stable reception of a channel? Can amplifiers with high gain figures (e.g., CM 7777) therefore magnify or even create the problem of second harmonic interference?

A more expensive FM Full-Band Stop Filter would be needed if there are multiple strong FM stations.

Is the FM trap feature, advertised by some (pre-)amplifiers, equivalent to an FM Full-Band Stop Filter?

[Jase88]

Originally Posted by schoenbe

Is the FM trap feature, advertised by some (pre-)amplifiers, equivalent to an FM Full-Band Stop Filter?

Depends on the specs of the FM band stop filter.

The Channel Master 7777 FM trap has about 20dB of attenuation. I believe the FM Band stop filter by Tin Lee of Toronto offers greater than 50dB of attenuation.

[JamesK]

Can amplifiers with high gain figures

All amplifiers have some amount of non-linearity, which is what causes distortion (harmonics) of the amplified signal. The higher the gain, the more likely this is to be a problem. Careful design can minimize, but not eliminate non-linearity.