Posted on 01/15/2023 8:12:13 PM PST by Paul R.
Is there an inexpensive way to determine if poor over-the-air TV reception is due to weak signal strength vs. multipath reflections? I am trying to troubleshoot a problem with degraded OTA TV reception that has not responded to installing new cable and connectors. The antenna is fairly new and in good condition.
With "Digital" OTA TV, the image cues are almost "all or nothing", with a slight transition: That is, the picture viewed goes from pristine (depending on the original video, of course), then there can be partial pixelization and audio dropouts, and any worse, the screen simply goes black and one gets a low signal or no signal message. There is really no way I know of* to discern from the picture if the problem is weak signal or multipath reflections. One can check "signal strength" (the "meter" and how to get to it varies from TV to TV), but it turns out this meter measures "signal quality", not the actual strength of the RF signal.
*I have some basic knowledge here, but I'm no expert, or I would not be making this post!
The old guideline of "the more height the better" applies primarily to signal strength, but, due to new construction near me, large metallic grain silos, I may well have a multipath problem as much or more than a signal strength problem: If multipath is the cause of my diminished reception, I "may" be able to address that without trying to go "over" the new silos (for which I don't have the resources.) From experience I know more amplification @ the amplifier is unlikely to help, and may make matters worse. The present antenna is a fairly directional design -- to do better would require most likely a top end antenna ($$$) and the silos are at enough distance that the signals reflecting off toward my antenna would likely still be in the beamwidth of the antenna anyway. In the past in this sort of (multipath) situation I've had more luck trying different antenna locations. But, then I had the guidance of being able to SEE the "ghosts" on the TV.
Not knowing for sure if my main problem is signal blockage, or multipath, complicates matters.
Is there an inexpensive way to get a better idea of signal strength (or degree of multipath distortion?)
Perhaps better put (that last):
Is there an inexpensive way to get a better idea of digital TV signal strength (or degree of multipath distortion?)
Your receiver typically has a decibel meter. Mine have, and you can get this typically by doing a channel scan. It will tell you how strong each station is by bars or numbers.
Multipath is not an issue if you have a strong signal.
go higher.
I discovered this while having problems a few years back with signal from a fairly strong transmitter (only) several miles away, that was still pixelating (as viewed): The "meter" showed low signal. I queried the TV's manufacturer and they clarified that the meter indicates signal quality, of which strength is only one factor.
I am an electromagnetic compatibility engineer. To determine if the problem is weak signal or multipath interference takes a device to measure field strength at the specific frequency like spectrum analyzer or a receiver with a field strength meter, and a highly directional antenna.
That said, it is highly unlikely that the problem is multipath. The interfering multipath signal would have to have a signal less that 3dB less than the primary signal. It is not that it can’t happen, but it is unlikely.
The new metal structures could block the signal to a degree and may have interrupted the line of sight between the transmitting antenna and the receiving antenna.
TV signals, even VHF (lower channels 2 - 13), are very much line of sight. Antenna height is critical. Lack of height can be mitigated by a high gain antenna, rotor, and a low noise preamp, and high quality, low loss RF cable.
If this was a recent problem, I have to pose the obvious question:
Have you contacted the broadcaster or whoever operates the transmission tower(s)?
My old RF technician friend (unfortunately passed many years ago) taught me that it is the ratio of the direct signal strength as received vs. reflected signal strength as received that matters most, given that the received signal is strong enough for good reception (barring strong multipath) to begin with. This agrees with observations:
I can take my 12v tv to open space 35 miles from the transmitter of the closest station giving me trouble, and pick it up nicely on rabbit ears on top of the tv. (Yes, the terrain is fairly flat.) At home I am <7 miles away from the transmitter (impossible to not be a much stronger rf field than the “35 mile” location) with a good antenna 30 feet up, and the signal pixelates.
For that “close” station, sometimes I actually get LESS pixelation with a temporary antenna on my ground floor, than I do off the outside antenna. But, then I can’t get the more distant stations.
Put another way, if the reflected signal is as strong as the direct signal, and both are strong, if I go up 10 ft., both will be stronger by the same amount, and the tuner still has to try to discriminate between the two.
I’m sure that if I went significantly higher than the silos, I’d have more direct signal and less multipath. But, that’d really be expensive.
Put the receiver at the antenna using something like a silicon dust box.
On top of this I use Plex to share and record shows
Yes, I remember that 3 dB capture ratio figure as being typical of cheap FM radio receivers. But, I probably do have some blockage and who knows what sorts of weird nodes could be set up by 4 large metallic structures...
I also remember how much reception could vary from room to room in tall apartment buildings in clusters (of buildings) even when signal seemed strong. (They were dorms some of my classmates lived in.) I actually had a high end FM tuner, Onkyo, IIRC, which had both a genuine signal strength meter and a dedicated output with which one could monitor multipath.
Anyway, even with some blockage I’m having trouble accepting the idea of a weak RF field from a 155 KW TV station (806 ft. antenna) under 7 miles away. The more distant stations... yeah...
Reception problems definitely increased when the silos went up. But, I highly doubt I’d get anywhere in getting them removed, nor do I want to start some sort of local legal war...
There’s also the red “snow” we get an inch or two of (from the silos) in the fall...
(Chaff from grain processing.)
:-(
Think we have talked about this before, but I am on FTA satellite in both Ku and old school C- band and they are broadcasting digital. All I can do is share what I have observed with signal strength and quality meters. These receivers have both. They give you the Ku band carrier signal reception strength separate from the individual channel frequency quality.
What I have observed is that even when I aim at a bird and get a good signal strength in a proper band I still cannot get digital lock until I fine tune the aim until I get a particular frequency quality up also. My signal strength can be 90+ yet until I get the quality meter up to at least 80+ the digital will not lock.
And it is not universal across the board. Even though all the channels have the same strength, the signal quality on each is different from each other even though they are coming from the same bird and most times even the same transponder. So what is happening is the signal strength meter is just detecting the overall Ku band strength, so the quality meter is what I have to use to pinpoint the digital frequencies for each channel.
There are so many transmissions out there close to a terrestrial system band that a signal meter might not do you much good anyhow. What I have to do is rely on just the quality meter. I put it on a known weak frequency and adjust until it locks. If it doesn’t lock then I know even though my strength might show good, I am still not getting enough of the digital frequency it’s self. So then I have to play with fine tuning the LNB/feedhorn H/V polarity or distance from the parabolic collector.
And because the wattage of the downlink on both is so weak by nature I have to be dead right on to get a digital lock. Even a wiggle in a stiff wind can pixelate the signal. So I guess what I am trying to share is that the quality meter is what you need to use anyhow. A signal strength meter has never helped me much once they went digital. Do you have neighbors who are already receiving? Someone who might know what the weakest channel would be in your area?
I always wondered if a dish could be repurposed to receive terrestrial broadcasts for those out in the boonies. C-band only downlinks at around 5 watts and then it is amplified. Maybe an signal amplifier would help your situation?
“I’m having trouble accepting the idea of a weak RF field from a 155 KW TV station (806 ft. antenna) under 7 miles away. The more distant stations... yeah...”
Well this tosses what I said in my previous post out the door. This sounds like interference from something in that same or close to that frequency confusing your receiver.
Good signal to noise (SNR) can mean as much as anything else. Have you added any noisy RF devices inside your house lately? Also, the higher you go with your antenna, the more bad RF interference you might be picking up from other sources.
A young man called “The antenna guy” devotes his entire youtube channel to over the air antennae and what broadcasters are doing as far as signal strength etc... he will have an answer for your problem I’m sure!
I know it sounds crazy, but digital is extremely sensitive to polarity for some reason. If this is a horizontal antenna and the broadcast is a vertical I would experiment with orientating the antenna vertical... lol
Never tried it, who knows?
Sorry for the delay to get back to this. Crazy day, and multiple vehicle problems as well...
My parents had a similar problem back in the analog TV era. They had two closer, strong FM stations in the same direction as a distant VHF TV Channel 6. My friend mentioned above made custom 2-notch FM filters for several houses in our neighborhood.
Sorry for the delay to get back to this. Crazy day, and multiple vehicle problems as well...
TV signals are more commonly (or at least used to be) only commonly horizontally polarized. But that may be changing:
https://www.channelmaster.com/blogs/free-tv/real-world-signal-propagation-what-you-need-to-know
That is, I might add, a VERY interesting article that I stumbled into, tonight. Note the section on multipath and signal polarization alteration by various objects. This silos I mention have CONICAL metal tops. Oh, boy...
Maybe I should lobby the owner (big operation wealthy farmer - their house is almost a mansion) to put up a tall receiving antenna with rebroadcast...
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.