Sources of Local Interference Affecting VHF Reception

[jimh]

Here is some data from a number of tests I have made to determine what locally-generated sources of interference to VHF TV signals were affecting reception at my location.

TEST METHOD

The test arrangement was a typical home computer, computer monitor, WiFi base station, and Ethernet hub. The antenna was a VHF Hi-Band Yagi located about six to eight feet away, in the same room. The antenna is positioned to point out the second storey window in the room and is bore-sighted on the TV station's location. The antenna is a seven-element Hi-VHF TV-band Yagi made by Televes.

The testing method uses a recent-model SONY Bravia receiver and its signal diagnostic screen. The receiver has a relative signal strength indication (RSI) using a scale of zero to 100. It also reports the signal-to noise (SNR) ratio of the demodulated ATSC 1.0 signal. The receiver is located on a lower floor, away from the computer gear and the antenna.

The VHF television signal being used in the tests is 188.990-MHz or CH-09, from CBET in WIndsor. Another VHF TV signal in my area is from WJBK on CH-7, which is very close to me with a ridiculously strong signal, completely immune to interference, and thus of no value for the test.

With the computer active, the monitor on and showing a normal screen presentation, the WiFi base station on, and the Ethernet hub active, there was NO reception of CBET-09 possible. With all computer related electronics off, the signal became rock-solid. The table below shows the progression in improvement from worst to best, as various gear was shut off. A watchable picture generally requires an SNR of 15 or 16.

TEST RESULTS

RSI (0/100)---SNR (dB)---TEST CONDITION
43/100----5 dB---Computer awake, monitor active, WiFi active, Ethernet Active; TV receiver has no lock and no picture or sound.
41/100---17 dB---Computer in sleep mode, monitor screen no-signal (amber inidcator), WiFi active, Ethernet active.
41/100---17 dB---As above, but monitor powered off; no significant change from no-display (amber) mode.
41/100---17 dB---Computer off, monitor off; no significant change from above.
41/100---19 dB---Computer off, monitor off, WiFi off, Ethernet hub off; improvement of 2 dB in SNR
40/100---20 dB---LTE filter added in antenna transmission line; improvement of 1 dB in SNR

SUMMARY
The primary source of interference was from the computer monitor, when it was active and its screen was rendering a typical computer display. If I was sitting at the computer desk, the presence of my body would reduce interference from the computer monitor enough to restore reception on CBET, but since I could not be in two places at once, I could not see the diagnostic screen to get the data from it. With loss of input signal from the computer, the monitor would go into blank screen mode.

DETAILS ON ELECTRONICS
The computer used was an Apple M1 Mac Mini. The monitor used was an HP24mh. The signal to the monitor from the computer was HDMI. The WiFi hub used was a Google NEST router. The Ethernet hub used was a TP-LINK six-port Gigabit switch.

HISTORY OF PROBLEM
Several months ago, I changed a setting on the computer to prolong the amount of time of inactivity before the computer would go into sleep mode. At that time, the original setting was to sleep in a very short interval of lack of activity. I was getting frustrated by having to re-log-in if I stopped typing or moving the mouse for a few minutes, so I increased the time-out length. This resulted in the screen staying active for much longer periods. It was only then that I began to notice that reception of CBET-09 was not possible. As this occurred in the late Spring, I initially attributed the loss of reception to be caused by air temperature changes and appearance of foliage on the many trees in the signal path. With the arrival of Fall, my interest in watching CBET resumed--I like to watch Hockey Night in Canada on Saturday. This prompted me to further investigate the loss of reception, which resulted in the discovery that the computer monitor was the source of significant interference.

[Round Six]

Glad to hear you resolved it. I think I'll take an opportunity to mention this. My Roku controller does the same thing on Fox2. Unplug it and the picture is fine.

[syntheticexctasy]

Great data. I've always found that ethernet is generally noisy - especially long runs of it. If there are two active devices on each end of an ethernet cable, you can bet it's leaking all kinds of noise from ~20mhz-~800mhz. Now that I have decent enough coax and an antenna located far (and a few sheets of sheet metal) away from noisy ethernet gear, I don't notice it as much.

I do have one (of about 10 or so that I regularly use) monitor here that emits quite a bit of trash across HF and VHF while on. It's a pretty old Samsung unit (circa 2003) that I bought impulsively as a kid for an insane amount of money, and thus refuse to let go of until it's dead

Back in the day when I housed everything in one room and used leaky old coax, I had much better results running everything I could via wifi, believe it or not. Noise levels weren't measured in any scientific way other than by ATSC decodes and by ear. I do a lot of FM and TV dxing. From Petersburg MI, CBET can be had most of the time, but tropospheric conditions can hamper/help that at times. This is with a giant old TV/FM logi-yagi combo antenna about 25' in the air. It doesn't have a great UHF section, so I have more fun at lower freqs (until Tropospheric conditions allow otherwise).

[km1125]

Interesting data. I though this might be more about cheap LED lights and CF bulbs! (those seem to be big issues in radio RF reception!).

I'm surprised to see the LTE filter make any difference at all on a VHF signal. I could understand seeing differences in UHF, but there's a lot of bw between the LTE signals and the VHF band.

[syntheticexctasy]

Interesting data. I though this might be more about cheap LED lights and CF bulbs! (those seem to be big issues in radio RF reception!).

I'm surprised to see the LTE filter make any difference at all on a VHF signal. I could understand seeing differences in UHF, but there's a lot of bw between the LTE signals and the VHF band.

even if harmonics aren't generated on the output signal, they can certainly be generated inside the local receiver.

sirius/xm had a similar problem for a while when LTE was first being deployed. one of the vehicles I own still has a pending TSB which outlines having the satellite radio antenna replaced with one that is more able to reject the LTE sigs. anytime I drive within sight of an LTE tower with said vehicle, the SXM signal breaks up/drops. personally, i've never tried an LTE filter on anything, as I have always lived far enough from the nearest tower that it couldn't possibly be an issue (hell, I can't get VZ data service most anywhere on my property except the second storey). ATT is the only game in town (close by) and it's far enough away to not be a problem. I am a frequent user of CB also (an even LOWER frequency) where I can pick out LTE interference when nearby any tower as well. Within a half mile, clear site from most LTE towers, interference is detectable almost on any band.

[jimh]

Re LTE filtering: I have a neighbor whose adult son lives with him. The son seems to spend a lot of time with his car parked in the driveway yakking on his cellular telephone before going into the house. This puts his vehicle about 50-feet away from my TV antenna, directly in line with the main lobe. I bought the filter on the hunch that the interference was related to his almost nightly tendency to spend 30-minutes in the car before going into the house, which I attribute to him wanting to keep his conversation private from the rest of the occupants in the home.

[jimh]

ASIDE: I have a second television receiver in the form of a Channel Master DDR. This receiver also can report data about the received TV signal, giving a metric in relative signal strength (0 to 100) and in "quality" (also 0 to 100). Interpreting the measurement of signal "quality" was somewhat nebulous, until I realized that the receiver was reporting the signal-to-noise ratio as a power ratio, not as a deciBel ratio. For example, a "quality" rating of, say, 80, indicated a power-ratio of 80:1 for signal to noise. Converting this to deciBel gives 19.0-dB.

dB = 10 x log(P2/P1)
dB = 10 x log(80)
dB = 10 x (1.903)
dB = 19.0

With this insight, I observed that the reported SNR between the Sony TV and Channel Master DDR were very similar, confirming my suspicion of how the DDR was reporting signal quality. It then occurred to me that by using the DDR as the measuring device, I could get a finer resolution of the signal strength, with changes of 0.1-dB being possible, whereas with the Sony TV the SNR was reported to only the nearest whole deciBel number.

Because the "quality" rating can only go to 100, which corresponds to a SNR of 20-dB, the DDR is more useful for measuring marginal signals. Anything over a 20 dB SNR gets reported as "100" quality.

I may repeat the testing procedure using the DDR as the SNR measuring device. This should remove some ambiguity when the SONY TV SNR measurement value would flicker between whole numbers.

[bmw]

The biggest source of RF interference I've located to date (on the AM band anyways) is emanating from some newly installed electric car battery chargers that my local city installed at the city park. My car radio reception goes to crap at least 1000 feet away and when I drive right by them, It is so strong I almost wonder if it could damage my radio's front-end. I don't know if they're not properly grounded or what, but they're an enormous source. So that'll be another nail in AM radio's coffin as these things become more commonplace.

[Plate Cap]

The biggest source of RF interference I've located to date (on the AM band anyways) is emanating from some newly installed electric car battery chargers t

Lots of powerline interference nowadays...DTE or Consumers used to jump right on it when it was reported. Not so much now. Usually a bad insulator or crimp.

[Turkeytop]

Recently I've been getting a lot of noise on the MW and SW bands. I finally tracked down the source.

Above my desk there are two 48" lights. When one of the fluorescent tubes failed last month I replaced them both with the new type of LED replacement tubes. Tonight I tried switching them off. The noise went away immediately. They really emit a lot of noise.

[paul8539]

Write it up with more data, and you might be able to get it published in a well known magazine, such as IEEE Broadcasting, WIRED, Computer, or some such magazine.

[jimh]

Above my desk there are two [lamps]. When one of the fluorescent tubes failed last month I replaced them both with the new type of LED replacement tubes. Tonight I tried switching them off. The noise went away immediately. They really emit a lot of noise.

Emission of radio-frequency interference (RFI) from lamps using LED bulbs is well know in the marine radio field. VHF Marine Band (156 to 162-MHz) antennas are often located at the top of a ship's main mast, where typically there will also be a navigation lamp. Several navigation lamps using LED bulbs were noted for producing significant interference.

This problem was so common that in 2018 the United States Coast Guard issued a Marine Safety Alert to inform mariners about it. See

https://www.dco.uscg.mil/Portals/9/DCO% ... 091109-630