I have an interesting situation.

In the direction my antenna points, there is an intersection with a traffic light. When cars are stopped at the light, my reception on some channels becomes garbled (blockiness, loss of audio). When traffic is flowing, or not present, the reception is fine. Other channels do not exhibit the problem.

I am guessing that the stopped cars are reflecting the signal, creating a temporary multipath problem.

Does anyone have suggestions for fixing this?

Details:

Channelmaster CM 3016 antenna. Mounted on 10ft mast on rooftop. Total height above ground approx 30ft. Mounted at peak of roof.

Cars stopped at intersection are around 100-120ft away. Antenna is aimed almost directly towards the cars. Antenna is roughly leveled (ie. not pointed down towards cars, but parallel to ground).

TVFool says that I am in signal shadow from a distant hill (1.5 miles away). Antenna height for direct view to antennas would be 100ft. All broadcast antennas are at same location.

Although cars could cause multipath, I'm guessing that it's more likely that it's ignition noise that's causing you're problems.

Not sure what to do about it except to raise the antenna and perhaps point the antenna up a few degrees to try to get the noise source as far into the null as you can get it.

I have read others recommendations of pointing towards the peak of the hill, since the radio waves will travel in a straight line from the ridge line.

In addition to the other fine suggestions you might try pointing the antenna slightly away from the intersection. Your antenna should have about a 30 degree beam width to allow for up to 15 degrees off center each direction. Like Mac, my guess is not multipath, but modern vehicle RFI or possibly the signal lights and traffic detection system itself. Is it worse with many stopped cars than just one or two?

I have an interesting situation.

In the direction my antenna points, there is an intersection with a traffic light. When cars are stopped at the light, my reception on some channels becomes garbled (blockiness, loss of audio). When traffic is flowing, or not present, the reception is fine. Other channels do not exhibit the problem.

I am guessing that the stopped cars are reflecting the signal, creating a temporary multipath problem.

Does anyone have suggestions for fixing this?

I vote for multipath, because I have a very similar situation. My 4-bay bowtie antenna, which is near ground level, points across a road with heavy traffic. When I have my Sadelco 719-E Signal Level Meter connected to the antenna I can see the signal level vary above and below the resting point as the traffic goes by. As long as the signal level (no amp) stays above about -10 or -15 dBmV, I can keep a good lock; when the signal drops below about -20 dBmV, I often get a freeze and/or tiles. When I tried a higher elevation for the antenna the signals were stronger but the multipath became worse; probably because the time delay between the direct signal and the signal reflected off the car tops was greater. You would have to go very much higher for your antenna not to pick up the signal reflected from the cars. My suggestion would be to try vertical stacking of two antennas (or a 4-bay like the CM 4221 which has inherent vertical stacking of the bays for UHF) which would narrow the vertical beam angle to reduce the signals coming from the car tops.

If you still have the ability to receive analog you may want to check out those signals to see if it's ghosting (multipath) or getting sparkles or static bars (noise) when you are having problems on Digital.

Channelmaster CM 3016 antenna. Mounted on 10ft mast on rooftop. Total height above ground approx 30ft. Mounted at peak of roof.

A 3016 is not that directional. Try an antenna with a more gain and therefore a more narrow pattern. You'll get more signal and less multipath simultaneously.

Perhaps, if lower-cost antenna variations/shifts won't help, the solution is one of the 5th/6th-generation tuners (see this forum) designed to minimize multipath problems. -- John

It does seem to be worse as more cars are stopped.

I do have an analog TV tuner, so I will hook it up and test, this evening. As mclapp suggested, this may diagnose which problem is occurring.

If in fact you're picking up signals from the intersection (stopped cars, metal detectors or noisy computers, etc); then perhaps you can install a "shield" below your antenna. I am envisioning something like a "dish" in blocking signals to the antenna rather than focusing them toward the receiving element(s).

However, my idea is closer to a last resort than something I'd try first. A better antenna would be my first choice.

If in fact you're picking up signals from the intersection (stopped cars, metal detectors or noisy computers, etc); then perhaps you can install a "shield" below your antenna. I am envisioning something like a "dish" in blocking signals to the antenna rather than focusing them toward the receiving element(s).

Your "shield" idea has merit and suggests a possible solution: The peak of the roof itself could be used as a shield if it is running in the right direction. If the antenna could be mounted a distance behind the peak, just high enough for the TV signal to clear but low enough so that the antenna did not have a direct view of the cars, then the signal reflected from the cars would be blocked.

This would be more effective for UHF than VHF because UHF signals behave more like light rays. See Choosing a mouting site, VHF and UHF diffraction at:

http://www.hdtvprimer.com/ANTENNAS/siting.html

If in fact you're picking up signals from the intersection (stopped cars, metal detectors or noisy computers, etc); then perhaps you can install a "shield" below your antenna. I am envisioning something like a "dish" in blocking signals to the antenna rather than focusing them toward the receiving element(s).

However, my idea is closer to a last resort than something I'd try first. A better antenna would be my first choice.


A shield came to my mind when I first read the posting. If it were an indoor antenna the shield would be an easier scheme. If it was UHF only it would be an easier scheme. An outdoor uhf/vhf antenna would be hard, anything put up has a wind load to be figured in, it might be possible but impractical.

I agree that putting a shield up would be last resort.

You need to determine if the interference is in fact multipath or man-made. Many traffic light controllers create a LOT of interference, particularly on HF frequencies. This interference does change with the lights' status. It could be a case where your nearby intersection is creating far too much interference due to defective equipment. My thinking is that if its affecting VHF frequencies there's something not right with the light controller and a call to the FCC needs to be made.

First check to see if you have problems late at night when there is no traffic.

donnyjag may be right

I connected an analog tuner last evening, and made some observations:

I see a very faint ghosting on about 1/2 of the channels. The only really noticeable artifact is a vertical line that runs top to bottom about 5% in from the left edge. I can't see any "double images" of picture content, so if it is ghosting, it is extremely faint.

The signal gets noisy when moving up into the higher UHF range, such as channels in the 50's.

Channel 56 is fairly strong and clear, so I watched what happens when traffic collects at the intersection. Sparkles appear, and tend to be in a band that scrolls up the screen. The band is about 20% of the screen height, and goes across from edge to edge. It is not a huge amount of sparkles, but I can see a difference between a clear street, and a dozen cars.

So, it looks like my problem is a generally noisy signal, compounded with EFI from the car ignition systems. I will look into finding a method of tilting the antenna to aim a little upward, as a first solution. That may reduce the EFI. The digital tuners seem to cope with the noisy signal, as I get a clear digital picture most of the time, and it only scrambles when the EFI puts it over the edge.

Some other notes:

The VHF signals are very strong and clear. (This is good news, since the 2009 cutoff will move some channels down to high VHF (7+), and eliminate the high UHF.)

I was wrong when I said the mount is at the peak of the roof. It is actually behind the peak. The mast is strapped to a vent pipe. So, I could try lowering the antenna to use the roof as a shield.

I see a very faint ghosting on about 1/2 of the channels. The only really noticeable artifact is a vertical line that runs top to bottom about 5% in from the left edge. I can't see any "double images" of picture content, so if it is ghosting, it is extremely faint.If you are getting this type of image on a VHF channel, it could be leakage from your cable TV provider. From your description, you're getting 2 signals, one correctly, and the ghost is slightly delayed from the original. If cable puts a channel on its own channel (OTA 7 is cable 7 for example), that could be what you are seeing.

Otherwise it sounds like your antenna might not be aimed correctly and/or you are getting a significant reflection off of something. Fortunately, modern ATSC receivers are able to handle ghosts like this.

Regarding using the peak of the roof for a shield; signals will more easily penetrate wood than something metal. So the roof may attenuate your interference some, but not enough. Adding metal at the peak of the roof might increase the attenuation enough to be successful.

Regarding the other post about RF interference from the intersection; I totally agree with that. Most intersections have metal detectors to detect when autos are present and microprocessors running the lights. Stuff does age, get out of tune or simply fail. Perhaps some tech did not put a shiled back onto a piece of equipment there (to solve a heat problem?) and basically creating a "transmitter".

One of my hobbies is listening to scanners (radios), and many models have features that can be used to find such radiation or RF signals. It might be interesting to use such a scanner out there listening when you do get the interference and when you don't.

Good luck.

I connected an analog tuner last evening, and made some observations:

I see a very faint ghosting on about 1/2 of the channels. The only really noticeable artifact is a vertical line that runs top to bottom about 5% in from the left edge. I can't see any "double images" of picture content, so if it is ghosting, it is extremely faint.

The signal gets noisy when moving up into the higher UHF range, such as channels in the 50's.

Channel 56 is fairly strong and clear, so I watched what happens when traffic collects at the intersection. Sparkles appear, and tend to be in a band that scrolls up the screen. The band is about 20% of the screen height, and goes across from edge to edge. It is not a huge amount of sparkles, but I can see a difference between a clear street, and a dozen cars.

So, it looks like my problem is a generally noisy signal, compounded with EFI from the car ignition systems. I will look into finding a method of tilting the antenna to aim a little upward, as a first solution. That may reduce the EFI. The digital tuners seem to cope with the noisy signal, as I get a clear digital picture most of the time, and it only scrambles when the EFI puts it over the edge.

Some other notes:

The VHF signals are very strong and clear. (This is good news, since the 2009 cutoff will move some channels down to high VHF (7+), and eliminate the high UHF.)

I was wrong when I said the mount is at the peak of the roof. It is actually behind the peak. The mast is strapped to a vent pipe. So, I could try lowering the antenna to use the roof as a shield.


Those scrolling sparkles will really cause problems with a digital signal. That type of noise is tough to get rid of other than eliminating the source (not an option in your case) The slight amount of ghosting you are mentioning shouldn't come into play at all if your receiver is fairly new.

If you must point your antenna through the intersection maybe a vertically stacked antenna array of some sort to narrow the beam width in the vertical direction may help. Tilting the antenna up away from the traffic may also help.