Ham radio transmissions can interfere with television reception. This is called television interference, or ham radio TVI. Consumer adoption of cable television has significantly reduced ham radio TVI. But the introduction of IPTV, or television over phone systems, has brought ham radio TVI back, often with a vengeance. Here’s why.
Before cable television, people received television “over the air”. Typically, consumers used rabbit ears or rooftop antennas to pick up local television. When a nearby ham radio or CB operator transmitted, there was a chance of TVI in two different ways. The first source of interference is overloading. In this case, the radio signal enters the front end of the TV and just overloads it; the picture disappears. The second source of ham radio TVI is harmonics. In this case, multiples of the ham’s transmitted signal cause lines of interference in the picture. (See above, left.) There are two standard tools for reducing or eliminating these problems. First, you can place a high-pass filter on the TV antenna input. This will reduce the level of shortwave radio signals that can get into the TV. Second, you can place a low-pass filter at the output of the ham transmitter. This will reduce harmonics leaving the ham station before they get to the television.
You can read a fairly simple technical article about ham radio TVI here if you are interested. But basically here is how it works: a television signal is an electromagnetic wave. A ham radio signal is another electromagnetic wave. When two such signals are on the same frequency, they interfere with each other. Plain and simple.
Do you have or remember an old TV with a channel selector knob? Did you ever wonder why TV channels started at “2” rather than “1”? Well, before 1948 there was a channel 1. In the early days, regulators thought that the channel 1 frequencies could be shared by television with police and other public safety services. That did not work out so well, too much interference. So television channel 1 disappeared.
The introduction of cable television significantly reduced ham radio TVI. For the first time, television signals travelled all the way to your set in a shielded cable. No more rabbit ears. Cable television expanded rapidly during the 1980’s. Bit it actually started in Canada and the United States in 1948. The early motivation for cable television was to improve TV service in more remote communities. Since most TV came from big cities, smaller communities needed a better way to receive distant signals. Cable became very popular in Canada as it provided a way to receive American networks. These could not be received directly over the air.
With cable television, all but the worst cases of ham radio TVI disappeared. The shielding of a coaxial cable, properly installed, reduces the power of the potentially interfering ham radio signal by a factor of one million (60 dB).
Ham Radio TVI and Digital Television
Since 2000, society has switched to digital television. Today, your favorite TV program arrives at your house as a stream of highly compressed data. So, there should not be any ham radio TVI since television is now data rather than an electromagnetic wave, right? Wrong.
The television data needs something to carry it to your home. That “something” is an electromagnetic carrier wave, some form of radio signal. Think of WIFI moving data to your computer, or cell towers sending data to your phone. Cable and IPTV are basically the same. They use a form of radio signal to send the TV data to your home. And not just TV data, but also Internet and phone data.
The cable TV companies use a standard called DOCSIS. This standard provides for a whole range of radio frequencies for sending television, phone and internet data back and forth. The telephone companies use a standard called VDSL. Same idea. A whole range of radio signals to carry data to and from your home.
So, what does this mean for ham radio TVI? Here is what I learned a few years ago.
After years without any ham radio TVI, I switched to TELUS Optik, a form of IPTV/VDSL from my phone company for my triple-play of television, Internet and phone service. Very soon, we noticed that our television stopped working whenever I transmitted on certain shortwave frequencies. This was very annoying, as we were still using the same shielded coaxial cables within the house. Where was the interference coming from all of a sudden?
I will spare you all the details of me trying different things, installing filters, pulling my hair out – and cut to the chase.
It turns out the cable television and telephone companies use different radio frequencies for transmitting and receiving data. DOCSIS sends signals in the very-high and ultra-high range (VHF/UHF). VDSL uses the shortwave radio frequencies (HF) which is where I was transmitting. And, because of the poor shielding at the IPTV receiver, my radio signal was entering it. Being a second radio signal on the same frequency, it caused ham radio TVI. (More technical details are provided at the end of this article.)
The graphic above illustrates the frequency bands for downstream television signals to your home using cable versus IPTV. For comparison, the popular ham radio HF transmitting frequencies are also shown. They share the same spectrum as IPTV but not cable.
So, after my 3 year contract with TELUS Optik ended, I moved back to Shaw Cable and my ham radio TVI disappeared. Bottom line, if you are an amateur radio operator that uses HF, you are more likely to have ham radio TVI with phone-based television service than with cable television.
Technical Note: The industry refers to the ham radio TVI problem as “RF Ingress”, or unwanted signals getting into their system. Both cable and phone companies use digital notch filters in an effort to prevent ham radio TVI. When a ham signal “ingresses” into the IPTV/Cable equipment, it is because of poor shielding. This might be caused by a broken cable or connector, or just simply a poor shielding design in the equipment. When the ham radio TVI interferes with IPTV, the system tries to use error correction on the data. Over time, say 10-20 seconds, the digital error correction becomes overwhelmed, and something the industry calls Bit Error Rate (BER) increases to the point that the picture will “pixelate”. (See top right pictures, above.) Eventually, either the ham stops transmitting and the picture returns to normal. Or, the BER becomes so bad that the picture just freezes. Keith Witney, VE7KW, has a good presentation on these and similar issues.