DTV Interference on VHF Channels 4–13

Such second-order distortion products may be generated by strong local FM radio signals in the front end of DTV receivers and downconvertors.
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In the June 10 issue, this column reported on a possible DTV reception problem for Channels 7–13. This would be due to interference from second-order distortion products; second harmonics of FM signals and beats between pairs of two FM signal frequencies, which create a new spectral component falling in the high VHF band.

FM INTERFERENCE POSSIBILITIES

Such second-order distortion products may be generated by strong local FM radio signals in the front end of DTV receivers and downconvertors. I called this FMI.

There are 38 TV channel allotments in the low VHF Band. Third-order intermodulation products (2* F1 - F2) generated by pairs of FM signals, which overload DTV receiver front ends fall into channels 4, 5 and 6. F1 is lower than F2.

For example: if F1 = 90.1 MHz and F2 = 95.1 MHz the third-order product is at 85.1 MHz in Channel 6. If F1 = 88.1 MHz and F2 = 107.9 MHz, the third-order IM (68.3 MHz) falls in Channel 4.

The maximum authorized power of low VHF band signals may be may be quite low (5 to10 kW). In that same community there may be two or more FM stations with as much as 100 kW ERPs radiating in a horizontal plane, and it’s possible they also radiate up to 100 kW with vertical wave polarization. FMI can affect Channels 4–13, not just Channels 7–13 as reported earlier. However the vast majority of DTV allotments are in the 174–216 MHz band compared to the number in Channels 4, 5 and 6.

One of my readers, Terry Howard of WSIU, commented that my column did not mention amplified indoor antennas as devices that can generate FMI. When I wrote that column, none of our local stations had yet switched their DTV signals to their analog VHF high band channels.

Since the Great Transition I was able to run experiments that proved how right he was. Terry also reported that an FM trap fixed the Channel 8 reception problem he had observed. Thank you, Terry.

When I started to write this column it occurred to me that third-order inter-modulation products could cause FMI to low band VHF Channels 4, 5 and 6. It takes at least two to generate intermodulation products. These are of the form: 2* F1 – F2 where F2 > F1. So channels 4, 5 and 6 can only suffer FMI where there are two or more local FM transmitters.

Another of my readers, Stan Knight, generated a spreadsheet showing the FM carrier frequencies allotted to the Portland, Ore. market. His spreadsheet supplies all the second-order distortion products that can be generated in overloaded Amplified indoor antennas, DTV downconvertors and DTV receivers. Many thanks to Stan for this spreadsheet.

Second harmonics of FM carriers are shown in light type, while each of the far more numerous second-order intermodulation products (F1 + F2) are shown in bold type. Not only are these sum frequency components more numerous, they are also 6 dB stronger than the second harmonics (the ones in light face type).

Figure 1 gives the number of second harmonics and the number of beat frequencies for pairs of FM signals in channels 7–13 near Portland, Ore. What is important here is that these distortion products tend to fall in the middle of the high VHF band, not just in Portland, but also in most other cities.

For example, Channel 10 is subject to 63 such distortion products, while Channels 8 and 12 are each subject to 31. These numbers are indicative of the fact that high band VHF channels near the center of the band are more likely to suffer FMI than those near its edges.

I believe that the total noise power in a given channel is what counts. Not all FM radio stations radiate 100 kW, but there are many operating with ERPs above 10 kW.

While an FM signal is “narrow band,” the numerous second-order distortion products in major markets make FM1 a wideband signal. Fig: 2 can serve as a model for you to generate your own market’s FMI data in order to understand the probabilities that exist in your market for FMI. Fig. 2 is a portion of Stan’s spreadsheet.

EXPLORING POTENTIAL DTV INTERFERENCE SITUATIONS

First, identify all local broadcast FM frequency allotments. I did this with my spectrum analyzer. Then, list them in the left most column and also in the top row of your spreadsheet. Double each allotted frequency and put that frequency in light type where the row and column have the same frequency. Then calculate the beat frequencies (F column + F row) and place these frequencies, in bold type where the F column and F row intersect. Stan color coded these to identify all distortion products that fall in each channel. I highly recommend that you follow this practice.

When you have completed your FMI spreadsheet, you will be able to determine which local high band VHF stations could suffer from FMI in your market. If your DTV channel is found to be subject to FMI, you can buy a 75 ohm FM band stop filter to determine whether FMI is causing your reception problem.

Where your market includes Channel 4, 5, and/or 6 DTV allotments, a second spread-sheet can be created.

AMPLIFIED INDOOR ANTENNAS

So far, I have tested only a few “amplified indoor antennas” and also some indoor antennas with a separate preamplifier. More to follow. Please don’t ask about specific brands or model numbers, but they are widely distributed products. One of these has a fixed gain, assuming it is switched to the “on” position. When it isn’t powered, it doesn’t pass the antenna signal to its output port.

Having two passive indoor antennas, I put one where the amplified unit was tested and received all local DTV stations. This proves that an amplified antenna is not needed here. Amplified antennas are very small and probably would not have provided enough gain on all stations without the amplifier operating, so any bypass feature would be of little help.

But, perhaps the signals here are too strong here for amplified antennas. My two indoor passive antennas are MegaWave brand devices. I also bought one of the MegaWave branded amplifiers when I purchased these antennas five or six years ago. With the MegaWave amplifier I could receive all local stations, and my spectrum analyzer showed very little, if any, FMI on vacant Channels 7, 9, 11 and 13. This shows that there are some amplifiers that are linear enough to not generate FMI. But how is a consumer going to know which are good and which aren’t so good?

As part of my testing I purchased an amplified indoor antenna from a national chain of retail stores. This one has a gain control feature, but no bypass. I was surprised to find that it worked quite well here when the gain control was set for maximum gain. However, if the gain was reduced, the amplifier generated FMI and reception abruptly failed.

Another amplified antenna was found by reader Howard to oscillate under certain conditions when adjusting its “rabbit ears.” Of course when it oscillates, it’s performing an illegal act and becomes quite useless to the viewer who has no idea that he is operating an illegal transmitter. Recently the FCC has found other instances of oscillating amplified TV antennas.

For some years, I have advocated the use of a low noise preamplifier at the antenna for “fringe area” DTV reception. I purchased a Channel Master model 7777 mast-mountable low noise amplifier unit, and thought that I should see if it could operate over a 13 mile, line-of-sight path. I was gratified to find that it works well with the signal levels at my present location. I saw no FMI in its output spectrum. So now I know that low noise pre-amplifiers are available that can operate without generating FMI.

CONCLUSIONS

So, where does this leave us?

My advice is to find out whether a reception problem involves the use of any amplifying devices. These would most certainly include so-called distribution or line extending amplifiers, especially in apartment houses, motels, or anywhere that multiple receivers operate from one antenna. While the RF distribution system in a multiple-unit building may have served well enough for analog TV, it’s unlikely that it can cope with much weaker DTV signals.

If there’s an active indoor antenna, temporarily replace it with a good passive indoor antenna. If that doesn’t solve the problem, add a really linear low noise amplifier such as the Channel Master 7777. If you have FMI, an FM band stop filter such as the Microwave Filter Co. model 3367-FM will confirm that this is an FMI problem.

Acknowledgements: I wish to thank Stan Knight for preparing the spreadsheet mentioned in this column.

It was a considerable effort, but well worth the trouble.

Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via e-mail atcwr@bootit.com.