Television Notch Filters
TV Interference -
As stated, TVi is very difficult to remove, and understand why we have it in the first place. It is essential to eliminate all known factor's, one by one. Test measurements noted before carrying out the station refurbishment showed that interference occurred with only a few watts of transmitter power. After refurbishment, no detectable interference was present with 100 watts, on any TV channel.
Traditionally it has always been good practice to fit at least one good quality low pass filter (LPF). The purpose of fitting a filter at the transmitter output, is to reduce or hinder the passage of any RF above 30 Mhz. Unless the manufacture provided a graph, or details of its response, you will be in the dark as to what it really does. At best, fitting one is very misleading and unless it is tailored for your requirements will not effect a cure. I have provided a table of amateur bands and harmonically generated frequency's, that fall within Band One TV channels. See Excel table
In the early days, sensitivity of TV sets were very poor, several hundred micro volts needed for a good quality pictures. My latest TV purchase, (TVi Monitor) is an LG model from Korea, which produces a viewable picture with only a few micro volts.
At my home location, I have measured the TV reception signal to be around 200 micro volts, on the end of a pair of rabbits ears, the type of antenna normally attached to the rear of the TV set in this area. We now have our fist set of known measurements that can be used in the design of our specific filters.
To understand the effectiveness of our filter, usually expressed in Decibels relative to one watt, it is necessary to subtract the filters insertion loss at the specified frequency, and convert the result into micro volts. We can then visualize a better picture of the progress that we are making and the necessary insertion losses required relative to our TV interference.
Decibel measurements are relative in expressing a given power or voltage ratio. I have chosen to work in pure micro volts relative to 50 ohms, as this is a tangible figure which can be measured, or visualized in our case at the TV antenna plug. If you own a receiver that covers VHF the S meter is already calibrated in micro volts, and will serve as a very useful tool.
I have constructed a working table that carries out all conversions, and is easy to see that with a given transmitter power, how many micro volts are radiated from our antenna after fitting our filter. An example of a standard situation is that with a power output of 100 watts the published figures shown in my TS940 service manual, is that all harmonics are -30Db down on carrier. This then leave 1 watt radiated into the surrounding air at 56Mhz before filtering. If we convert 1 watt into 50 ohms into micro volts the lever entering our television set is 223.6 micro volts. See DB Conversion Table
Remembering that our measured TV reception signal level is 200 micro volt, it is no wonder that we have TV interference. If the exercise is repeated for 1000 watts, then the interference to our TV receiver is a staggering 7 milli Volts.
Using the Excel spread sheet to calculate the filter loss required, is easy. By inserting loss required to take the harmonic down to say 50 micro volt with 1000 watts, shows we require a filer insertion loss of -120 db. Taking a best guess that our old LPF inserts typically only -60db we can now see why our filter have very little, or no effect in reducing TV interference.
We could insert several filters in series, but without knowing the exact specification, is of little use. I personally ordered two high quality filters from MFJ USA. Which had no effect in curing my interference. When I had both analyzed with a spectrum analyzer, the maximum attenuation occurred well outside any harmonic's generated by my transmitter.
The filters arrived without any specification sheet. The internal construction being open layout, implied that the filters were unlikely to achieve any significant attenuation anyway. The insertion loss on ten meters was in the order of several dB's not a good show for MFJ. The filters were returned for a refund.
The problem arose due to none compartment screening of tuned circuits, and lack of a suitable enclosure, to contain the RF at power levels. The same filter made under better engineering condition, and better work shop facilities, I fell is an excellent filter worthy of perusing at a later date.
My second attempt at making a filter had to be better, so set out to find a suitable RF tight enclosure, which was a major contributing factor of the failure of my first filter. Also a design that did not require home made high voltage capacitors.
Constructing Coaxial filters
I started with one stub (in the program) cut for 56Mhz and added inductance matching to both in and output ports. This improved the notch depth, then added another section, and another. With three stubs the notch at 56 Mhz is around 100 db, not bad, but still short of the required -130 db. I then added a small value capacitor across each inductance and the rejection, when exceeded -200db (in theory)
One nice effect, was that with the stub cut a few hundred Kilocycles either side of 56.7 Mhz the band pass and rejection notch did not alter. Staggering the stubs a few Mhz up or down only alters the band pass width at 56Mhz. Effectively broad banding the response, still well down in the - 150 dB's region.
The inductance matching for input and output ports and inter-stage coupling only effected the VSWR insertion below 30 Mhz and had no effect on the rejection notch. See Coaxial filter design and response.
Test Set Up
Inserting the coaxial filter into the stations antenna showed excellent VSWR especially on the 10 meters band. TV interference at around 600 watts was clear. Regretted under certain circumstances above 600 watts interference is still present. This is described in the next chapter concerning the rusty bolt effect.