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What to do, if your neighbour comes banging at your door just in that moment you want work the DX of the year ? Here is a solution. The only thing you have to do is to persuade your raging neighbour to let you improve his rotten TV system ...
In any normal case there is no need to put any filter in front of the TV or BC receiver. But in many cases people buy a cheap wide band amplifier and put it behind the antenna. In my case - and this is almost unbelievable - a professional antenna-man had built in such a wide-band device AND declared the receive system to be good - even after the advice, that there was a ham antenna 15 m away.
The filter I designed shall pass the TV frequencies beginning with 175 MHz. A 5 pole high-pass will do the job. The filter is fed via 22 pF capacitors. All components are SMD (surface mounted device) components, even the inductors. This means that the Q is relatively low.
The insertion loss at 175 MHz is around 2.5 dB, at 220 MHz I measure 1.5 dB, and in the UHF Band at 470 .. 690 MHz I see around 1.2 dB (without the 432 MHz stub). Introducing the 70 cm stub results in about 7 dB attenuation at the lower UHF channels at 480 MHz, but decreases to less than 2 dB at 550 MHz.
The band stop of these frequencies is excellent:
28MHz and below is better than -60 dB. 50 MHz is also below -55 dB.
144 MHz is problematic: Because of the low - Q of the inductors the attenuation is only about 18 dB for a series resonant circuit to cut 144 MHz. Here I made one circuit at the input and one at the output of the whole filter. This gives an attenuation of about 25 to 30 dB.
Do NOT use a quarterwave stub, because it will cut out frequencies in the TV bands with its harmonics.
On 432 MHz you have to add a quarterwave stub at one port, if you have need to do so. I really RECOMMEND to use Teflon type cable like RG400 or RG142. This has high Q and is quite narrowbanded.
Normally you do not need to place a blocking device. Sometimes however the amplifier devices are so broadbanded, that a bandstop becomes necessary. Here I recommend to make the 70 cm stub, because its 3rd harmonic cuts 1296 MHz.
Up to now I made 5 pieces of this filter. All do have the same performance. I use a simple experimenters' board with a pad distance of 1/10" (2.54mm) and I cut the board to 25 * 25 mm (1" * 1"). All components are SMD (surface mounted device). I used the small 0603 size, but larger 0805 or 1206 will do the job too. Try to keep the connections between the components as short as possible. I placed a soldable plug and socket for the European IEC norm on the sides of the board. If you do not have them, you may well take a short run of 75 Ohm coaxial cable with this type of connectors.
432 MHz Stub
I use to cut a 15 cm length of RG400 Teflon coaxial High-Q cable. I solder it as short as possible close to one port of the filter. I use to trim the open length of stub by putting the filter in front of a handheld rig on 430 MHz and a tiny RF meter behind. By carefully cutting the length you will see the power decreasing from 1 Watt to the 1 Milliwatt region or less. Stop cutting if the power tends to increase again. With RG400 cable you can easily achieve 36 dB of attenuation; with RG174 you will get around 20 dB, but VERY broadbanded. RG 225 Teflon is a not too bad compromise. I could not transmit with more than 200mW on a 6 element on 70cm before. After inserting the filter, EME power levels were no problem.
Time efficiency ...
If you have everything at hand, you can build 5 or more of the TVI stops within one hour.
I have shown a way to minimize frustration with with the neighbours and to maximize fun on the ham bands ... and this at very low cost. :-)
Have fun and 73
Text and All Images are Copyright by Christoph Petermann DF9CY
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