View Full Version : Transmitting 6 meter filter?
06-03-2002, 05:21 PM
since I'm about to get on 6 meters for the first time ever after being licensed for 40 years, should I insall a Ch 2 TV filter in the output of my transmitter? I have built various filters to reduce TV signal strength on various channels and I'm closely situated to a neighborhood cable tv line and don't want to have any ingress of my 6 meter signal on channel 2. Does this interference happen, or am I getting ahead of myself? I have the parts and know how to build a 2 or 3 section filter to tune to channel 2 and eliminate any RF that may be a potential TVI problem. I would rather eliminate TVI by my filter, rather than creating trouble or an awareness of neighbors with TVI and installing high pass filters. As I mentioned, the filter would be located on the transmitter. Thanks Paul K0YNE
06-03-2002, 08:22 PM
There are several low pass filters that have a very good cutoff frequency of 52 MHz including the Drake TV-1000LP which is good for up to 200 watts output on 6 meters.
With cable TV there is a very good chance that you will not have any problems since the vast number of channel 2 problems come in fringe areas with fundamental overload of the TV front end. Since the cable signal level is quite a bit higher than an off-the-air signal, then you shouldn't have any problems at all.
I run a Heath SB-110A (around 80 watts output) on 6 meter SSB / CW and do not have any problems on any of my cable connected TV sets (3 each) or with any of the neighbors, virtually all of whom are either on the cable or on one of the "dish" type setups. There is a channel 2 in this area, but it is an educational channel that is fed in parallel with channel 13 which has a much higher power level. Thus there are virtually no one looking at it. Also, the cable companies change the frequencies of the channels so that what is actually on channel 2's frequency will probably be something else. In this area it is the TV guide channel that no one watches for any length of time.
If you are planning on running 1500 watts output, then that may be a different matter. However, I don't have any problems with 1400 watts output up to 10 meters with TVI.
06-03-2002, 09:44 PM
Also, let's face it: The world's best "Channel 2" filter probably won't improve (reduce) radiated emissions from a commercially manufactured 6m transmitter that occur within Channel 2's video bandwidth, at all, since typically there aren't any emissions there to reduce.
How would a 50 MHz transmitter generate a signal at any level on 56 MHz? A spurious mixing product resulting from 50 MHz + some unidentified 4 MHz local oscillator that it mixes with? What-? The answer is: There already is no emission from six meter transmitters within Channel 2's band, so a filter that notches Channel 2 by 1000 dB is likely to do absolutely nothing.
The spurious emissions from six meter rigs are all likely to be -40 dBc, and occur primarily at the second and third harmonics (100 and 150 MHz). -40dBc from a 100W transmitter is 10mW.
TVI resulting from local 6m transmissions is 99% (or perhaps 99.9%) fundamental overload due to the receiver's inability to discriminate a 50 MHz signal from a 56-62 MHz signal. As such, any filtering really must be done at the receiver, not the transmitter.
06-04-2002, 02:29 PM
Tnx for the words of advice. I just remember back in the 60's, hams would remark that TVI did occur on channel 2 televison and was the problem with 6 meters in those days. So I was merely basing my concerns on what I remembered, and if precautions were to be made. I plan on using only 100 watts of power into a yagi or quad of some type. I also have tv field strength meters that may indicate if RF will be a problem. Yes, I know other frequencies coming out of the tranmitter are to be 40 db down, but you never know. Thanks again, and I can go on 6 meters with little fear now. 73, K0YNE. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
06-04-2002, 04:28 PM
If you're using a modern, commercially manufactured transmitter, your signal is very likely to be squeaky-clean; however, that doesn't mean you won't generate some TVI!
I've seen cable systems so poorly shielded that they may as well have been built using zip cord. That is the real problem, not your transmitter.
Of course, if you use a beam antenna, and get it high above all your neighbors' "heads," so to speak (that is, get the beam 20-30' or more above their rooftops, so you're not "firing" into their equipment), TVI is far less likely.
73 & hope to catch you on six!
I wonder if you really mean 10 or 11 meters instead of 6 meters. #11 meters has its 2nd harmonic right in channel 2 tv, 54 mhz and 10 meters, in and around 56 mhz. #Both of which would be very bad and you would want to add an outboard filter to your rig to knock the second harmonic down another 80db like the Bencher YA-1.
Both of those bands play havoc with an unprotected TV.
WB2WIK is exactly correct, that most if not all 6 meter TVI problems are from fundemental overload and not some weird harmonic or mixing product.
06-04-2002, 11:57 PM
Your garden variety low-pass filter has always worked for me. Even in the days before cable and sat. TV.
73 de Craig......N7CPC http://www.qrz.com/iB_html/non-cgi/emoticons/wink.gif