[TenTec] Low Pass Filter

[JAMES HANLON]

Bob,

Let me first apologize for not replying sooner.  I get the digest from the reflector, so I have some delay before I get to see your messages.

You asked, "IF the transceiver had harmonics and those were transferred to the linear, 
would they not be amplified by a factor of ten? (at  least where the 
harmonics fell into the range that the amp might be able to work on them) 
Then, if they were filtered they might not be so quiet as to be 
insignificant?"

I think the answer to your message is "it depends upon the characteristics of your amplifier."  If your amplifier has a broadband response that extends into the range where the harmonics exist, then yes it would amplify the harmonics as well as the fundamental signal.  In that case, eliminating the harmonics before the amplifier would make some sense.  If the amplifier's response is limited to a single amateur band, however, it would not amplify the harmonics.  

Think for a minute about what most Low Pass Filters are designed for and intended to do.  They generally have a cutoff frequency somewhere between 10 and 6 meters.  So any signal presented to them below their cutoff frequency will pass through them with little or no attenuation.  Above their cutoff frequency they provide a very poor match to signals trying to get through, and those signals get reflected back from the filter interface.  So, if you are working on, say 40 meters, and there are harmonics at 14, 21 and 28 mc presented to the filter, they will pass through the filter.  If the load beyond the filter is a reasonable match to those signals, for example if it is a multiband vertical antenna that looks like a 50 ohm load on those bands as well as on your 40 meter frequency, then those harmonics will radiate even though you have a low pass filter in the line.  But if you had an 8th harmonic at 56 mc, within TV channel 2, that would be attenuated by the filter.  So the result would be that you might have some lower frequency signals that bothered your fellow amateurs on 20, 15, and 10, but you wouldn't have a signal that would interfere with your neighbor or your family watching channel 2.  

So, the bottom line is that a single filter in the output of your amplifier is probably a good insurance policy to prevent TVI.  It will not effect your signal in any of the ham bands below its cutoff frequency, nor will it attenuate any of your harmonics or other spurious signals below its cutoff frequency.  An additional filter between your transceiver and your amplifier is probably overkill, but it won't do any harm.  

If you have TVI, you can get a good idea of whether the problem is being caused by your transmitter by observing a few of the TVI's characteristics.  If it is being caused by a signal from your rig, it will effect ALL of the TV sets in the area, decreasing as you go further away from your transmitter.  If it is a harmonic or a parasitic oscillation on a single frequency, it will effect only the specific TV channels where it lands.  In those cases, a low pass filter on your rig should help.  On the other hand, if some TV sets are effected but others are not and they are in the same vicinity, then the problem is not a signal in the TV range coming out of your transmitter but rather it is a case of "fundamental overload" in the effected TV set.  Your amateur band signal, not in the TV channel, is just overpowering one or more stages in the TV and that stage is responding by generating its own set of spurious signals and harmonics or is pushed out of its normal bias range into saturation or something similar.  The only way to cure that kind of "fundamental overload" TVI is to reduce the level of your signal at the TV set so that overload no longer occurs.  You could do that by reducing your power or by relocating your antenna to reduce the level of your signal at the TV set.  But if you don't want to do those things, the TV set owner could also reduce the level of your signal by installing a high pass filter in his antenna or cable lead that attenuated signals below it cutoff frequency, usually somewhere below channel 2.  In particularly difficult cases the set owner may also need an RF filter in his 120 volt power line, a ferrite donut core with the line cord wound through it for several turns might be enough.  And in extreme cases the set itself might even need some additional shielding.  All of those things would be the responsibility of the set owner, not yours.  

Just about all modern transceivers are designed so that they do not radiate signals in the TV range.  So I would expect that any TVI these days is of the "fundamental overload" variety.  In most cases a Low Pass Filter on a modern rig is not necessary, but it won't do any harm.  So if it makes you feel better, go ahead and get one and put it in the line.  At least you can tell your neighbor that you have one and that the problem is now his, not yours.

73,

Jim, W8KGI