Hello all,
One of the common sense rules involving transmitting on the FM band is not to cause interference to other users of the spectrum, and this is also made clear in the FCC rules. In order to avoid interfering it is helpful to know what can happen to cause interference.
Hello all,
One of the common sense rules involving transmitting on the FM band is not to cause interference to other users of the spectrum, and this is also made clear in the FCC rules. In order to avoid interfering it is helpful to know what can happen to cause interference.
On another board, I replied to a post where the author was stressing the importance of avoiding interfering with other spectrum users. I thought it would be of general interest to post a lightly edited version of my reply here and I hope this is the case.
When I wrote this I had in mind a scenario where a FM kit is connected to a power amplifier, but the technical information applies to any transmitter with or without an amplifier.
I offer some information regarding one of the part 15 FM operators "rules" to keep out of trouble, regarding interference to other radio services. Commonly on the amateur radio 2 meter band an amplifier is used to boost the power of a transceiver and some of what I offer is based on my experience there. It has been discussed here about filters which are used to supress harmonics and this is easy and effective, but other things cause interference. For example, if the power delivered to the final amplifier stage is not "rock solid" and properly bypassed or if it is not properly neutralized the amplifier can oscillate at almost any frequency on its own. Usually this is in the MHz. range and this adds to and subtracts from the operating frequency. Harmonic or low pass filters will not remove this and worst of all your signal will sound fine and you won't even know it is happening. I know this from experience where I received a call from the Maderia, Ohio police dept. telling me I was interfering with their 155 MHz. channel while operating mobile on the 146 MHz. ham band. I was five miles from the Maderia PD. I also receive a call from a ham at about the same time who was 8 miles away and he told me I was interfering with other hams off my chosen frequency. I had no clue that this was happening even though I was operating with a legal power (10 watts) and with a carefully checked amplifier. This problem was solved when I junked the amplifier.
You can interfere with TV and radio reception by causing "front end" overload regardless of the filtering. Your signal can radiate directly into the IF stages of receivers. Your signal can leak into a loose cable TV connection and be sent down the line. Any nonlinearity in the antenna system or in anything nearby can cause harmonic radiation regardless of your filtering. Your signal can mix in this nonlinearity with other signals and appear anywhere in the spectrum. I have experienced all of these except cable leakage on the ham 2 meter band. I even managed to halt a mainframe computer with my 2.5 watt handheld transceiver. (try to explain to the IT guys that the transmission was legal).
So, by limiting the radiated field strength on part 15 FM, the FCC is minimizing the chances of interference if one of the conditions I mention happen since your signal and thus your interference will be very weak and range limited.
By operating at 1 watt, etc. the chances of unintended interference are increased and though you have the best of intentions you can violate the no interference rule and not even know it. Keep in mind that police, fire, EMT, and aircraft use the bands even in the wide open spaces out in the country.
Just some information that I hope is useful.
Neil