Frequency modulation deviation increases with the amplitude of the audio being fed into the transmitter. If the audio level is too high, the transmitter will over-deviate (the carrier frequency swings more than it should). To reduce deviation you need to reduce the audio level reaching the transmitter — for example, speak more softly or move slightly away from the microphone so your voice is quieter at the mic.

Memory aids:

• FM deviation is proportional to the amplitude of the modulating audio (louder = more deviation).

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If a nearby amateur transmitter is so strong that a broadcast AM or FM receiver cannot reject it, the broadcast radio can pick up the amateur transmission even though it is outside the AM or FM band. This effect is receiver overload: the front end or intermediate stages of the broadcast receiver are driven into a non-linear region and the unwanted strong signal is heard.

High microphone gain or an overloaded audio amplifier in the transmitter can cause distortion or splatter, but they do not move the transmitted signal into a distant broadcast band. Likewise, setting FM deviation too low makes the received audio quiet or distorted, not cause the signal to appear on an unrelated broadcast frequency. The root cause in this case is the broadcast receiver's inability to reject very strong out-of-band signals (overload).

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All three listed phenomena can cause radio frequency interference.

Fundamental overload - This occurs when a received signal is so strong that it overloads the receiver's front end or intermediate stages, preventing proper reception of the desired (weaker) signals.

Harmonics - If a transmitter's output is not a pure sine wave, it contains harmonics (integer multiples of the fundamental frequency). Those harmonic signals can appear on other frequencies and interfere with receivers operating on those frequencies.

Spurious emissions - This is a catch‑all term for undesirable transmissions from a transmitter other than the intended signal, including harmonic emissions, parasitic oscillations, intermodulation products, and frequency conversion products; such emissions can interfere with normal reception.

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Low RF output from a modern solid-state rig is often caused by high SWR.

With a high SWR, power is being reflected back from the antenna system. Solid-state radios don’t like that, so they usually reduce their own power (“fold back”) to protect the finals. That shows up as “low power output” on a meter.

Why the others don’t fit:

• Noise figure affects how well you hear, not how much power you transmit.
• Linearity affects how clean your signal is, not its raw power level.
• Low SWR is what you want and won’t cause low power by itself.

So if your radio suddenly shows low power, a bad match / high SWR is a prime suspect.

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The non-amateur over-the-air radio is being overloaded because it is receiving signals in the amateur band. A filter at the antenna input of the affected receiver will block or attenuate the amateur-band signals before they reach the receiver's front end, preventing overload and interference.

Putting a filter on the amateur transmitter does not help because the transmitter would still radiate signals; the radio would still pick them up. Changing the transmitter mode (for example FM to SSB) or reducing its bandwidth does not prevent the receiver from receiving energy in the amateur band, so those changes are unlikely to stop the overload. The only effective fix in this situation is to stop the unwanted signals from entering the victim receiver by using a suitable filter at its antenna input.

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If you can't keep your signal out of your own TV, it's unlikely that you can keep it out of your neighbor's. It's much easier to experiment with your own TV to determine whether your station is the source of the problem.

First make sure that your station is functioning properly and that it does not cause interference to your own radio or television when it is tuned to the same channel as the affected receiver. If your equipment does not cause interference to your own equipment, the problem may be in the neighbor's antenna, cabling, or their TV/cable system (for example, your signal could be getting into their cable feed line).

Tuning your receiver to the same channel as the affected device helps determine whether the problem is caused by fundamental-frequency radiation from your transmitter or by harmonic radiation. If harmonics are the issue, they can often be reduced with filters at the transmitter or by correcting spurious emissions.

Memory aids:

• "If my TV hears it, so will theirs" — test with your own equipment first.
• Tune to the same channel to distinguish fundamental-frequency interference from harmonics.
• If your setup is clean, inspect the neighbor's antenna, cabling, or cable-system connections.

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The best way to eliminate the interference is to not let it in your radio. Just like if you have a noisy roommate, the best way to get rid of the noise is to keep them out of the room in the first place!

A band-reject (notch) filter removes the frequencies used by the FM broadcast station before the signal reaches your transceiver, preventing the strong nearby broadcast from causing interference or front-end overload.

Memory aids:

• FM station plays music; "Band" reject filter

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Start by checking your own station to make sure it follows good amateur practice — proper grounding, good connectors, decoupling, filtering, and correct transmitter operation reduce the chance that the problem is on your end. If your station is in order, cooperate with your neighbor to locate the source of the interference. A practical method is to have the neighbor turn off suspected devices one at a time (computers, peripherals, LED drivers, switching power supplies, etc.) until the interference stops. Once the offending device is identified, politely discuss the problem with the neighbor; most people are willing to help if they understand the situation. You can also explain that intentional or negligent emissions that cause harmful interference are not allowed under FCC rules, and suggest remedies such as fixing, filtering, or replacing the device.

For more information see: http://en.wikipedia.org/wiki/RF_interference

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The correct first step to resolve non-fiber optic cable TV interference caused by your amateur radio transmission is to be sure all TV feed line coaxial connectors are installed properly.

Poorly installed or loose coaxial connectors can act like antennas and allow strong RF signals from your transmitter to enter the TV system directly, bypassing any built-in filtering. This is especially common with cable TV systems that use coaxial cable but are not properly shielded or grounded.

Before adding filters or other devices, always check the basic physical setup. Make sure all connectors are tight, clean, and properly shielded. This simple step often solves the interference issue without needing more complex fixes.

Once all connections are confirmed good, and if the interference still occurs, then additional filtering (such as high-pass filters) may be considered depending on the frequency involved.

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Each of these problems can make your audio through an FM repeater sound distorted or unintelligible.

If your transmitter is slightly off frequency: When the transmitter is not exactly on the expected receive frequency, the repeater's receiver (or the listener's receiver) may not demodulate the signal correctly. That mismatch can make the audio sound garbled, low in volume, or distorted because the receiver is operating away from its optimum tuning point.

If you are speaking too loudly or too close to the microphone: Talking too loudly or positioning your mouth too close to the mic can overdrive the transmitter's audio input. Overdrive causes clipping and excessive deviation on FM, which produces distorted audio and can trigger processing in the repeater that makes speech less intelligible.

If you are in a bad location: Multipath reflections, terrain shielding, or other local propagation problems can cause fast fades, phase cancellation, or weak signals at the repeater. Those effects can make the received audio break up, sound hollow, or be otherwise unintelligible.

Any one of these issues — off-frequency operation, microphone overdrive, or a poor location — can produce distorted or unintelligible audio through an FM repeater.

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Distorted transmit audio is often caused by RF getting into the mic line and feeding back into the radio’s audio circuits.

A clip-on ferrite choke on the microphone cable helps block RF on that cable, so the mic only carries audio, not stray RF. That can clean up distorted or “RF crunchy” audio, which is why that choice is correct.

Why the others don’t help:

• Extra feedline to “fix” SWR doesn’t solve distortion (and is bad practice).
• Power-cycling the radio rarely fixes RF feedback problems.
• Cranking the squelch only affects received audio, not what you transmit.

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