What is the difference between a low pass filter and bandpass filter in a radio?

Johnson Francis | February 3, 2026 | Amateur Radio (Ham Radio) | No Comments

In radio circuitry, filters are the “gatekeepers” of frequency. They ensure that your receiver only processes the signals you want to hear and that your transmitter doesn’t bleed interference into other bands.

The fundamental difference lies in which part of the frequency spectrum the filter allows to pass through.

1. Low Pass Filter (LPF)

A low pass filter allows all frequencies below a specific “cutoff frequency” to pass through while blocking (attenuating) everything above it.

Primary Use in Radio: You will most commonly find these at the final output stage of a transmitter.
The Problem: Transmitters often generate “harmonics”—multiples of your operating frequency. For example, if you are transmitting on 7 MHz, your radio might accidentally emit a weaker signal at 14 MHz or 21 MHz.
The Solution: An LPF with a cutoff of, say, 8 MHz will let your 7 MHz signal out but “trap” those higher-frequency harmonics so they don’t interfere with other services (like neighbors’ TV reception or aircraft bands).

2. Bandpass Filter (BPF)

A bandpass filter is more selective. It allows a specific range (or “window”) of frequencies to pass, while blocking everything both above and below that range.

Primary Use in Radio: These are essential in the “front end” of a receiver.
The Problem: A radio antenna picks up everything—from AM broadcast stations to local emergency dispatch and cosmic noise. If all that energy hits your receiver at once, it can overload the delicate internal circuits (desensitization).
The Solution: A BPF acts like a narrow tunnel. If you are tuning into the 20-meter ham band, a BPF will block the low-frequency AM stations and the high-frequency FM stations, allowing only the 14.0–14.35 MHz range into your radio’s mixer.

Comparison at a Glance

Feature	Low Pass Filter (LPF)	Bandpass Filter (BPF)
Passes	Everything below the cutoff.	Only a specific frequency “window.”
Blocks	High frequencies (harmonics).	Everything outside the desired band.
Common Spot	Transmitter Output.	Receiver Input / Intermediate Stages.
Complexity	Generally simpler (fewer components).	More complex (requires tuned circuits).