How to Buy Oscillators and Filters for a Ham Radio Enthusiast

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How to Buy Oscillators and Filters for a Ham Radio Enthusiast

Amateur radio operation, more commonly known as ham radio, is a long-standing tradition and amateur hobby which provides myriad individuals around the world with a way to socialise, communicate, and build relationships. Friends of ham enthusiasts who may want to find a unique gift for members of this community might consider purchasing a new filter or oscillator for a ham radio operator. These essential devices drive much of the quality of a transceiver and are coveted by members of the ham radio community.

Before initiating a purchase, however, individuals may want to learn about the equipment and how oscillators and filters work. Learning the technical details behind these components and how to make an informed selection can help facilitate the purchasing process when looking for an oscillator or filter as a gift. Consumers can also benefit from understanding some of the better retail options when it comes to purchasing oscillators and filters. While traditional vendors can offer some options, online aggregators such as on the online auction site, eBay, extend a wide array of choices to consumers.

What is an Oscillator?

An oscillator is a key element of a ham radio which plays a central part in both the transmitting of signals and in the receivers that accept incoming signals. Oscillators essentially work as amplifiers with a tuned circuit that can be an LC circuit-based or crystal-based component. The type of tuned circuit used in an oscillator basically defines the output frequency of the oscillator. There are three common types of oscillators used in ham radios, Colpitts oscillators, Hartley oscillators, and Pierce oscillators.

Colpitts Oscillators

Colpitts oscillators use inductors and capacitors to create the oscillator frequency. A Colpitts oscillator is a type of LC oscillator and has the distinguishing characteristic of leveraging a voltage divider made of two capacitors to create the feedback needed for oscillation.

Hartley Oscillators

A Hartley oscillator is another type of LC oscillator which uses two inductors, one of which is tapped, and one capacitor to function. Hartley oscillators work in all broadcast bands, including the range of FM bands.

Pierce Oscillators

A Pierce oscillator was derived from the design of the Colpitts oscillator, but uses a crystal to create a piezoelectric effect to generate its oscillation. It is used in a wide range of consumer electronics in addition to its application within ham radio technology.

The Advantages of Colpitts, Hartley, and Pierce Oscillators

Each of these oscillators extends a particular set of advantages to users looking to install one in their ham radios. The following chart lists the various advantages of each unit and how it can affect the performance of an amateur radio.

Oscillator Type


Colpitts Oscillator

Performs well at high frequencies; remains stable at high frequencies; has a wide operating range of 1 to 60

Hartley Oscillator

Output amplitude remains constant; rich in harmonics; simple structure

Pierce Oscillator

Extremely reliable frequency stability

Consumers looking to purchase an oscillator should carefully consider their needs and which component extends the greatest advantages with regards to their ham radio. During the purchasing process, individuals can also communicate with sellers to confirm the applicability of specific oscillators to their set-ups.

What is a Filter?

A filter is a processor which allows some signals to pass while tamping down or filtering out others. Filters in amateur radio, also referred to as Radio Frequency or RF filters, can be used for several different functions within a ham radio, including receiving and transmitting, with some acting as multi-purpose devices and others acting in application-specific ways.

Ideally, a filter should demonstrate no signal loss within the allowed pass band, while signals in the stop band above the cut-off frequency are rejected entirely. In practice, a filter never works with this much exactitude, hence, there is occasionally some loss in the pass band and some leak in the stop band.

Understanding RF Filter Frequencies

A filter allows signals through what is called the pass band. The pass band is the band of frequencies above or below a specific cut-off frequency. The cut-off frequency is sometimes referred to as the 'half power' of the unit or the '-3 dB frequency' due to the fact that it occurs at the point where output from the filter is at 50 per cent or -3 dB of the in-band level. The stop band of a filter includes the frequencies that the filter is meant to reject.

Types of RF Filters

There are four main types of RF filters: High-pass, low-pass, band-pass, and band reject.

High-Pass and Low-Pass Filters

A high-pass filter is one which allows signal flow above a specific cut-off frequency and attenuates the signal below that frequency. A low-pass filter functions in the opposite way. It creates a cut-off demarcation below which frequencies are permitted while attenuating the signal above that cut-off. Both high-pass and low-pass filters have complicated structures. Many amateur radio enthusiasts buy them ready-made, instead of attempting to engineer them.

Band-Pass and Band Reject Filters

A band-pass filter is one that allows transmission of a selected band of frequencies with limited signal loss and attenuates any frequencies either above or below that band. Band reject filters perform the inverse and are useful for neutralising unwanted signals within a given bandwidth.

Filter classifications

Within these four types of RF filters, there are specific classifications of filters which focus on specific performance needs. While there is a wide range to filter classifications, there are three common types that are particularly relevant in ham radio: Butterworth, Bessel, and Chebyshev.

A Chebyshev filter delivers rapid roll-off upon attainment of the cut-off frequency. This causes more in-band ripple, but if high in-band ripple can be tolerated, a Chebyshev can deliver very steep roll-off. A Butterworth filter delivers maximised band flatness, enhanced group delay performance, and as a result, lower overshoot with a lower level of stop-band attenuation in comparison to a Chebyshev filter. A Bessel Filter delivers ideal in-band phase response and solid step response. It is the best filter for maintaining wave shape.

Filter Applications

Filters in the receiver of a ham radio are used to optimise the incoming signal for specific applications, such as voice communication or morse code. The following chart lists some of the filters often used in receivers and describes their respective functions.

Receiving Filter Type


Front-End Filters

Good in heavy interference settings, such as near high-powered transmitters or multi-transmitter stations

Voice Filter (2.4 kHz)

Passes radio spectrum wide enough to give colour to the human voice

CW Filter (500 Hz)

Ideal for transmitting morse code

Narrow Filter (250 Hz or less)

Ideal with severe interference

High-Pass Filter

Neutralises interference from televisions and media players

This is just a summary list of some of the applications for RF filters in amateur radio. Amateur radio enthusiasts share and develop filter applications for specific needs constantly. When shopping for a filter, consumers can benefit from proactive communication with a vendor and within the ham radio community to assess the best application for a specific filter.

Buying Filters and Oscillators on eBay

The online aggregator, eBay, brings together numerous sellers to create a large catalogue of options when it comes to ham radio filters and oscillators. If you want to look through eBay's catalogue and find one of these items, go to the site and find the search bar located at the top of any page on the site. Enter a search term relevant to your needs and click 'Search.' This pulls up a list of items that relate to your keyword. If you see a listing which seems relevant to your needs, click on the title and review the listing in detail.

Evaluating Sellers on eBay

Evaluating the sellers on eBay is an important part of the purchasing process, especially when purchasing items as technically specific as filters and oscillators. If an item catches your eye, take a moment to look through a seller's list of past auctions and review customer feedback the vendor has received from others. This can help you determine whether a vendor has the right filter or oscillator for your needs.


Operating a ham radio is a hobby which brings a lot of joy to individuals in the UK and beyond. Ham radio facilitates communication between local, regional, and even international enthusiasts, creating a large network of amateurs. Individuals who want to buy something for a friend involved in ham radio can consider purchasing filters and oscillators, which can optimise the transmission and reception of ham radio signals.

Before undertaking this process, consumers may wish to learn a little about these devices and their applications within ham radio. Once individuals have determined which oscillator or filter meets their needs, they can benefit from accessing the diverse catalogue of options on eBay. On this aggregator site, consumers can find a wide array of items often sold by experienced members of the ham radio community itself. By searching the listings and proactively evaluating a seller, any individual can use eBay to find an ideal filter or oscillator to add value and enhance the performance of ham radio.

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