fredbox
This is the story of the Fredbox, a rig that first originated in 1974 in Cambridge. Recently, it has been restored to full working order to capitalize on the renewed interest in AM operation on 2m in the UK. For several years, local stations were contacted using a simple low-power 10mW AM transmitter, coupled with a super-regenerative receiver that first appeared in Practical Wireless in the late 1960s. This receiver design has been republished in a recent issue. The straightforward combination was utilized on the bench with a toggle switch for changeover. Initially, the antenna changeover involved unplugging the antenna from the receiver and connecting it to the transmitter and vice versa. At that time, the antenna was either a small dipole or an indoor Yagi, rotated manually. Integrating these two circuits into a compact handheld unit took just a couple of weeks. A small PCB was etched after finding a suitable enclosure, and the circuit functioned correctly on the first attempt. Colleagues were impressed by its compact size, leading to the construction of three additional copies. The first QSO between Fredbox units spanned approximately 0.3 km. Local contacts were frequent in the Cambridge area, with the most regular QSOs occurring on 145 MHz with a local disabled amateur, Fred, G8BWI. Consequently, the device became known as the Fredbox, dedicated to the memories of enjoyable times with Fred. He was known for his ability to engage in conversation, often allowing others to take breaks while he continued to speak. Notable results were achieved beyond Cambridge, particularly in Yorkshire and South Devon. In Yorkshire, the Fredbox facilitated QSOs from a location in Barnsley to Leeds, approximately 20 miles away. In Devon, it was used for several QSOs from Start Point to Portland Bill in Dorset, a distance of 60 miles, all while handheld with a whip antenna. The most remarkable achievement was a 100-mile QSO from Bolberry Down to Brittany in France, which occurred one fine summer morning. Due to these impressive results, an article about the Fredbox was submitted to the RSGB for publication in RadCom. However, the committee deemed it unsuitable for their readership, and the article was never published. It is reproduced in its original form in a linked PDF file, which may take some time to download on slower connections. One reason cited for the rejection was the minimal re-radiation from the super-regenerative oscillator on the receiver, which was believed to be inaudible beyond a few meters. As the 2m band became busier, operators shifted to FM and SSB, leading to a decline in AM use, and the Fredbox was stored away until this year. With a resurgence in AM operation on 2m, the Fredbox was rebuilt into the original enclosure, and it has been operational again in the Cambridge area, achieving a reception range of 76 km by G1HDQ during its last outing in Devon, confirming that it still functions effectively.
The Fredbox circuit is a compact AM transmitter and receiver designed for the 2m band, utilizing a 10mW output power level for effective communication. The transmitter employs a simple oscillator circuit that generates an amplitude-modulated signal. The super-regenerative receiver, known for its sensitivity, allows for the reception of weak signals, making it suitable for amateur radio applications. The integration of both transmitter and receiver onto a single PCB minimizes the size and enhances portability, allowing users to operate the device handheld with a whip antenna.
Key components of the Fredbox include:
1. **Transmitter Section**: A crystal-controlled oscillator that generates a stable frequency within the 2m band, modulated by audio input from a microphone. The use of low power ensures minimal interference and extended battery life.
2. **Receiver Section**: A super-regenerative design that amplifies incoming signals, allowing for clear reception of AM transmissions. The receiver's design focuses on simplicity and efficiency, making it accessible for amateur radio enthusiasts.
3. **Antenna System**: The device is compatible with various antenna types, including small dipoles and Yagi antennas, enabling flexible operation based on user preference and location.
The Fredbox's design emphasizes ease of use, portability, and effective communication over moderate distances, making it a valuable tool for amateur radio operators interested in AM operation. The device's successful performance over distances of up to 100 miles showcases its capability and reliability in real-world applications. The revival of interest in AM operation on the 2m band underscores the Fredbox's relevance in the current amateur radio landscape.This is the story of the Fredbox, a rig that first saw the light of day in 1974 in Cambridge. In the last few months it has been restored to full working order to enjoy the renewed interest in AM operation on 2m in the UK. For several years I`d worked local stations with a simple very low power 10mW AM transmitter. This was coupled with a super-re gen receiver that first appeared in Practical Wireless in the late 1960s. Incidentally, this receiver design has just been republished in a recent copy. The simple combination was used on the bench with just a toggle switch change-over. For some time the antenna changeover consisted of unplugging the antenna from the receiver then plugging it into the TX and vice versa. The antenna at that time was a small dipole or indoor yagi rotated by hand. Combining these two circuits into one small handheld took only a couple of weeks. A small PCB was etched after a suitable box was found and the circuit worked first time. People working along side me were so impressed by its small size that very soon 3 other copies were made.
The first Fredbox to Fredbox QSO was over about 0. 3kms. Local contacts were frequent around the Cambridge city area and the most regular QSOs were on 145MHz with a local disabled amateur, Fred, G8BWI. Because of this, the little box became know as the FREDBOX. I dedicate the circuit and the memories of those fun times to dear old Fred. How Fred could talk! Sometimes you`d start a QSO, then hand over to Fred, have your tea, and he`d still be talking away. Such good, fondly remembered, times indeed. The most exciting results took place away from Cambridge in Yorkshire and in South Devon. In Yorkshire the Fredbox was regularly used to make QSOs from my wife`s parent`s house in Barnsley up to Leeds about 20 miles to the north.
In Devon, it was used to make several QSOs from Start Point to Portland Bill in Dorset at 60 miles distant, all with just the rig handheld with a whip antenna. Then, on one occasion the best result of all - a 100 mile QSO from Bolberry Down across to Brittany in France.
I was soamazed that this happened, but it most certainly did one fine summer morning. I was so impressed by these results that I submitted an article on the Fredbox to the RSGB for publication in RadCom. The fact that several had been made with good results was testimony to its reproducability. Sadly the committee of the day thought it was "not suitable for its readers", so the article was never published.
It is reproduced in its exact form here (follow the link). As it is a big. pdf file please be prepared to wait a while for the download unless you have a fast connection. One reason cited was the amount of re-radiation from the super-regen oscillator on RX. This was very small and I do not believe it would have been audible beyond a few metres. Gradually the 2m band became busier and people moved over to FM and SSB. AM all but died out on the band so the Fredbox was consigned to the cupboard and rarely saw the light of day again until this year. With a small revival in AM operation on 2m the Fredbox was rebuilt into the very same box as I still had the box and the built PCB - see photo.
Ithas again been on the air in the Cambridge area and was heard at 76kms away by G1HDQ (using a whip antenna too)when last down in Devon, soits STILL works! 🔗 External reference
The Fredbox circuit is a compact AM transmitter and receiver designed for the 2m band, utilizing a 10mW output power level for effective communication. The transmitter employs a simple oscillator circuit that generates an amplitude-modulated signal. The super-regenerative receiver, known for its sensitivity, allows for the reception of weak signals, making it suitable for amateur radio applications. The integration of both transmitter and receiver onto a single PCB minimizes the size and enhances portability, allowing users to operate the device handheld with a whip antenna.
Key components of the Fredbox include:
1. **Transmitter Section**: A crystal-controlled oscillator that generates a stable frequency within the 2m band, modulated by audio input from a microphone. The use of low power ensures minimal interference and extended battery life.
2. **Receiver Section**: A super-regenerative design that amplifies incoming signals, allowing for clear reception of AM transmissions. The receiver's design focuses on simplicity and efficiency, making it accessible for amateur radio enthusiasts.
3. **Antenna System**: The device is compatible with various antenna types, including small dipoles and Yagi antennas, enabling flexible operation based on user preference and location.
The Fredbox's design emphasizes ease of use, portability, and effective communication over moderate distances, making it a valuable tool for amateur radio operators interested in AM operation. The device's successful performance over distances of up to 100 miles showcases its capability and reliability in real-world applications. The revival of interest in AM operation on the 2m band underscores the Fredbox's relevance in the current amateur radio landscape.This is the story of the Fredbox, a rig that first saw the light of day in 1974 in Cambridge. In the last few months it has been restored to full working order to enjoy the renewed interest in AM operation on 2m in the UK. For several years I`d worked local stations with a simple very low power 10mW AM transmitter. This was coupled with a super-re gen receiver that first appeared in Practical Wireless in the late 1960s. Incidentally, this receiver design has just been republished in a recent copy. The simple combination was used on the bench with just a toggle switch change-over. For some time the antenna changeover consisted of unplugging the antenna from the receiver then plugging it into the TX and vice versa. The antenna at that time was a small dipole or indoor yagi rotated by hand. Combining these two circuits into one small handheld took only a couple of weeks. A small PCB was etched after a suitable box was found and the circuit worked first time. People working along side me were so impressed by its small size that very soon 3 other copies were made.
The first Fredbox to Fredbox QSO was over about 0. 3kms. Local contacts were frequent around the Cambridge city area and the most regular QSOs were on 145MHz with a local disabled amateur, Fred, G8BWI. Because of this, the little box became know as the FREDBOX. I dedicate the circuit and the memories of those fun times to dear old Fred. How Fred could talk! Sometimes you`d start a QSO, then hand over to Fred, have your tea, and he`d still be talking away. Such good, fondly remembered, times indeed. The most exciting results took place away from Cambridge in Yorkshire and in South Devon. In Yorkshire the Fredbox was regularly used to make QSOs from my wife`s parent`s house in Barnsley up to Leeds about 20 miles to the north.
In Devon, it was used to make several QSOs from Start Point to Portland Bill in Dorset at 60 miles distant, all with just the rig handheld with a whip antenna. Then, on one occasion the best result of all - a 100 mile QSO from Bolberry Down across to Brittany in France.
I was soamazed that this happened, but it most certainly did one fine summer morning. I was so impressed by these results that I submitted an article on the Fredbox to the RSGB for publication in RadCom. The fact that several had been made with good results was testimony to its reproducability. Sadly the committee of the day thought it was "not suitable for its readers", so the article was never published.
It is reproduced in its exact form here (follow the link). As it is a big. pdf file please be prepared to wait a while for the download unless you have a fast connection. One reason cited was the amount of re-radiation from the super-regen oscillator on RX. This was very small and I do not believe it would have been audible beyond a few metres. Gradually the 2m band became busier and people moved over to FM and SSB. AM all but died out on the band so the Fredbox was consigned to the cupboard and rarely saw the light of day again until this year. With a small revival in AM operation on 2m the Fredbox was rebuilt into the very same box as I still had the box and the built PCB - see photo.
Ithas again been on the air in the Cambridge area and was heard at 76kms away by G1HDQ (using a whip antenna too)when last down in Devon, soits STILL works! 🔗 External reference