Domestic VLF Reception
Traditionally, high-quality reception of Very Low Frequency (VLF) natural radio signals has required traveling to remote locations far from civilization to find an environment free of interference from artificial radio signals. Many attempts have been made to hike into the countryside with a portable VLF receiver in search of a quiet listening spot. However, the experience often leads to disappointment, as the signals are typically overwhelmed by sferics, tweeks, and the constant hum from nearby power distribution systems. Even in the best locations, this hum is persistent, with occasional whistlers emerging above it, but not often enough to create a remarkable experience. The only viable solution appeared to be establishing a means of receiving VLF signals cleanly from home. Despite living in a rural area, the surrounding moorland is heavily impacted by power lines, which contribute to a significant amount of hum when the receiver is activated. The hum, a dense and distracting noise, obscures all but the loudest sferics. Beneath the hum lies additional noise from household electronics, such as televisions and computers. A successful home receiver setup has been achieved, allowing for continuous, hum-free listening to the Earth’s natural VLF activity. Although interference from machinery and adverse weather conditions still pose challenges, the reception is generally excellent, with clear and rich sound quality when played through a home stereo system.
The initial step involved identifying the optimal location for the antenna. Modifications were made to the portable receiver to provide a readout of field strength, leading to a mapping of E-field hum distribution in the area. The primary source of hum was determined to be an overhead power line carrying 12kV, located about 100 meters away from the property. A distance of approximately 150 meters from this power line was necessary to avoid significant hum interference. The hum field stabilizes at a relatively constant level beyond this distance, influenced by more distant power lines approximately four miles away. While several low-hum spots were found, they often coincided with weak desired signals, particularly in areas near trees or structures. The crucial metric was the signal-to-hum ratio, which proved challenging to measure accurately. Fine-tuning the antenna location was conducted through auditory assessment of the signals, focusing on the clarity of loud sferics above the hum.
Ultimately, a location was selected on neighboring property, about 200 meters from the power line. The next challenge was transmitting the received signals back to the house. A coaxial cable was run to the receiver, but connecting the coax shield to the receiver ground resulted in a significant increase in hum levels. This issue stemmed from a ground potential difference between the antenna site and the household ground. The receiver’s ground connection lacked sufficient low impedance to eliminate this interference, resulting in unwanted noise being added to the desired signal at the receiver input. To mitigate this, enhancements were made to the grounding at the antenna site, reducing ground impedance to around 100 ohms. However, this did not sufficiently address the problem, as hum signals from the house traveled along the coax shield, undermining the purpose of situating the antenna away from domestic interference.
Consequently, a wideband radio link was employed to retrieve signals from a self-contained, battery-powered receiver. The coax was kept insulated from the ground near the antenna and separated from the antenna rod to avoid interference. A small VHF FM oscillator was integrated to bridge the gap between the receiver and a small insulated antenna at the coax end. This setup provided a clear signal back at the house while maintaining the original signal-to-hum ratio. The scanner output was connected to a PC sound card for analysis, ensuring no overload occurred in the signal processing chain. After careful adjustment of gain settings across the receiver, scanner, and PC mixer, software was developed to implement a comb filter with a 20 ms delay, effectively reducing hum while maintaining signal integrity.
Further stages of filtering were added to address non-harmonic noise components, primarily sidebands of mains harmonics. A frequency domain transformation using FFT was employed to identify and suppress persistent frequencies that exceeded a tunable threshold, resulting in a cleaner signal. This new filtering approach significantly minimized the remaining power line noise, allowing clearer reception of sferics and whistlers. However, the system still captured unwanted hiss and spurious signals, particularly from strong sources like GBR at 16 kHz, which affected the receiver's dynamic range.
To optimize performance, a configuration was established with minimal RC low-pass filtering preceding the first stage, followed by multiple poles of both low-pass and high-pass filtering. Although thermal noise from filter resistors remained a concern, a suitable compromise was found that minimized receiver noise while still capturing VLF signals. A new receiver was constructed, incorporating features such as a moisture detector and low battery warning, which introduced a warning tone into the signal. A changeover relay, controlled by a 27 MHz radio signal, allowed for the receiver’s connection to be opened or closed, facilitating accurate calibration of the signal chain.
The thermal noise characteristics of the system were analyzed, revealing that the overall response aligned well with background noise curves documented in relevant literature. The receiver demonstrated improved performance, successfully capturing a wider range of signals, including those approaching the natural noise floor of VLF. However, the increased dynamic range began to impact the VHF oscillator, leading to potential overmodulation issues and affecting the hum filter's tracking capabilities. Further exploration of sophisticated downlink options was considered, including the use of wideband FM microwave ISM transmitter/receivers, but initial attempts to connect the coax directly to the receiver ground resulted in overwhelming hum and noise, indicating the need for continued refinement of the system.Traditionally, high quality reception of VLF natural radio signals has involved driving or hiking out to remote locations far from civilisation, in order to find an environment free of pollution by artificial radio signals. I`ve lost count of the times I`ve hiked out into the countryside with a small portable VLF receiver in search of a quiet spot
for a night`s listening. All I ever seemed to get is sferics and tweeks, . and hum, the ever-present signal radiating from the country`s power distribution system. Even in the best of spots this hum never goes away completely, in my part of the world at least. Now and again a whistler manages to surface above it, but I never contrived to be in the right place at the right time to hear anything spectacular. On the whole, it was quite a disappointing experience. The only course of action seemed to be to arrange the means of receiving VLF signals cleanly from home.
Although I live in a rural location, the moorland around here is crisscrossed by big powerlines, as well as smaller ones leading out to the many farms. So when the receiver is switched on, most of what comes out of it is hum - a suffocating, mind numbing hum, which smothers everything but the loudest sferics.
Just beneath the hum, there is another layer of noise - an unpleasant brew of signals produced by televisions and computers in the house. I`m glad to report that I`ve had a fair amount of success with setting up a home receiver, and I can now listen to a completely hum-free rendering of the earth`s natural VLF activity, more or less continuously.
At times there is interference from machinery and so on, and although the antenna is now almost immune to wind and drizzle, it is still wiped out by driving rain or hail. But most of the time reception is first class, and I can listen to the signals anywhere in the house.
They sound awesome played through the stereo in the living room - great clarity and depth. The first step was to find the best location for the antenna. I modified my portable receiver to give a readout of the field strength, and spent a day or so mapping out the distribution of E-field hum in my neighbourhood. It quickly became clear that the main source of hum was the overhead line at about 12kV which brings power to the property.
This terminates about 100metres away at a pole transformer, and comes the rest of the way in at 240v, still overhead. This is where a suburban location, in which powerlines tend to be buried, would likely be better than my rural site.
I found that I needed to be 150 m from this overhead line before the hum field was no longer obviously influenced by it. Beyond this distance, the hum field settles to a fairly constant value - a level set, presumably, by the proximity of the next most distant source of hum: some huge overhead lines about 4 miles away across the moors.
The hum level would also rise as I approached the house, but not too severely, and it appears that placing the antenna just 10 or 20 m from the house is sufficient to reduce direct pickup from the domestic wiring to a level at which the more distant hum field takes over. I found many spots where the hum was very low, but unfortunately, so was the wanted signal too! This occured beneath trees, or near to walls, fences, outbuildings and so on. The important figure of merit is the signal/hum ratio, which is not so easy to measure. I couldn`t think of a neat way to measure this, so I ended up fine tuning the antenna location simply by listening to the signals and estimating how often a loud sferic made it above the hum.
In the end I settled on a spot on my neighbour`s property, about 200m from the end of the 12kV line. The next step was to convey the received signals back to the house. I ran out a length of coax to the receiver, but found that as soon as I connected the screen of the coax to the receiver ground, the received hum level increased enormously. The problem seemed to be that the ground potential directly beneath the antenna differed from the domestic ground by several millivolts.
The impedance of the receiver`s ground connection was not low enough to shunt away this interference, and consequently, it became added to the wanted signal at the receiver input terminals. The answer seemed to be to beef up the grounding arrangements at the antenna site, so that interference coming from the house would be shunted away.
Putting in several ground stakes reduced the ground impedance to around 100 ohms, but this did not seem to improve the situation much. The hum signals coming out from the house along the coax screen must be at quite a low impedance, and I soon came to the conclusion that a prohibitively substantial grounding system would be needed at the receiver site in order to dispose of these completely.
Bringing a coax out from the house seemed to bring with it all the domestic interference that I wanted to leave behind, and defeated the whole point of siting the antenna some distance away from the buildings. Therefore I decided to use a wideband radio link to retrieve the signals from a totally self-contained battery powered receiver.
I could bring the coax up quite close to the receiver box without causing interference, providing I made sure that the coax screen was insulated from the ground near the antenna, and ensured that it didn`t come anywhere near the antenna rod itself. This was close enough to allow the use of a small VHF FM oscillator, which could bridge the small gap from the receiver to a little insulated antenna fitted to the end of the coax.
This oscillator gave a fully-quieting signal in a scanner back at the house, and preserved the original signal/hum level. The next step was to connected the scanner output to the PC soundcard line input and take a good look at what was coming in.
I should stress at this point that it is very important to make sure that nothing is overloading anywhere along the signal handling chain, and it took me a little while to choose the best gain settings for the receiver, scanner, PC mixer, etc. With that done, I made up a simple software comb filter set to 20mS delay. This took away most of the hum but gave the remaining signals an unpleasant drainpipe-like quality. Increasing the number of delay stages eliminated that problem but it became necessary for me to continually fiddle with the delay to keep the sharp notches lined up with the incoming spectrum of hum harmonics.
Typically I would have to tweak the controls about once a minute to keep it on track, which was pretty tedious. Luckily, it turned out to be fairly easy to make the filter keep its own track of the hum period, and the result was a software program along the lines of the demonstration code in Humfilt.
So far so good. The hum was removed pretty much entirely, it tracked well, and there was no unpleasant coloration of the sound by the filter. Unfortunately this revealed a deeper level of noise coming from non-harmonic components of the power line voltage.
These were not particularly strong, but they were at times numerous across the spectrum and consisted mainly of sidebands of the mains harmonics. Luckily most where well defined tones at more or less constant frequencies. To deal with this non-harmonic power line hum, I added another stage of software filtering which transforms the signal to the frequency domain with an FFT, averages each bin, and recognises any persistent frequencies.
Those that exceed a tunable threshold have their FT bins zeroed before the signal is reverse-FFT`d back to the time domain. This was immediately effective in removing virtually all the remaining constant power line noise, and the only thing that gets through now is broadband noise and the wandering tones coming from things like power tools and washing machines.
Now that I had a pretty clean signal I could hear quite a lot of sferics and tweaks and some quite clear whistlers. Sometimes in the early mornings I could hear a faint chorus of auroral risers, so things were looking promising.
Unfortunately I could also hear a lot of hiss, along with a considerable number of signals that had no business being in that part of the spectrum at all. The problem here is that GBR at 16kHz is a huge signal and the entire spectrum 16-25kHz was being neatly translated to the range 0-9kHz by cross-modulation in the receiver.
This is not specifically an issue concerning domestic reception but is a problem that any VLF receiver must overcome. To cut a long story short I tried various arrangements of front-end and eventually concluded that in my case: I eventually settled on a configuration with just a bare minimum of RC low pass filtering before the first stage, and the first stage is followed by several poles of both low pass and high pass filtering.
The noise floor is still dominated by the thermal noise from these filter resistors, and things could be even better if I used LC filtering instead. However, I was able to find a compromise of RC values which gave me a receiver noise which was substantially less than the genuine VLF background hiss coming in from the antenna.
Putting all this together, I built a whole new receiver which performed much better. The circuit diagram is available here. I added in a few features such as a moisture detector and a low battery warning - each of these inserts a warning tone into the receiver signal. Another very useful feature is a changeover relay that can open and close the receiver connection to the antenna.
This relay is commanded by a 27Mhz radio control signal and when activated, the receiver just sees the thermal noise from the 10meg bias resistor. This can be calculated quite accurately and allows a reasonable end-to-end calibration of the entire signal chain to be made.
See my notes on calibration here. The green line is the thermal noise of a 10meg resistor shunted by about 37pF. The actual receiver noise is lower than this because when the antenna is connected (the red line) the bias resistor noise is further shunted by the antenna capacitance. It is nice to see that the overall response matches the background noise curves given in What and Where is the Natural Noise Floor by John Meloy.
The pronounced dip at about 4 kHz is present, as well as the broad peak around 10kHz and the rapid rise in amplitude between these two is reproduced. The mess of hum harmonics is very prominent, as is a whole bunch of very strong MSK signals between 16kHz and 24kHz.
Considering the receiver has about 5 poles of low pass filter at a corner of around 10kHz, these signals are doing pretty well! They were the major source of intermodulation in the back end of the receiver, and in fact the GBR signal continues to set the limit to the overall dynamic range.
Using the new receiver was like emerging from a dark cave into bright sunshine as far as incoming signals were concerned. The background hiss varies constantly and indicates that the receiver is now delivering up everything coming in right down to the VLF natural noise floor.
However, with the new lower noise floor and the greater receiver gain, the dynamic range was begining to frighten the VHF oscillator. The louder signals would overmodulate the FM, sometimes almost dropping it out from the scanner, which had a knock-on effect on the hum filter tracking.
Turning down the gain simply meant that the noise floor of the scanner receiver used for the downlink began to compress the bottom end of the dynamic range. Not only this, but some remaining intermodulation was almost certainly occuring within the VHF downlink.
Basically, the dynamic range and bandwidth of the VLF signal was just too much for the simple wide band FM downlink. I did consider a more sophisticated downlink, using a pair of the cheap wideband FM microwave ISM transmitter/receivers which are common these days.
But before doing this I decided to have one more bash at the direct connection. As soon as I connected the coax screen to the receiver ground, a vast amount of hum and noise poured in. This was something like a hundred times the level of hum picked up from the antenna alone, and was totally overloading the whole system.
🔗 External reference
The initial step involved identifying the optimal location for the antenna. Modifications were made to the portable receiver to provide a readout of field strength, leading to a mapping of E-field hum distribution in the area. The primary source of hum was determined to be an overhead power line carrying 12kV, located about 100 meters away from the property. A distance of approximately 150 meters from this power line was necessary to avoid significant hum interference. The hum field stabilizes at a relatively constant level beyond this distance, influenced by more distant power lines approximately four miles away. While several low-hum spots were found, they often coincided with weak desired signals, particularly in areas near trees or structures. The crucial metric was the signal-to-hum ratio, which proved challenging to measure accurately. Fine-tuning the antenna location was conducted through auditory assessment of the signals, focusing on the clarity of loud sferics above the hum.
Ultimately, a location was selected on neighboring property, about 200 meters from the power line. The next challenge was transmitting the received signals back to the house. A coaxial cable was run to the receiver, but connecting the coax shield to the receiver ground resulted in a significant increase in hum levels. This issue stemmed from a ground potential difference between the antenna site and the household ground. The receiver’s ground connection lacked sufficient low impedance to eliminate this interference, resulting in unwanted noise being added to the desired signal at the receiver input. To mitigate this, enhancements were made to the grounding at the antenna site, reducing ground impedance to around 100 ohms. However, this did not sufficiently address the problem, as hum signals from the house traveled along the coax shield, undermining the purpose of situating the antenna away from domestic interference.
Consequently, a wideband radio link was employed to retrieve signals from a self-contained, battery-powered receiver. The coax was kept insulated from the ground near the antenna and separated from the antenna rod to avoid interference. A small VHF FM oscillator was integrated to bridge the gap between the receiver and a small insulated antenna at the coax end. This setup provided a clear signal back at the house while maintaining the original signal-to-hum ratio. The scanner output was connected to a PC sound card for analysis, ensuring no overload occurred in the signal processing chain. After careful adjustment of gain settings across the receiver, scanner, and PC mixer, software was developed to implement a comb filter with a 20 ms delay, effectively reducing hum while maintaining signal integrity.
Further stages of filtering were added to address non-harmonic noise components, primarily sidebands of mains harmonics. A frequency domain transformation using FFT was employed to identify and suppress persistent frequencies that exceeded a tunable threshold, resulting in a cleaner signal. This new filtering approach significantly minimized the remaining power line noise, allowing clearer reception of sferics and whistlers. However, the system still captured unwanted hiss and spurious signals, particularly from strong sources like GBR at 16 kHz, which affected the receiver's dynamic range.
To optimize performance, a configuration was established with minimal RC low-pass filtering preceding the first stage, followed by multiple poles of both low-pass and high-pass filtering. Although thermal noise from filter resistors remained a concern, a suitable compromise was found that minimized receiver noise while still capturing VLF signals. A new receiver was constructed, incorporating features such as a moisture detector and low battery warning, which introduced a warning tone into the signal. A changeover relay, controlled by a 27 MHz radio signal, allowed for the receiver’s connection to be opened or closed, facilitating accurate calibration of the signal chain.
The thermal noise characteristics of the system were analyzed, revealing that the overall response aligned well with background noise curves documented in relevant literature. The receiver demonstrated improved performance, successfully capturing a wider range of signals, including those approaching the natural noise floor of VLF. However, the increased dynamic range began to impact the VHF oscillator, leading to potential overmodulation issues and affecting the hum filter's tracking capabilities. Further exploration of sophisticated downlink options was considered, including the use of wideband FM microwave ISM transmitter/receivers, but initial attempts to connect the coax directly to the receiver ground resulted in overwhelming hum and noise, indicating the need for continued refinement of the system.Traditionally, high quality reception of VLF natural radio signals has involved driving or hiking out to remote locations far from civilisation, in order to find an environment free of pollution by artificial radio signals. I`ve lost count of the times I`ve hiked out into the countryside with a small portable VLF receiver in search of a quiet spot
for a night`s listening. All I ever seemed to get is sferics and tweeks, . and hum, the ever-present signal radiating from the country`s power distribution system. Even in the best of spots this hum never goes away completely, in my part of the world at least. Now and again a whistler manages to surface above it, but I never contrived to be in the right place at the right time to hear anything spectacular. On the whole, it was quite a disappointing experience. The only course of action seemed to be to arrange the means of receiving VLF signals cleanly from home.
Although I live in a rural location, the moorland around here is crisscrossed by big powerlines, as well as smaller ones leading out to the many farms. So when the receiver is switched on, most of what comes out of it is hum - a suffocating, mind numbing hum, which smothers everything but the loudest sferics.
Just beneath the hum, there is another layer of noise - an unpleasant brew of signals produced by televisions and computers in the house. I`m glad to report that I`ve had a fair amount of success with setting up a home receiver, and I can now listen to a completely hum-free rendering of the earth`s natural VLF activity, more or less continuously.
At times there is interference from machinery and so on, and although the antenna is now almost immune to wind and drizzle, it is still wiped out by driving rain or hail. But most of the time reception is first class, and I can listen to the signals anywhere in the house.
They sound awesome played through the stereo in the living room - great clarity and depth. The first step was to find the best location for the antenna. I modified my portable receiver to give a readout of the field strength, and spent a day or so mapping out the distribution of E-field hum in my neighbourhood. It quickly became clear that the main source of hum was the overhead line at about 12kV which brings power to the property.
This terminates about 100metres away at a pole transformer, and comes the rest of the way in at 240v, still overhead. This is where a suburban location, in which powerlines tend to be buried, would likely be better than my rural site.
I found that I needed to be 150 m from this overhead line before the hum field was no longer obviously influenced by it. Beyond this distance, the hum field settles to a fairly constant value - a level set, presumably, by the proximity of the next most distant source of hum: some huge overhead lines about 4 miles away across the moors.
The hum level would also rise as I approached the house, but not too severely, and it appears that placing the antenna just 10 or 20 m from the house is sufficient to reduce direct pickup from the domestic wiring to a level at which the more distant hum field takes over. I found many spots where the hum was very low, but unfortunately, so was the wanted signal too! This occured beneath trees, or near to walls, fences, outbuildings and so on. The important figure of merit is the signal/hum ratio, which is not so easy to measure. I couldn`t think of a neat way to measure this, so I ended up fine tuning the antenna location simply by listening to the signals and estimating how often a loud sferic made it above the hum.
In the end I settled on a spot on my neighbour`s property, about 200m from the end of the 12kV line. The next step was to convey the received signals back to the house. I ran out a length of coax to the receiver, but found that as soon as I connected the screen of the coax to the receiver ground, the received hum level increased enormously. The problem seemed to be that the ground potential directly beneath the antenna differed from the domestic ground by several millivolts.
The impedance of the receiver`s ground connection was not low enough to shunt away this interference, and consequently, it became added to the wanted signal at the receiver input terminals. The answer seemed to be to beef up the grounding arrangements at the antenna site, so that interference coming from the house would be shunted away.
Putting in several ground stakes reduced the ground impedance to around 100 ohms, but this did not seem to improve the situation much. The hum signals coming out from the house along the coax screen must be at quite a low impedance, and I soon came to the conclusion that a prohibitively substantial grounding system would be needed at the receiver site in order to dispose of these completely.
Bringing a coax out from the house seemed to bring with it all the domestic interference that I wanted to leave behind, and defeated the whole point of siting the antenna some distance away from the buildings. Therefore I decided to use a wideband radio link to retrieve the signals from a totally self-contained battery powered receiver.
I could bring the coax up quite close to the receiver box without causing interference, providing I made sure that the coax screen was insulated from the ground near the antenna, and ensured that it didn`t come anywhere near the antenna rod itself. This was close enough to allow the use of a small VHF FM oscillator, which could bridge the small gap from the receiver to a little insulated antenna fitted to the end of the coax.
This oscillator gave a fully-quieting signal in a scanner back at the house, and preserved the original signal/hum level. The next step was to connected the scanner output to the PC soundcard line input and take a good look at what was coming in.
I should stress at this point that it is very important to make sure that nothing is overloading anywhere along the signal handling chain, and it took me a little while to choose the best gain settings for the receiver, scanner, PC mixer, etc. With that done, I made up a simple software comb filter set to 20mS delay. This took away most of the hum but gave the remaining signals an unpleasant drainpipe-like quality. Increasing the number of delay stages eliminated that problem but it became necessary for me to continually fiddle with the delay to keep the sharp notches lined up with the incoming spectrum of hum harmonics.
Typically I would have to tweak the controls about once a minute to keep it on track, which was pretty tedious. Luckily, it turned out to be fairly easy to make the filter keep its own track of the hum period, and the result was a software program along the lines of the demonstration code in Humfilt.
So far so good. The hum was removed pretty much entirely, it tracked well, and there was no unpleasant coloration of the sound by the filter. Unfortunately this revealed a deeper level of noise coming from non-harmonic components of the power line voltage.
These were not particularly strong, but they were at times numerous across the spectrum and consisted mainly of sidebands of the mains harmonics. Luckily most where well defined tones at more or less constant frequencies. To deal with this non-harmonic power line hum, I added another stage of software filtering which transforms the signal to the frequency domain with an FFT, averages each bin, and recognises any persistent frequencies.
Those that exceed a tunable threshold have their FT bins zeroed before the signal is reverse-FFT`d back to the time domain. This was immediately effective in removing virtually all the remaining constant power line noise, and the only thing that gets through now is broadband noise and the wandering tones coming from things like power tools and washing machines.
Now that I had a pretty clean signal I could hear quite a lot of sferics and tweaks and some quite clear whistlers. Sometimes in the early mornings I could hear a faint chorus of auroral risers, so things were looking promising.
Unfortunately I could also hear a lot of hiss, along with a considerable number of signals that had no business being in that part of the spectrum at all. The problem here is that GBR at 16kHz is a huge signal and the entire spectrum 16-25kHz was being neatly translated to the range 0-9kHz by cross-modulation in the receiver.
This is not specifically an issue concerning domestic reception but is a problem that any VLF receiver must overcome. To cut a long story short I tried various arrangements of front-end and eventually concluded that in my case: I eventually settled on a configuration with just a bare minimum of RC low pass filtering before the first stage, and the first stage is followed by several poles of both low pass and high pass filtering.
The noise floor is still dominated by the thermal noise from these filter resistors, and things could be even better if I used LC filtering instead. However, I was able to find a compromise of RC values which gave me a receiver noise which was substantially less than the genuine VLF background hiss coming in from the antenna.
Putting all this together, I built a whole new receiver which performed much better. The circuit diagram is available here. I added in a few features such as a moisture detector and a low battery warning - each of these inserts a warning tone into the receiver signal. Another very useful feature is a changeover relay that can open and close the receiver connection to the antenna.
This relay is commanded by a 27Mhz radio control signal and when activated, the receiver just sees the thermal noise from the 10meg bias resistor. This can be calculated quite accurately and allows a reasonable end-to-end calibration of the entire signal chain to be made.
See my notes on calibration here. The green line is the thermal noise of a 10meg resistor shunted by about 37pF. The actual receiver noise is lower than this because when the antenna is connected (the red line) the bias resistor noise is further shunted by the antenna capacitance. It is nice to see that the overall response matches the background noise curves given in What and Where is the Natural Noise Floor by John Meloy.
The pronounced dip at about 4 kHz is present, as well as the broad peak around 10kHz and the rapid rise in amplitude between these two is reproduced. The mess of hum harmonics is very prominent, as is a whole bunch of very strong MSK signals between 16kHz and 24kHz.
Considering the receiver has about 5 poles of low pass filter at a corner of around 10kHz, these signals are doing pretty well! They were the major source of intermodulation in the back end of the receiver, and in fact the GBR signal continues to set the limit to the overall dynamic range.
Using the new receiver was like emerging from a dark cave into bright sunshine as far as incoming signals were concerned. The background hiss varies constantly and indicates that the receiver is now delivering up everything coming in right down to the VLF natural noise floor.
However, with the new lower noise floor and the greater receiver gain, the dynamic range was begining to frighten the VHF oscillator. The louder signals would overmodulate the FM, sometimes almost dropping it out from the scanner, which had a knock-on effect on the hum filter tracking.
Turning down the gain simply meant that the noise floor of the scanner receiver used for the downlink began to compress the bottom end of the dynamic range. Not only this, but some remaining intermodulation was almost certainly occuring within the VHF downlink.
Basically, the dynamic range and bandwidth of the VLF signal was just too much for the simple wide band FM downlink. I did consider a more sophisticated downlink, using a pair of the cheap wideband FM microwave ISM transmitter/receivers which are common these days.
But before doing this I decided to have one more bash at the direct connection. As soon as I connected the coax screen to the receiver ground, a vast amount of hum and noise poured in. This was something like a hundred times the level of hum picked up from the antenna alone, and was totally overloading the whole system.
🔗 External reference
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