Three models are available:

Model HS-400: below 5kHz-530kHz converts to 4MHz-4.520MHz

Model HS-400 GPS-REF: below 5kHz-530kHz converts to 4MHz-4.520MHz

Model HS-350: below 5khz-530kHz converts to 3.5MHz-4.020MHz

Compatible with Software Define Radios (SDR) and analogue radios .





The VLF-LF-MF Up-Converter enables reception of VLF mobile maritime communications, aviation and marine navigational aids such as NDB and DGPS beacons, DGPS reference stations, NAVTEX, Standard Time & Frequency stations, LF Amateur bands, European LW broadcast stations, monitoring the sounds of nature created by planet Earth (Natural Radio) such as sferics, hiss, tweaks, whistlers, Dawn Chorus, other not well known VLF radio atmospheric sounds, and many more.

Depending on local conditions, kind of antenna, noise, etc. it is advisable to use a variable attenuator in front of the converter in order to improve signal to noise ratio and better energy transfer between antenna and converter due the broad impedance matching effect.
A front attenuator have more or less noticeable effects depending on location. For example if you are living in the country side far away from noise sources probably you do not need one.
In urban areas were strong signals and/or noise are more noticeable a variable attenuator avoids saturating the receiver chain. Adjusting the attenuation to a proper value weak signals buried in the noise can be seen emerging

Since many countries are allocating the 472 kHz MF band for experimental use by Radio Amateurs, a growing number of them as well as listeners have become interested on VLF and LF bands.

The fact that few radios are available on the market covering properly frequencies from below 5kHz to 530kHz, convinced us to provide radio amateurs and listeners with the high-performance receiving VLF-LF-MF Up-Converter that we introduce here.

The VLF-LF_MF Up-Converter extents the range of any shortwave receiver from below 5 kHz to 530 kHz. It is connected between the antenna and a shortwave radio receiver. When the converter is power-on the <5kHz to 530 kHz band segment is translate to 3.500MHz to 4.030MHz, Model-350 or 4.000MHz to 4.530MHz, Model 400, allowing the reception of VLF, LF and MF bands.

For example, for converter model 400, if the receiver is tuned on 4.135.500MHz, you are receiving on 135.500kHz, just ignoring the digit “4” on the dial.

Alternatively most SDR radio control software lets customise the dial by the user, as an example see below page "PowerSDR configuration".

Although the performance of the models HS-350 and HS-400 is excellent for regular listening there are applications needed by advanced users that demand high accuracy reception parameters.

In this regard from early 2023 we introduced a new VLF-LF-MF converter model, the HS-400 GPS-REF featuring the connection of an external frequency reference source such as GPS Disciplined Oscillator, GPSDO, Rubidium frequency reference or similar high accuracy instrument.

This kind of reference instruments can achieve frequency resolutions down to pHz (10-12 Hz) and low phase noise in the range of sub-picoseconds RMS jitter.

We recommend the cost-effective Leo Bodnar Low-jitter GPS-locked precision frequency reference:

Notice that model HS-400 GPS-REF do not mount a local oscillator. A 4.00MHz external frequency reference is needed. (Not included)



All models front view.

Model HS-350 and model HS-400 rear view.


Model HS-400 GPS-REF rear view.




Introducing the VLF LF MF bands Up-Converter





The annual transmission event on the Alexanderson Day with the Alexanderson Alternator from 1924 on VLF band 17.2 kHz mode CW with the call sign SAQ, on 4 July 2021. Recorded by M0WWA in London on 4 July 2021.

Station equipment:

- Long Wire antenna.
- Longwire Antenna Impedance adapter ; HST-210N ;Heros Technology
- Antenna Galvanic Isolator HS-100B; for local noise suppression; HerosTech
- VLF-LF-MF up converter HerosTech HS-400, IF output 4MHz-4.5MHz.; HS-400
- Flexradio1500 as IF receiver,
- GPS reference clock from Leo Bodnar
- Power SDR 2.8.0 from KE9NS








"The Big Jump"

"Alexanderson Day" special transmission on 17.2 kHz from SAQ Grimeton radio station in Sweden was received in Granby QC Canada (FN35RK) on 28 June 2015. Distance 5.700 km (3.600mi)
This was the group effort of VE2CRG,the Granby Amateur Radio Club.
The radio station setup included a tuned 1.7m diamond loop, FET balanced preamplifier, Heros Technology VLF-LF Up- Converter model-400, multiband IF receiver tuned on 4.017 MHz and Spectral Analysis software Spectran v2.




Technical Specifications:


Compatible with any kind of Software Define Radios SDR and analogue radios.

Converter class: Superheterodyne. IF output mirrored up.

Type: Double balanced mixer.

Frequency range: Below 5kHz to 530kHz.

IF output:

- 4MHz-4.530MHz; model HS-400 and model HS-400GPS-REF

- 3.5MHz-4.030MHz, model HS-350.

Input/Output impedance: 50 Ω

IF rejection: 110dB typical.

MW band rejection: >80dB

Local Oscillator (Only models HS-350 and HS-400):

- Frequency: 3.500MHz or 4.000MHz, fundamental. According to the converter model

- Quartz crystal. Low phase noise.

- Operating temperature: -10°C to +60°C

- Frequency Stability ±5.00ppm or ±20Hz @ room temp (25°C)

- Ageing ±3ppm typical per year or ±12Hz.

.External Master Oscillator (Only model HS-400 GPS-REF):  

- The model HS-400 GPS-REF do not mount a local oscillator. A 4.00MHz external frequency reference is needed. (Not included)

- Frequency: 4.00 MHz.Recomended a GPS Disciplined Oscillator, GPSDO. Not included

- Input Signal Level: 1V (+13dBm max)/50Ω.

- Input Signal Connector SMA.

General (All models)

Power: DC 12volts/40mA

Enclosure: Aluminium

Size: 125mmx105mx55mm ( 4.921x4.133x2.165in)

Weight: 480g

Shipping Weight: 800g

Country of origin: United Kingdom

  VLF-LF-MF Up-Converter user manual. VLF-LF-MF Up-Converter architecture
Models HS-350 and HS-400 only.
VLF-LF-MF Up-Converter architecture.
Model HS-400 GPS-REF only.

Connecting the VLF-LF-MF Up-Converter to a SW receiver or transceiver (click on image to enlarge)


Connection to a generic receiver.

Basic configuration.

Front End plot

Preventing accidental transmission configuration.


The VLF-LF Up-Converter is suitable for use with active antennas, loop antennas and preamplifiers providing an output impedance of 50 Ω.

If an active antenna is powered through the coaxial cable a DC block device must be connected at the converter antenna Input in order to avoid damage the unit.

Beverage antennas may need a 50 Ω matching network device for best performance.

If the VLF-LF up-Converter is connected in line to a HF transceiver acting as IF receiver, take precautions to prevent transmitting any signal into the converter.



VLF-LF-MF Up-Converter design


Front End

Referring to the above functional blocks diagram, the 50 Ω antenna is connected to J1 ,BNC type connector; a Gas Discharge Tube protects the converter from transients that may come from the antenna.

Next the input filters section is a combination of a 110dB IF trap and a 7th order, 0.3dB insertion loss, Elliptic Low Pass Filter with a cut-off frequency of 500 kHz.

This section provides good performance right up to the start of the AM broadcast band with a sharp roll-off, giving a good rejection to MW broadcast signals avoiding introducing overload or intermodulation products in the receiver.


Front End plot

VLF-LF-MF Up-Converter front end response plot.
(Converter model 350)

Local Oscillator

A low phase noise, crystal controlled oscillator provides +7dBm signal level to the mixer

A high quality quartz crystal of 3.5MHz or 4MHz, according to the model of converter, is used. A trimmer capacitor is accessible by the user if fine adjust of frequency is required in times. A low noise linear fixed voltage IC regulates the voltage applied to the oscillator to enhance stability and avoid drift.

The oscillator is followed by a buffer stage to improve its stability. Next, an amplifier accommodates the signal to the required level. It connects to a 4 MHz, 7th order, Elliptic Low Pass Filter which cleans up any harmonics and assures that a pure signal applies to the mixer to minimize spurious responses.


The mixer stage is designed around a Mini-Circuits SRA-8+ double balanced mixer. It is rated to operate from 500Hz, giving and excellent frequency coverage. Designed for +7 dBm of LO power, it have a conversion loss of less than 5 dB at VLF LF bands, which is compensate by the post-mixer amplifier. A double balanced mixer has the advantage of high port-to-port isolation, which keeps the strong LO signal from degrading the dynamic range of the IF receiver. Every effort has been made to terminate the ports of the mixer in a proper 50 Ω impedance to maximize its performance.

The mixer IF port is most critical in terms of proper termination, for so, a diplexer circuit provides a 50 Ω resistive load to the mixer while passing the desired signal to the IF amplifier, with minimum loss.

Next, a Band Pass filter at IF frequency connects to a Low Noise Amplifier to compensate losses, delivering around 5dB of gain to the output, necessary to overcome losses from external cable tails, relays and connectors.


A carefully PCB and shielding design keep feed through signals of the HF receiving frequency down to a very low level; therefore, weak LF signals will not be interfered by strong HF signals on the same frequency.

The HF receiver used should have strong rejection of HF signals through paths other than the antenna connector. Most HF transceivers and receivers have good performance in this respect, but some SWL receivers do not and may be used with the addition of filters and shielding.



Recommended accessories


10KHz to 150MHz Antenna Galvanic Isolator.

Removes ground loops.

Rejects pick-up noise from the antenna feedline.

Specifically recommended to work on VLF,LF, MF bands.




0 - 71 dB Step Attenuator Ladder Configuration

The step attenuator is suitable for receiver front-end protection or as calibrated attenuator for receiving performance evaluation.
It is an inestimable tool for accurate control of amplitude levels.
In addition, it improves the impedance match between impedance-sensitive devices, such as amplifiers, oscillators and filters, lowering the VSWR of components connected in cascaded mode by providing isolation between the impedances.



General purpose LF-HF-VHF-UHF, Dual Port, by-Pass relay

- 100 Watts RF power rating

- DC - 1300MHz

It is designed to by-pass the Input and Output RF ports of any device in line with your transceiver or transmitter.
A typical application includes inserting between antenna and a transceiver or transmitter a preselector, preamplifier or filter device.
On transmission switchover, via PTT or Keying line, the Input and Output RF ports of the device are by-passed avoiding being overloaded.



  VLF-LF-MF Up-Converter options Price    
  IF output 4-4.5 MHz.. ; SKU: HS-400 £ 187.00  
  IF output 4-4.5 MHz.. ; SKU: HS-400 GPS-REF £ 180.00  
  IF output 3.5-4 MHz.. ; SKU: HS-350 £ 195.00  
  The set includes:
VLF-LF-MF Up-Converter, user manual in CDROM format, two BNC-BNC coaxial cable tails, power supply cable with EMI filter and auxiliary connectors.