Tagged: filter

Building an “HF Helper” for Improving RTL-SDR HF Reception

Over on YouTube Amateur Radio VK3YE has uploaded a video showing his 'HF Helper' project. The HF Helper is a tunable HF filter and attenuator that helps improve HF reception when in the presence of strong overloading signals. VK3YE writes:

Using an RTL-SDR.COM dongle (genuine model V4) and a computer with SDR Sharp you can get quite good reception of HF signals. However reception can sometimes be spoiled by overload from signals on or away from the desired reception frequency. The 'HF Helper' presented here can reduce these problems. And you can use it in conjunction with a QRP transmitter to form a simple transmitting station.

RTL SDR HF Helper improves reception

Also as a bonus, in a related video VK3YE also shows the RTL-SDR Blog V4 being used on SDR Touch on an Android phone for portable HF, VHF and UHF reception.

Hear HF VHF UHF on an Android phone

SignalsEverywhere: Measuring Filters with RTL-SDR, a Noise Source and Spektrum

In her latest video Sarah from the SignalsEverywhere YouTube channel describes how it is possible to use an RTL-SDR to measure RF filters when combined with a noise source and the Spektrum SDR software. In the video Sarah comprehensively explains how to set all the various parameters in Spektrum, before demonstrating a filter measurement with a noise source. Sarah explains how the power measurements may not be entirely accurate, however it is enough to get some idea about the shape of a filter.

Measuring Filters With RTL-SDR and Noise Source. Spektrum SDR Spectrum Analyzer

Frugal Radio: Filters 101 – Improving Reception for your SDR

Over on YouTube Rob from Frugal Radio has uploaded his latest video which explains RF filters in a concise and easy to understand way. RF filters are often used together with SDRs to block out strong interfering signals which can degrade reception at other frequencies.

In the video Rob goes over the different types of RF filters such as high pass, low pass, band pass and band stop, and shows a few SDRs that have filter banks built in.

Filters 101 : Improving reception with your SDR or radio - a look at RF filters

Building a Hydrogen Line Front End on a Budget with RTL-SDR and 2x LNA4ALL

Adam 9A4QV is the manufacturer of the LNA4ALL, a high quality low noise amplifier popular with RTL-SDR users. He also sells filters, one of which is useful for hydrogen line detection. Recently he’s uploaded a tutorial document showing how to use 2x LNA4ALL, with a filter and RTL-SDR for Hydrogen Line detection (pdf warning). 

Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike.

In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.

One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.

Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.

One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.
One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.

Discussion and Review of our RTL-SDR Blog Broadcast AM High Pass Filter

Early last month we released a new broadcast AM high pass filter product. The goal of the filter is to block out extremely strong broadcast AM signals (and other problematic LF/MF signals) in order to prevent an SDR from overloading. This is especially needed if you live close to AM towers.

Over on the Utility DX Forum files section, reviewer D. B. Gain has written an excellent review of our broadcast AM high pass filter (pdf), also explaining why and in what situations it might be needed. In the review he explains how broadcast AM propagation works, and how it can change from day to night. He also explains how devices with diode switches (used for switching RF circuits such as filter in and out electronically) can easily overload and contribute to IMD within the switches themselves. This is why a filter without any diode switches in front of it is usually the best solution for reducing strong RF energies.

In the review he then goes on to test the filter, showing some screenshots of the reduction is AM signal strength.

RTL-SDR.com Broadcast AM Block High Pass Filter Now for Sale

Back in October we released a broadcast FM bandstop filter for removing strong signals in the 88 – 108 MHz region. Today we’re releasing a new broadcast AM high pass filter (BCAM HPF) with a 2.6 MHz cutoff. The cost is the same as the BCFM bandstop filter at $14.95 USD including free international air shipping. Faster shipping options may also be chosen if desired. We’ll eventually have this product on Amazon USA in a few months too, but for now it is only available from our Chinese warehouse.

The filter comes in a 2.8 cm x 2.8 cm x 1.3 cm aluminum enclosure and uses female SMA connectors on each end. Included in the package is also a SMA male to SMA male straight barrel adapter.

Click here to visit our store

The Broadcast AM High Pass Filter
The Broadcast AM High Pass Filter

This filter is designed to eliminate broadcast band AM (BCAM) stations by attenuating (blocking) any signals below 1.7 MHz. In reality due to roll-off the filter is usable from about 2.5 – 2.6 MHz and above.

The broadcast AM band exists at around 525 kHz to 1.705 MHz. These signals are usually local, and if you live close to a transmitter they can sometimes be extremely strong. Broadcast AM signals that are too strong can overload your SDR or radio, causing poor reception in other HF bands too. The filter also helps attenuate any other strong VLF/LF/MW interference. Note that this filter is a high pass and not a bandstop, so it will also block VLF signals. Specifications are shown below:

Filter Type: LC High Pass Filter
3 dB Cutoff: 2.5 – 2.6 MHz
Attenuation: ~60dB
Pass band I.L: Typically well below 2 dB
Power Levels: RX power only, cannot pass DC

Insertion Losses
Insertion Losses
Return Loss
V3 Direct Sampling no Filter
V3 Direct Sampling with BCAM HPF
Insertion Losses Insertion Losses Return Loss V3 Direct Sampling no Filter V3 Direct Sampling with BCAM HPF

We’ve also uploaded a video below that shows a demonstration of reception when using an RTL-SDR.com V3 dongle in direct sampling mode together with the BCAM HPF. In the video we first compare reception against an upconverter (the Spyverter). It’s worth nothing that the upconverter can receive signals well even without the filter in place. Using the filter does still help the upconverter receive a little bit better but the effect is not shown in the video. Then we simply scroll through the spectrum and listen to a few signals.

RTL-SDR.com V3 Direct Sampling with HPF Demonstration

Building an “HF Helper” for Improving RTL-SDR HF Reception

Over on YouTube Amateur Radio VK3YE has uploaded a video showing his 'HF Helper' project. The HF Helper is a tunable HF filter and attenuator that helps improve HF reception when in the presence of strong overloading signals. VK3YE writes:

Using an RTL-SDR.COM dongle (genuine model V4) and a computer with SDR Sharp you can get quite good reception of HF signals. However reception can sometimes be spoiled by overload from signals on or away from the desired reception frequency. The 'HF Helper' presented here can reduce these problems. And you can use it in conjunction with a QRP transmitter to form a simple transmitting station.

RTL SDR HF Helper improves reception

Also as a bonus, in a related video VK3YE also shows the RTL-SDR Blog V4 being used on SDR Touch on an Android phone for portable HF, VHF and UHF reception.

Hear HF VHF UHF on an Android phone

SignalsEverywhere: Measuring Filters with RTL-SDR, a Noise Source and Spektrum

In her latest video Sarah from the SignalsEverywhere YouTube channel describes how it is possible to use an RTL-SDR to measure RF filters when combined with a noise source and the Spektrum SDR software. In the video Sarah comprehensively explains how to set all the various parameters in Spektrum, before demonstrating a filter measurement with a noise source. Sarah explains how the power measurements may not be entirely accurate, however it is enough to get some idea about the shape of a filter.

Measuring Filters With RTL-SDR and Noise Source. Spektrum SDR Spectrum Analyzer

Frugal Radio: Filters 101 – Improving Reception for your SDR

Over on YouTube Rob from Frugal Radio has uploaded his latest video which explains RF filters in a concise and easy to understand way. RF filters are often used together with SDRs to block out strong interfering signals which can degrade reception at other frequencies.

In the video Rob goes over the different types of RF filters such as high pass, low pass, band pass and band stop, and shows a few SDRs that have filter banks built in.

Filters 101 : Improving reception with your SDR or radio - a look at RF filters

Building a Hydrogen Line Front End on a Budget with RTL-SDR and 2x LNA4ALL

Adam 9A4QV is the manufacturer of the LNA4ALL, a high quality low noise amplifier popular with RTL-SDR users. He also sells filters, one of which is useful for hydrogen line detection. Recently he’s uploaded a tutorial document showing how to use 2x LNA4ALL, with a filter and RTL-SDR for Hydrogen Line detection (pdf warning). 

Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike.

In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.

One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.

Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.

One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.
One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.

Discussion and Review of our RTL-SDR Blog Broadcast AM High Pass Filter

Early last month we released a new broadcast AM high pass filter product. The goal of the filter is to block out extremely strong broadcast AM signals (and other problematic LF/MF signals) in order to prevent an SDR from overloading. This is especially needed if you live close to AM towers.

Over on the Utility DX Forum files section, reviewer D. B. Gain has written an excellent review of our broadcast AM high pass filter (pdf), also explaining why and in what situations it might be needed. In the review he explains how broadcast AM propagation works, and how it can change from day to night. He also explains how devices with diode switches (used for switching RF circuits such as filter in and out electronically) can easily overload and contribute to IMD within the switches themselves. This is why a filter without any diode switches in front of it is usually the best solution for reducing strong RF energies.

In the review he then goes on to test the filter, showing some screenshots of the reduction is AM signal strength.

RTL-SDR.com Broadcast AM Block High Pass Filter Now for Sale

Back in October we released a broadcast FM bandstop filter for removing strong signals in the 88 – 108 MHz region. Today we’re releasing a new broadcast AM high pass filter (BCAM HPF) with a 2.6 MHz cutoff. The cost is the same as the BCFM bandstop filter at $14.95 USD including free international air shipping. Faster shipping options may also be chosen if desired. We’ll eventually have this product on Amazon USA in a few months too, but for now it is only available from our Chinese warehouse.

The filter comes in a 2.8 cm x 2.8 cm x 1.3 cm aluminum enclosure and uses female SMA connectors on each end. Included in the package is also a SMA male to SMA male straight barrel adapter.

Click here to visit our store

The Broadcast AM High Pass Filter
The Broadcast AM High Pass Filter

This filter is designed to eliminate broadcast band AM (BCAM) stations by attenuating (blocking) any signals below 1.7 MHz. In reality due to roll-off the filter is usable from about 2.5 – 2.6 MHz and above.

The broadcast AM band exists at around 525 kHz to 1.705 MHz. These signals are usually local, and if you live close to a transmitter they can sometimes be extremely strong. Broadcast AM signals that are too strong can overload your SDR or radio, causing poor reception in other HF bands too. The filter also helps attenuate any other strong VLF/LF/MW interference. Note that this filter is a high pass and not a bandstop, so it will also block VLF signals. Specifications are shown below:

Filter Type: LC High Pass Filter
3 dB Cutoff: 2.5 – 2.6 MHz
Attenuation: ~60dB
Pass band I.L: Typically well below 2 dB
Power Levels: RX power only, cannot pass DC

Insertion Losses
Insertion Losses
Return Loss
V3 Direct Sampling no Filter
V3 Direct Sampling with BCAM HPF
Insertion Losses Insertion Losses Return Loss V3 Direct Sampling no Filter V3 Direct Sampling with BCAM HPF

We’ve also uploaded a video below that shows a demonstration of reception when using an RTL-SDR.com V3 dongle in direct sampling mode together with the BCAM HPF. In the video we first compare reception against an upconverter (the Spyverter). It’s worth nothing that the upconverter can receive signals well even without the filter in place. Using the filter does still help the upconverter receive a little bit better but the effect is not shown in the video. Then we simply scroll through the spectrum and listen to a few signals.

RTL-SDR.com V3 Direct Sampling with HPF Demonstration

Finding Cheap Pre-Designed PCBs for SDR Projects

Recently RTL-SDR.com reader Neil KM4PHK wrote in to us to let us know that he’s been having a good time searching for SDR related PCB’s over on OSH Park. OSH Park is a company that allows you to upload and share a PCB, and then have it cheaply printed and sent to you for construction.

Some useful RTL-SDR related PCBs we found searching through their shared projects include PCB’s for a SAW filter, a PSA4-5043+ based LNAan MGA-53543 based LNAa lowpass or bandpass filteran FM trap, an ADS-B filter with LNA and a bias tee. More projects can be found by searching the shared projects page for strings like “SDR, LNA, Filter, Bias Tee, ADS-B”. Neil also writes that although some projects don’t have instructions on their OSH Park page, usually searching Google will reveal them.

An example PCB for an LNA that can be found on OSH Park.
An example PCB for an LNA that can be found on OSH Park.

Creating a DIY 88-108 MHz FM Trap

One of the most problematic strong signals you can encounter is regular 88 – 108 MHz broadcast FM stations. They transmit at high power and can cause overloading and intermodulation problems on simple receivers such as the RTL-SDR. This means that FM stations can prevent you from receiving signals even when you are tuned far away from the broadcast band.

The simplest solution to reducing strong FM stations is to build an FM trap. This is simply a band stop filter that blocks frequencies between 88 – 108 MHz from entering your radio. Adam (9A4QV), the creator of the popular LNA4ALL and several other RTL-SDR compatible products has recently uploaded an article showing how to build a home made FM trap out of cheap common parts.

Adams article goes through and explains the design of a FM trap and how to use freeware software to aide in the calculations. The final FM trap designed by Adam uses just 3 common SMD capacitors and 3 hand wound coils. His filter attenuates more than 30dB in the 88-108 MHz range with an insertion loss of less than 1dB up to 1.7 GHz.

A DIY FM Trap
A DIY FM Trap