Thank you to Frank Sessink (PA0FSB) for submitting to us his document describing the K9AY loop antenna (pdf), which is the antenna that he successfully uses with his RTL-SDR for HF reception. The antenna combines magnetic (H) and electric (E) field reception in order to create a directive radiation pattern. Frank extends the idea by showing a method that can adjust the directivity electrically with some simple resistor switching.
The antenna that I use is for medium wave DX, specially to receive MW from USA here in Europe/The Netherlands. The antenna is a combination of a magnetic loop and a sense antenna for the E-field. The magnetic loop is directive, but has no front-rear ratio. The E-field antenna has omnidirectional sensitivity. The combination, in correct phase and amplitude, results in a front-rear ratio of more than 25 dB over the frequency range from 500 kHz to around 3 MHz. Higher frequency makes no sense, since skywave signals distort the ground wave directivity pattern.
A simple modification is used as directional antenna with remote control: two orthogonal loops that combine E and H-field in a simple way. I can make 8 selectable directions.
We're happy to announce the release of our new high performance low noise amplifier (LNA) for improving 1090 MHz ADS-B reception. The LNA uses a low noise figure high linearity two stage MGA-13116 amplifier chip and three stages of filtering to ensure that strong signals or interference will not overload either the amplifier or SDR dongle.
The LNA is currently only available from our Chinese warehouse, and costs US$24.95 including shipping. Please note that the price may increase slightly in the future, and that Amazon USA may not be stocked until March.
An LNA can help improve ADS-B reception by reducing the noise figure of the system and by helping to overcome losses in the coax cable and/or any other components such as switches and connector in the signal path. To get the best performance from an LNA, the LNA needs to be positioned close to the antenna, before the coax to the radio.
The gain of the RTL-SDR Blog ADS-B LNA is 27 dB's at 1090 MHz, and out of band signals are reduced by at least 60 - 80 dB's. Attenuation in the broadcast FM band and below 800 MHz is actually closer to over 100 dB's. In the LNA signal path there is first a low insertion loss high pass filter that reduces the strength of any broadcast FM, TV, pager or other similar signals that are usually extremely strong. Then in between the first and second stage of the LNA is a SAW filter tuned for 1090 MHz. A second SAW filter sits on the output of the LNA. The result is that strong out of band signals are significantly blocked, yet the LNA remains effective at 1090 MHz with a low ~1 dB noise figure.
The LNA is also protected against ESD damage with a gas discharge tube and low capacitance ESD diode. But please always remember that your antenna must also be properly grounded to prevent ESD damage.
Please note that this LNA requires bias tee power to work. Bias tee power is when the DC power comes through the coax cable. The RTL-SDR V3 has bias tee power built into it and this can be activated in software. See the V3 users guide for information on how to activate it. Alternatively if you don't own a dongle with bias tee built in, then an external bias tee can be used and those can be found fairly cheaply on eBay. Finally, if you are confident with soldering SMT components, then there are also pads and a 0 Ohm resistor slot on the PCB to install an LDO and power the LNA directly.
In addition please remember that this is a high gain LNA. It is expected to be used at the antenna side, with some 3+ db loss expected on the coax. However, if desired, it can still be used on the receiver side. If used on the receiver side or with a low loss run of coax, you will need to tune the RF gain on the RTL-SDR dongle. By default most software sets the RF gain to maximum. We recommend turning the RTL-SDR RF gain down to about 32 dB if connecting it directly to the dongle, otherwise the high input power may overload the dongle causing poor performance.
Specification Summary:
Frequency: 1090 MHz
Gain: 27 dB @ 1090 MHz
Return Loss: -16 dB @ 1090 MHz (SWR = 1.377)
Noise Figure: ~1 dB
Out of band attenuation: More than 60 dB
ESD Protection: Dual with GDT and ESD Diode
Power: 3.3 - 5V via bias tee only, 150 mA current draw
Enclosure: Aluminum enclosure
Connectors: Two SMA Female (Male to Male adapter included)
Dimensions:
46.5 x 32 x 15.6 mm (not including the SMA).
Including the SMA the length is 69.8 mm.
Testing
We tested our new LNA against another ADS-B LNA with filter built in that is sold by another company and the FlightAware Prostick+ dongle in an environment with strong out of band signals such as pagers, broadcast FM, DVB-T and GSM signals. The results showed that the RTL-SDR Blog ADS-B LNA gathered the most ADS-B packets. In the tests both LNA's were connected on the receiver side to be fair to the FA dongle. Improved performance could be achieved by moving the LNA to the antenna side.
Other ADS-B LNA vs RTL-SDR Blog ADS-B LNA Received MessagesFlightAware Prostick+ vs RTL-SDR Blog ADS-B LNA Received Messages
Checking in SDR# for out of band signals also showed that the RTL-SDR Blog ADS-B LNA significantly reduces those strong out of band signals, whereas the others have trouble blocking them out. Below we show the results as well as some measurements.
This RTL-SDR Blog ADS-B LNA can significantly improve ADS-B reception, especially if you are in an environment with strong out of band signals. Even if you are not, the low noise figure design will improve reception regardless.
Over on his YouTube channel Adam 9A4QV has uploaded a video showing how an LNA work to improve signal SNR on VHF, as long as the LNA is placed close to the antenna. Adam is the manufacturer and seller of the popular LNA4ALL low noise amplifiers.
On UHF and high frequencies an LNA can help by reducing the system noise figure, but on VHF this effect is small. But if the LNA is placed near the antenna then the LNA can still help significantly by overcoming any losses in the coax cable, filters, switches or any other lossy components in the signal path. It might also help create a better SWR match for the dongle and antenna. The video has some sound issues in during the demonstration part, but on his Reddit thread Adam writes:
So, antenna itself represents as handmade on-PCB winding made of two-side-foiled fiberglass size of 30x40 cm. Both 'windings' connected in the middle and thus winded to 'continue' each other.
Preamp located in metal box attached to antenna and connected via 1.5m S/FTP cable to another box with RTL stick. Note that some transistors soldered on PCB in upside-down - dot on layout means base.
Electrically preamp made as 3-stages balanced signal amplifier with low-input impedance and low-pass filter before input with cut-off at 15MHz. Such complications were required to reduce interferences and intermodulations. Antenna itself is more effective on long-medium waves, so preamp has higher gain on short waves (gain varies from 45db at 200KHz to 68 db at 10MHz - see attached freq responce pic). Getting more flat responce at lower frequencies is possible by increasing C10/C11/C12 to 22nF.
My implementation has some additional elements to make possible to adjust preamp's gain in few db's. But seems its quite useless so that details not included in this post. Anyway, its possible to reduce gain by increasing R6 to 500K.
Box with RTL SDR: I put both signal wires as 3 windings via ferrite ring with high permeability just before RTL chip. This noticeable reduced stray interference, that induced in that cable but doesn't affect differential signal.
The Vivaldi is a fairly well known ultra wideband antenna that is directional. It is fairly easy to build out of a PCB board, but requires some careful design considerations to work well. In the second post hexandflex goes over all the design considerations that he put into his Vivaldi incliding the feed design, substrate choice and additional improvements like adding corrugations and crafting the geometry for a lens effect.
The results show that the antenna works well as a directional antenna above 1.7 GHz, and begins to work more like a standard dipole below 1.7 GHz. Directional gain is greater than 5dB above 1.7 GHz, and becomes negative below 1 GHz. Although hexandflex notes that the gain below 1 GHz is still reasonable, and probably still better than any untuned monopole.
Hexandflex has put up a small number of Vivaldi antennas that he's produced up for sale on Tindie for US$18. Currently he has a limited batch of units to sell, but notes that he may run additional batches if they are popular.
Over on his YouTube channel Tom Stiles (hamrad88) has been experimenting with and reviewing our multipurpose dipole kit. Tom is a ham radio YouTuber who runs a show that produces content often, so we encourage you to subcribe to his channel if you're interested. Tom reviewed our dipole kit over a series of 5 videos which we link here [1: Discussing the product], [2: Unboxing], [3: First ADS-B Tests], [4: Second ADS-B Tests], [5: Third ADS-B Tests]. We post have embedded video 2 and 5 below.
In his testing Tom finds that using the antenna in the vertical orientation improves ADS-B performance. This is expected as ADS-B signals are vertically polarized, and so the antenna should be too. By using the included suction cup mount Tom is able to get the antenna attached to his window which improves reception by getting the antenna as close to the outdoors as possible. This is an expected use case for the antenna, and it's good to see that good results are being had!
If you're interested in the set please see our store at www.rtl-sdr.com/store, or use the links provided in Tom's videos. We also have a tutorial and use case demonstrations for our dipole kit available at www.rtl-sdr.com/DIPOLE.
He uses a FlightAware dongle, which is an RTL-SDR optimized for best ADS-B reception when placed directly at the mast/antenna. For an antenna he uses the FlightAware ADS-B antenna, which we've reviewed in the past and found to be one of the best value ADS-B antennas available on the market. To process the data, a Raspberry Pi is used and it is powered via power over Ethernet (POE). If you didn't already know, power over Ethernet (not to be confused with Ethernet over powerline) is simply running power through unused wires inside an Ethernet cable. It is a convenient method of powering remote devices and giving them a network connection at the same time. The whole package is enclosed in a waterproof case, and the antenna attached to the top.
Putting the RTL-SDR and computing device at the antenna removes any loss from long coax runs, and the POE connection provides a tidy cabling scheme. The FlightAware dongle is a good choice for mounting directly at the mast or antenna because it has a built in low noise figure LNA. If using coax cabling instead, and keeping the RTL-SDR and Raspberry Pi inside, then it would be better to mount an LNA at the mast and power it through the coax via a bias tee.
Over on his blog VK5QI has shown how he has was able to re-purpose an old radiosonde into a wideband active L-band antenna. Radiosondes are small packages sent up with weather balloons. They contains weather sensors, GPS and altitude meters and use an antenna and radio transmitter to transmit the telemetry data back down to a ground station. With a simple radio such as an RTL-SDR and the right software, these radiosondes can be tracked and the weather data downloaded in real time. Some hobbyists such as VK5QI go further and actually chase down the weather balloons and radiosondes as they return to earth, collecting the radiosonde as a prize.
VK5QI and his friend Will decided to put some of his radiosonde collection to good use by modifying one of his RS92 radiosondes into a cheap active L-band antenna. They did this by first opening and removing unnecessary components that may interfere such as the main CPU, GPS receiver, 16 MHz oscillator, SAW filters and balun. They left the battery, LDO's, LNA's and Quadrifilar Helix GPS antenna which is tuned to the GPS L-band frequency. Finally they soldered on a coax connector to a tap point on the PCB and it was ready to use.
They then connected the new antenna to a RTL-SDR V3 and fired up GQRX. They write that their results were quite promising with several Inmarsat and Iridium signals being visible in the spectrum. VK5QI also used gr-iridium with the antenna as was able to decode some Iridium signals.
Modified Radiosonde L-Band Antenna connected to a RTL-SDR V3.
