Thank you to Jean-Marie Polard (F5VLB) for letting us know about his work in creating underground "earth probe" antennas that work for both RX and TX between 0 - 14 MHz, and are especially good at VLF and below. He writes:
Can't install an antenna at home? Madame refuses the masts, taut son? One solution, The Earth probes antenna.
Our group (https://www.facebook.com/groups/earthprobes/) started in January 2019. At first everyone made fun of me, the professionals called me crazy and today with more than seven hundred members, we installed underground antenna systems and the results are there.
Between 0 and 14MHz, in transmission and reception, it works!
This system dates from 1914/1918 but has been brought up to date.
It doesn't take much to get started, just the urge to try.
To access the Earth Probes and VLF.ULF.ELF groups you'll need a Facebook account. The groups contain a number of research papers documenting the concept, and the photos section. From the photos, an earth probe antenna appears to consist of two long grounding rods spread over a distance, or a grounding rod and long buried wire, combined with a balun.
CENOS are a company specializing in 3D modelling and simulation software for induction heating applications. However, they are now branching out and are creating software for antenna design and simulation. Final pricing of the software is not yet advertised, but they write that it has been made affordable thanks to "open source algorithms". Hopefully it will be affordable to hobbyists, but judging by the heat simulation software pricing it may not be (although they offer to software free to students, researchers and teachers).
However, it appears that they will soon be running a beta testing program that should hopefully be free to use during the testing phase. You can sign up to their email list and wait for their announcement on their website.
A Discone is a type of antenna that is designed to be resonant over a wide range of frequencies. Most antenna designs only really receive well on a few resonant frequencies, but a Discone is resonant over a much wider frequency range. This makes it a good partner for RTL-SDR and other SDR units as many SDRs tend to have wide tunable frequency ranges. With a wideband antenna like a Discone connected to an RTL-SDR one can scan over the almost entire tunable frequency range without needing to change antennas for each band. The drawbacks to a Discone however is that the antenna gain is not very high, and that it makes the SDR more susceptible to out of band interference. They also tend to be fairly expensive and difficult to build.
The Discone antenna is remarkable in that it is capable of receiving and transmitting over a wide range of frequencies with good matching. Because of this, it is a good match for SDR receivers such as the popular RTL-SDR sticks.
The only really tricking thing about making a discone is that the disc has to be balanced at the very top of the cone, which is mechanically awkward.
The two parts here allow the cone to be solidly clamped and provide an adequate base for the disk. There also two holes for bring the coax centre and braid out to the disc and cone. The base part has a socket at the bottom for 25mm (1 inch) plastic conduit for mounting
This antenna illustrated is designed for 400MHz and up, and as such transmits well on the 70cms amateur band, US and UK PMR channels and 23cms. It also receives aircraft ADS-B signals very well. I used .3mm plastic sheet for the cone and 2mm plastic for the disc, and then covered them with aluminium weatherproof tape. Be sure to check for continuity across the tape stripes.
The screenshot is of a calculator by VE3SQB which can be downloaded from http://www.ve3sqb.com/ if you want to make attenna's for other ranges.
If you're interested in building wideband antenna there is also the planar disk antenna (pdf) which can be built out of pizza pans.
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.
Thanks to Manuel aka Tysonpower for submitting to us his extremely cheap ADS-B antenna build. Manuels ADS-B antenna consists of a simple SMA connector with flange and some wires cut to the correct resonant length for 1090 MHz ADS-B. This ground plane design has been around for years on the internet with atouk’s guide being the most commonly used, although atouk’s design uses a larger SO-239 connector instead. Manuel takes the design one step cheaper by using cheap single core copper wire for the elements, and a low cost SMA connector. The wires are soldered onto the SMA connector flange so you will need to know how to solder to complete the antenna.
Manuel has uploaded a video which shows the build steps for his cheap antenna in a step by step guide. We note that the video is narrated in German, but there are English subtitles.
Akos the author of the radioforeveryone.com blog has recently added two new articles to his blog. The first post is a comprehensive guide to setting up your own ADS-B station. The guide focuses on creating a system that is easy to use, has good performance and is value for money. In the post he shows what type of computing hardware is required, what software can be used and what RTL-SDR dongles work best. He also shows what choices are available when it comes to amplification and filtering to improve signal reception and goes on to talk a bit about adapters and the antennas that work best for him.
EDIT: It’s been pointed out in the comments by antenna experts/enthusiasts that the 1/2 wave ground plane antenna described by Akos in his tutorial may not be technically correct. A 1/2 wave antenna has a huge impedance which requires some sort of matching. Without matching there is going to be about 10 dB of loss due to the mismatch, and so the antenna will perform poorly. We recommend sticking with a 1/4 wave design, which is essentially the same as Akos’ 1/2 wave ground plane antenna, just with the element lengths halved.
Over on Reddit user merg_flerg has uploaded an imgur post that carefully details a step by step guide for building a double cross antenna. A double cross antenna is great for reception of satellites like NOAA and Meteor since it has a sky oriented radiation pattern with very few nulls. This means that it can receive satellite signals coming from the sky well. Alternative antennas for NOAA/Meteor include turnstiles and QFH antennas, although the double cross antenna seems to have the least nulls, meaning that the signal is less likely to fade in and out as the satellite moves across the sky.
merg_flerg’s design is also modified from the standard design slightly, allowing it to become easily disassembled and carried within a backpack. At the end of his tutorial he writes that he gets much better reception with his double cross antenna than he does with his QFH.
In the post he demonstrates the final constructed antenna decoding a NOAA APT weather satellite image with an RTL-SDR and the WXtoIMG software. See our tutorial for information on decoding NOAA weather satellite images.
Recently we bought and tested one of Adam 9A4Qv’s ADS-B folded monopole antennas. This is a well thought out 50 ohm antenna designed for receiving ADS-B signals between 1030 and 1090 MHz. It has an omni directional radiation pattern (receives from all directions in the horizontal) and 3.67 dBi gain.
This antennas main defining feature is that it uses a DC grounded design which eliminates the static electricity problems other antennas can have. This allows this antenna to be connected to a receiver 24/7 without having to worry about ESD destroying the front end of your receiver or LNA. It is also small enough to be able to be used as a desktop antenna.
The antenna is made of FR-4 laminate (PCB material) with a conductive layer covering the ground plane board and an upside down U-shaped trace in the vertical section. The antenna requires assembly and detailed assembly steps can be found on the antenna’s web page. Assembly of the antenna itself was straight forward just requiring two solder joints to be made to connect the vertical part to the ground plane. You will need to take care to ensure that the vertical antenna is completely vertical once soldered in.
The antenna also comes with no included coax cable and so a cable must be soldered on to the antenna first. The assembly instructions recommend using Teflon coax cable (such as RG316), however we didn’t have any on hand, so we just used regular RG174U and carefully soldered it on, making sure to not melt the inner insulation too much.
After fully constructing the antenna we ran a SWR test using an RTL-SDR and a noise source and found the antenna to have a SWR of approximately 1.46 at 1090 MHz resulting in about a 0.155 dB loss due to mismatches. A SWR value of 1.46 is very good for a receive only antenna like this. In one of Adams videos he used some higher quality coax and his tests showed the SWR of the antenna with a value of about 1.2102.
As for actual real world ADS-B performance we did not test it against any other ADS-B antennas, but it received aircraft from almost 350 km away from an indoor desktop location which we think is pretty good. In comparison the standard 11.5 cm stock antenna that comes with most dongles only had a range of about 250 km when placed in the same location. The maximum range of an ADS-B signal is around 500 – 600 km, which should be easily achievable with this antenna mounted outside on the roof.
Compared to other hobbyist ADS-B antenna offerings, Adam’s folded monopole antenna is one of the cheapest we could find, costing only 20 euros + 5 euros for shipping (~$26.5 USD total), although it does not come with a mount or weatherproofing. It is also one of the smallest, measuring just under 6 cm high and 14 cm in width and depth. Below we table Adam’s antenna with some alternatives for a comparison (prices converted from Euro to USD where applicable).
Note remember that as passive antenna gain is increased, the receive radiation pattern becomes flatter (which you may or may not want – you’ll receive better out towards the horizon but worse at higher elevations) and that an active antenna with an LNA is most useful when long runs of coax cable is used.