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.