Wi-Fi DX

Wi-Fi DX-ing is an activity of searching for distant Wi-Fi wireless networks during enhanced weather conditions, that allow radio waves to travel farther than typically.

Wi-Fi networks use 2.4 GHz or 5 GHz radio bands. Such high frequencies are characterized by high free-space losses and strong attenuation due to any obstructions. Therefore, the range of such signals is limited to several hundred meters or so. Of course, this can be greatly extended using external antennas, but as Wi-Fi uses only microwave frequencies at low power (typically 100-1000 mW), the range for longer distances is limited for line of sight applications only.

What’s interesting, under some favorable circumstances, the radio signals can be received much farther than expected. The phenomenon is the same as mirage, where rays of light are bent, effectively extending the optical horizon. Similarly, the sound waves can also be refracted and returned towards the ground under some conditions. Finally, this also applies to radio waves. 

Tropospheric propagation

The propagation of radio waves, especially the microwaves, depends on the conditions within the troposphere, which is the lowest layer of Earth’s atmosphere. Usually, the temperature decreases with height above the Earth. When there’s a thermal inversion, i.e. the opposite of what is normally the case, radio waves can extend their range beyond the horizon.

During special conditions, air layers may form some kind of a guide for the radio waves, providing tropospheric ducting. This phenomenon usually happens during periods of stable anticyclonic weather and affects radio frequencies in VHF, UHF and microwave bands.

In short, an enhancement in radio wave propagation is called tropo. General information:

  • Inversion layer may bend the radio waves back to the Earth’s surface, instead of going into the space.
  • Under special conditions, radio waves may get trapped into inversion layer, travelling far away in a form of duct. 
  • Tropo requires fairly flat terrain (but having the antenna at higher elevation is an advantage though).
  • The best results are obtained along the lines of an equal pressure (isobars).
  • The steeper inversion gradient, the better propagation strength.
  • Wind controls the propagation activity.

The most common mode of tropo propagation is caused by radiation inversion.

  • Occurs at night after warm and sunny day, when the ground cools off and radiates the heat to the cloudless sky.
  • Follows the topography of the land.
  • Increases normal levels of line-of-sight signals at longer distances.
  • Very high temperature amplitude between day and night indicates a possibility of good propagation.
  • Lack of wind initially limits the enhancement to short distances. Weak gusts are favorable.
  • Stronger gusts (> 4-5 m/s) mix up the air and weaken or even destroy the inversion layer.
  • Distances are typically below 100 km, increasing throughout night.
  • Best performance is achievable in the morning (if the wind is still weak). Sun heats up the Earth’s surface, but inversion is still present at some height for a short period of time.

Another type of propagation, based on the advection inversion, occurs where warm air passes over a cool surface. It has similar effect as radiation inversion, but the directions and distances change as the air masses moves further. It happens often after heavy rain (storm), when the ground is cooled down, the weather improves (clouds are gone, wind weakens) and warm air is moving in.

Air in a high-pressure system warms up and dries as it descends. Cool and moist air can get trapped underneath such layer, forming an inversion. In contrast to other modes, it can last all day and result in very long distances. This mode is based on subsidence inversion.

There are also other modes of propagation, like tropospheric scatter, which is present all the time. Small temperature and humidity variations can bring the signal beyond normal line-of-sight. This mode is not usable for Wi-Fi DX-ing though, as it requires very high power amplifiers (measured in kilowatts) and exceptionally big antennas (several meters long or more). Also, the variations in signal levels are very significant.

Upper layers in the atmosphere above troposphere do not affect the propagation of microwave radio waves.

For more reference see:

Tropospheric propagation forecasts are also available:

IEEE 802.11 standards

Wi-Fi devices configured in access point mode broadcast beacon frames, that include information about network, typically around 10 times per second. Wide channel widths significantly limit the sensitivity of wireless modules to few microvolts. The power of Wi-Fi transmitters do not exceed several hundred miliwatts, and the strongest devices reach up to around 30 dBm (1000 mW). The lowest available modulation depends on the used standard.

  • 802.11b (2.4 GHz only) uses DBPSK modulation on 22 MHz DSSS channel,
  • 802.11a/g (2.4 or 5 GHz) uses BPSK modulation with FEC 1/2 on 20 MHz OFDM channel (effectively 16.25 MHz for 52 subcarriers).

The practical sensitivity is at around -94 dBm for 802.11a/g and around 3-4 dB better for 802.11b (e.g. -97 dBm). See the sample specification of a Wi-Fi module – Ubiquiti XR2 or XR5.

The more recent Wi-Fi standards, like 802.11n or 802.11ac introduced additional features like multiple spectral streams or high order modulation (256-QAM), but because of backward compatibility, the beacon frames are still being broadcasted with the lowest modulation in a single stream.

Location and antenna

Location is very important, as tropospheric propagation favors flat terrain between antennas. Unfortunately, microwave frequencies are very strongly attenuated by any obstacles. The higher frequency, the higher attenuation. Therefore, antenna must be installed at a height over all nearby buildings, trees and other obstacles. The elevation angle at horizon should be clear at 1° or better for best performance.

Antenna is the most important matter. Generally, the higher gain, the better results can be obtained. There are many designs available on the market, single or dual-polarized. It is very easy to achieve very high gain on microwave bands, because of a relatively short wavelength. 

On the other hand, high gain comes with a very narrow antenna beam. Therefore, high gain antennas require also a rotator, with precise positioning (at least 1°). The antenna mast must be very well-verticalized in order to keep the same elevation angle in all directions. Very sharp beams may also require additional rotator for elevation angle alignment though. For instance, the largest parabolic dishes available on the market are rated at 34 dBi peak gain on 5 GHz and a beamwidth of 3° for -3 dB (half-power).

History of Wi-Fi DX

When I was young I used to spend holidays in the countryside. Unfortunately, in those days, the internet access was not so common. The mobile internet was only available with 2G technology (GPRS and EDGE) and was incredibly expensive. However, the 2.4 GHz Wi-Fi band was quite popular among Internet Service Providers using 802.11b standard with outdoor antennas. In 2005 I got an offset dish antenna, so I could be online also during holidays.

2006 – my first tropo on 2.4 GHz

From time to time, I was also scanning the Wi-Fi band to check what’s going on around. In the summer of 2006 I accidentally logged some odd networks with names indicating distant places. I knew some of them from car trips to my family. That was a very strange experience, because it was difficult to receive a signal at a distance of few kilometers and suddenly I found many networks at distances of 50-70 km and more, with no line of sight.

After that I learned about propagation and its possibility of extending the signal range due to special weather conditions. Later, I also bought an used antenna rotator. It was not the best fit for my antenna, because of huge clearance, but it was still better than nothing.

During the next summer of 2007 I was observing the propagation on 2.4 GHz band several times. Once I reached a distance of around 180 km. In late 2008 I replaced my previous antenna with Andrew 24 dBi grid, which was a real bargain ($15). The wind load was much lower, which was good for my weak antenna rotator.

Andrew MAG Grid 26T-2400 installed together with a rotator (2008-11-10)

Shortly I lost the interest in Wi-Fi DX because of poor results and lack of time. My antenna was installed only at a height of 7 meters above ground level and I was not able to put it any higher than that.

2015 – Wi-Fi DX on 5 GHz

The idea of Wi-Fi DX came back in 2015. Some students have established 5 GHz Wi-Fi connection record in Poland at a distance of 250 km between two mountain peaks. This event received a considerable nationwide media coverage. I was still wondering is it possible to reach such distances via tropospheric propagation beyond the horizon. Anyway, my experiences were positive, so I decided to take another try.

I’ve got an additional motivation, because there were no reports of Wi-Fi DX available anywhere. Of course such distance is not a problem for narrow-band emissions, like CW, SSB or FM. However, Wi-Fi networks use wide channels together with low power, which make the DX-ing much more difficult.

I built a simple and low-cost system for 5 GHz observations. I bought an used 31 dBi grid antenna and I installed it together with Routerboard Metal 5SHPn radio at a height of 8 m in a horizontal polarization. Unfortunately, almost all directions were blocked by trees or other obstacles and only a very narrow section was exposed for tropospheric ducting attempts. I also started to work on a software for wireless network scanning and logging called MTscan, dedicated for Mikrotik RouterOS based devices.

After some successful initial tests, I added a decent antenna rotator to this installation. Initially I was using it manually with 12V battery, just by swapping the polarity. Shortly I also designed my own controller.

Poynting K-GRID-003-06 5 GHz antenna rated at 31 dBi installed with a rotator

To my surprise, few months later, I logged many networks at 300+ km distances including… Czech Republic. Full report is available there: Autumn 2015: Extreme tropo duct 5 GHz Wi-Fi 300+ km. I also successfully identified and spotted my new record at 346 km, but it was just a single received packet. After that I decided to start saving some money and build a better setup for DX-ing to cover 90% of azimuth ranges, instead of just… 10%? 

Later, I also wrote software for rotator remote control and I implemented support for it in MTscan. Since then I was able to log exact azimuth of each received signal. I also added a special radar mode to my software, where antenna is rotating forth and back within a specified azimuth range. Therefore, my setup for Wi-Fi DX has become fully automatic.

2017 – new 18 m mast and antenna

During the spring of 2017 I did some initial tests at higher height. I installed smaller 24 dBi dual-polarization panel antenna with a built-in radio (Mikrotik QRT 5) at the top of a temporary mast with a height of 15 m. First tests during evening radiation inversion revealed some tropospheric signals, which were still unavailable on the previous antenna, 10 meters lower.

Mikrotik QRT 5 at top of the temporary 15 m aluminium mast (2017-04-01)

Almost two months later I was finally able to compare the reception during tropo overnight. Well, the numbers were clear – only 276 networks logged with 31 dBi grid antenna (8 m AGL) and 868 networks with small 24 dBi panel antenna (15 m AGL).

Overnight Wi-Fi DX testing at 15 m AGL (2017-05-28)

The horizon was still blocked with some nearby trees, so few weeks later I upgraded the mast to 18 meters. Such huge length required additional stabilization of the mast during lift-up though. As first I installed my camera at the top of the mast in order to take photos around.

Canon 550D (left) and Mikrotik QRT 5 (right) at top of the strengthened 18 m mast (2017-06-15)

The overnight results were excellent, because I was able to detect up to … 10× more networks from a given direction. The outcome was clear – a stationary mast with a height of around 18 meters is necessary.

18 m low-weight aluminium mast, lowered down (54×2 mm + 60×3 mm + 54×2 mm + 50×2 mm)

Over two months later, at the end of August, the stationary mast was finished and ready to lift up. It was made of thick-walled aluminium tubes: 80×5 mm + 70×5 mm + 60×5 mm + 50×5 mm. Initially, it had a small rotator and QRT5 panel antenna at the top. It was erected up using three manual winches without any problems.

The first day of the stationary 18 m mast (2017-08-26)

Within next week I was thinking about new antenna. The choice was actually quite difficult, as weight was very critical during mast lift-up. Finally, I ordered Rocketdish RD-5G31-AC on September 5, 2017, which is a 75 cm deep dish dual-polarization antenna with built-in radome, at just 7.6 kg. It required some modifications to install Mikrotik radio instead of Ubiquiti. Details together with many photos and a video are available there: Ubiquiti RocketDish 5G31-AC – short review.

In total, the top segment (antenna, rotator, mast) weighs 16 kg. The additional cross element with guy lines greatly stiffened the mast during lift-up (this idea was carried from the temporary mast). Since then, the mast survived strong winds, with gusts up to 31 m/s. Later I also added third level (the lowest) of mast guys to fully stabilize this construction.

The first day with Rocketdish RD-5G31-AC at 18 meters (2017-10-01)

2018 – revised rotator controller and continous Wi-Fi DX-ing

In 2018 I revised my rotator controller, replacing PWM speed control with a step-down DC converter based on LM2576, with a ladder of resistors that controls the voltage. I also installed all modules together in an aluminium box:

  • Routerboard 411U – runs OpenWRT, with rotator remote control server, the power is injected into Ethernet cable that goes to the antenna at the top of mast,
  • rotator controller – allows smooth speed regulation, azimuth pulse count, soft start, virtual limit switches, etc,
  • DC voltage converters – power supply (20 V) and rotator voltage control (4 – 14 V),
  • ethernet surge protector – in a separate metal box.

   

In the meantime I also was working on new features in MTscan software, like passive scanning using TZSP packet streaming (parsing beacons, probe responses, including reverse-engineering proprietary protocol beacons like Mikrotik NV2 or Ubiquiti Airmax AC). Recently, I have also implemented automatic geolocation based on Wigle.net and my own database based on wardriving activity (which is also supported in MTscan).

With my current setup, the Wi-Fi DX results are pretty insane. Only in 2018 I logged over 13000 networks up to 464 km. For a comparison, with the grid antenna installed 10 meters lower only 2000 networks were logged between 2015 and 2017. The most notable propagation reports are available below:

According to the observations, distances of around 300 km or more are reachable quite often. In year 2018 such conditions were available for… over 30 days. More information is available in my write-ups (currently only in Polish language, but Google Translate does a pretty good job):

The cumulative network list, which I have logged since 2015 is available as:

Map of received and identified 5 GHz networks, incl. foreign signals from Czechia, Ukraine and Lithuania

While almost all of the logged networks utilize outdoor antennas, it also possible to receive signal from indoor devices, under exceptionally good propagation conditions. There are many domestic wireless routers supplied by ISPs which are high power devices, especially on 5 GHz band. According to FCC tests, even above 500 mW (excluding antena gain). The possibility of logging indoor networks via tropospheric propagation has already been reported in a stationary manner at distance of over 200 km (see write-ups above). Furthermore, we have successfully logged some domestic routers during a DX trip using a smaller antenna (55 cm dish) at distances of around 141 and 126 km (UPC and Orange routers). For more information see: Early morning on Dylewska Góra.

Last update: April 2019