In unserem Blog möchten wir Ihnen wertvolle Beiträge zu Themen in Bereichen wie ADS-B, Mode-S, MLAT, Flight Tracking, Antenntechnik und 1090 MHz geben.
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Over the christmas holiday our Radarcape demo station has got a new yagi towards Munich airport EDDM/MUC in order to improve the tracking of ground traffic. The airport is around 30 km east from the location of Jetvision headquarters but a not fully free line of sight. The Radarcape demo station for this purpose is equipped with the 2 channel antenna diversity Radarcape. Such a Radarcape-2CH is equipped with two independant antenna inputs, one connected to an omni directional A3-ADSB antenna. The second input is connected to a home made 2.5m long yagi antenna.
Because there are no such long yagis for ADS-B on the market our Jetvision member Guenter has constructed this experiemental light weight (1kg), 2.5m long yagi 1090 MHz antenna with 27 elements. Its gain is 16.5dB (18.7dBi). This, within our experience, will truely be reached. In order not to be obstructed by other metallic elements on the mast the yagi is mounted on a bracket. Due to the light weight design mounting was even possible during a local storm.
The result of this is a drastic improve of the track of ground based vehicles at the airport, for example snow plowing trucks during winter time. The antenna also extends the range of the Radarcape demo towards east, where regularily 450 km distant aircraft can be observed. There is still some room for improvement, vehicles that are hidden between the buildings may be trackable with some more gain and picking up reflections. For that purpose an even longer yagi, with some 5-6m length, is in our thought.
If there is interest, we are willing to create these yagis as commerical products for customers, too. Please let us know on our support line (firstname.lastname@example.org)
Ground traffic on Munich airport while cleaning the runway at January 17th, 2018
Antennas at Jetvision headquarters:
Center: 2.5m long Yagi antenna, Very left: A3-ADSB for Radarcape demo, Very right: Active Diapason for Flightradar24 and for FLARM (dl4mea), unused experimental antenna.
We do see various offers for any kind of preamplifiers that are promising wonders, so let me give an explanation for the true important factors when using such devices. During my active time at ham radio I was very active in earth-moon-earth communication, that is sending signals to the moon and getting the echo back, up to 5.6 GHz, far beyond the ADS-B frequency of 1090 MHz. This is one of the most sophisticated operational modes in amateur radio, and needs high transmission power as well as most sensitive receivers. This knowledge was brougth into the receiver design of the Mode-S Beast as well as the Radarcape.
Now we do see a lot of preamps on the market. Most of the time gain is given for them, but the more important value of noise figure is missing. There are synonyms for both that can be understood easier:
You may quickly understand by these synonyms, once you have destroyed the black level of your picture into a kind of grey, you no longer never can see the small nuances of weak black symbols on your screen. It is the same with radio signals: Once you have destroyed the noise figure, you never will get it back. Even turning the brightness on (= adding gain), you never will recover the true cold black on your screen but instead simply amplify your grey even more.
This brings me to the cable: A cable is attenuation. With our video screen, this is similar to a milky glass in front of it. So you don't see the nice pure black any more but a little bit of grey. Of course, your bright symbols still appear readable. If now you insert an amplifier, the bright symbols will become brighter, but also the grey becomes brighter. But the weak nuances of black are lost.
However, if you amplify before passing the lossy section, you will amplify the nuances of black we're talking about, and they may be still readable behind the attenuation.
Even worse, electronic devices may become overloaded by the shiny bright parts. So this means with an amplifier after the cable and in front of the receiver, you may make things even worse. Only if your device is really deaf or has a lot of internal losses, an amplifier in direct front of the antenna input will improve the situation.
Our Active Diapason antennas for 868MHz (FLARM) and 1090 MHz (ADS-B) are equipped with an amplifier directly connected to the antenna element, which is the pure optimum. Their noise figure is somewhat 1 dB, a quite reasonable value for an antenna that partly sees warm earth within its diagram (less than, say 0.8 dB, only makes sense for space pointing antennas). They are sold with 20 m cable because the gain should not overdrive the receiver, and because this cable attenuation is completly knocked out due to the low noise figure.