ARLA/CLUSTER: ADS-B folded monopole antenna

[Carlos Fonseca - CT1GFQ]

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Interest check
This is not just another ADS-B cut and try antenna, this is the ADS-B antenna designed by a highly skilled professional. The antenna is designed to efficiently cover the range from 1030 MHz to 1090 MHz, so the two common used channels in the ADS-B system. After the initial tests with the prototype antenna from the picture I decide to hear your voice.

What makes this antenna special and different from the other designs that we have on the market ?
Simply, this is one of a DC grounded antenna, efficiently handle the static problems that may occurs on the antenna. This antenna came out as the answer to many e-mails received where the user experience the situations where the ADS-B receivers, DVB-T dongles, low noise preamplifier's were damaged due to static coming from the antenna. There are many sources of the static, like corona discharge, rain static, wind static etc.

There are several ways how to cure the problem. The simplest one is to disconnect the antenna, but most of the users are running the receivers 24/7 or remotely positioned antennas where disconnecting the antenna is not an easy option.
Most of the users rely on the pair of the protection diodes commonly installed in the equipment they are using. This looks like a good prevention but in reality this is a double-edged sword!
This par of diodes are potential source of the intermodulation that can spoil your reception to the levels where the properly installed antenna with the professional preamplifier's may result in poor reception. This is the most common problem if you are close to the strong radio and TV broadcast transmitters, or GSM towers. Setups with the high gain antennas and preamplifier's without the good filtering will amplify already strong out of the band signals routing the same to the pair of diodes at the receiver input. This signals crate a so called ghost signals that may interfere with your frequency of the interest.

DC grounded antenna will ground also the static and your equipment will be protected. At the same time, the "protection" diodes may be removed, hence the possible cause of the intermodulation. Folded monopole antenna is an old and well known design but not so common used. You can find that antenna in some professional rugged systems where static discharge is the main problem.


The goal was to design a cheap and at the same time efficient antenna. Another very important thing was to design the antenna that may fit into envelope for the cheap shipping. The antenna should meet the 50 ohms impedance as the standard and cover both ADS-B frequencies. Assembling should be simple and straightforward not requiring special tools or user skills. It should be light and most important the antenna should work properly.


We manage to have all that including the following characteristics:

1. Omnidirectional pattern 


In order to cover all sectors and directions, the 360 degrees omnidirectional pattern is preferred for the broadcast nature of ADS-B transmission. Depending on the location of the antenna the signals from the planes, their position on the map should be scattered evenly around the location without the gaps.




2. Vertical polarization 



The signals from the ADS-B transponders are vertically polarized, hence the receiving antenna should meet the same vertical polarization in order to receive the strongest signal for the best reception. The folded monopole antenna is vertically polarized antenna as can be noticed from the diagram.



3. Efficiency and the gain




The efficiency of the antenna is better then 99% and the calculated gain is 1.53dB or, for the marketing guys 3.67dBi. The gain was not an imperative when this antenna was designed but it came out from the design.




4. Impedance 

The antenna has the impedance of 50ohms at the resonant frequency. As the standard widely used 50 ohms is preferred over the 75 ohms due to variety coax lines, preamplifier's and filters that are designed for the same impedance. It is true that there is a large growing community using the simple DVB-T dongles designed with the 75 ohms input impedance but to keep the things simple we stick to the 50 ohms.

5. Impedance matching


The folded monopole itself has much higher impedance then required 50 ohms so the matching was required. The matching system used is made using the microstrip line on the bottom of the ground plane made of FR-4 1.6mm laminate. Good matching has achieved across the whole working frequency range. At the end of the mach there are pads where any type of 50 ohms coaxial line should be soldered.

6. Voltage Standing Wave ratio (VSWR)


The VSWR is quite flat for the frequency range of the interest. If 75 ohm coaxial line is used the VSWR will degrade due to minimal mismatch between the feeder line and the antenna impedance. This should not affect the efficiency of the antenna in large scale.



The antenna is made on FR-4 1.6mm laminate and it consist of two parts, the ground plane with the matching line and the vertical hairpin style monopole. The ground plane is 140x140mm in size, while the hairpin is 65mm high. To assemble the antenna it required only to insert the vertical hairpin into the catted slot on the ground plane and to simply solder the hairpin microstrip line to the match on one side and to the ground plane on the top side. Solder the coaxial line to the connecting pads and antenna is ready.
If used outside, it is advised to place the antenna into some plastic canister, (cheap food plastic canisters).
Such antenna should be placed on top of the metal mast in order to maintain the omnidirectional pattern.

Target price is 20 Euro. If there is enough interest, I can make the first batch in short period so it is up to you.Please leave your comments and suggestions bellow in order to valuate the project.

Thanks


73's de Carlos CT1GFQ
SKCC#466C www.skccgroup.com
REP#1406 www.rep.pt
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