Software defined radio, RF pre-amplifiers and mixers. Adventures into RF antennas and proof that theory <> practice ;).

ADSB Antenna Comparison

Back to Telecommunication Antenna comparison trials

Side by side time trials with UHF antennas.

Time trials testing multiple distentions of various concepts of equipment. In the case of the ARM micro processor (Raspberry PI) the test involves reliability and up-time. In the case of the antennas (a j-pole vs 1/4 wave) the reception range in all seasons. These antennas are placed in close proximity with both units feeding signal to the same SDR receiver setup. By permanently running this setup the effects of weather, humidity and even foliage on trees can be seen.

ADSB Antenna Results

Antenna comparison results

See the effect different antennas have on the ability to receive UHF ADS-B signals from passing aircraft.

Live view (as shown above): 1/4 wave dipole ADS-B Aircraft tracking ground radar can be seen at

In other articles I'll go into the details of building these and other UHF antennas.

1/4 1090 MHz ADSB Antenna

1/4 wave antenna for 1090 MHz

The quarter wave antenna is as the name states, a quarter of the wavelength long. For 1090 MHz with a wavelength of 269.228 mm it makes the active antenna element 68.807 mm or 6.8 cm long.

This type of antenna requires a ground-plane (virtual mirror reflection of itself) to operate, hence the four (or more) ground-plane elements at the base of the active element. These are bend down at an angle of 45 degrees for impedance matching to 50 Ohms.

Quarter wave antennas require a large volume of space to operate and are not ideal for use in cramped areas, nor do they produce any gain (signal amplification).

J-Pole 1090 MHz ADSB Antenna

J-Pole antenna for 1090 MHz

The J-Pole antenna, depending on how you look at it, is a full wave element, using a half wave stub for impedance matching purposes. The quarter wave (short) section is grounded while the three quarter wave session is the active element.

It is longer than the 1/4 wave, but requires less volume the short element is 68.807 mm the long element is 206.422 mm. The distance between the elements (+ or - for matching) is 6.730 mm. This antenna goes not require a ground plane, but does require a balun. The balun for this type of antenna matching is to prevent current flow on the transmission line.

J-Pole design antennas are ideal "pipe antennas", which can easily be inserted into a section of PVC pipe to protect them from the weather.

ADSB Antenna Comparison Tests

Using multiple different antenna designs in the same location at the same time with the same length (lossy coax) is an ideal way to test and check the characteristics of the antennas in use.

Antenna comparison tests

Antenna Impedance Matching

If you're not rich (I certainly am not), then you have to be clever. A simple way of making an adjustable impedance matching connection to assist in finding the sweat spot. This can be done using antenna LCR analyzers, which are expensive, or by monitoring the RSSI levels.

Antenna Impedance Matching

Antenna Impedance Matching Point

Where the feed-line is attached to connector is the matching point. Do note that using these common electrical wire connector blocks does have a small effect as opposed to connecting the feed-line directly. It is a good way to get you to the sweat spot.

Antenna Impedance Matching Point

SDR monitoring RSSI

Using SDR is another way to compare the performance of an antenna with another. If testing more than one antenna, do make sure you either have a separate SDR setup for each, or make sure the transmitter you are tuned into is stationary = not airplanes in flight!

SDR monitoring RSSI

Collinear matching stub experiment

Silly antenna experiments (did not work out at all). Trying to construct a UHF collinear dipole. Performance was not at all good and getting it matched was impossible. Probably tried this on a bad day, but have since not returned to this antenna design type again.

Collinear matching stub experiment

Collinear antenna build

End fed collinear antenna with three sections. Was hoping it would not need a large ground plane, but that was not the case for UHF. Getting the LC ratios for the in-line matching stubs also proved a challenge as the length of the stub is known, but the LC of the diameter of the drill bits I have at my disposal is not. Once figured out, this little antenna was quit a pleasant surprise, even though a ground plane was required to stop using the coax as part of the antenna.

Collinear antenna build