Antenna – GEPRC

The Triple Feed Patch antenna is a directional circularly polarized antenna that I created in collaboration with Robin Theunis, one of my co-workers. The main design goals were:

Good radiation efficiency. Most FR4-based patches have terrible efficiency because FR4 is very lossy at 5.8 GHz.
Good axial ratio, not just directly perpendicular to the patch but also off-center.
Cheap and easy to manufacture with high accuracy, even for a hobbyist.

(strictly according to Maarten Baer design requirements, learn more, please click: http://www.maartenbaert.be/quadcopters/antennas/triple-feed-patch-antenna/)

Center frequency: 5.8 GHz
Bandwidth: 660 MHz (5.47 – 6.13 GHz)
Matching: S11 < -20 dB, VSWR < 1.22 (at center frequency)
Axial ratio: < 1.3
Antenna gain: 9.4 dBi
Half power beam width: 55° (horizontal and vertical)
Radiation efficiency: 83.5%

These values are based on simulation, actual measurements are below (they are very similar). The center frequency may shift a bit between batches as a result of variations in the dielectric constant of FR4. Also, not all brands of FR4 have the same dielectric constant. Frequency shifts up to 100 MHz are normal.

Simulation model. The two plates are wave ports that act as stand-ins for the SMA connectors.

3D radiation pattern. Look at the 2D version if you actually want to see the numbers.

Vertical cross-section taken at various angles.

Measurements

VNA measurements (S-parameters). S11 and S22 indicate the reflected power at port 1 and 2 respectively. S12 and S21 indicate how much power is leaking from one port to the other.

This gives you an idea of the kind of variations you can expect for hand-assembled antennas. I suspect that most of the variation is caused by the surface-mount SMA connectors since those were soldered by hand, and it's difficult to accurately control the amount of solder when you can't even see the center pin. This would explain why there is a lot of variation in S11 and S22, but almost no variation in S12 and S21. Based on S12 and S21 (which should be more reliable), the center frequency appears to be close to 5.7 GHz, which I suspect is a result of inaccurate spacing of the PCBs. The 4 mm spacers which I used to construct these units somehow resulted in a final spacing closer to 4.2 mm in reality.

“Pagoda 2” is based on Maarten Baert optimized circular polarized antenna， Produced by GEPRC customize a 5.8 G antenna, is the scope of its bandwidth of 500 MHZ (5.55 to 6.05 GHz). Structure consists of three layers of 1 oz copper foil embedded PCB, only 8 grams of weight, use special heat shrink packaging, solid, durable, and beautiful. PCB materials used 1.0 mm FR – 4, 1 oz copper foil (strictly according to Maarten Baer design requirements, learn more, please click: http://www.maartenbaert.be/quadcopters/antennas/pagoda-antenna/)

Pagoda 2 has natural advantages, axis ratio is below 1.3, standing wave ratio of 1.2.Can be used for transmitting antenna and receiving antenna (transmit and receive all use pagoda2 antenna is better).

Image: pagoda2-rad-3d-l80-wp-30db-small.png

3D radiation pattern. Look at the 2D version if you actually want to see the numbers.

Simulation model.

Image: pagoda2-rad-vert-l80-wp-small.png

Best case, vertical cross-section taken at various angles.

Image: pagoda2-rad-vert-l80-nowp-small.png

Worst case, vertical cross-section taken at various angles.

For comparison, here's a simulation of the traditional cloverleaf antenna using the same simulation software and setup:

Simulation model.

Image: cloverleaf-rad-vert-l80-wp-small.png

Best case, vertical cross-section taken at various angles.

Image: cloverleaf-rad-vert-l80-nowp-small.png

Worst case, vertical cross-section taken at various angles.

The 'best case' and 'worst case' correspond to different assumptions about what happens to signals that travel along the outside of the coax. In the 'best case' simulation, these are completely absorbed at the other end of the coax. In the 'worst case' simulation, they are completely reflected back towards the antenna. The latter case usually results in standing waves on the outside of the coax, which causes the coax itself to radiate and interfere with the radiation pattern of the antenna. This is why the radiation pattern isn't smooth anymore. The reality will be somewhere between these two extremes.