The 1.3 GHz Ground Relay Station
I installed a 1.258/1.280 GHz video system on my Fohdy as I can legally transmit on these frequencies with my amateur radio license, and they will give longer range than 5.8 GHz video. I decided to use a directional antenna at the receiver to maximize the signal strength and, since directional antennas in the 1.3 GHz band are too big to fit on goggles, built a simple ground station.
I chose the TrueRC X-Air 1.3 GHz turnstile/reflector antenna because it’s a good compromise between gain and bandwidth, offering at least 7 dBi gain over a 60° beam-width (see plot below). This means I can just point the antenna in the general direction in which I will be flying and as long as I remain within 30° either side of this, video should be good, without the need for an automated antenna tracking system. The antenna also provides gain similar to an omni (0 dBi) over a 120° beam-width, covering a wider patch of airspace for local flying. The X-Air 1.3 has a standard 1/4-20 UNC mounting socket so it can be mounted on a tripod as shown above.
TrueRC X-Air 1.3 Gain
In order to avoid having to tether myself to the ground station I used a low power (25 mW) transmitter to relay the video signal on 5.8 GHz. This will also allow me to operate some distance away from the ground station to prevent the third harmonic of my 430 MHz radio control signal from overloading the 1.3 GHz receiver.
Parts List:-
I built the ground station breadboard-style on the back of the X-Air 1.3 reflector. You can drill small holes in the reflector without significantly affecting the antenna’s performance, provided the the holes do not enter the raised dome in the centre of the antenna!
Rear view of the 1.3 GHz Ground Relay Station
The video receiver is mounted using M3 bolts – note that the top left hand mounting place has not been used, as the bolt would intrude into the centre of the antenna where the elements are. My X-Air 1.3 came with a semi-rigid coax that with right-angle adapters on each side was just the right length to connect to the video receiver, but I see that the current version ships with a longer flexible coax. This would also work, but the shorter coax makes a neater installation.
The 25mW transmitter is positioned at the top left to maximize separation from the receiver and the video and audio cables were glued to the reflector using cable glue after shortening the connector back-shells using a craft knife so they wouldn’t extend over the side of the reflector. I filed a notch in the outside of the antenna’s plastic cover to allow the transmitter to sit flush on the reflector with the antenna extending over the edge, and attached the VTX with double-sided adhesive tape underneath and a cable tie.
The battery (which must be 3S as the video receiver has a limited operating voltage range) is on the left to counterbalance the weight of the video receiver. It is attached using self-adhesive Velcro tape on the bottom and small battery/accessory strap. The XT60 connector is attached with double-sided adhesive tape and a cable tie. Because I connected the battery using silicone wire, which would not glue well, I routed the wire using cable ties.
Field testing with a 200mW VTX has shown no video degradation at 5km (3 mi) range and usable video with some degradation (monochrome and minor tearing) at 10km (6 mi).