# building a relay for 5.8Ghz?

I'm interested in building a relay for analog video sent from an R/C aircaft. The idea is to get around obstacles like hills and increase distance a bit.

Rather than just applying more power, I wondered if I could build something that would act as a relay station for everything from about 5700-5900Mhz.

I'm thinking about an amp in the range of 0.5w to 1Wa with a directional antenna hookup up to each side. Several of us would be flying behind a hill or obstacle, while the relay would be in a position that is LOS to both us and the aircraft.

I'd have to keep the input antenna from "seeing" the output antenna as much as possible, how much of an issue is this? Would two patch antennas facing away from each other work?

What issues would I need to be aware of with the amp?

Also, if is this easier or harder at different frequencies? Maybe 2.4Ghz, or 1.3Ghz or 900Mhz?

• I'm afraid we are not a design service here. To attract useful answers it is advisable to post your goal, then your attempts to accomplish it, then if you get stuck, phrase your question in a way that will fit the title. Mar 25 '18 at 5:35
• Another approach is to use a repeater. It listens on one frequency and transmits on a slightly different one. That makes it easier to isolate the two signals. They're quite common in the Ham radio VHF and UHF bands. Mar 26 '18 at 12:34

Let's work an example. Say the aircraft transmits at 1 watt, or 30 dBm, on 2.4 GHz. The aircraft is 500 meters away, giving a free space path loss of 92 dB. Between transmit and receive antennas perhaps there is 10 dBi gain, so the power received at the relay is:

$$30\:\mathrm{dBm} - 92\:\mathrm{dB} + 10 \:\mathrm{dB} = -52\:\mathrm{dBm}$$

Meanwhile, the relay is re-transmitting this signal at a power of 30 dBm. the isolation between the antennas really depends on the antenna design, but let's say you have a patch antenna with a null in its pattern that's 30 dB less than its peak gain. You have two of these which you position carefully so each is in the other's null, so you have a total of 60 dB isolation between them. That means the relay transmit power making it into the relay receiver is:

$$30\:\mathrm{dBm} - 60\:\mathrm{dB} = -30\:\mathrm{dBm}$$

Meaning, the signal from the aircraft the relay is trying to receive is 22 dB below the "noise" created by the relay's own transmitter. This will not work.

Somehow you need to introduce additional isolation such that the aircraft can be heard above the relay's own transmitter.

This could be done by filtering in frequency. The farther apart the receive and transmit frequencies, the less steep the filter transition must be. So a cross-band relay will be cheaper and simpler.

Alternating between receive and transmit in time is also possible, though perhaps not for a continuous analog video stream. With packetized digital video however, very reasonable.

• Thanks for working through an example and showing me how to tun the numbers myself.I was thinking about changing polarization between the input antenna and the output, that'll get me another 10-20dB between the two, which still isn't enough. Cross-band may be the right choice. Mar 25 '18 at 1:51
• Shouldn't the "10 dB" of combined antenna gain be explicitly stated as 10 dBi of gain? Mar 28 '18 at 10:31