u/Individual_Health_84

(needed to repost because video wasn't working )

Hi, I recently built this drone and created the 1.2GHz VRX ground station.

Setup:

The VRX is the cheap "Digital Display" with a custom band-pass filter, 20dB LNA and a homemade skew planar ~1.5dBi omni antenna. It's on the head high (you can see me at the beginning of the video)

2dBi dipole for VTX @ 25mW (It can go up to 1W)

More about the VRX here.

The furthest I've been was 250m completely without LoS. (the trees were very dense)

What do you think about the range?

Yeah its really my first real drone that isn't a fixed wing or isn't borrowed (forever) from my friends.

I decided to use 1.2GHz video system. On this band I can really fly in pitmode (0.1mW) even behind a tree, but it can go up to 1W

In a few days I'll try to compare it to O4 and 5.8 analog, since this frequency looks very promising for increasing the range.

The biggest problem I see is that it's really hard to get 900MHz link working but I'm working on that.

First, the background:

FPV drones often use analog video transmission on 5.8GHz. It's the most popular band so there are lots of good receivers out there. The best of them add the demodulated signal from 2 antennas based on weight assigned by RSSI/SNR to increase the performance (that increases the signal quality and range by A LOT) and also recreate the synchronization pulses.

There are also other bands like 1.2GHz, which of course offer much better range but are a lot less popular and therefore receivers (they use new old stock ICs used in analog TV back in the day) are very, very bad.

So why not try to create a dedicated up-converter which I can just attach to my 5.8GHz RX? Then I'll be able to use the additional signal processing from the 5.8 RX and also there will be no need to carry a tripod with a 1.2 -> 5.8 signal relay.

How much dB will I realistically lose? Is it worth it? I'm not sure if the FM deviation is the same across both standards - if not, is there anything i can do about it?

Or should I just try to create my own circuit which will sum the baseband signals from two off-the-shelf receivers?

So I've asked you some time ago to check the design of my filter in KiCad. Some of you told me it would be very bad because of the FR4. After cutting about 2.5mm it works pretty well, I think. -2.5dB loss at the center of the band.

I have put it after a 20dB 9037 LNA to receive a 1.2GHz FPV signal from my drone. Also I'm thinking about adding a band-rejection filter just after the antenna to be sure my control link isn't creating intermodulation distortion. Is it the most optimal way to do it or should I put this BPF before the LNA?

I'm getting into microstrip filters and designed this one using Marki calculator. I don't believe in this working. I am gonna use FR4 from JLCPCB. How many design flaws did I make?

It interferes with my 1.2GHz FPV Video link @ 1.28GHz. I receive it every ~10 seconds for less than a second, but if I move my antenna outside I can see that it lasts a few seconds (the tail end is weaker). It occupies about 10MHz from 1.27 to 1.28.

For $20 it gives so much fun 🤣

As an RF engineer I'm surprised that no one is doing that.

I know some people may argue that:

- Antennas are big - but how can you say that while using 900MHz ELRS dipoles, which are bigger?

- They interfere with control links - if you put a notch filter on the VRX and use a good enough VTX and don't blast 1W at 900MHz or 2.4GHz, nothing will happen.

- That's illegal - While it is illegal without a ham radio license, 5.8GHz @ more than ~15mW is also illegal without it.

- It may cause GPS interference - That's true, but with some good RF knowledge you can still make it work properly.

On top of that, 1.2GHz has some serious advantages:

- Superior penetration - lower frequencies go through walls like nothing.

- Superior LoS range - with all omni antennas you get similar range as with 10dBi on 5.8GHz.

- Less congested spectrum.