Micro Motor Community

Replace monopole antenna?


#1

My TinyFish FC has a single wire for an antenna. I want to make it longer for my build, but I cannot do that without changing the frequency.

Can I use a shielded antenna wire and solder the center to the FC RX and the shield to the lipo GND pad on my FC?

Will this work or do I need some extra circuitry, like the FC actually uses the shield signal for something on FC that have this configuration?

Thanks!


#2

You need a ground connection point within a couple mm of the signal connection point. If you can find that you can solder some coax on, and then make a simple dipole at the end of the coax.


#3

On the top left of the first diagram is where the antenna is soldered. Would using the buzzer- pad as a nearby ground and antenna pad as the signal work?

Here is the board circuit diagram: http://fishpepper.de/2017/03/21/tinyfish-fc-how-to-build-your-own-revision-0-2b/


#4

Generally the buzzer negative is switched so that won’t work. Don’t have my tinyfish handy at work but use a meter and probe the nearby caps, each has a gnd end.


#5

Is there anything else I should be worried about?

Replacing the monopole with a dipole won’t increase the voltage or current signal and fry the tiny RX chip?

I wonder why the creator of the TinyFish FC didn’t use a dipole if it increases the range considerably?


#6

There are no rx I know of that use a dipole only on small vtx. It’s possible to get 500m with a correctly sited single wire antenna mounted on the board.


#7

It’s heavier and they cost time or money to make. I’ve found that I get an extra 10-25% range by making the frsky semi-dipole antennas on XM/xm+ RX units into proper dipoles. I did the same to the D8 frsky that just uses a single wire antenna and got a significant range increase.
But, it took 10-20 minutes of fiddly work. That’s money if you’re in the production business, and typically the bits with single wire antennae are small, light, cheap, and not aimed at big field flying anyway.

Dipoles -standard ones anyway- (rather than sleeve) are more fragile too, the two wires need to be pointed away from each other, if they get bent in you lose significant sensitivity in that direction.

It’s all a trade-off. Single wire non-coax is easy, fast, light, durable, cheap, and usually good enough.


#8

Oh! Well I meant the semi-dipole antenna, like the one of Frsky XM+ receivers. My main issue was getting the actual antenna bit out from under the lipo, since my small form factor requires the antenna to be right next to the lipo.


#9

I’m no antenna expert but I’m fairly certain the xm+ type are not any sort of dipole. Each antenna has a single radiating element at its tip, they are diversity monopoles. A dipole needs a reflector part somewhere, the coax is merely a shield.


#10

Okay. But the shield is soldered to ground, so I may be able to use a ground on one of the nearby caps?

I just need a way to shield my wire so I can make the wire long to get the antenna out from under the lipo, but the antenna bit the correct length.


#11

Any nearby ground should work. Make sure it’s actually ground/B- though and not switched in any way.

Technically there’s no such thing as a monopole, it’s a physics impossibility. There’s always a second “ground” element, it’s just a case of whether it’s tuned or not.
The single wire antennae are dipoles where the second element is the B- plane in the PCB.
The frsky antennae are dipoles where the last ~50-15mm of the insulation sleeve acts as the second element.
The big AM radio towers use the tower itself and/or the Earth as the second element.
Car CB antennas use the roof (trunk lid, etc) as the second element.

The frsky version self-tunes to an extent as the length of the coax itself is tuned, but it’s not as good at resonating as an actual 31mm element would be.
The whoop style B- plane is a mess as a second element, but it’s there.

It’s just like you can’t have a voltage without having a return path for that voltage, it’s all relative.

Some coax done frsky style to get the antenna out from between the PCB and the lipo would definitely help range. You could also desolder the antenna and solder it back onto the flip side of the board.


#12

Ah yes but that gnd element is not radiating merely giving something to push against kinda. I’d alway thought the reflector part of the dipole actually radiated too giving the increase in range perceived.
As I said I’m no expert, always learning. :wink:


#13

It actually does, strange as it seems.

Not my image:

Trick is, it’s not ground, there is no ground, only return. The signal pin goes positive and negative in relation to the return pin.


#14

Excellent explanation!


#15

My antenna ripped off along with the pad is there a way to solder a receiver to the flight controller? I’m somewhat new to the hobby might be a dumb question.


#16

What FC are we talking here, picture of the situation please :smile:


#17


The FC is the Lumenier tinyFISH Power Stack F3 16x16mm Flight Controller + 4A 4-in-1 ESC + FrSky Rx
The black antenna off to the left came off along with the pad I’m somewhat new to the hobby and was wondering if there was a way to solder a different transmitter.


#18


#19

You can solder a new antenna to the end of the tiny green/black striped component next to where the antenna was.
Falling that, you can use rx3 for an external receiver.
I think that’s a F3 FC, so it should be able to deal with inverted and non-inverted receivers.