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Pumpkin drop event

Fixing a broken RC antenna (Coaxial)

#1
Recently, the antenna on my new Turnigy 9X8CV2 receiver broke after its very first flight! The flight went well, I didn't crash, and yet, my antenna was ruined. After failing to get a hold of anyone from customer service, I decided to go ahead and try and fix it myself. I had never repaired an antenna before, and figured it would be as straight forward as soldering two wires back together, which I was later to find out was not true. So here is my recount of how I managed to repair the antenna, and perhaps it will help others out there who will have this problem in the future

Here is a picture of what the 9X8CV2 receiver looks like before it was repaired; I didn't take any pictures before I repaired it so here is a picture of it off the internet:

Turnigy 9xbcv2.jpg

Taking the back cover off was as simple as unscrewing two screws and popping it off

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Next, I pulled the board out of the case (I de-soldered what remained of the antenna before this picture)

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Now onto actually repairing and reconnecting the antenna. Here is the antenna

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What I didn't know about antenna up until this fix is that antennas are not just one wire. They are what is referred to as coaxial wires, meaning inside the wire there are actually two different wires insulated from one another, in this case, a signal and a ground. Upon stripping back the initial insulation, I was met by an outer layer of silver mesh wire. This was the ground. Upon peeling that back, there was clear insulation protecting the signal wire. This wire was very, very small and fragile, and so I didn't want to risk stripping it with conventional wire strippers. Instead, I took an x-acto blade and lightly, ever so lightly, rolled the wire back and forth underneath the blade, almost like one would roll play-do underneath a finger. The insulation came off without a hitch, and this is what it looked like after. I also separated the ground and signal lines here.

2014-05-17 16.05.12.jpg

Next comes actually soldering the wires into place. The pad further back is for the signal, and the larger one closer to the front is for the ground. I had to be very careful not to let any of the wires touch, as both connections are very close to each other and both wires are no longer insulated against each other.

2014-05-17 16.05.26.jpg

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And now, to insulate them against each other and to add some support to these already taxed wires!

2014-05-17 16.20.58.jpg

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The final result next to the case, not necessarily the prettiest thing in the whole world, but it works!

2014-05-17 16.26.48.jpg

As I began to assemble everything back together, I realized why the antenna broke in the first place.....

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The angle that that the antenna is going through is absurd!! Once I screw the top plate back on, the antenna is going to be crushed back into two back-to-back ninety degree angles! This is unacceptable. If anybody has this receiver, I feel this next step is a must whether or not your antenna wire has broken, because if it hasn't, it's only a matter of time before it does.

So to remedy this problem I simply cut out a groove in the case with a hacksaw that allowed the antenna to sit flat.

2014-05-17 16.31.04.jpg

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And with the board and newly attached antenna inside the case

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Now to make add some structural support, and make sure the antenna doesn't flop around (More hot glue!)

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Now, with case all back together, you can see the difference of where the antenna was, and where it is now.

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AAAAAANNNNND finally, I hot glued the antenna to the side of the receiver so that it wouldn't be eaten up in my quadcopter's blades.

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Good as new!

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As this was my very first forum post here, let me know what you guys think, and maybe what I can do better next time. Thanks for checking this post out! Hopefully you learned a little something along the way too! :D
 

nerdnic

nerdnic.com
Mentor
#2
I attempted this on my first RX. Everything went great but my range was cut down to like 50 feet. The signal was fine within that distance but I couldn't figure out how re soldering the connection limited me by so much.
 

stay-fun

Helicopter addict
#3
Nerdnic: That range is an indication that it seems like the antenna is not connected at all...

ThatRoboticsGuy: did you do a range check after your fix? (quite funny, I wanted to ask that before I saw Nerdnic's post... :p )
Nice post!
 

xuzme720

Dedicated foam bender
Mentor
#5
I think it's the section under the heatshrink that is the critical length. The Coax is not radiating, only acting as a waveguide and shouldn't affect the wavelength. I might be wrong on this though...
 

nerdnic

nerdnic.com
Mentor
#6
Nerdnic: That range is an indication that it seems like the antenna is not connected at all...

ThatRoboticsGuy: did you do a range check after your fix? (quite funny, I wanted to ask that before I saw Nerdnic's post... :p )
Nice post!
The thing is, though, that when the antenna was broken off I got 0 feet range. My hack job soldering improved it a bit but it's far from usable :-(
 
#7
I haven't done any official range test where I like walk away with the transmitter, but I've done my usual flying and its so far is no different than what it was before. I think its mainly because I lost very little antenna length when it broke, I only had to de-solder and get rid of a tiny portion of wire. The length of the wire is very specific for the frequency, so cutting away as little of the wire as possible is very important.
 

jhitesma

Some guy in the desert
Mentor
#8
The length of shielded feedline isn't as important as the length of the driven element and reflector - but it can still play a roll in performance of the overall radio system. I've noticed that a shortening of the feedline almost always results in a loss of at least some range on these so they're probably fairly sensitive to feedline impedance which shortening the feedline would affect. The RX uses the same chip as the modules being used for a bunch of DIY projects and the output stage is nearly identical to what's used on the ebay modules I'm experimenting with for a DIY RX. I just wish I had more 2.4g test gear to really see what's going on. At those frequencies things start to turn into a big of black magic to me :D

I've had better luck replacing the antennas with antennas off of other broken RX's than getting the stock 9x antennas to work with a shortened feedline.
 

earthsciteach

Moderator
Moderator
#10
This very thing is what finally pushed me to FrSky. I got the craps of soldering the rx antennas. I tried the hot glue thing. It didn't help for long.
 

jhitesma

Some guy in the desert
Mentor
#11
Yeah, the stock 9x antenna will just keep breaking even after it's shortened. It's just too heavy of an antenna for the wire it's on. Glueing it to the side of the RX is good physically...but it's horrible for RF and your signal range. There's just not a great solution other than swapping to a different antenna which has it's own downsides in that the 9x antenna is a pretty decent antenna. I just needs a stronger feedline.
 

joshuabardwell

Senior Member
Mentor
#12
For what it's worth, I put a piece of heat-shrink on my stock receiver's antenna after making the same repair. I think the issue is not so much the angle at which it passes through the case, but just that the cable itself is way too fragile. You can strip the outer insulation with your fingernails alone.
 

Vorlin

Junior Member
#13
Ex-Navy Electronics Tech here.

The short, short, short version is that all you really need to worry about is

A) Use the right length antenna

B) Just because your solder job caused things to be physically attached, this doesn't mean that it's a good electrical connection. Um, yea I am saying exactly that. Yes, you can solder things together so that your wire touches solder that touches your board and you still can't get a good connection though it. It's called a cold solder joint.

A) You don't need to worry about antenna length if you re-use your original antenna. When you impedance match an antenna it needs to be as long as "an increment" of the wavelength you're trying to tune into (1/4, 1/2, the actual length) and, of course, those wavelengths are different for each frequency.

K.I.S.S. and reuse the same antenna or buy one that's the right size. It will allow you to avoid the kind of math that gives engineers headaches.

B) The really, really short version here is that when you solder you just need to approach the idea differently. Soldering is NOT about melting solder onto your wires and connections. If you drop hot liquid solder onto cold wires and connections then the solder never really absorbs into the metal. It's like water droplets sitting on top of a completely dried out sponge and this idea of the liquid (water or your solder) not absorbing into the solid thing (sponge or your wires and connections) tells you a lot about why electricity can't flow through a "cold solder joint".

The fix is simple... realign your thinking. The right way to do a solder joint is to touch your wire and connector to each other and then touch the soldering iron to both of them in order to heat them together, equally. **Be careful to never overheat the silver pads on circuit boards because they'll de-laminate and lift off the boards if you do.**

Let the things you're soldering together get warm for a couple seconds and only then do you touch the solder to the iron, melting it and then allowing it to run down onto / suck into your connector and wire. USE THE MINIMUM SOLDER POSSIBLE TO JUST COAT YOUR CONNECTIONS.

Why? Solder only melts if it absorbs heat. The more solder you use, the more heat you just sucked out of your connectors... the ones you just carefully heated up... and the colder they will be when the solder joint sets.

And if it's cold then you get a cold joint... which might be connected physically but electricity won't get through it any better than a single droplet of water on a hardened sponge. This is how you end up with 50ft range on a heli after soldering on an antenna.

If you have to redo a joint just remember three things.

1) Get all the old solder off that you reasonably can but don't go nutty about it. Warm the old stuff and wipe with a dry paper towel before it cools... do that 3x and it should be fine.

2) NEVER HEAT ANYTHING MORE THAN IS NEEDED TO JUST BARELY MELT YOUR SOLDER. (Should be no more then 5-6 seconds.) Overheat board connections at your own risk.

3) If you need to put wire through a hole that is covered by solder do not, not, not shove anything physically through the cold hole in an attempt to clear it out. You're far better off using a paper towel to get any excess off and then ignoring the solder. When you go to make the new connection the hardened solder in that hole will liquify almost instantly, making it easy to pass your wire right through the hole without destroying the board by trying to shove something through a blocked hole.

Good luck!