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How to make your own Crossfire RX antenna
#1
ATTENTION: There are 2 better choices if you are going to make your own antenna for the quad in the Crossfire system.

The Gen 2 (condom style) version is one choice.  In this video 

The Gen 3 (sleeve antenna) version is the best choice.  There is a tutorial HERE....

All three versions, the one in this tutorial and the two Generations above, are better than an Immortal T.



If you're a Crossfire user you know there are two of the issues plaguing us when it comes to the RX antenna.  How to mount the RX antenna and which antenna to use.  TBS states that the antenna should be mounted vertically for the best performance.  This is not because there is better power or such, because a horizontally polarized antenna actually works a little better for range.  That's why long range Ham radio antennas are almost always horizontally polarized.  The reason in FPV world why the antenna should be vertical is because when the antenna is horizontal the tips of the Immortal T antenna can point towards the transmitter causing a null in the signal and a failsafe.  This doesn't happen with a vertical antenna, so that’s why they recommend the antenna be mounted vertical.  The choices for antennas are limited, with the two most common being the antenna included with the RX, and the Immortal T.  Both are dipole antennas, but the Immortal T is formed into a T shape.  Well, there is another choice and that is to make an antenna yourself.  The advantages are you will have an antenna that will be vertical, it will be longer than the one that comes with the RX, so it will extend well above any possibility of the carbon of the quad blocking the signal.  This antenna design is the brain child of Mike Stevens.

Below is a tutorial on how to make this antenna.  I have made 4 now and I will never have any antenna than this one on my quads.  It performs excellent and is easy to mount.  I thought perhaps some here on the forum could benefit from this tutorial.  The pictures were done by a friend, Jeremy Mariscal.  He gave me permission to use his photos in this tutorial.  When he took the pictures, he didn’t provide a narrative, believing the pictures were sufficient.  In most cases I agree, but I will provide a narrative to give a little more information for those who might benefit from that.

Some of the things you will need are:

Wire cutters

Wire strippers

Soldering iron

Solder

Shrink tubing

The coax cable the antenna is made from.  That needs to be RG178 and have a female IPX/UFL connector on one end.  I get mine from this link.  They are good quality and the price is good.
https://www.ebay.com/itm/IPX-Female-to-I...1164101781

You will also need an antenna tube.  There are many choices for this, including the many vendors of antenna tubes.  I went with a suggestion by Kaitylynn here on the forum and used ¼" drip irrigation line.  I got a spool of 100 feet of it and it cost me less than 5.00.  I will never run out. 
And you will need some zipties to attach the tube to the quad.
 
So let’s get started:




[Image: tumblr_pee860NiIZ1x54b8qo1_540.jpg]
The parts of the antenna are the active element and the passive element or the ground plane.  In this picture you can see the black antenna which is the TBS antenna.  The light brown coax is  the wire that will be the new antenna.  On the black antenna, the passive element will be used on the new antenna.  If you don’t have a TBS antenna, you can make a passive element from a piece of the center conductor out of the brown antenna coax.   The passive element on the TBS antenna is the thinner black wire in the picture with the little bulb on the end.  In the next couple pictures we will remove this piece from the TBS antenna.

[Image: tumblr_pee860NiIZ1x54b8qo2_540.jpg]

First you will want to remove the shrink tubing from where the passive element is attached on the TBS antenna.



[Image: tumblr_pee860NiIZ1x54b8qo3_540.jpg]

Next, you will unsolder the passive element from the TBS antenna.  You can set the rest of the TBS antenna aside, it won’t be used.



[Image: tumblr_pee860NiIZ1x54b8qo4_540.jpg]

The new active element will need to be the same length as the passive element, which for the Crossfire at 915Mhz is 78mm.  For those of you operating at 868Mhz the length is 82mm.




[Image: tumblr_pee860NiIZ1x54b8qo5_540.jpg]

At this point, you can cut your new coax to the length you want your antenna to be.  This will be the overall length.  This is personal preference.  I tend to go longer than some, others make it shorter.  You just want to make sure it’s long enough that the carbon fiber of the frame doesn’t block it in any manner during flight.  The longer the coax the more loss.  But in reality given the realistic usable length possibilities, it won't make a difference, i.e. 6" vs 12"  



[Image: tumblr_pee860NiIZ1x54b8qo6_540.jpg]

Next you will strip the outer plastic sheath of the coax to 78mm.  You can strip it a little longer and then cut the active element later, your choice.



[Image: tumblr_pee860NiIZ1x54b8qo7_540.jpg]

Go ahead and confirm the length.


[Image: tumblr_pee860NiIZ1x54b8qo8_540.jpg]

Now remove the shielding of the coax.  For those who have never worked with coax, the shielding is a wire mesh that surrounds the center conductor of the coax.  Be careful when you remove the shielding as there is a thin plastic coating on the center conductor beneath it.  If that plastic coating is cut, exposing the wire inside to other parts of the coax, you may short the antenna and it will be useless.



[Image: tumblr_pee860NiIZ1x54b8qo9_540.jpg]

This is what it should look like when the shielding is removed.


[Image: tumblr_pee860NiIZ1x54b8qo10_540.jpg]

Next you will want to expose the shielding beneath the plastic outer coating.  Remove about 1/8” or 3mm of this plastic.  You will tin with solder this exposed shielding next.




[Image: tumblr_inline_pee85lGW5h1varx5q_540.jpg]

Go ahead and tin the shielding.  Do so carefully and quickly or you may melt the plastic protecting the center conductor.



[Image: tumblr_inline_pee85qy4Qq1varx5q_540.jpg]


Now you will solder the passive element you removed from the TBS antenna earlier.  (If you don’t have this piece, you can make one from the center conductor from part of the new coax you cut to size.  You will want to cut it to 78mm.  Then strip a small part of one end, perhaps 2mm.  Tin the end and attach to the same spot as we are with the TBS passive element)

Also, now is a good time to check to see if you shorted the antenna.  Use a multimeter to do a continuity check from the shielding to the center conductor on the IPX fitting.  If it's open you're good to go.



[Image: tumblr_inline_pee85spxcR1varx5q_540.jpg]

Now put shrink tubing on the connection of the passive and active elements and you’re done.




You can put the antenna in the antenna tube with the passive element pointing down.  There is the question that TBS says the passive element is to be 90* out from the active element.  In a perfect world that is how it should be done.  But in a practical application it’s not needed, and difference in overall performance between having the passive element 90* out from the active element, versus having the passive element 180* out, pointing down from the active element in the antenna tube, is insignificant.  Pilots are going out 6+ miles with this antenna and sustaining 99 on their Link Quality.  So this discrepancy is not an issue.



[-] The following 10 users Like Krohsis's post:
  • hawk01, Kevin2112, Oscar, Drone0fPrey, kaitylynn, SnowLeopardFPV, Banelle, sloscotty, Sugs, sirdude
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#2
Great tutorial John!  Answered a lot of questions for me!  Thanks for taking the time and effort to post it!  Smile
"Damn the torpedoes!!!  Full speed ahead!!!"
[-] The following 1 user Likes sirdude's post:
  • Krohsis
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#3
Yup, nice right up. Thanks for sharing that.
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  • Krohsis
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#4
Thanks guys.  Probably won't be the most used tutorial on the forum, but now it's here if needed.  My first tutorial, and now  I have a better appreciation for what Oscar and those of you who do this regularly go through.  It's rather time consuming.



[-] The following 1 user Likes Krohsis's post:
  • Drone0fPrey
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#5
(02-Sep-2018, 12:44 PM)Krohsis Wrote: Thanks guys.  Probably won't be the most used tutorial on the forum, but now it's here if needed.  My first tutorial, and now  I have a better appreciation for what Oscar and those of you who do this regularly go through.  It's rather time consuming.

Yup.....I've done a couple of reviews for Oscar and they are time consuming, but for me, rewarding. Smile
"Damn the torpedoes!!!  Full speed ahead!!!"
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  • Krohsis
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#6
Great tutorial  Thumbs Up Thanks for sharing.
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  • Krohsis
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#7
Very nice!
I was going to do a post about mounting those antennas Smile
Don't be a LOS'er, be an FPV'er :)  My Gear - Facebook - Instagram - Twitter
[-] The following 1 user Likes Oscar's post:
  • Krohsis
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#8
(09-Sep-2018, 01:20 PM)Oscar Wrote: Very nice!
I was going to do a post about mounting those antennas Smile

Please do!

Great tutorial, thanks for sharing.
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#9
Well done Kroh! Thumbs Up
The Obsession IS Real!
My Youtube and Instagram links
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  • kaitylynn, Krohsis
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#10
Hi, this is a really great tutorial! I have 2 questions:


1. I've a crossfire antenna but measuring both ends gives me 75mm on one side and 76mm on the other side. As I'm using 868Mhz (France), I'm supposed to have 82mm. Does it mean I should buy another antenna, in the hope it will be the correct length (82mm)? If it does not really matter, what should be the length of the coax shielding to remove? Should I stick to 76mm or would it be bad to have one end at 76mm and the other at 82mm?

[Image: 81wqC5p.jpg][Image: 5QgMENi.jpg]

2. I'd like to find an easy way to pack my quad in the backpack, which seems a bit complicated with a 30cm antenna, unless I bend the antenna 90° when packed. I was wondering about this configuration: crossfire receiver <--> UFL to SMA pigtail <--> SMA crossfire antenna (above tutorial). Would this configuration be OK? I'm concerned about the potential loss in terms of reliability?
Thx in advance for your help.
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#11
(12-Nov-2018, 01:13 PM)sebastiendamaye Wrote: Hi, this is a really great tutorial! I have 2 questions:
1. I've bought a crossfire antenna but measuring both ends gives me 75mm on one side and 76mm on the other side. As I'm using 868Mhz (France), I'm supposed to have 82mm. Does it mean I should buy another antenna, in the hope it will be the correct length (82mm)? If it does not really matter, what should be the length of the coax shielding to remove? Should I stick to 76mm or would it be bad to have one end at 76mm and the other at 82mm?

Not having pictures it's difficult to advise 100% correctly.  I'm guessing when you talk about the antenna you have, it's the dipole that comes in the micro rx pack that looks like the pictures of the black dipole above.  The two different antennas (868mhz and 915mhz) can be identified now with the newer versions by the color of shrink tubing at the union of the active element and the ground plane.  If I remember correctly, gray is 915mhz, black is 868mhz.  Given the 915mhz length is to be 78mm, you may have an antenna tuned to 915mhz, and quality control was a bit lacking.  I wouldn't think TBS quality control would be so bad they missed the mark for your antenna 6mm on a 82mm/868mhz antenna.  I have made 7 of these home brew antennas now.  Out of all those, I had a few dipoles that came from TBS with the micro rx 1-1.5mm too short (77mm-76.5mm) this is another reason to build your own as you can take more care in getting everything just right.  Even having the electrical length off by a couple mm will affect your signal when out many miles.  

If you like the stock antenna we are talking about, I would try to buy another one from TBS and make sure  you order the 868mhz version.  Your other two options are to modify the one you have to the correct lengths, or build one yourself like in my tutorial.  


2. I'd like to find an easy way to pack my quad in the backpack, which seems a bit complicated with a 30cm antenna, unless I bend the antenna 90° when packed. I was wondering about this configuration: crossfire receiver <--> UFL to SMA pigtail <--> SMA crossfire antenna (above tutorial). Would this configuration be OK? I'm concerned about the potential loss in terms of reliability?
Thx in advance for your help.

If you build the antenna yourself, you can make the feedline as long or short as you want, it doesn't have to be as long as mentioned in the tutorial.  The longer feedline is just to get the antenna up high enough there would be no way the antenna would be blocked by carbon fiber, and it gets the active element further away from on quad interference.  But you can make the antenna much shorter if you choose.  

As to your question about hooking the ufl to a sma, etc.  Yes, you can do that, but I would recommend against it.  Connection points are high loss points for signal.  So you could create an antenna setup that would give you very poor range.  Also it seems that configuration may create a high failure point in the event of a crash.  Perhaps if you want to stay with a longer antenna, but need to put it in a backpack, try an antenna tube that will flex enough to allow it to be bent over for transport.

Hope this helps a little.  If you have more questions, try to provide pictures if you can.



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  • sebastiendamaye
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#12
Hi Krohsis,

Thanks a lot for your detailed feedback, this was really helpful!

For the 1st point, I've edited my initial post to add pictures so that you see what I'm talking about. I confirm what you said, the shrink tube I've removed was grey. So that was obviously for 915Mhz. I'll buy a new antenna, making sure this time I take one for 868Mhz Smile


For the 2nd question, that's again, very helpful. Indeed, I was not really comfortable with an additional connector. I'll probably make a shorter antenna Smile
I've also received the below suggestion from Mike Stevens via the LR Hooligans FB group, which I really liked:


Quote:"It is how you mount it. There are 3D printed antenna tube holders that go on stand offs. If you have your antenna heat shrinked to the outside of a antenna tube, then simply pull the antenna assembly out of the tube holder and lay it next to the quad. Before flight, stick it back in".


Again, thank you very much!
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  • Krohsis
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#13
Great idea by Mike....glad to help....good luck.



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#14
Hi all,

I'd like to come back to my previous post... I've ordered a new stock antenna and this time again, grey heat shrink and 76mm length (915Mhz) instead of 82mm (868Mhz)... I immediately contacted the shop and they told me they directly contacted TBS to have a confirmation. They have been confirmed that there is now only 1 model with the grey heatshrink, that supports both frequencies, as depicted on the below picture:

[Image: 1DSC00463-ga.jpg]

Now, my question is how long should I cut the other side to use 868Mhz? Same length as the black cable... 76mm? It's really weird, isn't it?

Thx for your help.
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#15
I had a discussion with SnowLepoardFPV a few days ago about inconsistent antennas with the FrSky R9 system.  Seems they have issues producing antennas the correct length.  Here is that discussion.... http://intofpv.com/t-frsky-r9m-r9mm-ante...th-mystery  Weird that TBS is getting flaky with their antennas now.  

Not sure what is going on with TBS and their antennas.  Have you flown with the new antenna that is one size fits both frequencies?  Does it perform well enough for you?   If it works, use it.  The reason to build your own is to get the most performance out of the antenna as possible, and to make the feedline a little longer to get the antenna up and away from the quad. 

As an option, do you have a couple of the TBS antennas that you could play with and cut one one way and one the other and see how they perform?  I wouldn't put an antenna on and off the receiver a lot as that connector is not designed to take that kind of use.  But you could do it a few times and likely not damage it.

Or you could cut the active element to the 82mm length on your existing TBS antenna, and if the passive element is shorter than 82mm, you could make a longer one with some small gauge wire (unshielded) (18-22ga) an cut it to 82mm.  You can leave the plastic sheath on the wire, just don't use a wire that has metal shielding like coax.

And as a final choice, you could make your own antenna from ground up as in the tutorial.  That's what I recommend.  All my home built antennas have outperformed the TBS antenna that comes with the receiver and the Immortal T as well. 

Check back in if you choose to play with the TBS antenna, let us all know what you learned so we can learn too.

Good luck!

EDIT:  I talked to a customer service representative at TBS today.  He confirmed in a PM that TBS was running the dipole in the receiver pack and the Immortal T in the middle of the two bands.  They feel this is ok for most pilots.  I would agree with that, but if you want to get the best range and least effect by interference, being as close to the base frequency with your antenna length is the way to go. That means 78mm for the US and 82mm in the EU.   

Not sure what is going on with yours though, as 76mm is way outside the area they are suppose to be manufacturing them.  .



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