FlightAware Discussions

Coco (Coaxial Collinear) Antenna - Tips & Tricks

These are important tips for making a successfull CoCo.

1. Coax and Velocity Factor

1.1. You must know exact VF (Velocity Factor) of the coax your going to use to build a Coco.
1.2. You should only that coax for which you know the exact manufacturer and model number, and obtain datasheet from manufacturer or seller for that particular make & model number of coax.

1.3 Use VF only from manufacturer’s/seller’s datasheet/specifications. No guessing, or estimation of VF, or using VF figures given on various tutorials on web sites or youtube.

1.4. Off the shelf coax from Super Markets, Satellite TV Supply Stores, etc normally dont have any datasheet or specs available. Some large stoees do provide specifications on their web site, but these specifications do not have Velocity Factor specified. Do NOT use such coax for making a CoCo.

2. Type of Braid of Coax: Soldered vs Push-Pin Construction

Coax are available with braid made of either Copper or Aluminum or Steel. The copper braid is preferable as it allows soldeing, while aluminum or steel braid cannot be soldered, and only a push-pin type of element assembly can be used. The push-pin method of construction is mechanically weak, as movement of coco during installation or use can result in slightly pulling out the pins, resulting in substantial performance drop. The soldered construction is free from this shortcomming.

3. Calculating length of CoCo elements

The length of element should be calcuulated by following formulae

Length of element in mm = Velocity Factor x 275/2

4. Accuracy

Accuracy is the king, both in using accurate VF to calculate element length, AND in cutting & assembling the elements. An error of a millimeter or two can result in poor performance.

Posted by @theresjam in another thread:

https://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/

Read the bit about characterising notch filters this will tell you how to work out the velocity factor of your coax.

Also if building a co-co dont bother making a push together one solder it.

Posted by @nu3e in another thread

Much coaxial cable is of unknown origin. Even what appears to be legitimate cable may be counterfeit. For an application reliant on velocity factor, such as element lengths in a coaxial collinear antenna, knowing the actual velocity factor (VF) is essential.

Measuring coaxial cable VF is relatively straightforward with a vector network analyzer (VNA). The following example was performed at HF, but the exact same technique can be used at higher frequencies, using a shorter length of cable. Coaxial cable VF does not usually change much with frequency, so it isn’t necessary to perform the measurement at 1090 MHz.

Caveat: in order to obtain an accurate VF measurement it is essential to establish the VNA measurement plane, aka calibration or reference plane. For new VNA users, this excellent tutorial explains VNA calibration.

Although coaxial cable VF measurement can be performed with a lower frequency VNA, such as the original NanoVNA used in the above tutorial, I would encourage ADS-B hobbyists to obtain a VNA capable of at least 1090 MHz so that completed antennas can be measured. With a 2-port VNA, filters can be characterized as well. I believe the NanoVNA V2 is currently the least expensive 2-port VNA suitable for 1090 MHz (it works from 50 kHz to 3 GHz).

Some of the Large number of Flavours of Coco Found on the Web

This drawing does not cover all configurations of Coco available on web/ youtube. Lot of configurations on web / youtube do NOT use sleeve balun at feed point of Coco.

I have made this drawing in May 2014, and have posted it in Planefinder forum

 

2 Likes

Some personal experience with coco

For measurements I used information from this site - https://www.balarad.net. I did not use any lambda/4 protection stubs and etc. since antenna is indoors. I connected coco directly to short piece of coax -> filter -> LNA -> SDR -> Raspberry (initially just coco -> SDR -> Raspberry). I think I went from 6 to 8 and then to 10 elements probably without significant gain in reception just to fill PVC pipe I used to hold coco.

Setup

This is from today when I disconnected coco in PVC tube and connected new coco for testing. Antenna sitting by the 3rd floor window facing SE. House extends in NW direction with two other attached townhouses directly in NE and SW directions. Directly in SE direction midrise building partially seen on photo. Another 3 story townhouse right behind that white wall across backyard.

Build #1

Belden 1189A from this site.

10 element coco. Snapshot taken 2 days ago (09-22-2020).

Build #2

Ordered piece of Canare-L-5CFB coax here.

6 elements coco. Snapshot from today (09-22-2020).

Results

Today overall was so-so day around here and I see stats for nearby sites dropped approximately in line with drop that I experienced (09-20 vs 09-22).

Conclusion

IMHO coco is a very repeatable and simple build. All you need to know velocity factor and have a simple ruler. This will do.

Suggestion

Don’t use coax with solid copper center conductor. It is PIA to remove insulation (so easy to cut copper) and even more PIA to connect elements (copper bends even by just going through electric tape). If you have a choice, use coax with steel conductor (Belden in my case).

3 Likes

All those who have build a successful CoCo are invited to kindly post details of their CoCo with photos, as @vkirienko has done above. Thanks.

Today I executed small antenna performance test against global plane population. I opened my local tar1090 and ADSB Exchange global web views in side by side browser windows (with same scale). As far as I can see my antenna tracks between 50 and 80% of the planes shown on ADSB Exchange global page within 100 miles radius (excluding planes on the ground). As I suspected SW and NE views are pretty limited. But overall IMHO not bad for indoor antenna with limited view.

1 Like

Does anybody know what theoretical gain these CoCo should produce? I finally pulled a trigger and got ADSBx version of 5.5 dBi antenna, run some tests and have question above. :slight_smile:

I don’t see how this helps. Until there is a dual-band receiver at least.

If you have a dual band antenna followed by a splitter to feed two receivers, you are down (at least) 3dB on each port. Your 5.5dB antenna is now a 2.5dB antenna which is less exciting.

The alternative is a diplexer, but as far as I know, no one is making a commercial unit for 978/1090MHz. As the two freq.s are close together, good component selection and construction techniques would be critical to make it worthwhile.

Diplexers are not complicated, but the two frequencies are close together, so the cross-over needs to be sharp and on freq.

https://forum.flightradar24.com/forum/radar-forums/technical-matters-hardware/5986-show-describe-your-fr24-hardware?p=113345#post113345

 

 

I don’t have a splitter so it still should be a 5.5 dBi. Isn’t it?

Yes!
The antenna will perform well for either frequency. It just starts to get complicated when you use one antenna for both.

@abcd567 did you mean to show it should be 9dB for 16 or 6dB for 8 element CoCo?

I dont understand what exactly the drawing says, as it is in German language. @wiedehopf will be able to tell.

I had some worked up from the same company that makes the cavity filters a few years ago:


That was the final specs, had some scope shots somewhere as well. 2 samples were $239 shipped, then they would end up being $49/each shipped (with DC pass), or around $41/each shipped in lots of 100.

Then I was also working up LNA’s:

These guys would end up around $18/each in lots of 100 and I was working towards some other integrated filter variations of this dude until I realized it would be a fulltime job and I was never in this to make money.

The only thing I couldn’t find worth a damn was a decent dual-band antenna. I sourced and tested quite a few too - it sort of blew the whole idea up in the end.

1 Like

Those are really impressive specs. Definitely ‘next level’ over the simple diplexor I was thinking of.

That said, I’m so glad UAT is SEP *

*- SEP - https://en.wikipedia.org/wiki/Somebody_else's_problem

Well It is my problem as well, but I solved it simply by using two antennas, one for 1090 other for 978.

Actually both were trimmed/tuned to 1090 Mhz. On the left one in the photo I added a piece of wire to increase its length by few mm, tuning it to 978 Mhz.

By far the easiest solution.

As above, until there is a dual band receiver (or you are somhow limited to one external antenna), I don’t see the benefit of the dual band antenna.

If you live where mobile phones are operating around 950MHz, this antenna cause you more problems.

@abcd567

hi abcd
found this data for a cable at a store nearby.
this company posts a whole lot of data but do not have the knowledge to evaluate it
I think Velocity Ratio 85% same as Velocity Factor ?
any thoughts on that cable ?

https://www.cavel.it/en/prodotti/1/1/02/SAT752F

ed 1. on top it says Rg Ref : RG 6+ more info on RG 6 for those interested
https://en.wikipedia.org/wiki/RG-6

Most likely YES. They are using “Velocity Ratio” instead of more common term “Velocity Factor”

 

It is copper foil, copper braid, copper conductor - VERY GOOD
Insulation is PEG (PolyEthylene Glycol). Never heard about it.

PE (PolyEthylene) has VF= 0.69
FPE (Foamed PolyEthylen) has VF= 0.80~0.87 depending on how much foaming (mixing of nitrogen or air to PolyEthylene) is done by the manufacturer. More the Air/Nitrogen, higher the VF. If Air/Nitrogen is zero, VF=0.69, If Air/Nitrogen is 100%, :wink: VF=1 (100% Air/Nirogen means an air insulated cable)

1 Like