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

 

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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).

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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.

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