# Coaxial Collinear (CoCo) Antenna - Hyps & Facts.

There are lot of common beliefs about making a Coaxial Collinear antenna. Some of these are real, while others are either imagination or over blown facts. I decided to start this thread to find out the truth. I invite all members who are good in theoretical knowledge, or have good measurement skills & equipment, or have made a DIY CoCo and have good/bad experience of making their CoCo to contribute their views.

As a first attempt, I ran a simulation of 8-Element CoCo made of coax having VF=0.83.
The element length = 0.83 x 275/2 = 114 mm.

To see the effect of error in cutting, I ran a sweep varying element length from 100 mm to 125 mm.
The resulting plots of Gain, SWR, and pattern are given below.

Two things are very noticeable:
**(1) Contrary to common belief, an error of up to ± 2% in cutting the elements do not have any significant effect on either Gain, SWR, or Radiation Pattern.
(2) The best SWR occurs at ≈ 95% of designed length i.e. at 108 mm (SWR ≈ 1.3) instead of designed 114 mm (SWR ≈ 3.3).

Please note that simulation results sometimes may be erroneous, therefore cannot be considered authentic unless verified by experiment, testing, & measurements.**

The software used is 4NEC2

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Could you provide us with a link to the used .nec model file for these simulations?
/paul

OK, I have now uploaded my simulation file “CoCo 8 element FPE insulated.nec”.
http://1drv.ms/1i5fGj8

Hi ab cd,

Interesting plots. Some while ago you made a series of CoCos of different element lengths. Does the theory support your practical observations? Your plots suggest the element length of ~95% of theory.

Over on Plane Finder forum you posted

(3) The correct length of ¼ wave whip : The 1090 Mhz wave in air is 275 mm. Hence ¼ wavelength is 68.75 mm which is rounded to 69 mm. When RF current travels in a wire, the length of wave is slightly less (1% to 5%), and depends on diameter of wire use. The figure of 65.4 mm originates from ARRL book used by hams, and mainly pertains to lower freqencies of 440 mhz & below (uhf, vhf, hf).

At the ADS-B frequency, does the copper wire affect the propagation of radio waves? Does 4NEC2 take this into consideration?

 Does the 95% hold true for lower frequencies?

I made series of CoCos from a coax of unknown VF. The only thing I knew that it is RG6 with FPE insulation, and the VF lies somewhere in the range 0.8 to 0.85. I made CoCos using different VF in this range i.e. from 110 mm to 117 mm, and one of these (114 mm) performed better than others (but still worst than Cantenna & Spider). From 114 mm figure, I inferred that my unbranded coax’s VF is 0.83.

By the way, I have substantially improved the 114 mm coco by inserting a 1.5 pf Capacitor into the feed line at 95 mm from feed point. This shows that SWR of 114 mm Coco was very high.

Today’s simulation shows that 95% of calculated length gives better SWR. I am going to make one more CoCo of element length 95% of 114 mm (=108 mm) to see how it performs compared to the 114 mm one.

Over on Plane Finder forum you posted

(3) The correct length of ¼ wave whip : The 1090 Mhz wave in air is 275 mm. Hence ¼ wavelength is 68.75 mm which is rounded to 69 mm. When RF current travels in a wire, the length of wave is slightly less (1% to 5%), and depends on diameter of wire use. The figure of 65.4 mm originates from ARRL book used by hams, and mainly pertains to lower freqencies of 440 mhz & below (uhf, vhf, hf).

At the ADS-B frequency, does the copper wire affect the propagation of radio waves? Does 4NEC2 take this into consideration?

(1) Yes, at all frequencies, propagation in wire is slightly slower than the air, hence wavelength is shorter, but the amount of reduction depends on frequency & dia of wire.

(2) Yes NEC does take into consideration this effect, but too much quantitatively. I remember when I simulated for a dipole, it gave minimum SWR at 63mm instead of 69mm limb length. When I practically tested, I found that 67 mm for un-insulated wire, and 66 mm for insulated wire gave me the best result, but the difference in performance between 69mm & shorter length, though noticeable, was small.
EDIT
Got some new information about 5% reduction in length of a dipole as below:
(1) Practical wire elements are not the same length as the free space value.
This is due to capacitive end loading (‘End Effect’) which makes an antenna element appear to be slightly longer than it actually is. So a half-wave element needs to be slightly shorter (usually about 5%) than a half wavelength in free space.

(2) The end effect changes the complex impedance of the antenna/feedline system so that the resonant frequency occurs at a shorter physical length than the one half wavelength predicted by theory without end effect. The end effect is effectively modeled by an additional lumped capacitance.

 Does the 95% hold true for lower frequencies?

The 95% figure for CoCo is something new I found only today, and I dont know if it is frequency dependent. I am also not sure if it is something real or a simulation error, unless a prototype is made & put to trial.

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THE ACID TEST
I already have a CoCo made of RG6 (VF=0.83), with element length 0.83 x 275/2 mm = 114 mm.
Yesterday I made two additional CoCos from same coil of coax, one with element length 114+3 mm = 117 mm, and other with element length 114-3 mm = 111 mm.
I then put to trial run 111 mm coco side by side with 114 mm coco: result 111 mm coco poorer than 114 mm coco.
I then put to trial run 117 mm coco side by side with 114 mm coco: result 117 mm coco poorer than 114 mm coco.
I then put to trial run 114 mm coco with a 1.5pF Capacitor inserted at 95 mm from feed point: result 114 mm coco’s performance increased greatly.

**Observations: **
(A) Optimum Element Length
The simulation shows optimum element length = 108 mm (= 0.95 x VF x ½ λ),
The test above shows optimum element length = 114 mm (= VF x ½ λ)

(B) SWR
The simulation shows SWR <1.5 at optimum element length
The improvement in 114 mm coco by inserting a capacitor shows that the SWR at optimum length is higher than 2.5

Conclusions:
I think there are two possible reasons for this descrepancy between simulation & trial run:

(1) The software has limitation of accuracy for complicated antennas like coco.
OR
(2) There is some error/shortcoming in the model I have made.

What version of CoCo you checked? From coaxial cable completely - without the copper tubes? The Central wire in each element was in isolation or clean? The antenna was shorted at the top? At the top of the antenna you made quarter-wave pin or not?
Improving reception from the injection of a capacitor in a feeder line says wrong about agreeing to the output resistance of the antenna with the feeder. On the other hand the introduction of the capacitor leads to rupture by current and increase the chances of breakdown of the receiver input with static electricity from air.

Hi ab cd.

Try upside down plecement of 108 mm CoCo

Best regsrds,
Serge

I made the CoCo from coaxial cable completely - without the copper tubes.
Don’t understand the question “The Central wire in each element was in isolation or clean?”. Please explain.
The antenna was NOT shorted at the top.
There was NO quarter-wave pin at the top of the antenna.

On the other hand the introduction of the capacitor leads to rupture by current and increase the chances of breakdown of the receiver input with static electricity from air.

The static buildup is problem even without capacitor.

Improving reception from the injection of a capacitor in a feeder line says wrong about agreeing to the output resistance of the antenna with the feeder.

Inserting a capacitor in feed line at a specific point slightly less than 1/2 wavelength from feed point can bring the impedance of antenna to a value (R₀ + j 0) = characteristic impedance of feed line. The exact value of capacitor & distance of insertion point from feed point depends on antenna impedance & feed line characteristic impedance. This can be explained better on Smith Chart.

I take your point. Without shorting from the top you get the antenna with high impedance output. So you have to put a piece of cable and a capacitor to better align with the feeder. We use a different approach and a different CoCo. May be use only coaxial cable cut 112mm or use a hard copper tube and copper wire without insulation, missed exactly in the center of any element - we use the closed top of the antenna. It will work differently. A different distribution of currents and voltages. And about the coordination with the feeder line is a joke that when the length of the feeder is greater than 10 wavelengths with our “quality” UHF cable at these frequencies already SWR will strive to 1. And no joke - FR24 complements your receivers antenna that is shorted. In recent dry thunderstorm at night last week I burned down the port on the router, but the FR24 receiver is left alive.
P.S. Look at the photo - antenna time-consuming. Only the case of fiberglass tube would be better.
P.P.S. Even to reduce losses in CoCo from coaxial cable can be waived from the connector on the antenna and the last antenna element to solder directly to the feeder cable.

Today I made following changes to my 8-element coco:

(1) Removed capacitor. The performance dropped from range = 200 nm to range < 50 nm.

(2) Added at the top, a ¼ λ piece of coax (½x114mm=57mm), core & braid shorted at top. I also soldered a ¼ λ whip (68mm, VF=1) to the shorted top. There was an improvement in coco’s performance, but not much (range < 100 nm).

(3) I then inserted the capacitor in the modified coco’s feedline, and it’s performance increased to range =250 nm, i.e. a little more than the original unmodified arrangement with capacitor.

Conclusion
Adding a shorted ¼ λ piece of coax, and a ¼ λ whip to the top of Coco, does ease-out the swr and improve the performance of coco, but not much.

On the other hand, inserting a capacitor at an apptopriate distance from feed point eases-out the swr to a much bigger degree, and hence gives bigger improvement.

I have done no impedance matching on my self built 8 segment COCOs and have good results on every build I have done.

Are you saying I’m doing it wrong without impedance matching? I can give some of your ideas a try and post comparison results between my two antenna feeds.

Cheers!
LitterBug

No, you are not doing anything wrong by not using impedance matching.
The Cocos you have built already have impedance low enough to be close to impedance of the system (75 ohms). That is why these perform good.
If you try to add a capacitor, it will most likely give only little or no improvement.

For cases like my built of coco, where performance is poor (which is mainly due to high impedance of coco), the impedance matching is effective & beneficial.

I can give some of your ideas a try and post comparison results between my two antenna feeds.

Go ahead. It will be interesting to find out how adding a capacitor affects an already good coco.

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2 abcd567
Thank you for believing in shorted CoCo. She has the right to live. It’s a pity that failed to reach a radius of 300 miles. My kit from FR24 takes to 270 miles. The four elements of a Franklin antenna at the second radar takes up to 240 miles. The difference is also small. Only in the rain Franklin stops working because of water drops in a quarter-wave parts. When communicating with other spotters was suggested that to make multi-element CoCo with taking 300 miles is very difficult even with devices for measuring resonance and SWR, too much will affect. The main rule is to set the antenna to the roof on an already-configured fat UHF cable. To adjust the input impedance of the antenna under the wave resistance of a cable with an error less than 20%. And if we evaluate the overhead of the design, assembly and configuration of CoCo, we understand that it is easier to buy ready-made factory performance. And homemade CoCo greater than 4 elements is very easy to make working WORSE than Franklin antenna.

P. S. I wonder what will show the simulation of the shorted antenna in 4NEC2 program in terms of input impedance and gain?
P. P. S. This weekend I will still try to make 2 versions of my Lazy CoCo and compare the area of the receiving antenna with an open top and shorted with quarter-wave vibrator pin. The upper part will not be soldered, but will link pins through the thin insulator.
P.P.S. Anti-static resistor to top of open CoCo savepic.org/6612957.htm

CoCo is very tricky.
For some it brings a lot of planes, while for some others it brings a lot of frustration

The four elements of a Franklin antenna at the second radar takes up to 240 miles. The difference is also small. Only in the rain Franklin stops working because of water drops in a quarter-wave parts.

Franklin was my default ADS-B antenna for a long time (click here), till I thought of, and made, a wide-bodied descendant of the old timer “coaxial antenna”. I subsequently named the newborn as “Cantenna” (click here). Since then, Cantenna is my default & benchmark antenna.

P. S. I wonder what will show the simulation of the shorted antenna in 4NEC2 program in terms of input impedance and gain?

Please see my following posts in Flightradar24 forum “best antenna”, posted 2 years ago in Sep 2013
1. post # 821
2. post #823
3. post #840

P. P. S. This weekend I will still try to make 2 versions of my Lazy CoCo and compare the area of the receiving antenna with an open top and shorted with quarter-wave vibrator pin. The upper part will not be soldered, but will link pins through the thin insulator.
P.P.S. Anti-static resistor to top of open CoCo savepic.org/6612957.htm

Happy experimenting!

.

I eat my own hat. Admire. Saw some original solutions and realized once again that it is very easy to do wrong Franklin and it will work great than efficiently configure CoCo. But the desire to experiment is still not gone! Not all options are checked

About the different antennas. I don’t believe in the great influence of the input impedance and SWR of the antenna is operating only on reception on a frequency of 1090 MHz and with a cable length of more than 10 meters. For me, the main characteristics will be the reception range and the ratio between “Positions reported” and “Aircraft seen” in the daily statistics in the user profile FlightAware. It will be the number of received packets per one aircraft. When the same call parameters dump1090 with antenna Franklin I have a ratio in the range of 70-80, and when using CoCo 8 elements is not less than 90-95. A great boost! According to the statistics of reception on FR24, the reception area is equal from both radars factory from FR24 with the standard KIT factory made collinear antenna and a homemade antenna CoCo with RTL on RaspberryPi. The antenna Lazy CoCo assembled without soldering. Elements connected “by inserts” the exposed central conductors. Originally cut 7 pieces of 15cm and 1 piece of 13cm. Of 15cm was cut with a braid on two sides by the edges 19mm to the central wire to leave work item length 112mm. By one pin element is glued on a piece of tape by perforated himself. Element from the workpiece 13cm circumcised only one side is top item. At the ends before gluing tape definitely need a knife to strip insulation up to a light color so they don’t catch thin wires and bits of foil with braid shield. The connection elements are the introduction of 19mm protruding pins of the two elements into each other under the braid shield. When connection elements are important to keep a stepped design so that elements are inserted at the top and bottom alternately and all the connections were in two lines at the top and bottom. The lower element may also be connected by inserting the lead-in cable. At the output of the reduce cable did 2 turns of cable to get the throttle shutoff and symmetric inductance. The top element is not short-circuited (2 abcd567 i will testing it for one week). It is clear that this antenna will not withstand the rain, so put it in a fiberglass tube with closed top on a separate mast. Coaxial cable for this Lazy CoCo get a cheap RG-6U with soft outer isolation that the prongs of the elements with little effort entered under it and formed a good enough electrical connection. I doubt that this antenna will last more than a year, but as the project quick and lazy making collinear with good characteristics it can be done in less than half an hour.

Hi All,

There is still one problem to be addressed, where does the antenna ‘start’ and the feed line ‘end’ ?

There needs to be a method of decoupling the feedline, otherwise every time you move the coax the antenna pattern will change, making true A/B comparisons very difficult.

It also prevents the feedline from being an active part of the antenna structure and unintentionally modifying the intended radiation pattern. It can also help to reduce the pick-up of unwanted noise by the feedline.

A 1/4 wave long decoupling sleeve (or cone) should be connected to the coax outer 1/4 wave down (approx 0.95 of the free space value to allow for end effect) from the first coax section crossover. The decoupling sleeve should be at least 4 times the diameter of the coax cable, preferably more. A coke can (or similar) cut to the correct length would work in this application.

Regards,

Martin - G8JNJ

Do you Earth the shield on your coax feedline?

Cheers!
LitterBug

COCO → Amp → feedline → Earthing block → feedline → power inserter → Dongle

Hi LitterBug,

What would you earth the shield to ?

If it is for electrical safety then any connection to equipment that is in turn connected to the mains by means of an earthed three pin connector will provide this function.

If you mean a ‘RF’ earth as part of the antenna system then this concept is meaningless at microwave frequencies where a wavelength is measured in cm.

A decoupling sleeve is basically a 1/4 (or odd multiples of 1/4) wave long resonant tube. The open end is at a high impedance, so the end that is connected to the coax will be at a low impedance (forming a virtual RF earth). Note that it doesn’t need to be connected to an actual ‘earth’.

In the case of a CoCo the antenna operates as a series of 1/2 wave balanced radiating elements which are connected in parallel with each other. The decoupling sleeve provides a balanced (antenna) to unbalanced (coax) transition. By mounting the 1/4 wave decoupling sleeve 1/4 wave down from the first coax crossover, the decoupling sleeve and coax section make up the first 1/2 wave collinear element. Signals flowing on the outer of the coax on the underside of the sleeve ‘see’ a 1/4 wave section with an open end (the edge of the decoupling sleeve). This forms the virtual RF earth that I mentioned previously.

Note that as the decoupling sleeve and coax section make up the first 1/2 wave collinear element, there should always be an even number of elements in total, including the top section.

The top section can either be a 1/2 wave open circuit coax section with a length of about 0.95% (end effect) of the free space value, or a shorted 1/4 wave (with VF for the coax applied) topped with a 1/4 wave wire section with a length of about 0.95% (end effect) of the free space value.

The shorted 1/4 wave version can help reduce the level of unwanted interference from Broadcast and Cellphone transmissions on lower frequencies.

Regards,

Martin - G8JNJ