Simple collinear antenna in Sep "Silicon Chip"

[VK4GHZ]

...but we do have to be careful not to make the subject appear so technical that others, especially newbies, are put off building and trying simple projects.

+1
Whether we like it or not, society is dumbing down, so overly-technical content can have counterproductive side effects of putting people off right from the start.
It can be a good learning exercise to make mistakes along with the way, without having to strive for perfection from the beginning.

[VK2XSO]

I've never had much success with coaxial colinears. The problem is I find them very difficult to tune or construct on frequency.
The article inspired me to drag out the VNA and start playing with them again.

[VK2AVR]

Don't write impedance matching off in receive. In the scenario I described, Mismatch Loss is 6dB which wipes out the potential gain of a properly phased and isolated antenna of that type.

Hi Owen,

Is the mismatch loss primarily in the transmission line or input to the receiver? Or both?

From researching satellite antennas it seems to be the case that an antenna with "some" gain is good, to assist with receiving birds near the horizon when they are further away and the radio waves have more atmosphere to travel through. Birds overhead can be received easily because the path is shorter and less "stuff" (technical term) in the way so you can get a good signal with less gain. This might explain why Wayne's antenna seems to work, because the overhead reception is easy it's long distance that's the challenge.

...but we do have to be careful not to make the subject appear so technical that others, especially newbies, are put off building and trying simple projects.

+1
Whether we like it or not, society is dumbing down, so overly-technical content can have counterproductive side effects of putting people off right from the start.
It can be a good learning exercise to make mistakes along with the way, without having to strive for perfection from the beginning.

I think the Silicon Chip article in essence is a quick "afternoon hack" design to get people into the idea of ADSB etc. So although it's a compromised design for the reasons Owen pointed out, let's not crucify it for not being technically perfect. If it works, it works. Maybe someone here can design a better collinear that is easy for a newbie to put together in an afternoon?

Although I know what Wayne and Adam are getting at, I would also like to make the point that we should not be dumbing down the hobby overall. Yes, we absolutely should make it accessible and easy for people to enter. But as they start to understand more, the hobby should be able to provide detailed technical knowledge as well. If we keep dumbing everything down and glossing over the details we'll eventually reach the point where our overall knowledge decreases, and that would be a bad thing!

[VK2JDH]

Whether we like it or not, society is dumbing down, so overly-technical content can have counterproductive side effects of putting people off right from the start.
It can be a good learning exercise to make mistakes along with the way, without having to strive for perfection from the beginning.

From little acorns do oak trees grow.

If the project was to buy $100 of ally, tools etc no one would build it for that project. But 2-3 M of coax and a pair of sidecutters many more will give it a try. If it seems to work better than their existing one they might experiment and build a bigger and better one.

[VK6WAX]

I've been following the Silicon Chip (SC) series of articles on DVB-T dongles and it was the fascinating Flight Radar article which had me searching the interwebs for suitable antennas.
The pricier commercial products seem to favour the co-linear theme and either prompted by that, or more likely sheer luck, I found an article by Jose I. Calderon, DU1ANV, titled "Development of VHF Colinear Antennas, Matching and Feeding Techniques".
The coaxial version is at the bottom of the page under the heading "Other Unique Features of Colinear Antennas and variations". The issues of impedance matching and common mode current are addressed and could possibly be added to the SC version.
Cheers
Andrew
vk6wax

[VK2OMD]

...DU1ANV..."Development of VHF Colinear Antennas, Matching and Feeding Techniques"... The issues of impedance matching and common mode current are addressed and could possibly be added to the SC version.

Fig 11 has several problems that prevent effective in-phase current distribution on the radiator. It inherits those problems in following W7LPN's 'improvements' to WA6SVT's implementation.

The articles by DU1ANV, W7LPN, and WA6SVT do not give credible measurements of gain or models to substantiate their explanation or claims... just like SIlicon Chip really.

Owen

[VK6WAX]

Fig 11 has several problems that prevent effective in-phase current distribution on the radiator. It inherits those problems in following W7LPN's 'improvements' to WA6SVT's implementation.
The articles by DU1ANV, W7LPN, and WA6SVT do not give credible measurements of gain or models to substantiate their explanation or claims... just like SIlicon Chip really.
Owen

If I've understood your comments correctly Owen, it would seem a better alternative would be the original Franklin Antenna:

A side-by-side comparison with a coaxial co-linear would be interesting. I really should learn how to drive those antenna programs!
Cheers
Andrew
vk6wax

[VK2OMD]

Fig 11 has several problems that prevent effective in-phase current distribution on the radiator. It inherits those problems in following W7LPN's 'improvements' to WA6SVT's implementation.
The articles by DU1ANV, W7LPN, and WA6SVT do not give credible measurements of gain or models to substantiate their explanation or claims... just like SIlicon Chip really.
Owen

If I've understood your comments correctly Owen, it would seem a better alternative would be the original Franklin Antenna:

FranklinAntenna.jpg

A side-by-side comparison with a coaxial co-linear would be interesting. I really should learn how to drive those antenna programs!
Cheers
Andrew
vk6wax

Explain to yourself how the antenna in your diagram works.

Note that the single feed point in your diagram is a high voltage low current feed point and the structure that is below it is an impedance matching arrangment to transform high Z down to 50 ohms for the coax feed.

Now chop a quarter wave off each end and analyse the current and charge distribution, for that is equivalent to what W7LPN has done, and DU1ANV recommends.

WA6SVT's radiator design is equivalent in some ways to your diagram, but the elements are not free space half waves, but 2/3 of that length, except for the end quarter wave sections which are free space quarter wave. (He also feeds the array directly at multiple points.)

Silicon Chip's idea is that you can connect one end of the collinear array to the coax shield in your figure.

So, if you take your figure which has a rational explanation of how it operates as an in-phase collinear array, you will see that the things that each of the other 'designs' do is to break fundamental aspects of the rational design.

Nevertheless, large numbers of hams have built these things and proven that they 'work' whatever 'work' means.

Owen

[VK6WAX]

Thanks Owen, it is becoming clearer. Antennas are a bit of a dark art to me.
And now having taken this thread off topic, I'll leave it to others to hopefully add their findings from constructing the SC antenna.
Cheers
Andrew
vk6wax
(Edited: Bad keyboard-brain connection!)

[ZL1RS]

I've never had much success with coaxial colinears. The problem is I find them very difficult to tune or construct on frequency.
The article inspired me to drag out the VNA and start playing with them again.

Me also. Half of Friday was spent trying a couple of these coaxial colinears on the 2m band ... but not the SC version because it is so obviously flawed (three transpose connected halfwaves connected to the top of a halfwave length of feedline extension just doesn't make sense).

Both the DU1ANV and WA6SVT resonated in the 2m band (X=0) but their feed impedances were a bad mismatch to 50 ohm cable ... the WA6VST feed Z was around 300 ohms (much like Owen's analysis), the DU1ANV feed Z around 13 ohms.

I think I just wasted several meters of otherwise perfectly good coax ...........

Thanks Owen, it is becoming clearer. Antennas are bit of a dark art to me.
And now having taken this thread of topic, I'll leave it to others to hopefully add their findings from constructing the SC antenna.
Cheers
Andrew
vk6wax

Like Andrew, I would like to hear from anyone who has also actually built any one of the colinears referred to in this thread.

73,

[VK2XSO]

I did the same thing today. I scaled it down for 2.45GHz and made it from soldered sections of UT-141.
The result was lousy. It had a vSWR of 2.5:1 or worse across the band.
The feeder is only 20cm, so the return loss is expected to be high.

For comparison I tried a simple 1/4 stub with no ground plane, and the result was a narrow 1.8:1 vSWR.
I'm not giving up on it yet. I'm going to try it again in a few days but with a longer feeder cable (to fudge the SWR) and a lower frequency in open space to reduce reflections.

One of my favourite examples on this subject is mobile phone antennas. The kind found on mobile phone towers. The omni directional antennas are sometimes thrown out and I find them fascinating inside.
They're usually some form of collinear coaxial antenna. But they're center fed and broadband and sometimes have a tunable section at some point, though not tunable enough to bring them up to 23cm.

I'll take another apart if I remember next weekend and take some photos for the entertainment and education of the crowd.

[VK2OMD]

In the words of Crocodile Dundee "you call that a collinear", what about this...



(Photo by G8JNJ.)

Note:
- it permits and excites in phase current distribution, at not point where there needs to be a high charge gradient is there a conductor to spoil it (all elements are end fed half waves);
- it has 20 almost half waves, so there are 20 high impedance feed points effectively in parallel which helps get closer to 50 ohms (it appears to have no additional impedance matching);
- the fat sleeve at the bottom is more effective in reducing common mode feed line current than a small diameter one; and
- most of it is silver plated.

It quite possibly works, and the silicon chip jobbie bears almost no resemblence in either functional design or implementation quality.

Owen