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Collinear design and questions

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Joined
Mar 29, 2012
Messages
20
#1
Hi guys,

I want to share with you a schematic of a 70cm antenna, designed by PCTel, built in Russia and sold by a business in UK. More about the model here :
http://www.saraelektronik.com/pdf/204_SIGMA Antene catc-datasheet-v1.pdf

Although it performs well, it has a design flaw which makes the fiberglass rod come out of its base due to vibrations. Also, PCTel confirmed that it has been discontinued.
Among many types of collinear designs I've seen, this ones looks and performs great for repeater installations.
Because I collected the "remains" of the dead antennas, I opened one up and measured the elements then drew the schematic available here :
http://www.xpander.ro/Sigma_Collinear_Design_draft1.pdf

Now, I could build it on my own and correct the flaws but I was wondering if any of you can answer a few questions :

1. If somebody knows or understands, how does the matching works? I believe it is made by tuning A2+B+C1 elements? The upper element looks like λ/2 @ 439Mhz

2. How can I adapt the schematic for 2m band?

3. If the gain is 5.2dBd, a 0dBd version can be obtained by using only two λ/2 elements and keeping the end-fed the same?

I will start my experiments in the spring and will get back with details, but for now I just want to understand how it works and how can I adapt the same design to different frequencies and gains.

Thank you.

Regards,
Razvan
 
Joined
Jun 30, 2006
Messages
7,664
Location
So Cal - Richardson, TX - Tewksbury, MA
#2
Its hard to understand your drawing and how the elements are connected, but there are common variations of this antenna that have been built since the 1960s. The common theme is using 1/2 wavelengths of coax connected in and out of phase to make the stacked collinear antenna. Most variations have a 1/4 wavelength whip on top and an 8 element version is good for about 6dBd of gain and it takes 16 elements to get 9dBd gain.

Some designs use a 1/4 wavelength 'bazooka balun" or coaxial sleeve to decople the RF from the feedline and usually the antenna side of the sleeve is connected to the ground side of the feedline with the end facing towards the radio left unconnected.

I have some instructions on building one in an old book from the 1970s and here are a few similar instructions: Omni-Gain Vertical Collinear Antenna by WA6SVT

This one claims 9dB gain but with only 8 elements it would be about 6dB gain: Build A 9dB, 70cm Collinear Antenna
prcguy


Hi guys,

I want to share with you a schematic of a 70cm antenna, designed by PCTel, built in Russia and sold by a business in UK. More about the model here :
http://www.saraelektronik.com/pdf/204_SIGMA Antene catc-datasheet-v1.pdf

Although it performs well, it has a design flaw which makes the fiberglass rod come out of its base due to vibrations. Also, PCTel confirmed that it has been discontinued.
Among many types of collinear designs I've seen, this ones looks and performs great for repeater installations.
Because I collected the "remains" of the dead antennas, I opened one up and measured the elements then drew the schematic available here :
http://www.xpander.ro/Sigma_Collinear_Design_draft1.pdf

Now, I could build it on my own and correct the flaws but I was wondering if any of you can answer a few questions :

1. If somebody knows or understands, how does the matching works? I believe it is made by tuning A2+B+C1 elements? The upper element looks like λ/2 @ 439Mhz

2. How can I adapt the schematic for 2m band?

3. If the gain is 5.2dBd, a 0dBd version can be obtained by using only two λ/2 elements and keeping the end-fed the same?

I will start my experiments in the spring and will get back with details, but for now I just want to understand how it works and how can I adapt the same design to different frequencies and gains.

Thank you.

Regards,
Razvan
 
Joined
Mar 29, 2012
Messages
20
#3
Thanks for your input, prcguy. So, the bottom part is called "decoupling"? I have seen models (and I know the links you pointed) but using tubes and other metals. This antenna uses only coax cable, which makes it easier to make.
Some pictures :
(decoupling)

(A + B connections)

Any input for my questions, please?
 
Last edited:
Joined
Apr 28, 2016
Messages
313
Location
Nankin Township, Michigan
#4
I've built this one below, with 4 half wave pieces instead of 8. Only difference is I didn't use radials, I put 4 toroids on the coax instead, pretty much the same as the toroids in the link that prcguy has. This worked out well, I put it inside some schedule 20 pvc. Because it was for 440, I don't think it was too long at all. I had it at 30 feet, and was full quieting into a repeater 40 miles away.

http://peak.biosfarm.ro/~yo4tnv/doc/misc-ham2/How to build a high gain antenna.pdf
 
Joined
Mar 29, 2012
Messages
20
#5
Golay, the idea is to avoid using toroids, radials, etc. Most collinear designs are using similar "decoupling" mechanism and element connections (50Ohm cable, center to braid).
This one, as can be seen in the image, uses a "bazooka baloon" type for decoupling, as prcguy said and mixed elements (50 ohm braid to 75ohm center). The problem is that I can't understand how to adapt it for other bands and also how to reduce the size without affecting the decoupling.
 
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