7dB colinear calculations

FYI, I tried it out on a SOTA activation at the weekend. 

I was about 300mASL, and had a QSO with a station 160Km away with only 5W. So the gain / radiation pattern must be pretty good !

EI5IKB wrote:Yes, thanks a million. 

FYI, i've built it using 15mm polypipe as a former for the phasing coil. I used narrower wire, as it was all I had, which gave me 7 3/4 turns per phase on the coil. I drilled the pipe all the way through and used tie wraps to position the wire. 

Did you measure the field strength (if so how?) or was it calculated?

Hi again,
Yes, I did measure the field strength. Most of my projects have a theoretical start, but I do a lot of "hacking" to get them to perform as I want them, and this is one of those projects. Tp measure the field strength I put an antenna in a tree about 80 to 100m away (about 10m AGL).

I used 1-watt or so and rotated this antenna, supported up a fiber-glass flag-pole. Measurements were done using an HP spectrum analyser with 100m roll of cable to the test antenna.

I remember this project well as it took a whole weekend and Maj-Lis was bit worried when I was climbing the tree, and later pulling flag-line to lift and rotate the antenna.

Your construction method sounds good. I thought that it would be difficult to access and repair when finished, so I didn't want to risk any connectors/joints in the wire elements/coils. That is why I used one continuous length of cable to make the elements.

One small caveat, when I first put it on-the-air I made the mistake of testing it on a repeater channel. I brought up four repeaters on the same channel, simultaneously !! In Sweden there are not as many repeaters as there are in England. So be careful of other band users. Those extra few dB make a big difference to an omni-directional antenna.

BR Harry

Yes, thanks a million. 

FYI, i've built it using 15mm polypipe as a former for the phasing coil. I used narrower wire, as it was all I had, which gave me 7 3/4 turns per phase on the coil. I drilled the pipe all the way through and used tie wraps to position the wire. 

Did you measure the field strength (if so how?) or was it calculated?

Hi,
That is a project from the past.

When the project was originally built I had to update the construction of the phasing due to effects I found when testing. I updated the pictures but not all the text - sorry!!

The number of turns had to be reduced, but remember that there is also a space between the coils due to the bending radius around the center-pin. Also, one half of the coils is wound in the opposite direction to the other half, which seems to have had a large effect on the phasing.

The original design I "guestimated" that coiling the wire would increase the inductance by some 20%, but in reality it increased it a lot more.

My original thought so 1/2-wavelength should have been modified but in reality the phasing coil is really just a transformer where the second coil gives the opposite phase to the first coil input. It doesn't have to be 1/2 a wavelength.

The pictures are correct, but the text needs updating. When I built the original antenna the beam was not horizontal, but with the windings as shown in the pictures it became near perfectly horizontal at about 120 m distance.

I had intended to update the text, but as often happens, this is a job for "later-on".

As regards the element length, a calculated 1/2-wave would be real if the element was suspended in free-space, but by adding coils to the ends you add capacitance, which shortens the element length by about 4%. This is normal with all antennas.

An "inverted V" for HF, for example, has the ends of the elements close to the ground, which means that the elements should be about 5% shorter than calculated.

Have I answered you questions?

I have some questions on this article http://sm0vpo.altervista.org/antennas/7db-colinear.htm
I'm trying to understand the physics behind the numbers, so 

"Measure the wire diameter, WITH INSULATION and multiply by 16. Mine is 2.5mm so my coils will be 2.5mm x 16 = 40mm long."

what is the basis for this calculation? Why 16? Is it based on inductance calculation? Or is it just a  greater multiple of the number of total turns calculated below?



"The stub will be coilled up, so calculate your 1/4-wave and multiply by 0.8 to get the true length. With 26mm Diameter formers, 
145MHz, this is exactly 408mm, which just happens to be exactly five turns."

Again, whats the basis for these calculations too? Where does 0.8 come from? Is it dependent on the dielectric of the former / VF of wire?
Also my calculations give a quarter wavelength of 414mm (150/145/2* 0., not 408mm.

And finally, "Element wire lengths are 96% of a calculated 1/2 wavelength (150000 / MHz = mm)" Where does 96% come from?


Thanks in advance.