Odd antenna Design

[Laxplurr06]

Today I opened an 800mhz antenna that was not performing as well as I had hoped. I found this inside the tube and was hoping that someone would be able to explain to me how this antenna works.

The center conductor is connected to the brass tube that is located in between the others, and small piece of coax at the top. The outer conductor is connected to the other brass tubes as show in the picture above. The center conductor is indicated by the blue lines, and the outter conductor by the red lines.

 

[mancow]

it's a collinear type. Google that in images and you will see similar designs.

http://www.rason.org/Projects/collant/collant.htm

 

[kb0nly]

Yep collinear...

A collinear antenna is just basically stacking elements of a measured wavelength to get gain. Its like having a bunch of 1/4 wave antennas stacked to make one big antenna.

 

[Laxplurr06]

Is a certain amount of quarter waves needed, or could they keep being stacked on top of each other to get more gain? I am curious because I would like to try and make one for 470mhz, or possibly try and increase the gain on this one.

 

[mancow]

That page I linked to has the details on how to build one and the relationship between number of elements and gain.

 

[mpddigital]

They are not hard at all to construct but length of the elements and construction are critical to getting the antenna turned to the freq you want to operate. The ones we have used operate best on a fixed frequency rather than using it over multiple bands.

 

[lmrtek]

collinear designs use electrical 1/2 wave elements stacked
A doubling of the amount of elements will give a 3db increase at the expense of course of vertical beamwidth

 

[Laxplurr06]

Thanks, I might try that.

 

[WQFM513]

one of those for 462.xxx at about 9db gain would be 6.8 feet tall.

 

[prcguy]

At 462MHz a 5dBD version about 7ft long and a 10dBD gain version is a good 20ft long.
prcguy

one of those for 462.xxx at about 9db gain would be 6.8 feet tall.

 

[kb2vxa]

There's more to it than simply stacking elements. In between are phasing stubs to keep the active antenna elements in phase or they would cancel each other out resulting in no gain.

 

[prcguy]

This design just stacks 1/2 wavelengths of coax (1/2 wave w/velocity factor) and the phasing is done by swapping the center conductor and shield at every element. There is usually a 1/4 wavelength coax balun at the feed and a 1/4 wave whip on the top of the last element.

Because the phasing and distance between elements is not optimum it takes eight half wave elements to give about 5dBD gain where it only takes 4 elements in an exposed dipole array to get 6dBD gain.
prcguy

There's more to it than simply stacking elements. In between are phasing stubs to keep the active antenna elements in phase or they would cancel each other out resulting in no gain.

 

[WQFM513]

i recently built one for my gmrs repeater. used schedule 40 pvc as the radome. the wind took it down pretty quick. 70+gusts. i decided not to repair the collinear and went with a simple whip type with a ground plane. another one i also made.
my measurements brought the 9db collinear to 2100mm (6.8 feet) and it worked great. nice gain, swr at 1.2:1 and it could "hear" stations very clear, loud, and out pretty far.



½ wavelength of coax = 300 / F / 2 * V

Where F = Frequency in Megahertz

V = Velocity factory of Coax

300 / 462.6250 / 2 * .66 = .2139 meters or 213.9 millimeters

1/4 wavelength radiator = 300 / F / 4

Where F = Frequency in Megahertz

300 / 462.6250 / 4 = .1621 meters or 162.1 millimeters

213.9 X 8= 1711.2
162.1 X 2= 324.2
1711.2 + 324.2 = 2035
2035 + about 2 inches of radome length for comfort = 2100 mm
2100mm converted to inches =82.67
82.67 inches = 6.8 feet

 

[prcguy]

You show 8 1/2 wave elements, that would make around a 5dB gain colliniar not a 9dB.
prcguy

i recently built one for my gmrs repeater. used schedule 40 pvc as the radome. the wind took it down pretty quick. 70+gusts. i decided not to repair the collinear and went with a simple whip type with a ground plane. another one i also made.
my measurements brought the 9db collinear to 2100mm (6.8 feet) and it worked great. nice gain, swr at 1.2:1 and it could "hear" stations very clear, loud, and out pretty far.



½ wavelength of coax = 300 / F / 2 * V

Where F = Frequency in Megahertz

V = Velocity factory of Coax

300 / 462.6250 / 2 * .66 = .2139 meters or 213.9 millimeters

1/4 wavelength radiator = 300 / F / 4

Where F = Frequency in Megahertz

300 / 462.6250 / 4 = .1621 meters or 162.1 millimeters

213.9 X 8= 1711.2
162.1 X 2= 324.2
1711.2 + 324.2 = 2035
2035 + about 2 inches of radome length for comfort = 2100 mm
2100mm converted to inches =82.67
82.67 inches = 6.8 feet

 

[WQFM513]

my sources figures said 9db gain. the one i built did out preform my 5db gain antennex omni. just couldnt handle the wind out here.

 

[prcguy]

One of my sources is the older the ARRL antenna handbooks which published the internal construction for the original Phelps Dodge Stationmaster antennas which became Cellwave then something else. I've also disassembled several 5dBD Stationmaster antennas which have eight half wave coax sections soldered with opposing center and shield plus the 1/4 wave sections on the top and bottom, exactally like the ARRL antenna handbook.

How tall was your Antennex? If it was a 5dBD model it would be about 7 1/2 ft tall. If it was shorter it was not a 5dBD collinear antenna.
prcguy

my sources figures said 9db gain. the one i built did out preform my 5db gain antennex omni. just couldnt handle the wind out here.

 

[WQFM513]

i never said anything about the antennex being a collinear. i said 5db gain antennex omni. it is a stacked element stainless whip.

 

[prcguy]

Is that the NMO mobile antenna with a black coil in the middle about 40" long? If so they are not anywhere near 5dBD, thats why your 8 element collinear would work better.
prcguy

i never said anything about the antennex being a collinear. i said 5db gain antennex omni. it is a stacked element stainless whip.

 

[WQFM513]

yes it is an nmo mobile with a black coil between elements. it is 32 5/16 oal. it sure seems to be a 5db gain antenna. works as well as advertised.
i can activate and receive the repeater between royal city and yakima washington from my current location. thats roughly 65 air miles. 45 watts 462.700 standard gmrs offset. about 5 feet above ground level.