DECEMBER 12, 2021
“Build a VHF/UHF Omni-directional gain antenna”
The design of this antenna has been around since 1954, parts of the data shown here is from a 1990 article in 73 magazine by Mike Collis WA6SVT.
I originally built this antenna in the early 80’s to use for 434Mhz ATV operation working simplex with stations in Los Angeles transmitting fast scan amateur television from my QTH in San Diego, California.
The aspect I like most about this antenna is that you can make the gain from 3.5db, 6db, 9db or 10db.
Originally my ATV antenna sections were made out of RG-58/U coax, comprising of 21 half-wave length sections giving the antenna 10db of gain.
The same formula is used if you want to build an omni-directional 146Mhz antenna, 444Mhz antenna, 927Mhz antenna.
I did built a 2.4GHz omni antenna for a room-to-room TV video link system ( this worked VERY well!) extending the range from about 150ft to over a half mile.
Today we are going to build a 3.5db 927Mhz omni antenna to work the local repeaters around the valley. This will be used on a 25 watt mobile as a base radio.
This antenna is an end-fed vertical omni collinear antenna, consisting of 1/2 wave length sections and a 1/4 wave top section including a 1/4 wave whip element at the top, (More on the top sections later)
The main elements are constructed from ½ wavelength sections of rigid coaxial cable that you build out of the brass tubing and the center conductor from RG-6 foam coax.
It will be mounted in a piece of 200PSI PVC pipe to serve as the antenna’s radome and can be attached to a tower leg, pipe mast (using hose clamps), hung from a tree branch or bracketed to the edge of the roof.
The number of 1/2 wave sections soldered together is what determines the gain of the antenna. There is also a brass coupling sleeve at the base which is used to couple the feedline and adjust the SWR.
The materials/ tools list.
Soldering iron with quality 60/40 solder
Tubing/ pipe cutter
Razor blade or sharp knife
RG-6 coax (cable TV cable)
Brass hobby tubing 11/64″ x .014″ ID (7/32″ OD) 1/2 & 1/4 wave sections https://ksmetals.com/
Brass hobby tubing 11/32 x .014″ Coupling sleeve https://ksmetals.com/
#12Ga copper wire (Top 1/4 wave element)
RG-142 Teflon coax or your choice of feedline
1″ 100PSI ( Biege color) PVC pipe and misc fittings to suit your method of install. (Preferred)
**NOTE ABOUT PVC – The dielectric properties of this 100 PSI beige PVC is considerably different from white PVC piping of a higher pressure value and will RADICALLY de-tune the antenna by more than 15 Mhz ( Any antenna, this collinear…J-Poles…Slim-Jim will be affected) if white PVC is used.
Also the fittings are not compatable between the two types, the 100 PSI fittings are the same beige color as the pipe.
The formula to determine the length in inches of the 1/2 wave sections is 5904/Freq ( in Mhz) x VF.
VF is the Velocity Factor of the coax from the manufacturers spec. sheet.
The velocity factor of the 75ohm RG-6 coax is .83 according to the manufacturers spec sheet.
Ex. 5904/927.00 X .83 = 5.27″ Each 1/2 wave section of brass tubing will be cut to 5.27″, 1/4 wave section will be 2.64″
I have found this to be a good way to get rid of the sharp edge created when using a tubing cutter.
A few spins while holding the tapered nose of a pair of needle-nose firmly, will give you a smooth edge and bring the inside diameter back to what it’s suppose to be.
The velocity factor of the 75ohm RG-6 coax is .83 according to the manufacturer spec. sheet.
Remove the outer jacket and shield from the RG-6 coax and make sure it fits inside the tubing.
Cut lengths of coax center conductor approximately 7/8″ longer than the brass element tube.
Center the coax element in the brass tube as seen is Figure 1 in #2 photo above.
Using a knife, being careful not to nick the center conductor, cut the dielectric so that a 1/16″ of the white foam sticks out past the end of the tube. This should leave about 3/8″ of center conductor exposed on each end for soldering. Note Figure 2 above.
Being careful to keep the whole antenna as straight as possible, solder the prepared elements together by soldering the center conductor of each element to the outer conductor of the next element as seen in figure 2 in #2 photo. You will end up with transposed connected elements.
(Top section #1) The last element is ¼ wave long, exactly ½ of the measured length of the ½ wave length element.
Short out the top of this section by bending over the center conductor and soldering it to the brass tube as seen in figure 3 above. A ¼ wave whip is connected to the top of the shorted out ¼ wave coax element.
The whip is a true ¼ wave long (no velocity factor correction) and can be constructed out of small diameter brass rod or 12Ga copper wire.
Make certain that the full ¼ wave extends above the point where the coax section is shorted out by cutting the rod a bit longer and soldering this excess to the brass tube.
The 50 ohm feedline can be any length. I use RG-142 coax with an “N” connector attached. Strip off at least a half wavelength of shield on the other end of the feedline.
Leave about an inch of shield sticking out of the vinyl jacket for soldering to the brass tube (coupling sleeve).
Make a true ¼ wave long (no velocity factor correction) decoupling sleeve out of a piece of 11/32″brass tubing.
Using some excess shield material, or some other acceptable manner, solder the decoupling sleeve to the feedline outer conductor at a point exactly ¼ wavelength down from where the feedline attaches to the first ½ wave element.
Attach the exposed end of the feedline to the bottom of the collinear by connecting the center conductor of the feedline to the outer conductor of the antenna and vice versa.
Cut a piece of fiberglass or 100PSI PVC pipe to become the radome for the antenna.
Slide the antenna carefully into the pipe and cap off the top.
Drill two holes near the bottom of the radome pipe and pass Ty-rap through and around the feedline below the decoupling sleeve to support the weight of the antenna.
Pull the Ty-rap tight until it holds the feedline tightly against the radome cover.
You are ready to fire up the your collinear!
Tune up and Operation.
Find a clear area, free of obstructions. Mount the antenna to a pole making sure to clamp the antenna to the mast at a point below the decoupling sleeve area.
Attach a wattmeter or SWR bridge to the antenna.
If the SWR is over 1.5:1 you can adjust the decoupling sleeve slightly up or down for the best reading. When you are satisfied with the SWR solder the coupling sleeve to the braid.
Mounting your collinear on top of your tower will give you an omni-directional pattern. If you desire a Cardoid pattern, or if your only option is side mounting, you can mount the antenna to the side of the tower with one or two brackets. Make sure the bottom support is attached to the antenna below the decoupling sleeve, and that the top support is mounted 18″ or more above the top of the whip.
Mounting the collinear ¼ wavelength away from the side of the tower will give you about a 2 dB increase in the frontal lobe of the pattern. A spacing of a ½ wavelength will increase the signal 2 dB at 90° angles from the frontal lobe. Both patterns give a null in the direction of the tower.
ADDITIONAL INFO –
Since this antenna is end fed, the available signal for radiation is the greatest at the bottom of the antenna and diminishes as the signal goes toward the top….
Why? As the bottom element receives the signal, some of the signal is lost to the next element because some of the energy was radiated (this is an antenna) and now is not available to be fed into the next element, all the way to the top.
Also, some of the energy was lost due to dielectric losses in the element, just as a piece of feedline has loss. This loss is not available for rf radiation from the antenna.
VARIATIONS TO THE ORIGINAL WA6SVT ANTENNA PLAN BY Kevin Custer W3KKC http://www.repeater-builder.com/antenna/uhf-coll.html
This antenna should handle the worst that Mother Nature can throw at it. The original has performed admirably at the ATV repeater site on Santiago Peak at an elevation of 5670 feet (about 1730 meters) for many seasons. Mounted on the tower it blends right in with the commercial antenna installations.
This is the same scheme that was used to build the Station Master and SuperStation Master antennas that commercial repeater owners used on transmitter sites. ( ** note ON COUPLING SLEEVE)
If you look around or ask any Two-Way service shops if they have any old Station master antennas laying around, often times they will give them away at no or little cost.
By taking the antenna apart and discarding the old antenna, the fiber glass radome will make your home brew antenna look just like the Big Boys!
GOOD LUCK ON YOUR BUILD!
DE WB6AMT EARL
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