Lets build a vertical antenna for 6 meters today by modifying an old Radio Shack CB antenna. Most mobile CB antennas these days are quite short with a hefty base loading coil. These will also work, but will require playing with the inductor a bit to get the antenna to “tune” on 6 meters, and radiate instead of being a dummy load.
First of all – the basics. We are going to build a 1/4 wave elevated ground plane antenna for non-mobile use. The formula to figure the starting length of a 1/4 wave antenna is 246 / frequency in megahertz. So for 50.200 MHz and SSB operation we would want an antenna that is 58.8″ long. This is the actual length of a quarter wave. The VF (Velocity Factor) of your feedline and diameter of the radiator will determine the actual length, which will be somewhat shorter than the above figure – probably in the range of 55-56″.
The inductor on the CB antenna makes the antenna electrically longer than its actual radiator length. CB is 11 meters, so the inductor obviously adds too much electrical length for 6 meters. The antenna I used has a 53″ stainless steel whip. So I am very close to the ~55-56″ of radiator length I will need. I just have to eliminate the inductor. I did not think to take a photo of this, but it is very simple. I cut the plastic covering off the inductor, cut the wire on one end and unwound it. The inductor consists of two chrome-plated copper ends with a fiberglass coil former in between. After unwinding the inductor I stretched it end to end, cut the wire and soldered it to end where I cut it. This effectively leaves a piece of wire (now part of the radiating length) connecting the two ends of what was an inductor together. Those two ends are handy because as I will show in a subsequent photo they can be used to mount the antenna to a bracket and adjust the stainless steel radiator.
This is a photo of the modified inductor. The coil of silver plated copper wire next to it is what I unwound from the inductor. I put a piece of heat shrink tubing over this outfit to prevent water from getting in between the fiberglass coil former and the chrome-plated copper ends of the unit:
Do not throw away that silver plated wire that you remove from your inductor. It is handy for fixing inductors in radios, making your own inductors for other projects, or even wiring up chassis-mount SO-239’s, etc. on tuner or radio projects.
The rest of of the antenna construction is very simple. I made a steel bracket to mount it to a mast. Aluminium or other material could be used for this as well. It will be mounted using common u-bolts shown in the photo:
This next photo shows the various parts of the antenna laid out in order on the floor of the shop. Note that this antenna was designed for either L-bracket mount (such as mirror mount on a truck) or thru-roof mount with a hole in the roof of the vehicle. So it has a SO-239 with a stepped insulating washer for the center conductor where it goes thru the big hole in the L-bracket:
This photo shows the base of the antenna assembled. As always, when building any antenna always check your connections and joints on the assembly to make sure you have continuity where you should have, and it is insulated where it should be. This photo also shows a tape measure alongside the antenna. The radiating length at present is 58″. The antenna will radiate from the base of the previous loading coil to the tip of the stainless steel whip, and the length will provide plenty of extra length for tuning VSWR:
This last photo shows the base fully assembled with the ground plane lugs installed and a sample ground plane element in one of the lugs. The ground plane mounting lugs are common electrical lugs available in the electrical dept at Menards or a hardware store.
From there it is a simple matter of mounting the antenna, running a feedline to it and tuning it up for lowest VSWR at the desired frequency.
Some notes on the theory, ground plane and mounting the antenna:
This antenna should be mounted at least one full wavelength above ground (22 feet). It’s gain is approximately equal to a resonant 1/2 wave dipole in free space. The higher you place it, the more real-world gain it will have over a dipole. The ground plane elements should be a full 1/4 wavelength long – approximately 58″ – and they will end up slightly longer than the radiator. On this particular antenna I am using 3/16″ steel rods for ground plane elements and they will be bent down approximately 45 degrees – commonly called the “droopy ground plane” 1/4 wave. It does not matter what material you use for the ground elements. Steel, aluminium, copper – they all work.
Somebody thought up a myth once, and it is commonly recited, that the more ground plane elements you have the more gain the antenna has, and that you have to have at least four of them. This is a myth because it is untrue. You only need one ground plane element for the antenna to tune and be resonant. And you can actually run no ground plane elements if you don’t mind using the coax shield as a common-mode current carrier for the ground plane. One ground plane element will cause the antenna to be somewhat directional. Three will make it as purely omni-directional as possible. Adding more elements does absolutely nothing.
The 1/4 wave vertical simulates a 1/2 wave dipole by “mirroring” the dipole’s ground element in the ground plane. However, unlike a dipole, all the radiation from a 1/4 wave vertical is concentrated ABOVE the ground plane. So it has more real world gain than a dipole because it achieves a lower radiation takeoff angle than is possible with a dipole, either horizontally or vertically polarized. And the 1/4 wave vertical wastes less radiation straight up into the sky than a dipole does. Remember that gain is only concentrating your radiation where you need it to go. Gain does not make your antenna put out more power, or give it any magical power increase.
Since this is an elevated ground plane antenna, the ground plane radials should be insulated from the mast and tower. This means using either an insulator on the mount, or a non-conducting mast like heavy PVC electrical conduit or fiberglass for mounting. The shield of the coaxial feeder should have a good RF ground on the station side of the feeder.
Why build a vertical? Why not? The 1/4 wave vertical is one of the most forgiving antennas there is and has very good performance for its size and wind loading. Vertical polarization is used exclusively on 6 meters in the FM portions of the band. Horizontal polarization is predominantly used for SSB and weak signal work because of the predominance of yagi antennas and arrays with serious 6 meter operators. However, for local communications out to 50 miles or so using omni-directional antennas vertical polarization is better than horizontal. It is easier to concentrate the radiation on the horizon where it needs to be with a vertical than it is with a horizontal.
You will have mixed results working DX with a vertical antenna on 6 meter SSB. Close-in contacts out to 250 miles will probably be weak not be heard if they are using horizontal polarization. With E-layer and tropo ducting propagation it won’t make any difference and there are plenty of folks who use everything from 1/4 wave verticals to J-poles and Slim Jim antennas on 6 meters.
So to sum it up, the advantages of the 1/4 wave vertical are easy to build and tune. Very low wind loading compared to yagis and loops. Good radiation pattern for local communications on vertical polarization, and long range DX. Easy to mount the antenna very high on your tower because it is a relatively small antenna. Allows you to switch back and forth from 6m SSB weak signal work to 6m FM at will, using the same antenna.
The disadvantages are that it is not so good at short (up to 50 miles) and medium range (50-250 miles) if the other station is using horizontal polarization. And this antenna will not put you in with the “big guns” on 6 meters that communicate just about anywhere they want and bounce signals off the moon to Europe at will with stacked arrays of yagi’s with 1500 watts going to the array.
Final note:
The 6 meter Holcombe repeater, 52.810 -1,700 kHz offset PL 110.9 is back up and working, according to reports. That repeater has coverage all over Barron County, as well as several other counties. According to Mike Metzdorf W9MM that repeater has some “serious range”.
Have fun!
73,
— Chris AC9KH
