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Fabricating CB antennas - Ham Radio Library

Fabricating CB antennas

Citizens Band Frequencies and CB Antenna Requirements

 

Citizens Band frequencies are at the top end of the high-frequency band, or stated another way, just below the bottom end of the VHF band. Here is where base-loaded CB antennas usually work well. A mobile antenna should be placed in the middle of the vehicle roof to make best use of the ground plane. The longer the CB whip, the better the performance. The more you load an antenna, the shorter it gets, and the lower the radiation resistance. Lower radiation resistance means less signal output and input. Select the tallest antenna possible and put it as high as possible.



Phased Two CB Antenna System


You might also consider a phased two CB antenna system to give an additional 3dB of gain. You can purchase "co-phase" coaxial cable assemblies, or make your own using a coaxial cable T-connector and two equal lengths of RG8AU coax. Make sure that each side of your co-phased coaxial cable feed lines are identical in length, down to a fraction of an inch, but be sure to avoid a precise half-wavelength on 11 meters for your phasing harness. Mount the antennas on either side of your vehicle. The best performance will be to the front and rear of the vehicle, with slight nulls to each side of the vehicle.



Unloaded Whip


The easiest way to get the strongest signal out on Citizens Band is to use an unloaded whip CB antenna. You may purchase these stainless steel or fiberglass whips at any CB Radio store. Mount the whip as high on your vehicle as possible. Because it has no loading coils, it will offer unity gain without loss whereas you might expect a loss of 1 to 2dB from a loaded CB antenna.



VHF and UHF Bands


On VHF and UHF mobile radio bands, the whips are so short that they may be phased vertically in a collinear array to achieve increased gain on transmit and receive. Why send half your signal into the sky when it can do more good down close to the horizon? You can spot a VHF or UHF collinear high-gain antenna by the loading coils in the center of the whip.

For VHF operation, a 5/8-wavelength CB antenna offers a low take-off angle and may achieve increased gain of approximately 3dB over a conventional 18" spike on the roof. You can spot these antennas easily by a single yard-long whip and a gray or stainless steel loading coil at the base.

For UHF operation at 400-500 MHz, a popular mobile CB Antenna is the collinear halfwave-over-halfwave whip. It's about 20" tall with a sealed phasing coil in the center, and offers increased gain over a little 611 spike. A car with one installed on the roof usually will still fit into a residential garage.



Do-it-Yourself CB Antennas


There are lots of CB antennas out on the market today, all kinds of different shapes and sizes. Because of this competitive market, companies are always trying to put out a better product for a cheaper price. Commercially-made CB antennas are usually easy to put up and maintain. However, you might want to try your hand at making an antenna. You can make an one out of readily-available parts that will work as well or better than some commercially-made CB Antennas.

You will need an SWR meter to check out the Antenna after building it.



 

¼ Wave Ground Plane CB Antenna


This CB antenna consists of a driven element and four radial wires that act as a ground. The driven element receives the transmit energy from the rig.

 

Parts List

104" (264 cm.) [I 02' (259 cm.)] piece of aluminum pipe or conduit

¾" - 1" diameter

Two U-bolts, same size as pipe

One sheet metal screw

One J-hook

Four "egg" ceramic insulators

408" (10.22 m.)[400'"(10.2 m.)] of 16 gauge wire

Rope, enough to guy the ground radial, depending on the height of the antenna

A couple of two-by-fours

Electrical tape

Silicone sealer to cover coax connection


 


1/4 Wave Ground Plane CB Antenna

The inside conductor of the coax is connected to the aluminum pipe by means of a screw into the bottom of the pipe. See the detail drawing on this page. Coat this connection with sealer or cover it with tape to protect it from corrosion.

All vertical antennas need to be grounded in some way. A mobile CB antenna uses the car body as the ground. On this CB Antenna, the four radial wires are used as the ground. This is called the ground plane of the antenna.

The braided wire which forms the outside conductor of the coax is soldered to all four radial wires. The wires must be exactly 264 cm (104') long [259 cm(102") long] (¼ wavelength).

Remember that the inner conductor and outer braid of the coax must not touch each other, nor can the radials come in contact with the driven element. The radials slope down at about a 45 degree angle in different directions, and are tied to the insulators. Rope or nylon cord is then tied to the insulators and used to hold the radials out. They can be attached to anywhere convenient; trees, a fence, house, etc.

If you are short on room for such a radial system, you can use 104" (radials) and 102" (driven elements) pieces of aluminum tubing, or suspend the wires on PVC pipe, bamboo, or 1" x 2"' wooden sticks. It's a must to check the SWR when done. It should be lower than 2, and ideally lower than 1.5 or 1.3.

A ¼ wave ground plane CB antenna made from wire can be suspended from a tree. We have talked to stations over 40 miles away using this antenna up about 30 feet high, running a mobile rig for a base.



Coax Cable Vertical CB Antennas


For a quickie CB antenna, a vertical dipole (1/2) wave can be made right from the coax itself You take your coax and very carefully, without nicking the braided shield, strip 102 inches of the outer insulation jacket off one end. After removing the outer jacket, start bunching the shield down the coax from the end. Now, where the outer jacket and the shield meet, separate the braided shield enough to get the inner conductor out through the hole in the braid. Pull all of the inner conductor through and stretch it and the braid out. Be careful not to skin any of the insulation off the inner conductor. Now attach an antenna insulator to the end of the inner conductor. Measure the braided shield. Cut it off at about 106" and attach an antenna insulator to the end. The total length of the inner and outer conductor should be about 17 feet (1/2 wave). You can haul it up to any height you want with a string or rope attached to the insulator on the center conductor. It's a good idea to coat the end of the coax cable where it separates with some kind of waterproof sealer. This keeps water from seeping into the coax, which could cause a high SWR.

Just attach a coax connector on the end of the coax going to your transceiver and you are on the air. The SWR should be 1.5 or better, if cut to the proper length. It'll get out about as good as a ¼ wave ground plane CB radio antenna if you get it up high and in the clear. Do not hoist this antenna up next to a metal pole, because the metal will interfere with the antenna's operation and cause a high SWR.



Using a Mobile CB Antenna for a Base Station


A mobile CB antenna can be used as a base antenna by mounting it on the top of a metal pipe. The metal pipe serves as ground connection for the antenna, taking the place of the body of the vehicle. Remember to run a separate ground wire to a proper grounding rod for lightning protection.



Building a Vertical Gain CB Antenna


This is an easy antenna to build and find the parts for. If you follow these instructions and have it come out looking like these pictures, it should have a very low SWR and appreciable gain.

You can use any number of pieces of aluminum pipe so long as they are ridged and fairly thick-walled so as not to become bent and broken in a strong wind. The pieces should be gradually smaller, one being able to fit inside the next. Cut two 2' grooves with a hacksaw down the outer pieces of aluminum and put a hose clamp around them. Now adjust the antenna to 22 ½ feet (6.85 meters) and tighten the hose clamps down to hold it all together. This 22 ½-foot vertical element can now be mounted with two U-bolts on to the 2" x 6' board. This board should be treated or painted to protect it from the weather.

Put a bolt through the piece of wood a few inches below the vertical element. Here you should fasten one end of the 6 foot (183 centimeters) piece of copper wire or tubing, the outside braid of the coax and each of the 104" (264 cm)[102" (259 cm)] long, stranded wires. These are called the ground radials and should be tied off with string (not wire) at a 45 degree angle away from the bolt. The ground radials and the braid from the coax can be soldered together or can be crimped together with a crimp connector that fits the bolt. The other end of the 6' copper wire is bent and fastened to the vertical element. The end of the center wire of the coax is then twisted onto this in such a way that it ca be slid up or down along the copper wire and soldered after adjustment.


The SWR of this CB antenna is adjusted by sliding this connection. You do this by keying your rig up on channel 20 and sliding this connection up or down until you have the lowest SWR. In our experience, an SWR of 1:1 or 1 was easily reached on channel 20 with a low SWR throughout the 27 MHz band.

Be sure to cover the end of the coax real well with a moisture proofing sealing compound so no moisture can get in it.



Fishing Pole Ear--A Homemade Mobile CB Antenna


While the next project won't necessarily save you any money, you might want to give it a try if you are interested in understanding more about how fiberglass whip antennas do their thing. We have talked 30 miles mobile-to-base on one of these home brew fishing pole antennas using a $20 barefoot rig.

 

Antenna Parts

One 7-foot fiberglass fishing pole with hollow base One piece of steel rod 4 to 6 inches long-right diameter to slip into base of pole One ¼" x 1" machine bolt (threads to match mount) 12 feet of enamel #18 gauge wire Some good epoxy glue


 

Antenna Mount Parts

One 114' x 21/z" bolt and nut (same threads as bolt on base)

One longer-than-usual ¼" nut

Two plastic insulating washers

Three metal 5/8" diameter washers

One large terminal lug


First you've got to get yourself a fishing pole, 6 to 9 feet long. If you already have an old one lying around, you can clip off the line loops and cut the handle off.

We did some shopping and found that a finished fishing pole as long as we wanted was at least as expensive as a newly-manufactured CB antenna. But then we discovered a sporting goods store that sold unfinished fiberglass poles 7-feet long for $6. The kind we found was a black hollow tapered pole with about a 1/8" inside diameter at the base.

The next step is to hook something to the pole so you can screw it to a mount on your vehicle. The way we did it was to get a piece of scrap steel rod near the inside of the base of the hollow pole. (If the pole is not hollow you will have to figure out another way of hooking to it.)

Grind a slight taper to match inside taper of the fishing pole. Braze bolt to rod. Use 1/4" x 1" steel bolt. Epoxy the steel rod inside the base of the fishing pole.

There are many commercially made CB antenna mounts that you can buy at electronic parts stores that could be used to hold your fishing pole ear. If you elect to buy one of these mounts, get one with a spring so that if the pole encounters a stray tree branch, it can bend instead of break! If you are going to use this antenna with a store bought mount, the threads on the bolt on the bottom of the antenna should mate with the hole in the top of the mount's spring.

 

Tuning the Fishing Pole Ear

Now it's time to get into the electrical part of the antenna. The fishing pole is not the antenna; it's just a prop that holds the wire up. So we need to wind a wire around the fishing pole in such a way that will make it tune the 27 MHz CB band

If your antenna is shorter than 9 feet long, you can tune it by winding a coil around the pole. On our 7-foot ear, we found that it was necessary to make a coil of four turns ¼" apart about 2'- 3' up from the base of the pole. Tightly wrap the wire in a spiral up from the base of the pole. Make the distance between windings as wide as possible below and above the coil. If you follow these particular dimensions you should be close to being tuned up. To really make the ear a perfect match, however, you'll need to use an SWR meter to check the SWR. You may have to modify the coil spacings or add or subtract a turn from the coil to get it just right.

Different lengths of CB Antennas will also work, but different coil windings will be necessary, so if you have a different length pole you'll have to use a meter to tune it up. You just have to dive right in and try different numbers of coil windings. The longer the pole, the less center coil windings; the shorter, the more windings necessary. It takes some playing around and trial and error.

We used enamel coated wire, the kind used in motor windings, generators, transformers, etc. It's best to use enamel-coated wire so that the coil turns can't possibly short to each other.

At the bottom, wrap the wire around the 1/4" bolt. Be sure to scrape the enamel coating off the wire and clean the bolt for good contact. Solder that wire to the bolt or use a nut to hold the wire onto the bolt so that it makes a good electrical connection.

We covered some CB antennas we made with a thin coat of fiberglass (which you can tint any color you want). They looked pretty good but the fiberglass chipped off some of the antenna tips because they were mounted fairly high up and got tangled in the trees. You can prevent chipping by putting some shrink tubing over the tip. Shrink tubing would be another possible way to hold the wire on the fishing pole. It's available at most electronics shops.

 

Building a Fishing Pole Antenna Mount

If you are really adventurous, you can also build your antenna mount. The main point to understand when making an antenna mount is that the radiating element is not supposed to ground out to the body of the vehicle. This means that the bolt the antenna hooks to must be insulated from the metal body of the vehicle.

Your homemade mount must use good insulating washers, because if the bolt shorts to the vehicle body it could possibly blow out your rig's final RF power transistor.

One source of home brew insulating washers is the main output terminals of junk alternators or generators. The washers should be made out of some type of plastic. The best kind of insulating washer has a shoulder around its hole. You drill a hole in the vehicle body, big enough to allow the shoulder to fit through. This holds the bolt away from the metal body.

If you can't find a plastic washer with a shoulder, it is possible to use two flat plastic washers with a little piece of plastic tubing in place of the shoulder. Make sure the plastic is tough enough to not get cut by the edge of the hole in the vehicle body.

Your base CB antenna system may be put up 20 feet higher than the highest point of the building or tree on which it is mounted; however, the highest point of the antenna must not be more than 60 feet above the ground. There are additional restrictions on an antenna system located near an airport. Consult FCC Laws Part 95, Subpart D for your particular situation.
sistance means less signal output and input. Select the tallest antenna possible and put it as high as possible.