A 2 meter / Marine RG-6 Coax Version of the Vertical Bazooka.
Article by DE Sanders, W4BWS/HC4
I have produced the stick antenna for
hams and commercially by SKYLANE Products for 2 meters and Marine VHF band
since 1982. They are usually made from RG-58 coax and housed in a white
PVC housing for weather protection. The drawing above is
courtesy of VE3VDC from his article:2
meter Vertical Bazooka Ant. on
There is much to recommend this design but I will not discuss the basic design here as it has been covered many other places. The design has been proven beneficial for hams even with many variations in materials. This is the core of Amateur Radio; to experiment, build your own and learn about antennas.
I believe there is one basic error in the design information presented by these authors. This may be a misunderstanding of coax. When they refer to the velocity factor (VF) to multiply the lower section, they only used RG-58 or RG-8 coax and referred to the velocity factor of the coax used. However if you use RG-6 or other foam dielectric coax with a VF of approximately .80, you will be too long.
The VF they should have referenced is not the VF of the center conductor insulation dielectric but the VF of the outside jacket of the coax. That is the insulation that interacts with the outer sleeve. Fortunately, I discovered that on RG-58 and RG-6 this VF is about .66. So that is the shortening factor to use for polyethylene insulated coaxial cable outer jacket.
I found a need while in Ecuador by cruisers wanting a standby VHF Marine antenna in case their main antenna failed or was damaged. Also many are also hams and wanted dual use as 2 meter and marine antenna.
In Ecuador the only coax readily available was RG-6 TV coax cable. This posed several problems. The shield was aluminum wire and does not readily accept solder. The braid is loose, with about 40 to 60 percent coverage, over an aluminum foil. Thus making the sleeve was a problem. There were no PL 259 style connectors for RG-6 coax.
How To Build The VHF Coaxial Sleeve Dipole
Recalling the many commercial antennas I had made using copper tubing as the sleeve, I decided to use aluminum foil for the sleeve and eliminate the dissimilar metals and the need to solder the shield.
A 13-foot (4 meters) length of RG-6 coax was prepared by first removing about 24 inches of the black outer covering. Be careful to not cut the braid or foil removing the covering. I found a Xacto knife or sharp razor blade would cut a clean groove down the length of the jacket and around the circumference. The jacket peeled off nicely.
I cut and removed all but 6 inches of the braid and foil. I then cut a 14-inch strip of aluminum foil 3 inches wide. This strip was placed with the 14-inch length along the coax. This was wrapped around the outer jacket below where the jacket was removed and rolled around until the whole length was tightly wrapped. I used black plastic tape to secure the foil tightly every 4 inches, not a critical dimension, beginning 2 inches below the top end and leaving a few inches at the bottom for trimming.
I applied a small amount of anti-oxide compound to the top 2-inch bare aluminum foil. Then I doubled the braid back over the coax and aluminum foil and pressed it tightly against the foil. The coax foil shield was then folded down similarly along the foil. Using about a 4 inch wide by 4 inch long piece of aluminum foil, I wrapped this over the braid and foil of the coax that was folded over. This wrap should be as tight as possible. This made a good electrical connection between the coax braid, foil and the aluminum foil along the coax. Tape the foil at the top tightly to secure it.
I bent the top of the coax center conductor back on itself so it could be used as a hanger for testing. I hung the assembly on the side of a 5-foot piece of 3/4 inch PVC and prepared the far end of the coax for connection to the wattmeter and radio.
Connecting the RG-6 to a connector
There are several choices available for the connector to the radio. The PL259 with adapter for RG-59 coax and type "F" TV cable connectors are the most used types. I'll discuss these types.
I found that the PL259 connector with the adapter for RG-59 coax would fit well on the RG-6. So one antenna was made this way by stripping the coax normally after installing the adapter on the end of the coax. The foil and braid was folded back on the adapter shell and trimmed at the shoulder.
The center wire insulation was removed and the connector body installed. The adaptor was screwed into the body tightly and the center wire soldered to the center pin. These adapters were designed for use without soldering. Despite several articles about soldering the braid to the adapter, they work well without soldering. I usually apply black tape over the cable and adapter for strain relief.
Type "F" connectors are readily available and work well up through 450 MHz up to 500 watts. In the USA and several other countries there are adapters available for type "F" to PL 259. This is perhaps the easiest connection to make on the RG-6 coax.
Since there were no PL259 connectors or adapters available in this part of Ecuador, I used the type "F" on the coax. I prepared a 6 inch length of RG-6 by putting a type "F" connector on one end and stripping the other end about 3 inches. After stripping the insulation from the center wire, solder a brass screw that fits securely in the SO239 radio connector to the coax center wire. This is wrapped with black electrical tape for insulation. The braid was twisted and wrapped around the outside of the radio connector and secured by an adjustable cable clamp around the connector shell. Then using a double female barrel splice, the two "F" connectors were connected. This worked very well at 2 meters and VHF Marine band.
For testing, I made adapters from a couple PL259 connectors with about 6-inch lengths of RG 6 coax installed and an "F" connector on the other end of the coax. With 2 barrel connectors I could attach the antenna coax to the wattmeter and the wattmeter to the radio.
Set PVC pole and antenna in the clear, away from metal and set the transmitter to 146 MHz for 2 meters, 156Mhz for Marine, or a compromise of 148 or 149 for dual band use. Adjust the antenna by clipping 1/2 inch at a time off the center conductor wire. I was able to get 1.2:1 SWR with a 22-inch length at 146 MHz. This length also was 2.5:1 at 156.475, Channel 69.
A second antenna was built and adjusted for lowest SWR at 149 MHz and showed less than 2:1 SWR at 145 and 156 MHz. This was considered adequate for dual band use. With 25 watts forward and 2 watts reflected, most any 2 meter or VHF marine radio will perform normally.
After adjustment the foil on the antenna lower section was over wrapped tightly with black electrical tape.
The Antenna Housing
If wrapped tightly the antenna will fit snugly into a length of 1/2 inch PVC. I considered this too tight for general use, but it makes a lighter antenna for backpacking or emergency use.
I used 3/4 inch PVC 5 feet long. At the upper end I cut a notch in opposite sides to fit a piece of bamboo skewer. The end of the antenna is folded over about 1 inch and hangs on this bamboo.
Cut the bamboo flush with the PVC and secure it and the antenna with a bit of hot glue or RTV.
Cap the end of the PVC with a pipe cap, which has a hole drilled in the top center. A screw eye is screwed into this hole and sealed with RTV inside and out. The eye can be used to haul the antenna up into a tree or up a mast. Secure the cap on the PVC pipe with RTV.
At the bottom of the PVC pipe fit another cap with a hole drilled in the center to pass the coax. It is a good idea to put the cap on the coax before you install the connector. Again secure the cap and around the coax with RTV.
This completes the assembly of the antenna. I usually use two adjustable hose clamps, about 6 inches apart, to secure the PVC mast to a support. Plastic tie wraps or tape can secure he coax also.
About 50 feet (15 meters) of RG-6 or RG-8 makes a good feed line. Since RG-6 is all that is available here, I apply a type "F" connector on both ends of the 50-foot coax. A double female at each end allows connection to the antenna and radio with the adapter as described above or a "F" to PL259 adapter. For weather protection, wrap the coax fittings with black electrical tape after assembly.
For a fixed station a variation that may improve performance is to cut the PVC so it ends a few inches below the lower sleeve shield end. Install a plastic Tee fitting and install the left over PVC as a lower support. The coax should exit through the Tee fitting and a short pipe stub. The PVC cap fits on this stub.
The coax should be wrapped about 8 turns around the support pipe to form a current choke. Tape the coil and the feed line then drops down. Some builders report this improves the radiation at a lower take off angle and helps prevent feed line radiation. I have not tried the coax coil as I have not found it to be a problem.
The antenna can be made as a roll-up emergency antenna by not putting it is a housing. A loop in the top of the radiator will allow tying a string or nylon cord to hang it up or pull it into a tree. It can be taped to a window or to a bamboo or fiber fishing pole. Use your imagination.
Enjoy your Sleeve dipole antenna.
This article shows ham ingenuity at it's best. As mentioned in the article, many ham radio connectors that we see as very common every day items are almost impossible to get cheaply when you are in another country. The author shows how we can overcome almost any "problem" when we use our brains! N4UJW
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