6.5 dB of Gain on 2 Meters
Bargin Hunting Again
As I have said in previous articles I think I am typical of many Hams that just can’t pass up a “Bargain” at the swap meet, garage sale or the local ham radio shop! Have you ever noticed, seemingly overnight, what you have collected over the years? That five-pound hybrid transformer has only made a good paperweight since the swap meet two years ago but the 2 Meter, 6.5 dB ground plane described in this article will make a noticeable improvement in your fixed station 2-meter operation! If you like to “Home Brew” your projects, this one will not become your next paperweight! If you are new to weekend ham radio construction projects or to 2-meter radio operation you will find this project quite straight-forward and a good and useful place to start.
Some time back Radio Shack purchased “Tech America” and began “RadioShack.Com” as their online store offering all of their standard catalog items plus products from Tech America. It is one of the Tech America products, now being offered as a close out item, that is the basis for this project. The item being sold (catalog number 940-0674)***, at a 50% discount, is a 6.5 dB “MOBILE” antenna that is approximately 9 feet (2.8 m) in length and uses a UHF (SO-239) connector for mounting and its electrical connection. It was a simple matter to add the necessary “L” bracket, “U” clamps and radial system to convert this over sized mobile whip like antenna into a very useable, very high performance Ground Plane station antenna. Please see Photo 1 (above) and Photo 2 (below) for details.

Construction
The “L” bracket was fabricated by using one half of a 3.5 inch x 19 inch (8.9 cm x 48.26 cm) blank standard aluminum rack panel. The panel thickness is .125 inches (32 mm). This panel material was used because it was available in my “Junk Box” and provided the size and strength required for the wind conditions in this area of Arizona, USA. The minimum tower design wind force here is 75 MPH (120 Km) and I wanted the bracket to be rugged enough to handle these wind conditions. You may substitute comparable materials in this project as you choose. The bracket layout and hole pattern is shown in Photo 3 and Photo 4 after bending the 90-degree angle allowing the antenna and radials to be mounted on a rigid flat surface and providing two “U” clamps for mounting. This angle was formed by placing the completed plate in a large vice and forming the 90-degree angle using a mallet and a hammer. Be cautious, using this method, not to fracture the metal plate at this point by over-stressing the bend. A local metal shop might be talked into making this simple bend for you using a metal bending “brake” for a perfect job. The “U” clamps specified indicate a 2-inch (5.1 cm) inside dimension but you are free to select the proper size to match the diameter of the mast chosen. Always allow a slightly larger clamp than the pipe diameter. Use 1.5-inch (3.8 cm) clamps as an example if you are going to attach this mount to a 1.25 inch (3.175 cm) mast.

When this antenna was designed as a mobile unit the vehicle served as the ground plane under it. A typical mobile installation is usually a poor substitute for a proper ground plane but with this design we have the opportunity to construct a much more effective ground plane to work against. Four 20-inch (50.8 cm) long radials were added to the mounting bracket to serve as the ground plane. A .125-inch (3.175 mm) brazing rod was used because it was available at the local hardware store. I have had very good luck with it in other designs and it is very easy to work with. Again you may substitute solid aluminum or stainless steel rod or tubing with comparable results. All of the mounting hardware shown is stainless steel. The slight increase in cost is well worth the benefits gained. The mounting bolts .25 inch x 20 x 1 inch (2.5 cm), washers, lock washers and “U” clamp assemblies will not rust or corrode over the years and you will eliminate any metallurgical problems due to dissimilar materials on your antenna.
Assembly

The brazing rods were cut to a length of 21 inches (53.34 cm) and a loop with a .25-inch (6.35 mm) inner diameter was formed on an end of each rod. A stainless bolt and flat washer was then inserted through the formed loop and on through the top of the mounting bracket, then fastened in place using both a flat and lock washer and the mating nut. Separate the four radials equally spaced from each other on the mounting bracket forming a 90-degree angle to each other. Be certain the mounting surface of the bracket is clear of any paint, oil or other oreign matter prior to assembly to assure a solid, gas tight, connection.
This antenna as received is designed to be mounted to an SO-239 style connector. To provide this mating connection you have two mounting-connecting methods to choose from. If you wish to be able to fasten your coax cable to the completed antenna using a PL connector you should use a bulkhead style UHF female-female (barrel) connector. This is fastened to the mounting bracket through the center .625-inch (1.59 cm) hole using two nuts and star washers that come with this connector. Space the antenna side of this feed-through connector in a manner that will allow a solid connection and is not so high above the bracket surface that a proper weather seal is not formed. As an alternate mounting solution you may use a standard SO-239 chassis connector bolted to the bracket using four each number 5×40 screws, lock washers and nuts. Then solder your coaxial cable directly to the underside of the connector. Be sure to mount one solder lug under a mounting screw or through a fifth hole drilled for it alone through the bracket using a fifth screw, lock washer and nut. Solder the braid of your cable to this lug. If this method is selected be certain to coat the exposed underside of the connector and solder joints using RTV or other sealing compound to weather proof your connections.
Mounting and Test
Attach the antenna to the mounting bracket using the SO-239 style connector you have selected. Seal the base of the antenna/bracket junction using RTV or other sealant for weather protection. Attach your coaxial cable (if you are using the barrel connector method) or lay out your solder attached cable in preparation for attaching the mounting plate to your mast. Side mounting this antenna to a tower is not recommended as it will severely affect the feed impedance and radiation pattern of this antenna. The completed antenna assembly should be mounted as high as practical and in the clear at least several wavelengths from other objects as would be normal installation practice with this type of antenna. After mounting is complete and your coaxial cable** is installed you are ready to use your new, omni-directional antenna boasting a gain figure of 6.5 dB.
I have constructed two of these low cost high performance antennas to-date: one using the direct solder connection method and the second using the bulkhead connector for cable connection. My preference is using to bulkhead connector for the slight additional cost. There is also good argument to support direct solder connection as well. This choice is up to the builder’s preference. Both antennas built performed in the exact same manner right out of the box without any attempt to make any adjustments which I do not believe you will find necessary.
Using an MFJ-259B analyzer both antennas indicated their resonate point at approximately 146.40 MHz. Their VSWR did not exceed 1.5 to 1 across the 2-meter band! On-the-air tests comparing this antenna to a unity gain ground plane antenna (my test standard), a 5/8 wavelength ground plane antenna produced better than expected performance. The tests were conducted by mounting each antenna, one at a time, to a twenty-foot (5.5 m) tall antenna test stand and obtaining on-the-air readings from several stations involved in the test. The receiving stations ranged between twenty and forty miles (32 to 64 Km) distant. All methods of test provided evidence that the newly constructed Radio Shack (Tech America) antenna greatly outperformed the unity gain or 3.3 dB gain 5/8 antenna models. A J-Pole antenna was also tested and fell in line just below the 5/8 wavelength ground plane. I was also able to work through the Dolan Springs Arizona repeater which is located about forty miles from my home, behind a mountain, that I had never been able to use in the past and was not able to raise using my 5/8 wavelength gain ground plane. This station cannot even be heard at my home QTH on the unity gain ground plane!
Conclusion
In the last two months that have passed since the construction and use of these antennas they have consistently proven themselves to be well worth the $35 or so I have invested in them. Due to their original design for mobile use they appear to be able to hold up very well in heavy winds. My maximum wind speed to date this time of year has only been gusts of 45 MPH and the whip like construction of the radiating element just bends with the wind and springs right back.
Technical Notes
Use care when selecting the brand and type of coaxial cable used with your 2-meter or higher frequency amateur radio station. “All coax is NOT created equal”! There are even great differences from one manufacturer to another on cable that has the identical part number. Stick with name brands such as Belden and others of similar quality. Your coax feed line is no place to pinch pennies and short-change your station performance. From 144 MHz on up in frequency never use RG-58 unless it is a very short length such as needed in a mobile installation. At 2-meter frequencies never use more than 20 to 25 feet (6 to 7.6 meters) of RG-58, RG-58 Foam or RG-8X. If possible avoid them completely. Use only good quality RG-8 Foam as a minimum. Belden 9913-F will show a great improvement over RG-8 Foam cable due to its lower loss characteristics. Please remember, don’t throw away some or all of the antenna gain this project will provide for you in your coax line. Also keep in mind that a 3 dB loss in your coax cable will cut your transmit power to one half and reduce your receive sensitivity by that same 3 dB figure. So while you are installing this new antenna think about treating yourself to a new roll of high quality, low loss, brand name coaxial cable.
Reduce Your RF Noise
This antenna is designed as two 5/8 wavelength antennas co-phased one above the other which accounts for its high gain. The antenna is fed directly by the 50-ohm coaxial cable and is not “Shunt Fed”. This means that the radiating element is NOT at DC ground potential which may allow the noise level, under certain circumstances, to be higher than if the radiator was at ground potential. There would also be some additional lightning protection gained by having the antenna at DC ground potential.
Fortunately this problem can be overcome through the addition of an easily constructed choke mounted under the “L” bracket. However, to use this technique, you must select the SO-239 as your mounting connector, not the UHF barrel connector. This will allow the RF Choke, to be described, to be soldered directly under the “L” bracket between the center conductor and the braid (ground) soldered connections. To construct this RF Choke close wind 12 to 14 turns of #14 solid insulated wire around a .5-inch (1.27 cm) form (such as a drill bit shaft of that size). After removing the coil from the winding form wrap two or three loops of waxed lacing cord, fishing line or other method to hold the coil close wound. Strip and tin the ends of the choke back about .25 inches (6 mm) and solder in place as described above. This modification will only show the most minor effect on your VSWR reading if any at all but will bring the antenna radiator down to DC ground potential with its obvious advantages. In my installation, boasting a very high level of power line noise, there is a significant reduction in background noise with this modification in place.
2 Meter Mobile Antenna
(From radioshack.com) Catalog No. 940-0674
This is a Premium mobile antenna constructed to exacting standards. It has the same quality as other national brands at significant savings and fits UHF mounts. 144-148 MHz Frequency Range, Gain: 6.5dB, Length: 96.9″, Maximum Power: 200W. To order call: “800-THE-SHACK”
Parts List
Antenna, Mobile, 6.5 db gain | Radio Shack P/N 940-0674 | Used as received |
4 ea. 1/4 inch x 20 SS Bolt | Mounting Hardware | Stainless Steel |
4 ea. 1/4 inch Flat Washers | Mounting Hardware | Stainless Steel |
4 ea. 1/4 inch x 20 Nuts | Mounting Hardware | Stainless Steel |
4 ea. 1/4 inch Lock Washers | Mounting Hardware | Stainless Steel |
4 ea. 21 inch Lengths | .125 inch (32 mm) Brazing Rod | (Other materials may be used) |
4 ea. 5 x 40 .5 inch (1.27 cm) | Stainless Steel Screws | (When using the SO-239 conn.) |
2 ea. “U” Clamps | Sized to fit your mast | Stainless Steel |
1 ea. “L” Bracket | See Construction Detail | Aluminum or Stainless Steel |
1 ea. UHF SO-239 Connector | See Text For Details | UHF Feedthru may be use |
20 inches of (50 cm) Insulated Solid Copper Wire | See Text For Details | Only needed if RF Choke is desired |
Originally posted on the AntennaX Online Magazine by August Hoecker, W8MIA
Last Updated : 18th May 2024