902 MHz Quagi

Art W0BA

I have heard there will be a transverter kit released on 902MHz soon. Antennas for this band are not particularly plentiful so here’s a 10-element quad-yagi (Quagi) project that stands on the shoulders of giants in the antenna design space.  Dave (W6OAL) and I have had success with close spaced (0.125 wavelength) quad reflector to quad driven element spacing and wide spaced yagi directors.

This antenna is constructed on an aluminum boom 3/4 inch square and four feet long.  Unlike many proponents of Quagi construction I use metal booms. They tend to weather well and take hardware more solidly. It is small and light enough to end-mount, so we will be leaving enough boom material behind the reflector to attach your U-bolt or other mounting hardware.  Mounting at the end is desirable for horizontal or vertical polarization, and running the coax feed along the boom and out the back of the antenna works best.

Parts list:

1 X Square tube 1/16″ thick 3/4″X3/4″X4′           $15.95

8 1/8″ diameter brass or aluminum rods 6″ long            $8.80

3′ #14 Solid copper wire (can be insulated)         $2.12

2′ 3/8 wood dowel or arrow shaft                         $4.00

8 Rust resistant 3/8″ machine head wood screws           $1.79

Directors are cut from the brass or aluminum rods, and must be close to the specified length.  Use bare (or insulated) copper #14 solid wire for the Reflector (R) and Driven Element (DE) so you can solder directly to it. Lengths in the table are in inches.  Spacing of the elements on the boom are measured from the reflector.

Use a drill press to make the job of aligning the elements easier. That’s another reason to use a square boom. This can be, and has been, done with a hand drill and a step bit. Fasten all the elements in the boom with small screws (Stainless steel would be best).  I used some anodized screws from my miscellaneous box of hardware.

All right then, here we go;

902MHz is 33.236cm (1.0904′)(13.0848″) and utilizing a 5% increase for the reflector we get 13.73904″. Call it 13.75″. Cut your loop reflector for 14″ so you can overlap the ends 1/4″ and solder them together. Form the director into a square and you will find the square root of the sum of the squares still works to find the element support length of about 4.85″.

Call that 5.25″ to give us some additional length to attach the reflector. The element supports (spreaders) are non conductor 3/8 wood dowel or, as I did, old fiberglass arrow shafts. Here are the components of our first element (the reflector) for your viewing pleasure. Do not attach it to the boom at this time.

We are off to a great start so let’s get into the meat of the project with the driven element. The Driven Element is #14 solid copper wire so you can solder the coaxial cable or connector directly to it. We will be cutting this element precisely to the specified length and stripping the ends to allow a connector to be soldered in place. Since we will be working in the diamond orientation (to allow the more durable two support (spreader) configuration) and feeding the driven element at the bottom care should be taken to have the ends at the corner of our driven element. Cut another set of 5.25″ support spreaders for the driven element.

Next we cut the 1/8″ diameter directors to length. I generally use a big pair of diagonal cutters for aluminum or the score and snap method for brass.

(Yes, I used recycled aluminum elements. When they are polished and have their ends rounded no one will know. . . .chuck them in a drill and bring on the 220 grit. . . )

Let’s turn our attention to the boom. Since this is the most expensive component of the antenna measuring twice is a good idea. Six inches from the end is a good place to mark the reflector as it will leave plenty of room for a U bolt or mounting plate.

Element lengths and distance from the reflector are in inches. You can use a tape measure for this. Just strike off an inch and add an inch to the measurement to be as accurate as you can. Better accuracy translates directly to better performance. After marking your distance from the reflector run a scribe line along the boom or accurately measure 0.375″ at each point to make your punch mark for each element in the center of the boom. The driven element and reflector have two holes each. One on the top of the boom and the other on the side to accommodate the spreaders. Place these two holes on either side of your mark with just enough boom between to allow the wire element to be suspended between them right on the mark.

Time to break out our trusty hole maker. I’ll use a drill press for all the pilot holes. One of the most obvious signs of a “homebrew” antenna is elements out of alignment. Don’t worry if you only have a hand drill. You can do it. Just be careful and measure both sides of a given element hole to assure it is the same distance from the edge of the boom as its mate on the other side. Drill and deburr the element holes just big enough to fit your elements and spreaders. Then put a smaller (determined by the size of your anchor screws) bit in your drill and make your director anchor screw holes on the side from the element holes (90 degrees from them) such that the screw digs into the director and keeps it in place. (Don’t install the elements or anchor screws yet though, wait for it. . .)

This part is optional but I like to do it. It’s time for a bit of finish. Chuck the linear elements in your hand drill and take some steel wool to them or use a finish sander with 220 grit paper in it. Make them shine. Make the boom shine too except it will require more elbow grease and probably a Fosters as chucking it in a drill won’t work. You won’t have another chance to conveniently put a shine on your antenna. We are really close to finishing the antenna and you will want to show it off to visitors to your Shaque. Oh yes, if a polished boom is not your cup of tea you can put some 220 grit sandpaper in your finish sander and go over the boom to give it a brushed aluminum finish. That’s what I did as I was out of Fosters. Here are the parts ready for assembly.

I started with first. ‘slid them into place and measured to assure they were centered. Then marked and Gorilla glued them.

After about 45 minutes the glue set up the directors are installed and anchored with the screws. Here’s a picture of the antenna from the bottom. The screws could be cut off to make an exact fit with the screw head flush to the boom, but I didn’t.

The horizontal spreaders are now installed and Gorilla glued in place.

Don’t worry about the glue expansion. We will clean that off with a razor blade later.

Now attach the director to the spreaders such that it is suspended between them right on the mark that we scribed on the boom a while ago. I used number 22 solid wire to tie the director to each of the spreaders at the corners creating a diamond shape with respect to the plane of the antenna. Another method is notch the REF & DE spreaders, place a small hole behind the notch, run a length of magnet wire through the hole and secure the element wire on each side of the notch.

The driven element is connected the same way with the open end at the bottom of the diamond. Now you are ready to connect your pig tail (coax with a connector on it) to the open ends of the driven element. With the measurements given my antenna was 1.1:1 on 902.1 and 1:1 at 901.8. Remember, your pig tail connections will add to the length of the driven element and you may need to trim them as close as you can to keep the DE resonant at 902.1. Here is a picture of the DE and R portion of the antenna after initial testing.

Clearly there is cleaning up to do and refinement of the attachments to the spreaders. However, at this point we have a high performance antenna that is ready for on the air testing. SWR is 1.1:1 at 902.1. and WAG gain is 12dB. The SWR profile is shown below.

The degree of refinement and addition of mast or tower mounting hardware is left for the builder to customize to their particular taste and installation.

Here is the length and spacing chart.

Route the coaxial cable along the boom to the rear of the antenna.  Be sure to seal the antenna end of the cable to keep out moisture.

On the air testing went well. Signals 80 and 40 miles distant were armchair copy. Aiming was fairly demanding indicating a narrow beam width, somewhat surprising for a 10 element antenna. Front to side and front to back rejection was also impressive. The hybrid close spaced quad reflector and driven element with wide spaced linear directors design proves itself once again.

I look forward to hearing you on 902.

Note: If this project is beyond the scope of your workbench or time to apply to such things, Dave (W6OAL) is the proprietor of Olde Antenna Labs and would be pleased to give a quote to build this antenna.