2 Meter 3 Element Quad
Direct coax feed!
The Quad antenna, sometimes called the Cubical Quad, is an antenna which is used by many amateur radio operators. We owe it’s development to Clarence C. Moore(patent no. 2,537,191), W9LZX, (SK), an engineer at HCJB,
(a shortwave missionary radio station in the Andean Mountains). He developed it to resolve issues with large coronal discharges in the thin air with a beam antenna. The quad was born!
Moore describes his antenna as “a pulled-open folded dipole”.
The 3 element 2 meter (designed for 146mhz) quad in this project has a very small footprint (boom length) of about 32 1/4 inches and according to the computer guru’s out there, it has about 9.45dBi gain with only three elements.
9.45 dBi equates to 9.45 – 2.15 = a whopping 7.3 dbd gain with over 20db front to back ratio and a 2:1 swr bandwidth of 3.9mhz!!!!!!!
Let’s see a standard dipole do that!
So with 3 elements on a 32 inch boom….you’re more than multiplying your effective radiated power by 5 times! If your 2 meter rig is getting 50 watts to the 3 element quad (assuming no line loss)…the station on the other ends “thinks” your putting out over 250 watts! That’s hard to beat on such a short boom and only 3 elements. If your rigs puts 75 watts to the antenna, that’s a whopping 375 watts + erp! More than enough to do some serious repeater “Kerchunking”. Put a 5 watt handheld on this antenna and it acts like over 25 watts erp out!
The standard 3 element yagi gain is about 4 to 5 dbd and is usually longer in boom length compared to the quad and sometimes difficult to tune for best performance.
Other than the gain for it’s size and unlike most quad designs….
This 3 element 2 meter quad design has a major plus going for it!
The feed point is about 53 ohms….a great match for direct feeding with 50 ohm coax…no baluns, no fuss, no muss.
So how do we build it?
There are many ways to build a quad antenna and the difficult part of constructing the boom, attaching the support arms, etc will be left up to your ham ingenuity. Most builders of VHF quads use PVC or wood as the boom, and wooden dowels, PVC, fiberglass, or some other non-conductive material for the spreader (support) arms. Wire is used for the element loops.
You be your own judge and build the antenna out of material that suits your construction skills and talents….experiment.
Below are the exact measurements for building the 2 Meter 3 Element Quad.
Refer to the drawings and the Quad.BAS screenshot supplied below for the layout and lengths.
To convert to inches, multiply by 12. Design above for highest gain at 146mhz
ELEMENT LENGTHS FOR #14 WIRE AT 146MHZ:
This design only presents a good match to 50 ohm coax using #14 wire and using the spacings and lengths below….do not change unless you want to experiment! If you want to experiment with the design program, see details below.
Reflector total length in inches = 86.76 inches (86 3/4 inches)
Driven element length total inches = 83.05 inches (83 inches)
Director element length total inches = 79.33 inches. 79 3/8 inches)
Reflector to driven element = 14.25 inches
Driven element to director = 18 inches
Spreader arm lengths:
If you can put any faith in computers, the lengths were calculated:
Reflector spreader bar = 30.67 inches
Driven element spreader bar = 29.36 inches
Director element spreader bar = 28.04 inches
BUILDING NOTES: Allow yourself a minimum of 31 inches from tip to tip for the spreader supports. If that should happen to be too long, you can always cut off to fit when done. Remember this is from one end of the “+” thru the boom to the other end.
It is best to layout the individual loops for the quad in an exact square pattern so they won’t move. Just take the element length from above, divide by 4 for each side of the “square”. Then measure from one corner to the other and add an inch or two. Don’t “cut” yourself short!
Do not change the total length of the loops unless you have some really strange swr readings when done. Do not change the element spacing on the boom!
According to the calculations, you should have an extremely good match to 50 ohm coax by direct feeding.
You must fit the supports (spreader bars and boom to the length of the loops…not the other way around. The size wire, lengths and spacing of the elements are fairly critical for the near 50 ohm impedance of the feed point. Your measured impedance may be slightly different due to your construction techniques, etc, but you should be close according to the program used. See below for the program information and a source to download it. It is loaded with other interesting design programs.
Do not allow the wire elements to sag. They must be secure and in as near a square pattern as possible on the spreader bars.
The quad can be fed as in the drawings below for vertical polarization suitable for FM repeaters, mobiles, etc….or…..turn the feed point to the bottom or the top for horizontal polarization for ssb work etc. It makes no difference which side the feed point is on for vertical polarization.
Quad shown above in Vertical polarization configuration
Note: The quad can be mounted with the top and bottom sides horizontal to the earth, but then it still must be fed from one vertical side in the center of one side for vertical polarization or from either the top side or bottom side for horizontal. This makes for difficulties with supporting the feedline and extra measures must be taken to prevent strain on the coax connection to the loop. Either way you mount it. it should perform the same.
Feed point end detail on spreader bar support
Attaching the feed line for the driven element:
There are many ways to attach the feed line to the driven element while allowing good support for the feed line and also the wire loops on the end.
One example above on the driven element is to drill 4 small holes big enough for the loop wire to pass thru and twisted back on itself giving some strain relief. Then the coax center and shield is extend out just enough to twist onto the loop ends, soldered and then the coax ends and the solder connections sealed from the weather. The feed line can be nylon tied to the spreader bar, taped, etc. In doing this, remember to keep the exact lengths of the wire loops unchanged. The length of the radiator loop , (the driven) starts at the coax connection center conductor, continues around the loop and attaches to the shield. Don’t leave three or four inches of center conductor or shield extended before it attaches to the wire loop. This will detune the antenna because this added length becomes part of the radiator!
The other element loops can be simply run thru a hole drilled all the way thru the end of the spreader bars at the appropriate distance from the boom while maintaining the exact loop length for each element and the square shape of the antenna. Attach them securely to the spreaders, twist together the ends and solder. You should have 4 equal length sides on each element of this 3 element quad. Looking at the finished quad from the side should look something like this…. |-|-|
You may also want to add a small 1:1 coax “air” choke consisting of about 3 to 5 turns with the end of your coax feed line wound into a coil in series with the feed line near the feed point to help eliminate feed line radiation similar to the “Ugly Balun”….your choice. It might be advisable to us a short length of PVC heavy enough to support the finished antenna mounted to the boom as a “mast” extended below the antenna, then attach it to your final support mast. This keeps any metal mast out of the field of the antenna which could possibly distort it’s pattern. Use a rotor if you need to “aim” it.
You should not have to “tune” this design….unless the computer program has errors in the calculation results. The lengths above were taken directly from a GWBASIC program called: