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 The VHF Quad Beam


My Homebrew, 4-element VHF Adjustable Antenna

Closeup of antenna
Four Element VHF Quad
center frequency 146Mhz
T

his is the VHF beam antenna that I used to work in my attic at another location. It is built entirely of wood. There are four diamond shape loops which are stretched apart by wooden dowels. These "crosses" rest on top of two pieces of wood (joists) through which are placed bolts with wing nuts. You may adjust, or tune, the beam spacing by moving the loop elements and then tightening the bolts to hold them in place. This allows easy adjustment until you have the beam tuned up and working properly. (Notice the small blocks that are clamped by the joists, and which also attach to the loop crosses. See below for more details...)

This beam is turned by a rotator and can move 360 degrees. It was normally aimed to the North and was "looking at" Tewksbury and Andover, MA. The frequency was 145.630. It is currently not in service, but am hoping to re-activate it as a "roving" test antenna . (I have never measured the antenna for gain, but it should yield at least 6db and possibly a bit more.)

NOTE: For a broader discussion of antennas used in packet radio, please see Packet Radio Antennas. Here many types of VHF and UHF antennas will be listed...



Construction Notes

The inspiration for this homebrew project came from the The ARRL Antenna Book, the 1997 Edition. The article was entitled "A Portable 144-Mhz 4-Element Quad," pages 18-30. This looked like a great antenna design, and as it turned out, I needed an indoor attic antenna but only had limited space for the beam rotation. So the relative compactness of this design (four elements) caught my eye.

NOTE: The above article does an excellent job of describing the suggested parts and methods of assembly, so I won't try to summarize, but rather will point out where my approach differed.

Electrical Parameters:

Just in case you can't get a copy of this article, here are some of the electrical specifications for the wire lengths: the reflector, the driver, and the two directors. Three formulas specify these lengths in feet:

To activate the VHF 4-element Quad calculator, press: and enter the center, or mid-bandwidth, frequency that you want for the antenna.
  • reflector = 1046.8 / f MHz

  • driver = 985.5 / f MHz

  • director(s) = 973.3 / f MHz


My spacings are |ref-------17"------|drv------13"-----|dir1-----11"-----|dir2

It tunes up quite well on 146 with an impedance of 50 Ohms. (Note: the antenna is transmitting to the right with the reflector on the left and the two directors facing to the right.)

It is recommended that you tune the driver to 146 Mhz since it is fairly broad in its bandwidth about 2 Mhz at the 1.5 SWR points. The article also suggests that you space the elements between 0.14 and 0.25 lambda. It does not give any hard and fast distances for these spacings. That is one reason I made the elements adjustable along the boom! You may have to experiment and test with an SWR meter to determine your best spacings and impedances. It took me a while to get the tuning right on this antenna, delving into other books on quad design, but I did finally get it right onto resonance!


 
 Additional Working Notes:

What is the point of this "adjusting?" If you have been working with antennas for a while, you know that the length of the driver element is what determines the center resonant frequency. Changing this length is the only way to "tune" the antenna. The design, as it stands, is not tuneable after the wire has been cut. So when we move the elements on the beam, what are we changing? Two parameters:

     1.  The Feedpoint Impedance
     2.  The RF Gain of the Antenna

and, the resonant frequency remains uneffected unless we add a tuning or matching device or re-cut the driver. It's quite evident why more gain is always a sought-after commodity, but why "fiddle with" the impedance?

Well, as we know, the right impedance match does effect the rf output since a "bad" match means heavy losses in the feedline. Any gains in the antenna could be offset by those feedline losses. Or, suppose you want to feed the loop with 75 Ohm line or something unusual. Then you can keep adjusting the element spacing to obtain the best SWR on your meter. (You might even be able to find a match for 300 Ohm low-loss TV line! Lower impedances are more likely though...)

As you work with this antenna, it becomes clear that there is a relationship between the right impedance match and gain. When the impedance "locks in," i.e., your lowest SWR reading, you are usually at maximum gain...



If you are really ambitious and have room for a 6 foot boom, try the and enter your frequency. (This is purported to have a gain of 11db's!)
  • reflector = 1071 / f MHz

  • driver = 998 / f MHz

  • director(s) = 973 / f MHz


NOTE: I have not constructed this antenna! This informative article was published in QST Magazine, the January 1995 Edition. It was entitled "A Five-Element Quad for 2 Meters" pages 67-69, written by Jim Reynante KD6GLF, and seemed to be a very good design with amazing results! It may offer some added insights when buiding your own VHF quad, no matter how many elements you decide on.

Comments: Each subsequent director after the first one is 3% shorter in total wire length than the previous one. The "fixed" spacings, as specified in the article, are:

|ref-----17"----|drv-----13"----|dir1----16"----|dir2----19"----|dir3

(If you employ the adjustable "joist-boom" approach as described here, there is no reason why this 5-element antenna should not work just as well as the 4-element one; and you will have the added fexibility of adjusting the feedpoint impedance right where you want it, as well as having a high gain antenna that is actually very portable!)

The Support System:

Two requirements became evident when designing this antenna:

  1. Variable Tuning (Impedance & Gain)
  2. Compact Size

I wanted to move the elements relative to the beam! This would permit a kind of "tuning in place" while keeping the beam fixed above its rotation point in a very limited space. This led to the idea of sliding the elements on two "joists," which could act like a clamp, until the best tuning position was found. Also, the entire set of elements should be able to be moved forward or backward to allow for rotational clearance.

The Pedestal or Base of the Beam Mount
Pedestal mount supports beam "joists"
clamped by a center wing nut
The "pedestal" mount seen here to the left, where the vertical support intersects the joists, also allows for some adjustment of the placement of the entire joist assembly. This pedestal, or mount assembly, sits on top of the vertical pole support which is attached to the rotator motor at its base. At the top of the mount, there is a small "stud," sticking up about an inch and a half, that fits between the joists from the underneath side. The beam's position can be adjusted via several holes along its joists and then bolted into place. (In this illustration, there is only one other hole to the left of the center, bolted hole.) And, I used diamond-shaped elements instead of the square shape since the horizontal dowels could also rest on the joists adding more support to the elements.



Closeup of antenna
Close-up of the block/slider
showing its connections to the dowels
which form the struts for the wire support system
Small blocks fasten the dowels where they cross and form "sliders" that the joists can clamp onto. The close-up photo on the right shows one possible solution to this problem of affixing a rectilinear shape to a circular shape. In this example, I drilled through the block/slider the long way to hold the vertical dowel in place (with wood glue.) It was a tight fit, and there is a flathead nail visible near the back that "pinned" the dowel in place while it was gluing. The horizonal cross piece sits in a groove with a depth of about half the diameter of the dowel and is also held by a nail, offering both stability during gluing and a little extra grip to keep the dowel secure. This is just one approach to the problem. I also tried making a block using only grooves to avoid excessive weakening of the material, and again lots of waterproof glue. (A typical size for this block is about 2 inches long by 5/8 of an inch thick, and about 1 inch for the bearing surface. Use whatever you think is going to be appropriate for your strut strength. If you were scaling this design up to, say for example, a 6m antenna, then much larger blocks would be needed...)

In addition, the antenna wire itself helps to stiffen the dowels making the strut elements more rigid. Here is an overview of the entire antenna in the illustration below...


A Side View of the Adjustable Quad Antenna

Side view of antenna
Note:  The antenna's transmit direction is to the left...

Once I had approximated the locations of the elements, I used three (3) bolts to clamp the joists together. You need to drill holes about every foot or so, to get the right pressure on the blocks/sliders. Sometimes you have to drill a "special" hole if it conficts with an element spacing block...

You may build this antenna "support system" out of any size wood dimensions that you like, or that is convenient, i.e., material you have laying about. I used two pieces of strapping for the joists about 4 feet long. The wooden dowels are 3/8 inch diameter. Again, you may choose to make it much more rugged. I also did not weatherproof the structure since it was intended for indoor use. The "mounting blocks" were cut from the strapping stock. The vertical pole was either closet rail or a broom handle. This is really improvizing!

Final Touches:

If you intend to use this antenna outside, and once you have it tuned up to your satisfaction, you should drill the mounting blocks and bolt them in place. You may leave the other bolts in as well since this will add stability under wind loading and rotational torquing. And be sure to weatherproof it completely since wood ages quickly when exposed to the elements.

And a closing observation. I just happened to use wood because it was handy and easy for me to work with; but selecting appropriate materials, such as light hard woods, plastics, or aluminum fixtures, or experimenting with new non-conductiong materials, could produce a very light-weight, rugged quad that would be a great antenna to add to your "farm," or one that would make a great portable emergency unit, which could be broken down and re-assembled very quickly...



(Courtesy KBNorton Computer Systems)