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Introduction

Making a simple antenna for 40 Meters is not very difficult. That is, if you have the space. A standard center fed dipole dipole for 40 Meters needs around 67 Feet of space. But, what if you only have space for a 20 Meter dipole, 33 Feet? If this is case, than you have several options.

  1. You could just forget about 40 Meters and work the higher frequency bands, 20 Meters on up.

    What? And miss out on all the fun dodging the the short wave broadcasters in the evening.

  2. You could create a Inverted-V type of antenna and raise the feedpoint on a mast.

    This is a possible alternative, but for this particular case, you would need a 28 Foot center mast and the apex angle would be less than optimum. This may cause some signal cancelation and give you a radiation pattern that you don't want.

  3. You could shorten the dipole arms to fit the space and use a loading/matching coil in the center.

Shorty 40 Antenna

Item number 3 is what this page is about. Jact Sobel, W5VM (which is now assigned to Vernon Dyer), had at one time described a shortened dipole center fed with a loading/matching coil at the feed point. A drawing of which is below.

Initially, this seems to be a different approach than the shortened dipole designs, detailed on my Short Dipole page, but it isn't. If you were to design a short antenna, with the Short Dipole page, using a distance of 0 for B it will come up with a design that requires two inductors. With the B distance being 0, the inductors are butted up against each other forming one large center inductor. The only real difference is that a standard shortened dipole keeps the inductors separate and has a balanced feed point. On the other hand, the Shorty 40 ties the inductors together and feeds the antenna directly from un-balanced 50 Ohm coaxial cable.

Assuming that the two coils are an equal number of turns, and that the wires attached to each side are equal in length, the center of an antenna should be a zero voltage point. This makes a handy place to tie your coax shield. You could wrap several turns of wire around the coil in the center and feed it that way. But I couldn't begin to tell you how many turns to use or what the feed impedance would be. Each turn of the coil, as you move away from center, provides you with a different impedance and a possible match. By attaching the center of your coax to one of the coils turns, you should be able to find a good 50 Ohm feed point. This then gets around the balanced to unbalanced conversion effort (balun), that would be required and you were center feeding or link feeding..

Each element arm is 18 Feet 6 Inches (5.638 M) long. The loading/matching coils consists of 30 turns of #12 SWG (#10 AWG) enamelled copper wire wound on 2.5 inch (63.5 mm) diameter PVC tube 6 inches (152.4 mm) long. The winding pitch should be about 6 turns-per-inch (25.4 mm). Although the picture doesn't show it very well, the shield of the 50 coaxial cable is connected to the center of the coil. The coax center conductor is connected to a point 2 or 3 turns away from the center, to a point which gives the lowest SWR. This point may take some experimenting, depending on which section of the band you wish to operate in.

Other Bands - Other Inductors

The approach for the Shorty 40 is not limited to 40 Meters. Nor is it limited to the loading coil specified in the section above. The same approach can be applied to a 40 Meter antenna with a different different wire, different length, and a different loading coil (diameter, turns, length, etc.). Perhaps you need a portable antenna for 60 or 80 Meters, but don't want to pack a lot of heavy wire. The calculator below can possibly help you with your design.

This antenna is a dipole with lumped-constant loading. At heights of 30' to 100' (10 to 30 meters), it will work well, as long as its physical length is longer than 0.2 λ. For maximum efficiency use the largest practicable diameter wire for both the loading coil and the dipole legs.

Start with entering the Center Frequency and your Available Space (Length A).

If you enter a length that is too short, for efficient operation, the Minimum Length will be highlighted in RED. You can still use the antenna but it may not be very efficient.

PVC Pipe
Pipe Size Outside Diameter
3/8"0.675" (17.145 mm)
1/2"0.840" (21.336 mm)
3/4"1.050" (26.670 mm)
1"1.315" (33.401 mm)
1-1/4"1.660" (42.164 mm)
1-1/2"1.900" (48.260 mm)
2"2.375" (60.325 mm)
2-1/2"2.875" (73.025 mm)
3"3.50" (88.900 mm)
3-1/2"4.0" (101.600 mm)
4"4.50" (114.300 mm)

If you enter a length that is greater than the space required for a full size antenna, the Full Size listing will be highlighted in RED. This indicates that you don't need a loading coil. Another indication that the entered length is too long is, the calculation for the loading coil will go negative, and all the rest of the loading coil information will be useless.

Just about any diameter coil form can be used but you should choose the form size so that the coil length does notexceed twice its diameter (L/D Ratio <= 2.0). If you enter a diameter that creates a coil with a L/D Ratio > 2.0, increasing the diameter will decrease the L/D Ratio. At the same time, increasing the diameter will reduce the number of turns required. On the right is a small table that lists the PVC Pipe sizes and their actual outside diameters.

Its OK to use insulated wire for a close wound coil, but avoid close-wound coils of enameled wire. You can also use bare wire that is spaced at its own diameter.

The equations behind the calculator, initially calculate optimum coil dimensions. It then rounds the Number of Turns up, to the nearest even number, and recalculates. This makes the ends of the coil turn up on the same side of the coil form. And it makes it easy to find the exact center for attaching the Coax Shield.

Connect the Coax Shield to the center of the coil and then offset the Coax Center Conductor as many turns as required, for best SWR.