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Website updated: June 25, 2008  1:23 AM.

Copyright 2008 Charles Elliott's Web Site & Content - All rights reserved.
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Click for Big Stone Gap, Virginia Forecast

 

KF4VDF's Antenna's


Quads, Beams, Wire Dipoles
J Poles.....and others.

 

 

 

Dipoles

There exists many variations to the half wavelength dipole, I will describe a couple of them on this page. They are the Inverted "V" and the Half Wave Sloper. I am not going to discuss how these antennas function, I will however describe what they look like and how they are built.

Although the antenna lengths can be determined by using the same equation as a dipole (468/Frequency in MHz), you will find that the lengths are actually a bit too long. This works fine though since you can just trim each side back to reduce your match.

Here is an example of how to use the formula. For the 30 meter band, the frequency that you might select is 10.125 MHz. Using the equation above, you will find that the total length of wire required is 46.22 feet (468/10.125), this is about one half of a wavelength. This length is then divided by two (23.11 feet), giving you two quarter wavelength sections. Each section will make up one half of the antenna and they are connected at the center by an insulator to the two antenna wires. Soldering and weather proofing the connections will provide for a solid connection and long life for the antenna.

As with most antennas, these antennas can be supported at the center with anything that may be handy: a mast, a tree, a tower, or any other structure. Some sort of insulator must be used to isolate the antenna wire from the structure itself.

Each antenna may be fed either with coax or ladder line. If you use the formula for the half wave antenna, you will be able to use 50 ohm coax without any matching device.

 

The ends of the wire are not placed directly on the ground, they should be suspended several feet above the ground (10 feet or more to prevent anyone or anything coming in contact with the wire.) A peg and a length of rope can be used to anchor it to the ground, as illustrated below.

Results from both of these antennas are very good and many amateur radio operators throughout the world have used these antennas.

 


The Inverted "V" 
is exactly that, the antenna looks like a upside down "V" when erected. Each wire should be at a 45 to 60 degree angle from the center point. Illustrated below is an inverted "V" antenna.

Half Wave Sloper

Half wave Sloper is also a very simple wire to construct. The feed point for this antenna is at the center insulator. The wire slopes to the ground at about a 45 degree angle. Excellent results can be obtained from this antenna. Illustrated below is a half wave sloper.

 

Design Your Own Dipoles And Inverted Vees


Most people are familiar with the center fed, half wave dipoles and inverted vees that are so very popular. But did you know that there really is no exact formula for computing the length of these antennas? The problem is that there are several factors that affect the resonant frequency of any antenna (including these two types.) Some of these factors are: the height above ground, the diameter of the wire, nearby structures, the affects of other antennas in the area and even the conductivity of the soil.

If you've ever played around with some of the antenna design programs you may realize that the formula for these types of antennas vary from about 476/f Mhz to 490/f MHz depending on the band and the height above ground!

Fortunately there is a standard formula that can be used as a starting point in your design. For a center fed, wire dipole, this formula is 468 / frequency in megahertz. Although, at my location I've Always have had to shorten the antenna in order to obtain a 1:1 match.

At least in theory, an inverted vee should be about 5% shorter than a dipole at the same frequency.

This page uses the standard formula, 468 / f MHz to calculate dipole lengths. You may change this number if you feel you know a better number to use as a starting point.


Enter the formula for the antenna calculation

Divided by Freq MHz

Percent smaller for the Inverted Vee

Your dipole's total length is feet
Each leg of the dipole is feet

Your Inverted Vee's total length is feet
Each leg of the Inverted Vee is feet

 

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