Here is a simple antenna calculator for two popular forms of ham radio HF wire antennas: the horizontal dipole and the inverted "V".
Enter your desired frequency (MHz) of operation (i.e. 3.55). If you have no particular preference within a given ham radio band, then simply enter its center frequency (i.e. 7.15 for the 40 meter band).
To fully understand the results obtained by this calculator, please take a few minutes to read the explanation below it.
Here is how to interpret the resulting wire lengths given by the calculator.
Be prepared to trim the ends of the inverted V dipole if the final frequency of resonance ends up being too low for your needs when the inverted V is installed in its permanent position.
The most widely used formula to calculate the approximate overall length of wire required for a dipole is:
468 / frequency (MHz) = length of wire in feet.
The antenna calculator above uses this formula as a starting point to calculate wire lengths for the dipole. The results are conveniently displayed in inches, centimeters, feet and meters.
This formula to obtain the length of a half-wave dipole antenna will give a good ballpark value to start with.
However, the actual resulting frequency of resonance and feed-point impedance of a dipole will depend on:
When each side of a dipole slopes down from the feed point, it is commonly called an inverted V.
The inverted V results in:
Some say that the inverted V should be cut 4-5% shorter than the dipole! But if one were to actually do that, the inverted V would resonate at an even higher frequency!
The formula used by the calculator to compute the wire lengths for the inverted V is based on the formula for a half-wave dipole. It is adjusted to take into account the special characteristics of the inverted V.
In the case of the inverted V we must add - to the list of environmental variables influencing the half-wave dipole - the angle between the two legs of the inverted V.
The angle below the two sections of a horizontal dipole is 180 degrees. As the two sections of the dipole are lowered below the feed point, the angle between the two legs decreases:
NOTE: if you start by giving the calculator your *desired* frequency of operation, the inverted V - when installed in its final position - may be still end up too short or too long, depending on the environmental conditions mentioned above.
If the angle between the two legs of the inverted V becomes less than 90 degrees, the radiation patterns from each leg of the inverted V begin to interact and cancel each other to some extent.
Therefore, the angle between the two legs of an inverted V should not be less than 90 degrees.
Remember, an inverted V requires slightly more wire than a horizontal dipole for a given frequency of resonance.
The antenna calculator above will provide appropriate wire lengths for the inverted V and the dipole for a given frequency of your choice.
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