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Overview

This calculator can be used to design a quarter wave ground plane antenna with radials. Based on rothammel dl6wu you may want to combine it with a folded dipole. But it s not for an antenna on pcb. This range is commonly used for wireless systems 802 11b g rfid systems satellite dish antennas dish feeds and beacons. For information on connecting a biquad antenna to a wireless radio, have a look at the page on using wireless antennas. When using a biquad to establish a link to another wireless device, you should ensure the polarisation of the biquad is the same as the antenna you are connecting to.

This sector antenna was made from a piece of thick copper wire formed into a 'bowtie' shape (the biquad - with sides 32 millimetres) and soldered to a round N-type connector. The N connector was then screwed into a steel disc about the size of a CD (Compact Disc).

The original prototype (shown) is still in use two years later, and has survived severe storms.

Construction

The connector - N-type

N-type socket round 'through bulkhead' came from R.S. number 112-0773. Packaging says Telegartner (Manufacturer presumably) Type N-Einbaubuchse J01021A1084 Tel +49 (0) 7157/125-0 Fax -120

The biquad ('bow-tie' bit)

The 'bowtie' section was made from a length of copper at least 400 mm (0.4 metre) long. The wire came from standard twin-and-earth stiff household mains wiring. By holding either end of the wire with a pair of pliers and giving it a sharp tug the wire was straightened. Leaving about 20 mm before starting, the bowtie shape was folded into the wire using the pliers, with the side of each square being 32 mm. The edge of the pliers was useful to obtain right angles. Both ends of the wire end up in the same place once the 'bowtie' is formed. These were soldered together and bent 90 degrees out of the plane of the bowtie, ready to be soldered to the casing of the N-type connector. The 'bowtie' then had an extending piece of wire soldered to it at the point where there is a 90 degree bend joining the two squares together, ready to be soldered to the centre pin (solder bucket) of the N-type connector.

Soldering

We worked out where the rear disc (groundplane) would sit when the N-type connector was affixed to it, and then both extending wires we cut so as to stand the 'bowtie' off the back plate by 18 mm. The centre pin was easy to solder because it has been tinned by the manufacturer. Soldering the other extending wire to the casing was not so easy. It required the surface roughing with sandpaper, and then tinning with a lot of heat until the solder flows. Once the casing and the end of the wire extension was tinned soldering was easy, while the connector body was still hot. At this point the dialectric (the white bit around the centre pin) started to go a bit soft. We were careful not to move the pin, and ensure it was straight before it cooled.

The 'bowtie' and N-type connector assembly, before fitting to the ground plane.

The ground plane

We drew around a compact disc on to a sheet of galvanised steel (about 1 mm thick - but none of use know our steel gauges - think it's 8 gauge) - and marked the centre. We used a pair of tin snips to cut out the circle. Using a 20 mm cone cutter, we drilled a 16 mm hole on the centre of the disc (by putting the 16 mm washer from the N-type connector over the tip of the cutter before drilling the hole stops at exactly the right size. We de-burred the edges of the disc with a fine file.

Assembly

We screwed the 'bowtie' and connector assembly into the hole in the ground plane, tightened it up with two spanners, and then carefully adjusted the 'bowtie' wire until the plane of the 'bowtie' was nicely parallel to the ground plane.

Weather proofing

Diy Biquad Wifi Antenna Calculator

We have used no weather proofing on these antennas yet. It needs some kind of plastic cap over the front.

Mounting

The antenna mounted on the U-bolt assembly.

These antennas need aiming. We mounted the antenna on a u bolt assembly using the bottom two bolts only. Thus we drilled two holes in the ground plane at the top. Ensuring the bolts protruded only a nut width, and using 2 nuts behind and 2 nuts in front, secured the ground plane to the lower u-bolt

The U-bolt assembly was clamped down hard on the bar, pointing in the required direction.

The antenna mounted on the pole, showing cable tie at the bottom as strain relief for the cable.

Cost

£5.50 for the N connector. The wire and steel disk were recycled. The u-bolt mounting assembly was around £3.00

Warning

Apart from the fact it works really well, no-one has yet popped on their lab-coat and done any high-brow tests on this 'homebrew twig', and of course manufacturers recommend you don't do anything which they don't recommend, or attach non-proprietary stuff to their stuff. Of course.

Initial tests

Initial tests show that the antenna has a beamwidth of roughly 70 degrees in elevation (vertical) and azimuth (horizontal) planes, and 12 dB gain in the signal to noise ration over the pcmcia card.

Copyleft

Author: Dave Gough

Copyright (c) 2002 Psand Limited. Permission is granted to copy, distributed and/or modify this document under the terms of the GNUFree Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled 'GNU Free Documentation License'.

Here is a simple antenna calculator for two popular forms of ham radio HF wire antennas: the horizontal dipole and the inverted 'V'.

(Updated June 5, 2021)

The Antenna Calculator
for
Half-Wave Dipoles

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.

Interpreting The Results

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 Half-Wave Dipole Antenna

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:

• The height of the dipole above ground;
• The conductivity of the ground below;
• The dielectric constant of the ground below;
• The presence of buildings, trees, metal structures (tower) nearby.

The Inverted V Dipole Antenna

When each side of a dipole slopes down from the feed point, it is commonly called an inverted V.

The inverted V results in:

• A more omni-directional radiation pattern than that of a dipole.
• A lower resonant frequency for the same length of wire as the dipole.
• A lower feed point impedance than the horizontal dipole (for feed points at the same height above ground).
• Some loss in bandwidth.
  

Some say that the inverted V should be cut 5% shorter than the dipole. I chose to make it about 4% shorter.

Biquad Wifi Antenna Calculator

The antenna calculator above will give you a wire length for the inverted 'V' which will be about 4% shorter than that of a dipole at the same desired frequency of operation. This will give you some leeway to trim the wire ends back if the inverted V resonates too low for your purpose when installed in its final position.

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:

• The resonant frequency of the inverted V is lowered (see the theory behind this at the bottom of this page).
  
• The influence of the ground conductivity and dielectric constant becomes an increasing factor.
  

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 antenna is too long (resonates below your desired frequency) then it's not a problem: you just trim each leg a little at a time until you reach lowest SWR.
  Note: To avoid cutting the wire, I usually just fold back each end of the antenna wire on itself and tape it tight if it's insulated wire - or twist it back on itself if you are using bare wire.
  
• But if the antenna is too short (resonates above your desired frequency) then it's a little more trouble to add wire to each leg to bring down its frequency of resonance where you want it.
  

To avoid the latter outcome, the antenna calculator above is set up to compensate somewhat for local adverse environmental conditions. The length it will calculate will likely be a little too long. You will only have to trim it a few inches at a time to bring the frequency of resonance up to where you want it.

For more detailed information on ham radio HF wire antennas, please visit this section of our Web site.

Furthermore...
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 less wire than a horizontal dipole for a given frequency of resonance.

The Theory Behind The Antenna Calculator

A dipole antenna is a resonant circuit at a given frequency. The same goes for an inverted 'V' version of that dipole.
However, when we lower each leg of a dipole antenna closer to the ground, we
introduce additional capacitance - which arises from the closer proximity of the antenna legs to the ground.

Consequently, for a given frequency of resonance, the legs of an inverted 'V' must be made slightly shorter - by about 2% each (overall length of wire about 4% shorter) - than a horizontal dipole.

The formula proves it!

When capacitance (C) is increased, the inductance (L) must be decreased if we are to arrive at the same frequency of resonance as before.

If you don't want to bother doing calculations with the formula, the antenna calculator above will provide appropriate wire lengths, to start with, for the inverted V and the dipole at a given frequency of your choice.

73 de VE2DPE
Claude Jollet
7, Rue de la Rive, Notre-Dame-des-Prairies, Québec, Canada J6E 1M9

QTH Locator: FN36gb

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