Purpose of this article: This article describes the basics of the dipole antenna for HF/VHF

 

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Why a dipole?

  • Possibly the simplest and easiest to construct
  • A good antenna to experiment with and offer flexible deployment options
  • Very cheap to source the parts

 

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Antenna accessories from NBS Antennas – a local VK small business

https://www.nbsantennas.com.au/antennas-and-parts.html

 

Antenna accessories from Rippletech – a local VK small business

http://rippletech.com.au/html/baluns.html

 

A handy online calculator to help with your measurements

http://www.66pacific.com/calculators/dipole-antenna-length-calculator.aspx

 

 

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Construction or installation notes

 

Steps to Building a dipole

  • Calculate the lengths of wire required.
  • Decide what layout options will work for your QTH (Steer well clear of power lines, telephone lines, TV antennas)
  • Try to get the antenna as high as practically possible.
  • What options do you have to hold the dipole in the air?  Mast? Trees? Buildings?
  • What type of feeder will you use Open Wire (Ladderline) or Coax?
  • What will you use as insulators for the centre feed-point and ends?
  • How will you weather-proof the open joins to the wire antenna elements?

 

Tony from Rippletech describes how to build a simple dipole and sells simple but effective kit.

http://rippletech.com.au/html/insta-balun.html

 

 

 

 

Things to consider when building a dipole

  • Brian from NBS Antennas provides some good insulators to help with your dipole antenna.  You could also make your own centre feed-point insulator by cutting/sawing an acrylic cutting board to size.  Drill and file holes for the wires, feedline and rope.
  • Whilst a bit messy, you can use silicon sealer and/or self-amalgamating tape to provide a level of weather proofing.  You could also build a small box to house the connectors
  • You could buy a commercial balun which will make you antenna system more efficient.  Tony VK3TZ from Rippletech provides a great local product.
  • Ask for help at your local club.  Simple dipoles make great club projects and learning opportunities.
  • Tuning your dipole will depend on your local site.   Surrounding objects, earth conductivity and other factors will mean that you may need to adjust the lengths of your wire elements.  So, start with your wire elements being a little longer to begin with.

 

Dipole Construction

 

The basic construction of the dipole is two elements each 1/4 wavelength long, fed in the centre by a transmission line (as shown in the figure below).

 

 

The total length of the dipole is given by;

 

143/ Frequency (MHz) = total length in metres 

or

468/ Frequency (MHz) = total length in feet 

 

Example

A dipole for the SSB portion of the 40 Metre band, cut the dipole for the frequency of 7.130 MHz. Using the formula, calculate the total length of the dipole (a half wavelength).

 

143/7.13 = 20.056 total length in metres or

468/7.13 = 65.64 total length in feet 

 

You may have noticed that the Metres formula came up with a figure close to half (a half wavelength) of 40 Metres (a Full Wave). This is handy to double check your calculation.  

 

An Inverted “V” dipole configuration is 2 to 5% Shorter.

 

The calculated length is halved and becomes the two “arms” of the dipole antenna.

 

Make the dipole elements from 16 gauge or larger copper wire, Stainless steel also works but the length may change slightly. Stranded wire is generally better since solid wire can stretch under tension. As shown in the figure, you’ll need some kind of insulator at the centre and at both ends of the antenna. You can buy insulators designed for this purpose or you can just fabricate something on your own.

 

As shown in the figure above, the coaxial transmission line may be attached right to the antenna wires directly without a connector. The coaxial centre conductor attaches to one of the ¼-wavelength elements and the ground side shield or braid attaches to the other segment. 

 

You may also purchase special centre insulators that have a coaxial connector integrated into them so the transmission line can be easily attached. And of course, there are many commercially available dipole antenna products ready for a simple coaxial cable connection and minor trimming or “tuning” to proper length.

When a dipole antenna is fed with a coaxial cable from the transceiver, the coaxial cable outer braid/shield may become part of the antenna, radiate and allow RF into the shack. A Transformer commonly known as a Balun, is designed to match the “Balanced” dipole to the “Unbalanced” (hence the name BalUn) coaxial cable feedline will assist in preventing this.
 

 




When using a Balanced Feeder such as ladder line you will need to convert the impedance of say 600 Ohms to the 50 ohms your transceiver expects to see. This can be accomplished by using a balun or using the 600 ohm input / balanced line input on an Antenna Tuning Unit.

 

The name Antenna Tuning Unit is a misnomer as its purpose is to convert the antenna impedance to a value expectable to the transceiver in use.

 

More information is available on our Knowledge Base articles Feedlines and Baluns

 

Supporting the Dipole Antenna

 

Hang the antenna by attaching rope or cord to the end insulators. A pair of tall trees can work well, if they are spaced at a convenient distance. Or you may have to connect one end to your house and the other end to a pole. The main thing to keep in mind is that the antenna should not be in close proximity of large metal objects. For example, if you mount the antenna close to a metal rain gutter, it will severely detune the antenna and it will not work well. You’ll want to get the antenna up into the air about ¼ to ½ of a wavelength above ground, but you may have to settle for less than that.

 

Dipoles are commonly erected in one of three configurations: 1) flattop, 2) inverted V, or 3) sloper. The inverted V configuration has the advantage of requiring less horizontal space and only a single high support in the centre, although the ends should be kept high enough to be out of normal human reach. This configuration also provides a bit better signal propagation off the ends of the antenna than a flattop arrangement and is more omni-directional. 

 

The flattop configuration tends to provide slightly better gain, or signal strength in the broadside directions (right angles to the wires), but performance out the ends of the wires is poorest. 

 

The sloper also uses a single tall support, and is mildly directional off the end of the antenna.. In each case, route the transmission line away from the elements at a right angle or directly between the elements for the inverted V, , to avoid detuning the antenna.

 

In an urban environment every antenna is a compromise, so decide which dipole configuration will work best for your situation.

 

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