Your first dipole
The dipole antenna is an easily designed and made antenna usually used on HF, although can be (and sometimes is) made for VHF and UHF antennas, and in varying forms is also used as part of different designs of antennas â€“ i.e. as the driven element for directional antennas.
There is a relationship between frequency and the wavelength and thus the physical size of antennas. The frequency is often referred to according to their wavelength â€“ i.e. the 27 MHz band is known as 11 metres, the 144 MHz amateur band is known as the 2 metre band, etc.
To get the wavelength of a frequency, there is a simple formula â€“ that being 300 divided by the frequency in MHz â€“ i.e. 300 divided by 27 equals 11.11 â€“ which is where we get 11 metres (the wavelength) for 27 MHz CB.
A dipole antenna consists of two quarter wave lengths of wire for the desired frequency, one joined to the centre of the coax, and the other to the braid.
- A length of wire â€“ I have used insulated wire (Dick Smiths Electronics part number W2280), which will likely stretch as it ages, which means it will need to be trimmed if I want to keep it resonant on the desired frequency.
- One SO239 plug (Dick Smiths Electronics part number P2340)
- Coaxial cable
- Two PL259 plugs to suit the coaxial cable you will be using
- 4 small bolts (small enough to fit in the holes on the corners) and matching nuts.
With HF, unless you are running huge distances of cable, there is no problem with using coaxial cable like RG58 â€“ the losses at HF are relatively insignificant.
First off you need to decide what frequency that you want to construct the antenna for. In this case, I am constructing this antenna for the 40 metre (7 MHz) amateur band, and I have decided to centre it on 7.05 MHz. As such, we use the formula as above â€“ 300 divided by 7.05, which equals 42.55 metres (one wavelength), and then divide it by 4 to get a quarter wave, which in this case is 10.64 metres. In real life due to a number of reasons, a wire antenna is actually a little shorter than in theory â€“ however it is always easier to shorten than lengthen an antenna.
I have not bothered with a balun with this antenna, as it is for RX only. Rather than simply soldering the two pieces of wire to the coaxial cable, I have fabricated a mount for a SO239 chasis socket from a couple of pieces of perspex. I only had some thin perspex, so I have fastened a couple of pieces together to make a thicker piece for improved strength.
Next I drilled a hole in the perspex large enough for the socket to fit through it (a 17mm drill bit makes it easier), and a couple of holes for the bolts to go through to fasten the socket to the perspex, and holes for the wire to loop through to reduce the strain on the connector.
After securing the socket to the perspex, the next step was to solder the wire to the socket. Both wires were looped through the holes, tied off, and then soldered to the socket â€“ one to the centre connector, and the other to a lug that I secured via one of the bolts securing the socket to the perspex.
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