EXPERIMENTAL RADIO – Simple stuff- Build your own HF, Single or Multiband Dipole Antenna on a single feeder if desired.
Make your own wire antenna, spend your money on things you cannot make so easily–you can feed it with an open wire line & use an atu or a Balun at the radio end –however, and much simpler– you can use one of our baluns ( or any 1..1 balun) at the feed point i.e. the center of the antenna itself & then use an RG8U cable with excellent results.
For the elements the greater the diameter of the wire the more broadband the antenna becomes–I use 2 mm diameter hard drawn copper
Using E.G RG58 U coaxial cable is practical for building broadband dipole elements.
Some crazy guys call this system double bazooka ( BS ???) for goodness sake! It’s a broadband dipole -make it yourself so simple and easy
There are problems however in supporting such antennas because of the weight of the co-ax cable and the risk of water ingress in the longer term, there is no gain at all (no directional properties) existing over a standard dipole. Forget the specs on the package –again it’s more BS. We will address Antenna Gain in another article.
The balun should be rated well above the proposed power level for safety ( SWR spikes are the biggest risk)
Below you can see that a half wave antenna length is derived from dividing 468 by the frequency in MHz and further on in this article, how this is derived.
For multi-band operations, you can attach several elements to the same center balun or centerpiece–start with the lowest Frequency–prune to the desired resonance center frequency
Then add the next set of elements and prune to frequency–checking that the first pair of elements are still happy
Continue adding as required–space the elements apart as much as possible–horizontal or inverted vee configuration works well
Always keep the feed point as high as possible
Bear in mind that the ideal height for a horizontal dipole is half a wavelength above the ground –this is around 130 feet at 80 meters/3.5 MHz a bit impractical
However, as the frequency goes higher it is very practical to achieve the target height –at 40 meters/7 Mhz you are looking at 66 feet
However, at 20 meters /14 MHz its just 33 feet very practical as you see in the first drawing below for a 20-meter dipole at 30 feet
Below are drawings borrowed from Frederick R. Vobbe, W8HDU
20-meter antenna 30 feet off the ground, notice that the elevation or take-off angle ( green line) is about 35 degrees not bad at all-also understand that if the antenna was mounted higher the takeoff angle would reduce–that means far longer distance working-as the signal is refracted back to earth from a farther distant point and at a much more advantageous angle-the nearer the take-off angle gets to around15degrees, the greater the distance ( DX) you will achieve, anything less than 15 degrees takeoff angle is hard to achieve and can be counter-productive as near field obstacles then come more into focus
20-meter antenna 20 feet off the ground –sky warmer-say no more
A simple Dipole System is a pair of 1/4 wave elements -fed in the center -many ways to feed it as above.
Radio waves travel at the speed of light in free space ( 300 000 000 meters/sec & a little slower in a cable. Dividing this distance by the frequency in use or the desired frequency equals the wavelength at that frequency. The associated half-wave dipole antenna is half of this full wavelength. each element is a quarter wavelength This half-wave dipole is fed in the center via the feedline. Then there are a few considerations as follows:-
A good VSWR meter and a transmitter or an antenna analyzer are required for the next steps.
If the feedline is just a coaxial cable typically 50 ohms, the antenna quarter wave sections are adjusted equally until the least reflected power from the antenna occurs on the desired center frequency. Bear in mind the upper end of the coaxial feeder in the above case will almost always become part of the antenna & is non-radiating –just burning up some valuable RF power
However, should a good quality Balun or Choke balun be installed into the center point of this dipole –then the coaxial cable will not become part of the resonant antenna and the element now are better tuned to radiate all the RF power arriving at the feed point of the elements.
The other option is an open wire feeder typically 400-600 ohms -a good ATU at the Transmitter end–then the antenna can be tuned across a much wider range of frequencies with excellent results on the resonant frequency of the elements & good results on other frequencies. (there is so much that can be said here that it would form a separate article)
Formula–Dipole-use the formula below–cut a little longer to allow for pruning to resonance & for the insulators
Horizontal Dipole Beam pattern