First, please excuse my artistic abilities. The diagrams are basic but show the rudiments of this excellent aerial, which is ideal for the newcomer to HF and those on a small budget as well as those with little space to erect large arrays!
Please note that my calculations are worked out using feet and inches because that’s what I grew up with! The reader who understands the Metric System should have no difficulty in converting to Centimetres and Millimetres but this article will use Feet and Inches.
Before outlining the construction, I would just add that when I was first licensed, I tried various aerials, verticals; inverted V’s, inverted L’s and wire dipoles. I finally gave this design a go. It was a brilliant success and I had a confirmed (QSL Card) contact with CE0ERY Hector on Easter Island on 30/04/1984 quickly followed by VK’s, ZL’s, and a string of exotic DX throughout the Indian and Pacific oceans.
YES using this same design I am about to explain to you in the following paragraphs!
WHAT YOU WILL NEED
You will need some wire…I have always found that old or new house wiring cable is an excellent source of wire but will need to be stripped from the grey outer insulation. The red and black pair of wires needs to be further stripped from their coverings. If you don’t fancy this task, it will still work, BUT it’s better to use naked copper wire for best results.
Here’s how I strip wire in long lengths.
First I use side cutters and cut into the wire from the end, trying to keep close to the bare earth wire that is in the centre of the cable and which runs along the length.
Once I can get a grip on the end of this earth wire with a pair of long nose pliers, I twist a few turns around the nose of the pliers and gently pull. Soon there will be enough slack grey outer sleeve to allow it to be held in place with the feet standing on it firmly. I have also tied the end of the grey insulation (with red and black wires) to a door handle or vice handle.
Once the thin earth wire starts to strip it usually comes easily as long as a steady firm pressure is applied and sudden jerks are avoided. When the thin wire has been removed from the entire length, having acted like a cheese cutting wire, the red and black should peel out without any bother.
The 75 ohm twin feeder that I used was cadged from a BT worker that I spotted while driving home. I approached him and told him what I needed some for and asked if he might be able to oblige. I made sure that he spotted the couple of old One Pound notes that I held in my hand. He cut me off a generous length but refused to take my money. I told him I owed him, thanked him and rushed home to start making my aerial.
You will also need a small square or oblong rectangle of insulating material such as Perspex or Paxolin. I didn’t have any so I made do with some plywood which I drilled and varnished until I was able to obtain some better material later. It is also possible to use a custom centre piece complete with female coax socket for the feeder and two wing nuts (butterfly nuts) at each end of the T-piece where all the bare ends can be twisted together and connected. This is the quickest method and gives reliable results but regular swinging in any wind WILL eventually cause breakages at the connections! A proper centre piece with female SU socket is not too expensive but you’ll experience greater satisfaction if you make up the part yourself.
To avoid becoming confused, it is preferable to work with one pair of quarter wavelength wires at a time and to use a short strip of masking tape folded and pressed together over the end of a length of wire and marked in Biro 10 M and 15M and 20M as appropriate.
After you have ended up with pairs of wires, all cut to length ready for connecting to your centre piece, the next thing to do is to connect all the ends together and to the feeder.. Look at the enlarged diagram showing the centre-piece to understand how this is done. Basically, the end of each wire is passed through two holes, an overhand knot (one turn over the wire and through the loop) pulled tight will prevent the wire from being pulled out of the centre piece. Strip off about half an inch of insulation from each wire end after the knot has been tied and connect all the ends on one side of the connector centre piece and to this should be added one side of the 75 ohm Twin Feeder which is commonly used as telephone wire leading from telegraph poles to the house and which is a superb match when used as a feeder or Transmission Line for this aerial design.
After fitting, joining and soldering the joints, each bare wire and soldered joint should be covered by a blob of bathroom sealant or other rubberised substance which, when dry, should offer some protection from weathering. It will pay dividends to check and perhaps redo this each year with your annual inspection and maintenance. What do you mean, you don’t do annual maintenance?!
Although the final choice is completely up to yourself as to which band you have at top, bottom or in the middle, the results I achieved came from having the shorter 10M pair at the top, the 15M pair in the middle horizontal position and the 20M pair at the bottom working as an inverted ‘V’. The reason why this design gives such good results is because of this: At ten metres the 10M section acts as a half wave dipole but at the same time the 15M section also radiates some lobes and the 20M section acts as a full wave dipole on 10 metres in addition to the radiation from the actual 10M section.
Now that you understand how each section interacts with and reinforces each other section, it is time to look at how the individual lengths are worked out.
L (length) = 468/F (frequency in MHz)
/ = Divide
As an example, suppose you wish to operate the 20M dipole at 14,250 KHz (14.25 MHz).
Length (overall of half-wave) = 468 / 14.25 = 32.84 or 32feet 8 and a half inches.
(You will see that I have approximated the inches. Such a small inaccuracy will not make any noticeable difference!)
Divide 32.84 by 2 to give 16.42 feet, the length of each quarter wave section that makes up the half-wave dipole length.
Purists may wish to add an inch and a half to allow for the knots at the support points at each end. Where the bare end of a wire connects to an insulator such as an egg type insulator, pass the wire through and around the insulator then twist the end back over the main wire with a series of small twists or turns. (Bare wire!) The length in this case will be to the centre of the bend where the wire curves back on itself at the insulator and NOT to include ALL the total length of wire providing it does indeed connect back on itself and in effect is shorting itself out!
So there you have it. The formulae to work out your own individual measurements and the instructions for putting it all together.
PLEASE REMEMBER NEVER TO ALLOW ANY OF THE SECTIONS TO BE CUT AS A HALF WAVELENGTH FOR A BAND FOR WHICH ANOTHER SECTION IS THREE HALFWAVES!
For example, if you cut the lowest dipole for 40M, you would not wish to cut another section for 15M as they would interact giving a wrong feed-point impedance in addition to a skewed radiation pattern.
In practice there will be small differences in impedance and standing wave ratio (SWR) in each different location. This is due to conductivity of soil, height above ground, wire type and other tiny factors which we have all experienced in our hobby as Radio Amateurs from time to time.
In conclusion, I do not claim this as my design. Indeed, it can be found in varying forms in a number of books on Antenna Design. I have just presented it in a way that I hope may be attractive enough to cause you to give a try to making one. I have tried to explain in such a way that you can picture the finished aerial in your mind.
If you are unwilling to construct an aerial that resembles a Union Jack flag in wire, you CAN build it so that all sections lay alongside of each other BUT ONLY if the insulation is retained on each wire component. It won’t work quite as well as the described aerial but it will work.
I have used 75 Ohm Co-axial Cable in place of the twin feeder and it still worked surprisingly well. Once you are satisfied that it exhibits low SWR (standing wave reflection) at each frequency it is cut for, you can just get on and use it without the need for an ATU (antenna tuning unit) except for at the band ends. I am of the opinion that far too much fuss is made about SWR measurements that hardly deflect a needle of a meter. You can fire up into an SWR of 3.0 and there will be hardly any difference in the received signal at the other end of the contact!
Your Transceiver will soon let you know if things are not quite right!
I might have been extremely lucky, but I just made mine, stuck it up, ran the twin feeder to my transceiver (Yaesu FT 401-DX) and started using it straight away with good effect.
I DID have a good earth rod though, driven into the ground to a depth of six foot (6′) which I would advise you to have no matter which type of HF wire aerial you use. On the night that I worked Easter Island, I called many times without getting a reply. So I went and sprinkled a packet of salt over the top of my earth rod and splashed more than ten washing up bowls of cold tap water over the area. When I returned to my station and called again, I was answered immediately and Hector said how pleased he was to hear and work a ‘G’ station. We exchanged reports of 5/5.
The reader is expected to resolve his/her own problems and the writer does not accept any responsibility for wrongful use of the information provided. All information is supplied in good faith.
Get cracking now… and good luck with your CQ calls!
This article by Mel Fisher G4WYW, was found on http://www.southgatearc.org/articles/g4wyw/tri_band/tri_bander.htm