AN EFFECTIVE 3-BAND WIRE DX AERIAL
By Mel Fisher G4WYW
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!
Mel Fisher G4WYW
Watch out for another great antenna
design from Mel G4WYW - Coming Soon!
Other articles by Mel Fisher -
Anternna solutions for high-rise apartment dwellers - Click here
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