traditional W3DZZ trap dipole. The antenna system will almost certainly require
some impedance matching to suit rigs with solid state power amplifiers - again, this is just
like the W3DZZ antenna.
Antenna purists will often tell you that an antenna of this type requires radials, or a
sophisticated earth system for optimum performance. But in practice the antenna works
very well with a modest earth system, although this is dependant on soil conditions.
Fortunately, for me in the south western area of Scotland we're 'blessed' with more
than our fair share of rain, together with a very clay-rich soil, usually just below the surface.
Because of the rain and soil blessing, a good r.f. earth is assured with only a single one metre long
earth rod. But should you have a rather more sandy soil in you area, you may require to install longer
earth rods or an earth mat to achieve an acceptable r.f. earth. It's very much cases of 'suck it and see'
- though not literally of course!
The prototype antenna was constructed from a heavy enameled copper wire (2mm diameter or 14 s.w.g.)
obtained free as an end of roll gift from a local armature winder. There's a singe 7Mz resonant trap to make
it more efficient on that band.
The general layout of the antenna and the theoretical lengths of the antenna are shown in the illustration Fig.1.
The Antenna is fed with 50W coaxial cable, with the coaxial screen connected
to the earth rod. This connection is secured using a 'Tenby' earth clamp intended for earthing water pipes
and available from most d.i.y. centers.
The centre core of the coaxial cable is connected to the antenna via a
single 15A 'chocolate block' connector. After double checking these
Connections and continuity, the whole area is covered in Denso tape.
to waterproof it. No balun or other matching network is needed for 3.5
and 7MHz as the antenna's feed point impedance is close to 50W
The 7MHz trap
is constructed from 11 turns of RG58 coaxial cable wound on a 100mm piece of 40mm diameter
plastic drainage piping as shown in the illustration Fig. 3.
and the photograph Fig. 4.
type of trap the coaxial cable acts as both capacitor and inductor and is capable of working at power levels
in excess of a kilowatt.
It's imperative that screen and centre cores of the coaxial cable are parted as close to the point the
cable passes through the hole in the pipe as possible. This is to ensure the correct
value of capacitance and inductance. The centre core of one end of the coaxial cable is soldered to the screen
at the other end.
As with the feed-point, the ends of the coaxial trap and other joints must be weather proofed. The
capillary effects of coaxial cable are legendary and water ingress will total ruin your trap.
Any sealant must be of the non acetic type, (i.e. it doesn't smell like vinegar) to
prevent cable corrosion.
The height at which the antenna folds over from vertical to horizontal
is not critical but generally the higher it is the better. Extra height, not only
aids the DX performance of the antenna but also significantly reduces the
amount of horizontal space required.
Tuning the antenna is quite simple but it is imperative that it is done in the correct order. Firstly cut both
sections of the antenna about a half a metre longer than the dimensions shown in Fig. 1.
To start the tuning operation, begin on 7MHz
and trim the wire length at
the end nearest the earth connection 50mm at a time until the lowest s.w.r.
is achieved. I managed an indicated s.w.r. of less than 1.2:1 over the whole
Then move to 3.5MHz
and repeat the process, but this time trimming the side of the antenna
furthest away from the earth
i.e. the side nearest the house in Fig. 1. The s.w.r.
on the 3.5MHz band should be less than 2:1 over the whole of the band
falling to about 1.2 to 1 at the point of resonance. So, it's worth setting the
lowest s.w.r. at the section of the band you normally use, if you have a preference.
The antenna can be made in a weekend with plenty of time left
over to work that elusive DX. Having a very low visual
visual impact most Local Authorities Planning Departments can be
Convinced that it is a 'minimalist' installation not requiring planning
permission, however this should be confirmed with your local planning office.
I've had a lot of fun using the antenna and thoroughly recommend it to anybody not having enough
garden room to erect a full size dipole in the optimum direction. So, why not
chat up your local electric motor rewinding company for the wire to
make your next antennas? Page 3
|Fig. 3: A cross-section of the 7MHz trap made from a length of 40mm diamter, plastic pipe and some RG58 coaxial cable. (see text for more detail).
|Fig. 4: A photograph of the finished trap, before sealing against moisture ingress (see text for more detail).