Inverted 'V' Trapped Dipole Antenna for 14MHz,
21MHz and 28MHz Amateur Bands.
On the Antenna page, it was stated that there was a pulley fixed to the top of the vertical
radiator in order to suspend a trapped dipole to the house. It was decided in May 2004, to
use this pulley to hang the Trapped Dipole Antenna as an Inverted 'V'. This would support
the weight better - would provide maximum height for the feed point (where the maximum
radiation takes place) and to give an omni-directional radiation pattern.
(When suspended horizontally, the direction of propagation is a figure of eight pattern).
The Trapped Dipole can be seen in the Inverted 'V'
configuration above - also the KW Balun at the top.
The KW Balun which has a
ratio of 1:1 and impedance
of 50 ohms is shown above.
The Trapped Dipole started off as a Mosley TD3jr and was purchased by G3NGD in the early
sixties. It wasn't very successful when new, and was put aside for many years.
Later, when John became more experienced, he decided to strip it down and find out why it didn't
work all that well. It was found that the connections inside the Trap were poor, and when cleaned
up, proved to work a little more efficiently. (There were warnings on the instructions not to open
the Trap Units, and also that the resonant frequency cannot be measured with a GDO).For those interested, John has given the constructional details below.
Fig.1
Both the 21MHz and 28MHz Trap coils are enclosed inside one
Aluminium tube - 343mm long and 30mm outside diameter.(Fig.1)The advantage of placing the coils inside the Aluminium tube is
that a parallel tuning capacitor isn't required - (The tube serves as
the capacitor).
The constructional details shown above is for the 28MHz Trap.
The coil for 21MHz will be similar, but wound to 10mm from the
end of the former. The two coils are fitted as shown in Fig.1 -
the 21MHz coil to the left, and the 28MHz coil to the right.
The aluminium tube length is critical because it forms part of the
length required to resonate the antenna on 21MHz.
The wire between the traps, Balun and egg insulators is
2.5mm copper (pvc insulated).
(The original dimensions suggested by Mosley were:
3' - 0¼" and 7' - 9½" in 1960 pre-metric units!)
SWR / Frequency graph for the 20 Metre Trap.
SWR / Frequency graph for the 15 Metre Trap. SWR / Frequency graph for the 10 Metre Trap.
