THE G2ATK MATCH -BOX MINIMISES TUNING ON ALL HF BANDS
G2ATK's Match Box
There are, and have been,
various antenna 'matching
units' offered to the amateur
and professional user which
the makers of same have specified that
it enables an antenna to be coupled to
a transmitter )that has an 500 output/
and the resulting s.w.r. will be less than
2:1 on ail frequencies between 1.8 and
30MHz.
However, no information is
forthcoming of what the various units
consist of in the way of components,
etc. The vast majority are also
encapsulated to prevent any visual
examination, so it can only be assumed
that somewhere in the enclosure is a
50 ohm 'load'. I decided to experiment to
see if I could find such a device that
worked. When built it had to cost less
than the [corn and Antiference units.
The last time I checked some three or
four years ago, prices were around
£100 plus!
Various Combinations
After many hours of trying various
combinations of circuits consisting of
inductances of different types, built
around a 50 ohm load, the result was
finally the circuit shown in Fig, 1. This
circuit was tested in two forms, ie. an
18 -turn trifilar wound 1.2mm (18s.w.g.)
enamelled copper wire on a Ti 30/6
toroidal former. The second form was
also wound with the same number of
turns and type of wire on a 1200/2
Torpid. The smaller version unit was
tested at my location where I have a
13.5m centre fed antenna with 13.6m
of 300 ohm feeders.
Trevor Talboys G2ATK passes on some
of his many years of eperience to help
you get the perfect antenna match.
The resistors are solderd into two pieces of
printed circuit board material
Fig. 2: A 40W 5012 non -inductive load can be made from 20 1 ki)
2W resistors as shown here.
The set-up al G2ATK is the maximum
length of antenna that the plot of land
allowed me to erect. The rear garden is
only 13m long and 113m wide. The
second, and larger unit, was tested by
my very good friend G40HJ at his
location. Although he only has a
21.4m centre -fed antenna, although
again fed with 13.6w of 3001 feeder.
In each case, one leg of the antenna
was used as a random length single
wire antenna, the resulting s.w.r. was
better than 1.5 to 1 on all bands.
Now look at the drawing of Fig. 2,
showing the method used to construct
the 50 ohm load that I've used on the unit.
I've experienced no trouble with this
setup when used with transmitters of
up to 100W s.s.b./c.w. output. This
method of construction allows plenty
of air to circulate over the load keeping
it at a reasonable temperature.
The switching circuit shown in Fig. 3 is
a simple one that could be built into
the box housing the Match -Box circuit
of Fig. 1 itself. But if you prefer a more
comprehensive switching circuit, I've
shown one in Fig. 4. This shows a later
witching arrangement that allows four
methods of using a centre fed antenna
with the 'Matchbox'.
It's well worth trying all of these
methods as owing no doubt to polar
'foot -prints', one of these methods may
be advantageous to and from the
station you are in contact with. This
more comprehensive circuit enables
different connections of the centre fed
antenna to be tried.
On 1.9 and 3.7MHz the best results,
from received reports, were with the
antenna connected as shown in the
diagram of Fig. 5. While on 7MHz and
upwards the better balanced feed of
Fig.6 is used. As I'm sure it's obvious,
the antenna is used in a balanced feed
set-up as shown the earth connection
merely ensures safety.
One drawback of the Match -Box is that
in use, there is an insertion (or through)
loss, which I've estimated at around
5dB. However, the convenience of not
needing an a.t.u. and its attendant
'fiddling' to get on -tune should appeal
to blind and disabled operators. And of
course there are other times when the
box would be also be useful, such as
there are inexperienced operators of
transmitters on board private yachts for
instance.
But enough talking about the Match -
Box, let's start building one/ Firstly the
unit should be housed in a metal case,
and I suggest that the 119 x 93 x 52mm
Eddystone die-cast box is ideal for this
Note phasing of
the windings
Ant 2
Ant 1
Fig. 1: The original - or simple G2ATK Match -Box circuit.
Fig. 4: A slightly more complicated switching circuit. See text
for more details.
24
PW - Antennas in Action, September 1997
Feeder
Antenna 1
Antenna 2
0
Fig. 3: A simple switching circuit giving either a top -loaded
combination for 1.8 and 3.5MHz (as shown) or a balanced
25.9m
12.95m
[VVT0684b1
Fig. 6: This is the 'normal' layout for use on 7MHz and
upwards.
antennas
maction
25.9m
12.95m
IWT0684-.;]
Fig. 5: A top -loaded vertical antenna for the lower two amateur
bands of 1.8 and 3.5MHz.
Loading resistor pack
Antenna 1
Antenna 2
Toriodal winding
and support
.T106132 j
Rubber feet
Fig. 7: The overall layout of the Match -Box in the recommended
metal box.
purpose. And in Fig. 7 I've shown a
layout that works in my case although,
I've not shown the insulated material
supports for the 50Q matching load.
The toroidal winding is made up using
three lengths of the enamelled copper
wire wound in a trifiliar manner around
the core.
To complete the windings three lengths
of the copper winding should be laid
side by side to form a 'flat' three wire
form. Then wind the 18 turns through
the toriodal core keepin the three wires
together but separated from the next
'turn' as much as possible. When
making the interconnections try to keep
the wires as short as possible.
The s.w.r. diagrams of Fig.s 8a and 8b
show the relationship between bands
and s.w.r. of the two prototype G2ATK
Match -Boxes. So you can see how
efficient the units are. A price well
worth paying to get such an even
loading without fiddling don't you
agree?
Silent Key - G2ATK
Ii's with regret that we have been informed that Trevor Talboys G2ATK the
author of the preceding article has died. However, we are grateful to his
family for permitting us to publish the article as a fitting tribute to his many
years of Amateur Radio experience.
1.511
1.41
9
cc
1 3:1
g 1.2.1
ca
W 1.171
1
.1
1.9
3.7
7.05
10.12 14.27 18.12
21.2
24.9
25.5
Frequency 1MHzi
1.5:1
1.4:1
cc 1.3:1
1.2:1
LA
C 1.1:1
1
:1
I Virro444
19
3.7
7.05
10.12 14.27 18.12
2
2
24.9
28.5
Frequency (Mild
Fig. 8a: The s.w.r.
curves of the
smaller Match -
Box.
Fig. 8b: The s.w.r.
curves of the
larger Match -Box.
PW - Antennas in Action, September 1997