The use of a light-weight mast and antenna, carried by
each Member in his
car at all times, is not a new idea. It allows Members to be deployed
directly from home or work to the scene of an incident, or to a desired
relay point. Members of at least one Group carried a plastic-pipe mast
and a modified Slim
Jim antenna in their cars in the 1970's. When fibre-glass Roach Poles
available, some other Groups carried them, and have developed ingenious
and fittings for them.
This Note is a brief introduction for the present
(2005) use of portable
masts and antennas for RAYNET, and suggests two easily made antennas
The greater antenna height using a mast, compared with
the height of a car-roof
antenna, gives a greatly increased chance of setting-up a successful
communication link. This applies in most terrain, urban or rural. While
the height-gain from 1.5m to 3m in urban areas is about 3dB (6dB in
open flat ground), getting the antenna above much of the natural and
man-made clutter by using a 5m mast often results in a gain of 8dB to
12dB or more in VHF/UHF signal strength. If on arrival it is found that
the car-roof antenna will not provide a link, the use of a modest mast
Points to bear in mind are firstly, that simple
light-weight antennas will
not have as much gain as the commercial vertical colinear antennas used
by Radio Amateurs on buildings, (but these require heavier masts).
Secondly, that feeder loss should be kept to a minimum, as with all
elevated antennas. Nevertheless, such simple antennas and light-weight
masts, with a feeder as short and low-loss as practicable, will
generally give greatly increased communication capability compared with
car-roof antennas, and are quicker to deploy and erect than heavier
masts and higher gain antennas kept in a central store.
What is comparatively new is the availability of
light-weight and inexpensive fibre-glass telescopic masts, closing down
to less than 1.2m (48in) in length.
They can be stored in most cars. Also, having a weight
of less than 1 Kg
they can be carried easily on-foot. The top section is very thin and
easily in the wind, but that section need not be used. So, for example,
a 6m telescopic fibre-glass mast without the top section will be about
in height, and sufficiently rigid at the top for a light-weight and
antenna to be used without signal flutter in the wind. This height is
a good choice, but similar masts up to 9m are available. A 6m
telescopic mast can be bought for as little as £10.
Carbon-fibre masts should not be used because they are
conductive, and this
restricts the choice of antennas.
As these masts are very light and easy to handle, it
is possible to tie
them to a fence, car, or other convenient object with string. It is
possible for an operator to hold the mast vertical with one hand while
the radio. Of course a light-weight stand of some kind is useful.
The majority of RAYNET communication is in the 2m
band, and some may feel
that a portable antenna for that band alone is sufficient. In that case
a suitable and easily-made antenna is one of the 2m Band Ribbon-J
Antennas such as that described by John Belrose VE2CV (1). This version
can be rolled up when not in use. It incorporates a choke in the
coaxial feeder, and this is desirable for most vertical VHF antennas.
The dimensions given there are for the 144-148 MHz band, so for the
144-146 MHz band the lengths can be multiplied
by 1.014. Larger and smaller versions could be made for other single
On the other hand, links on 70cm and 'Talk-Through' on
2m/70cm are not
uncommon now that most Hand-Held and Mobile Transceivers incorporate
bands. A dual-band antenna may therefore be preferred, if it is
and can be made easily.
Some Groups have made and use the "Townsman" Dual Band
2m/70cm antenna (2). This is a rigid antenna, and has a Patented
impedance matching arrangement at its base. The desired radiation
pattern on 70cm is achieved with a cylindrical metal sleeve over part
of the radiator.
A simple dual-band 2m/70cm antenna that is easy to
make, does not require difficult adjustment, and can be rolled up when
not in use, is the
"FBK 2/70 Portable Coax Antenna" designed by Frank
Bremer PA0FBK (3).
It is made of RG58 coaxial cable, with some parts of
the braid removed.
The braid is retained over part of the radiator, for the same purpose
the rigid sleeve in the case of the Townsman.
We have added an RG58 "tail", part of which is coiled
to form a choke.
This choke is to reduce currents on the outside of the feeder that
tilt the main lobes of the vertical radiation patterns (VRP) of both
up or down. It also avoids possible matching problems at the
This air-cored choke construction, winding the coax tail inside a PVC
(wound with 6 turns of the PVC cable jacket touching), is satisfactory
reliable. It avoids the cost and difficulty of obtaining a suitable
In the design of dual-band 2m/70cm vertical antennas
generally, the harmonic relationship makes it relatively easy to obtain
acceptable impedance matching on both bands, but it is not easy to
obtain the desired VRP for the 70cm band.
It is difficult to avoid an upward or downward tilt of
the main lobe, or a null in the horizontal plane. The braid on part of
the vertical radiator of the FBK antenna considerably improves the 70cm
The details of the FBK antenna are shown on the
attached graphic, together
with the tail/choke. In (3) PA0FBK mentions his previous use of the
Townsman. We have made the FBK antenna and have tested it for impedance
match and VRP on both bands, with the choke (4). We checked VRP's on a
large flat open area (Playing Field). Whilst the 70cm VRP is not quite
optimum, it does
have one of its maxima in the horizontal plane and this is within about
decibel of the others. After initially trimming the top, the VSWR was
below 1.5 at the centre of both bands, and below 2 at the band edges.
The antenna performance is considered quite acceptable for its purpose
on both bands. It is recommended that in the first instance the top
(unsheathed) section of this antenna be made about a couple of
centimetres longer than the figure given. By checking the VSWR in both
bands (with the antenna attached to a
fibreglass mast using sticky insulating tape), the top can be trimmed,
2mm at a time, for the best compromise VSWR on both bands. It is best
tape the antenna to the mast at only three or four points, so that it
not held tightly against the mast over most of its length and therefore
not be affected appreciably by the dielectric of the mast. If desired,
ends of the braid can be covered with short pieces of heat-shrink
a seal, taking care not to melt the polythene dielectric.
We have designed and made other flexible Dual Band
2m/70cm antennas, with the radiator centre-fed through the coax. These
work well, but they incorporate traps that require accurate setting-up,
and so are not as suitable for general DIY construction. No doubt other
designs will emerge.
Flexible antennas can also be used conveniently
without a mast. For example the top can be tied to a branch of a tree.
If operating near the top of
a building of several storeys height, it is sometimes possible to open
window and let the antenna dangle down outside from the window sill, or
to use the antenna inside the room. And so on...
If using any antenna on 70cm, remember that half a
wavelength is only about 35 cm (approximately one foot), and that there
may be a horizontal standing-wave pattern in space due to reflections,
as well as a vertical pattern. Moving the antenna horizontally a few
inches may then result in a
considerable change in signal strength. For fixed commercial
Point-to-Point links at similar frequencies, the antenna is often
mounted on a "swan-neck". Antenna position then is adjusted
horizontally and vertically during installation.
Technical Topics (Radcom May 1995)
B.Hewlett G3JAM (Wireless World February 1980).
Frank Bremer PA0FBK
Gerry Hussey G4PGZ assisted with the antenna tests.
Bernard Spencer G3SMW
Marlow April 16th 2005
Text modified 4th April 2008