4-Metre Band 5/8 Wave Vertical Antenna
This project began after a friend (Graham
M0GDE) found me a Phillips
FM1100 PMR radio suitable for conversion to the 4-m band. I
carried out the PA4DEN conversion procedure and then found myself in
need of an antenna with which to test and use the rig.
I first tried a folded-J made from 300 ohm feeder, but without much
I had a CB-style
steel whip antenna element with a 3/8 inch screw fitting at the base
(courtesy of my local emporium)
the length was just about right
for a 5/8 wave design on 4 metres. This appealed as it would
offer substantial gain and lower angle of radiation compared to a
simple quarter-wave ground-plane, and it seemed simpler to construct
than a folded
The project has taken a while to appear in hardware form because of
spent in designing, producing drawings and then finding someone
prepared to produce the parts in small quantities at a reasonable (!)
The picture shows a prototype (complete with red insulating tape on the
joins) being tested atop a small mast. For scale, the base of the
antenna is about 40 mm in diameter, the radials are about 1m long and
radiating element is about 2.4m long.
5/8 Wave Antenna Theory
This is a vertical, end-fed ground-plane antenna that incorporates
radial elements (3 in my case).
Radiation Pattern and Gain
The principal reasons for selecting the 5/8 wave length are the
increased collection aperure compared to a quarter- or half-wave
antenna, and the strong low-angle radiation lobe which offers
potentially high gain. These proprties come from the length of
the radiating element and its mounting position over a ground plane.
Various references (including the ARRL Antenna Handbook) show the
calculated ideal radiation patterns form a simple linear radiating
element as its length is increased. When the length is equal to
half a wavelength, the familiar "doughnut" radiation pattern
obtains, with maximum radiation orthogonal to the radiating element and
a gain of about 2.15 dB relative to an idealised isotropic
(non-directional) radiator. As the length is increased the
radiation pattern tends to become asymmetric with elevation angle and
to develop multiple "lobes" and nulls. It is found that a 5/8
wave element driven against a ground plane (so that it induces a
virtual "mirror" radiator below the ground plane) posesses a
partiicularly strong lobe at a relatively low angle of radiation
(around 16 degrees). The calculated gain with respect to a
half-wave dipole is approximately 1.5 dB, and the strongest radiation
lobe is angled closer to the horizontal than would be the case for a
half-wave or quarter wave antenna fed over a ground plane.
Further increases in radiator length produce more lobes and the gain
Impedance Match Strategy
Having selected the desired gain, the geometry of the antenna is fixed:
one is now faced with the separate (though related) task of matching
the antenna to its feeder. For convenience of mounting,
end-feeding is preferred.
The first point to realise is that the 5/8-wave antenna is not
self-resonant (5/8 is not an integer multiple of 1/4) so a signal or
travelling from one end to the other and back will not return in-phase
with the driving signal. In fact, the end of the antenna presents
a capacitive reactance at its operating frequency (i.e signals return
with a phase lag of about 90 degrees).
In order to build up a substantial current in the antenna, it is usual
to resonate it: this can be done by adding a series inductance at the
antenna base connected to ground to form a series L-C resonant
circuit (sometimes called a "trap" circuit"). A suitable match to
a typical coaxial feeder can then be
obtained by tapping into this inductance at an appropriate point above
the ground connection.
There is an alternative method which is a little simpler to construct
(it avoids tapping the coil). It is observed that a 3/4-wave
antenna (which does not have such a nice radiation pattern) happens to
exhibit an end-feed impedance very close to 50 ohms. Such an
antenna presents a good match to typical coaxial cables. The
5/8-wave element may thus be matched by making it appear electrically
but not physically
to a 3/4-wave element. This is
achieved in practice by adding a series inductance which approximates
1/8-wave of conductor coiled up so that it does not radiate
significantly. The coaxial cable is then attached directly to the
of the coil.
The design presented herein adopts the un-tapped series inductor
matching, Results from the prototype are presented below.
The antenna base (grey) is designed to fit inside the top of a standard
1.5 inch aluminium tubular mast (shown in dark blue). The base
has an RF connector (either SO239 or N-type) fitted: both
the connector and the cable are protected from weather to some extent
by the mast and the shielding effect also helps unwanted prevent
radiating currents on the outside of the braid of the coaxial
cable. The base supports three radials which are fabricated
from rods cut to about a quarter-wavelength at mid-band. The design
calls for hexagonal sections to be brazed on to aid in tightening and
retaining the radials.
The base section also supports two co-axial glass-reinforced composite
tubes: the inner tube serves as a coil former and provides some
mechanical support whilst the outer tube (shown in white) acts as the
main mechanical support and also as a weather shield. There is
sufficient space to fit a coil made from 3 mm or 1/8 inch
copper wire or microbore tube: this is intended as a way of minimising
RF losses from the skin effect.
The top section (gold) has a 3/8-inch tapped hole in the top that
accepts the threaded base of the whip antenna element (or any other
element with that fitting). It also has an axial hole and grub
screw arrangement that accepts and clamps the wire coming from the top
of the coil.
The prototype design model shows screw holes for joining the parts, but
a final design would probably use adhesive and sealant for a more
Stainless steel (316) has been selected for the antenna base,
radials and top caps; it does not have the conductivity of aluminium,
and is much heavier, but it is relatively cheap, highly robust,
resistant to corrosion, and cheaper / easier to machine than
titanium. The resistive losses were thought to be acceptable.
The radiating element in this case is a very flexible and tempered
grade of stainless steel (unknown alloy), although it too could have
been manufactured from 316 alloy rod. The resistance of this
element is relatively high so some loss is expected; its small
(tapering) diameter is expected to give a relatively narrow bandwidth
antenna. On a positive note, the element is extremely flexible
and unlikely to be damaged by wind.
The insulating sections (coil support and outer support / weather
protection) are made from high-quality glass-reinforced resin composite
"pultruded" tube. This material has very good strength and
(essential qualities when supporting such a long antenna in high
There is a potential for some corrosion of the aluminium alloy mast if
the junction with the stainless steel base becomes wet (250 mV
difference in electrochemical potential). In practice this is not
likely to be serious and can be remedied by anodising, aluchroming or
painting the aluminium mast section.
The antenna was mounted on a telescopic mast and raised well clear of
buildings to about 4m above ground. For reference measurements,
an identical assembly was constructed with the matching coil replaced
by a straight length of wire.
Measurements of impedance seen at the base of the antenna were made
using a Vector Network Analyser (VNA): the unit used was a "Mini-VNA
" which is
available from my local emporium
The Mini-VNA is a very versatile and useful piece of kit: in this case
it permits complex impedance (resistance and reactance with the correct
sign) to be measured at a series of frequencies within a desired
range. In combination with an excellent piece of software called "ZPlots
", the results can
be graphed and even represented on a Smith chart. The Smith chart
gives a particularly clear way to visualise how the matching process
Measurements were made using a short length of coaxial cable the
electrical effects of which (loss and phase shift) were allowed for in
The un-matched antenna shows a resistive impedance (left-hand vertical
axis) that is close to 50
Ohms within the 4m band (shown approximately by the two markers M1 and
M2) but it has a large capacitive reactance (capacitive because the
sign is negative, indicating a phase lag). The overall impedance
is approximately 50 - j75 Ohms in the band centre. This is roughly
in line with
expectations. It is worth bearing in mind that the measurements
include the effects of the quarter-wave radials which are cut to be
resonant in the 4 metre band but which will not provide a "solid"
groundplane at other frequencies!
The same data were plotted on a 50-Ohm impedance-type Smith chart
(above) with the markers set at either side of the 4 metre band.
The target impedance (50 Ohms) is located at the centre of the plot
(labelled "1"). The horizontal axis represents resistance in
multiples of 50 Ohms, running from short circuit on the left to open
circuit on the right. Reactance is represented by a deviation
below (capacitive) or above (inductive) the central axis. The
circle-segments which appear to emerge from the extreme right and cut
the outer edge of the plot are lines of constant reactance (with values
normalised to 50 Ohms). The circles which are all tangent to the
right-hand side are lines of constant resistance: the circle
that passes through the centre of the plot marks all points whose
resistance is 50 Ohms (only one of which - the centre point - is purely
For example, consider a frequency in the middle of the 4m band (between
the markers). The conventional plot shows that the resistance is
about 50 Ohms, and the two markers "straddle" the 50 Ohm circle
on the Smith chart. The reactance in the middle of the band
is approximately -75 Ohms, which is equivalent to -1.5 times the target
impedance of 50 Ohms. The two markers sit mid-way between two
circles of constant normalised reactance labelled "1-" (equivalent to
-50 Ohms) and "2-" (equivalent to -100 Ohms).
If any measured impedance plots
inside the green circle on this Smith chart, the SWR will be less than
2:1. The closer the impedance at the frequency of interest can be
brought to the centre point,
the better the match.
To match the antenna it is necessary to insert a series
inductance whose positive reactance cancels the negative capacitive
reactance of the antenna without affecting the apparent resistance.
On a Smith chart, a series inductance moves a point along a circle of
constant resistance by an amount determined by its inductive reactance
(which varies with frequency!). The second Smith chart
shows how the points defined by the two markers have been moved almost
to the centre of the diagram. The match is not quite perfect:
there is still a slight capaciticve reactance across the band and the
resistance has crept up slightly, possibly due to losses in the coil
(although the coil was substantial in this case!). The worst
match occurs at the lower end of the band (light blue marker) whilst
the match at the upper (FM) end is very good. (The small
discontinuities result from problems measuring the phase accurately -
the trace should resemble a smooth loop!)
Note that the other, out-of-band, impedances have also been affected by
the series inductance. They have all been moved around circles of
constant resistance by differing amounts as the inductive reactance
varies with frequency. The result is that the impedance plot has
been "looped" near the 50 Ohm point. This antenna would therefore
offer a reasonable match over quite a wide band of frequencies.
The impedances can be used to calculate the SWR to give a more
conventional if over-simple view of the matching process.
As noted, the best match occurs around the top of the band - slighty
more inductance is required to move the minimum SWR towards the middle
of the band. The SWR does not reach 1 because the resistance has crept
up just a little and the reactance was not quite fully cancelled out.
Note that it might be possible to improve the bandwidth somewhat by
using a larger-diameter radiating element: the thin, tapered vehicle
whip is expected to give quite a narrow bandwidth.
The antenna has been tested "on-air" using the "Parrot" (MB7FM) located
near Tring (at a distance of about 53 km). The antenna was sited
near Kingston in Surrey (my home QTH) and raised to about 5 m above
ground level clear of buildings. The feeder was about 25 m of
RG213 co-axial cable and the exciter was a converted FM1100 PMR radio
with its front-end filters optimised in the 4 metre band (which does
make quite a difference!). Using a power of about 25 Watts, the
"Parrot" receives and repeats my signal with smooth noise and good
readability. This is not a definitive test of
performace of course, but in the absence of proper gain measurements
it does show that the antenna is useable!
There is no new material in this project: the design and matching of
5/8-wave antennae is quite well known.
The project has taught me several practical lessons, both regarding
practical matching and also practical aspects of design and
construction. The result has been a practical prototype antenna
which will now be subjected to life testing and further tweaks, and
from which may come a more easily constructed "production" version.
This page is under construction. More details will be added
concerning the construction, adjustments and assembly of the antenna
and the evolution of the prototype.