Projects & Technical
The versatile end-fed wire
by Peter Parker VK3YE - first appeared in Amateur Radio, June 1998
A piece of wire of almost any length can be
used as an antenna on the HF bands. However, just because an antenna can be
made to work is no guarantee that it will perform efficiently. This article
will initially concentrate on the half wavelength of wire and its use as an
effective multiband antenna. Information on a simple antenna coupling unit and
tuning indicator for use with these antennas is provided towards the end of the
article.
Length
It was mentioned above that the actual
length of wire used in an end-fed antenna is not critical. However, some
lengths are easier to use than others, particularly if multiband capability is required.
Also, very short antennas (significantly less than a quarter wavelength on the
operating frequency) are inefficient, making it hard to put out a good signal.
A length of one quarter wavelength (ie 20
metres on the 80 metre band) is commonly suggested. Though such antennas do
work, an extensive ground system or counterpoise is required for best
performance. Ground systems can require considerable time and effort to install
and detract from the extreme simplicity of these types of antennas.
An alternative is to use a wire of one half
wavelength in length on the lowest operating frequency. An extensive earth
system becomes much less important. Indeed the author has had good results
whilst using no earth at all. However, for certain other reasons (explained
later) some earthing is desirable.
The antenna described here is forty metres
long, or a half-wavelength at 3.5 MHz. As mentioned before, a substantial earth
is not required. Because a half-wavelength piece of wire exhibits a very high
impedence at the operating frequency (and its multiples), some form of coupling
unit between the transceiver and antenna is required. Its function is to
efficiently transform the transceiver's 50 ohm output impedence to the
antenna's high feedoint impedance. Whether a wire antenna has a high or low
impedence is important because it affects the type of coupling unit required as
well as the need for an earthing system.
So what is the impedence of this antenna on
bands other than eighty metres? We already know that a wire that is a multiple
of a half wavelength exhibits a high impedence at the feedpoint. At 21 MHz (15
metres) a forty metre wire is approximately six half-waves long. On 28 MHz (10
metres) it is eight half-wavelengths. Similarly, our wire is several multiples
of a half wave on other HF bands such as 40 and 20 metres. This means that the
antenna will always have a feedpoint impedence appreciably higher than 50 ohms
and will not require much of a ground system on all bands. It is for these
reasons that 40 metres is a good length for an end-fed wire antenna for the HF
bands.
Benefits and limitations
Because it is fed at one end, people whose
house is near one boundary of the block will probably find this antenna easier
to put up than a half-wave dipole, which is fed in the centre. Another
advantage of this antenna is that no separate feed line is required. This makes
it particularly attractive for portable use as coaxial cable can be quite
bulky.
What are the disadvantages of this type of
antenna? The first is that it requires a matching unit to operate. Each time
you change band you will need to adjust this for best impedence match between
transmitter and antenna. Another risk with these types of antennas is RF in the
shack. Nevertheless, these two problems are not insurmountable, and the end-fed
wire is one of the most cost-effective multiband antennas available.
Erection of antenna
The antenna should be as high as possible.
Have as much of the wire as possible running horizontal, or nearly so. However,
if this is not possible, don't despair; your antenna will still work. The
antenna is not particularly directional, especially on the lower frequency
bands, so orientation is not that critical.
The type of wire used is also not critical.
Medium gauge stranded insulated wire has given good service in the author's
antennas. Ordinary egg-type insulators can be used to suspend the wire. As an
alternative to purchasing these new, insulators can be made from short lengths
of plastic water pipe or conduit.
Either trees, chimneys or specially-made masts can be used to support the wire. Two such supports are normally required for these antennas unless your radio shack is on a second or third storey. In many cases the second support can be a tree in the backyard. It is not necessary to climb this to mount the antenna; with a small lead sinker a fishing line can be thrown over a convenient branch. The sinker is then removed and the line tied to the antenna's insulator. While observing the sag in the antenna wire, pull the fishing line tight. Then release it a little and tie it off at a convenient point. Some sag should be allowed for in wire antennas to allow for movement of the supporting branch in the wind. Always observe the usual precautions about keeping the antenna away from power lines and public thorougfares.
Coupling unit and resistive bridge
The purpose of the coupling unit described
here is to transform the transceiver's output impedence of 50 ohms to the
higher impedence of the wire antenna. Between the matching unit and the
transceiver is a resistive antenna bridge that is switched in to aid the
adjustment of the coupling unit. Shown below is the schematic diagram for the
complete unit.
An L-match circuit consisting of one
adjustable inductor and one variable capacitor is used here. This is simpler
than most other antenna coupling units which require two or more variable
capacitors, a number of inductors and possibly a switch. This simple approach
is possible as the unit is only required to match a limited range of antenna
impedances.
The resistive bridge is used to show when
the L-match is properly adjusted. Using it is similar to a standard SWR bridge
in that you initially adjust the sensitivity control for full scale on the
meter and adjust the L-match until the reading on the meter is zero (or close
to it). However, the resistive bridge is unlike an SWR meter in that it does
not have a forward/reverse switch. Also, it cannot be left in line while
transmitting. Further information on operating the resistive bridge is given
later.
Photo 1 (not on internet version)
shows the completed unit. The variable capacitor adjustment is in the centre of
the front panel. To its left is the ten-position rotary switch for the
adjustment of the L-match inductor. The right-hand third of the panel is taken
up by the resistive antenna bridge. Below the meter movement is the
tune/operate switch and the meter's sensitivity control.
Photo 2 (not on internet version)
shows the inside of the L-match and resistive antenna bridge. The home-made
tapped inductor is mounted just behind the rotary switch. Alongside the coil,
behind the vernier drive, is the variable capacitor. Most of the remaining
space inside the box is occupied by a piece of matrix board that holds the
parts used in the antenna bridge. Because light weight was important, the
prototype is housed in a commercially-available plastic box. Note that to
accommodate the top of the vernier drive, some plastic has had to be shaved off
inside the top lid of the box. This may be visible in Photo 1.
The variable capacitor pictured is a rare
transmitting-type unit. Unfortunately, these can be hard to come by. However,
Daycom of Melbourne may be able to supply a suitable unit. An alternative is to
caniballise a variable capacitor from any valve broadcast receiver, or one of
the older transistorised sets. Unless you are using very low power (a few
watts), the small plastic dielectric types used in modern AM transistor radios
are not really suitable.
Hamfests, junk sales and the like are other
good sources for these capacitors, even if you have to buy the radio that goes
with it. Most variable capacitors that you'll see will have two or three
sections or 'gangs'. Simply use only one gang and ignore the rest. The actual
value of the variable capacitor is not important provided its maximum
capacitance exceeds 150 or 200 picofarads.
A vernier reduction drive and dial adds
greatly to the appearance of the finished product and makes adjustment easier.
The one pictured came from Dick Smith Electronics. However if your budget is
tight and you are unable to find suitable second-hand reduction drives, this
part can be omitted.
The rotary switch used was a salvaged wafer
switch having ten positions. The switch originally had several sections, so the
unwanted ones were removed and the rear of the shaft cut to size. It is
desirable to have a switch with as many positions as possible to allow more
precise adjustment of the coil. If you are unable to salvage a suitable switch,
Dick Smith stocks a small 12-position rotary switch. These are suitable at low
power levels, but the author has not tried them with 100 watts. If all else
fails, an alligator clip and wander lead will be just as effective as the
switch, though somewhat less convenient to use.
The tapped inductor is the other main
component of the L-match. The coil in the photograph was wound on a piece of
25-30 mm diameter plastic tube. Ordinary thin insulated wire was used in the
prototype. The number of taps needed is always one less than the number of
positions available in your rotary switch - thus the coil here has nine taps.
To make a tap, simply remove about 1 cm of insulation with a knife, form the
bare portion of the wire into a hairpin loop, twist and solder. Hold the iron
on the joint for only the minimum amount of time necessary to prevent the insulation
melting off the wire.
The following table gives the coil taps used
on the prototype. Note that the start of the coil is connected to the antenna
socket and variable capacitor and the wiper of the switch is wired to the
antenna section of the Tune/Operate switch.
The end of the coil whose taps are closest
together should be nearest the switch. The reason for this is that these taps
are likely to be used on the higher frequency bands, where the effects of stray
inductance are more significant. It is also for this reason that all
connections between the switch and the coil should be short and thick. The coil
is attached to the bottom of the case with a pair of bolts, nuts and 10 mm
stand-offs, which can be made from an old straight-sided ball point pen.
Transceivers with rotary band switches
normally have the lower frequency bands (eg 80 metres) near the
anticlockwise-most end of the switch's rotation and the higher bands (eg 10
metres) selected when the switch is turned clockwise. Similarly, when you turn
the VFO knob of you transceiver clockwise, the frequency selected will
increase.
The controls on the prototype behave in a
similar way. This is achieved by switching in the whole coil (which may be
required on low frequency bands) when the rotary switch is turned to its
anticlockwise-most position (position 1 in the table above) and successively
smaller portions of the coil as the switch is moved clockwise (position 10 on
the table above). These smaller sections of the coil will be required when operating
on higher frequency bands such as 10 and 15 metres.
The variable capacitor is configured in a
simailar way; as the reduction drive is turned clockwise, the capacitance is
reduced, and the unit is tuned to a higher frequency. However, it is important
to note that this cannot be achieved with some variable capacitors because a
clockwise movement in the shaft increases rather than decreases the
capacitance.
Most of the parts for the resistive antenna
bridge are mounted on a piece of unclad matrix board, which is mounted to the
case with screws and stand-offs. Component values are not particularly critical
except for the seven 27 ohm resistors. The function of these resistors is to
provide a reasonably constant 50 ohm load for the transceiver when the L-match
is being adjusted. For this reason they will be required to dissipate a fair
amount of RF power. Two-watt resistors were used in the prototype. This proved
adequate for use with a twenty watt transceiver provided the carrier was wound
down to 5-10 watts and tuning-up was completed in a reasonably short length of
time. Many modern 100-watt transceivers can be wound back to produce the few
watts required for this tune-up process.
No accidents have been had with the
prototype unit. However, if you routinely wish to use it with high power
equipment, and have a habit of forgetting to wind the power back, it should be
possible to replace each 27 ohm resistor with four two-watt 100 ohm resistors
to increase the unit's power handling capacity. Do not be tempted to use
wire-wound resistors - their power ratings may look attractive, but their
self-inductance makes them unsuitable for a project such as this.
The Tune/Operate switch is a medium-sized
DPDT unit. Again, this has given reliable service with 20 watt equipment.
However, it might be wise to use a larger type if you intend to use this unit
with 100 watt gear.
Other parts are not critical. The panel
meter in the prototype was salvaged from a non-working CB transceiver. The scale
was whited out (using correction fluid) and a new one written over it with
biro. This operation calls for a fair degree of manual dexterity - it is easy
to damage the meter movement if you are careless. If in doubt, leave the meter
as is. The variable resistor could also be a salvaged item; in this case the
volume control from a radio or a tape recorder will be fine.
A pair of binding posts was used for the
antenna and earth terminals. Use colour coding to avoid confusion. The
connection to the transceiver is either via a BNC or SO239 socket. Coaxial
cable should be used between this and the transmitter section of the
Tune/Operate switch to minimise stray capacitance and inductance. Either RG58
or RG174 will be satisfactory.
Adjustment and use
Adjusting L-type couplers is simple. Set the
inductance for maximum noise on the receiver. Then adjust the variable
capacitor to obtain a further increase in noise. Apply a few watts carrier and
switch to 'Tune'. Position the sensitivity control so that the meter is reading
full scale. Adjust the variable capacitor for a dip in the reading on the
meter. If it is not possible to get a zero reading, try a different combination
of coil and capacitor settings until this can be achieved. At this point the
system is tuned up, and the unit may be switched to 'operate'. This step
bypasses the resistive bridge and allows the full output from the transceiver
to reach the antenna. Note that when changing bands or making significant
frequency changes within a band, this process should be repeated to assure full
power transfer.
A counterpoise may or may not be required.
Because the antenna is high impedance, adding one will not normally boost
radiation efficiency or materially affect the settings of the L-match. However,
in some cases, going without a counterpoise can cause RF to get back into the
transceiver and spoil operation. A short length of insulated wire connected to
the earth terminal of the L-match minimises this risk. One or two metres is
usually enough.
In practice, the system described has proved
easy to use, and represents a good way of getting multi band operation from a
single length of wire. There are no lossy traps or feedlines, and the antenna
is easy to erect. Interstate SSB contacts have been made with this antenna on
both eighty and forty metres with powers of between two and twenty watts.
Though no detailed measurements have been made, performance on the lower bands
seems to be roughly similar to a half-wave dipole at the same height.
Theoretically there may be some gain off the far end of the wire on the higher
bands, but whether this is useful depends on the wire's orientation.
References and further reading
1. Moxon, LA HF Antennas for All
Locations, RSGB, 1982, page 154
2. The Radio Amateur's Handbook - 1977,
ARRL, 1976, page 599
3. Cook, R & Fisher, R Amateur Radio,
May 1997, page 20
4. Butler, L Amateur Radio, September 1997, page 15
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This page was produced by Peter Parker VK3YE parkerp@alphalink.com.au. Material may be copied for personal or non-profit use only.