Projects & Technical
A magnetic loop antenna for HF
October 1997's Novice Notes looked at
compact antennas that amateurs use to operate from confined locations. The
smallest antenna described for 80 metres was a magnetic loop. This article
provides all the details needed to build your own.
Description
Able to cover all frequencies between 3.5
and about 10 MHz, the loop described here is directional, does not require a
radial system, and stands just 1.8 metres tall. Most parts needed can be
purchased at a hardware shop. The antenna can be put together in an afternoon
and requires only hand tools to assemble. It should cost less than sixty
dollars to build.
Shown below is the schematic diagram for the loop. Note that the element is continuous except for a gap at the top across which the variable capacitor is wired. The feedline is connected to the bottom of the loop.
Also shown is the physical construction of
the antenna. The loop element is 1.5 metres square and is supported on a wooden
cross. To minimise losses, thick aluminium strip is used for the element. At
the top of the loop is a high-voltage variable capacitor. This is used for
adjusting the antenna to the operating frequency. Because of its narrow
bandwidth, the tuning is very sharp and a vernier drive has been added to make
tuning easier. Dimensions are not particularly critical, provided it is
possible to bring the loop to resonance on all operating frequencies with the
variable capacitor used.
Parts needed
The following materials are required to
build the antenna:
3 2m lengths of 3x20mm aluminium strip
1 1.8m length of 20x44mm pine
1 1.5m length of square (12x12mm) wood
1 polyethylene chopping board (medium or
large size)
1 150 x 80x4 mm piece of stiff high-voltage
insulating material (eg bakelite)
2 right angle metal brackets
1 20-400pF high voltage variable capacitor
1 6:1 vernier reduction drive (Dick Smith No
P-7170)
small length of coaxial cable braid
RG58 coaxial cable (any length) and PL259
plug
screws, nuts and miscellaneous hardware
Many of the above items can be bought at
hardware shops. The main exception is the wide-spaced variable capacitor. These
are almost unobtainable commercially, though you could try Daycom in Melbourne.
Other possible sources include old high power transmitting equipment, hamfests
and deceased estates. The exact value of the variable capacitor is not
particularly important, provided it is at least about 400pF. The capacitor used
in the prototype was a two gang 200pF unit with 2mm spacing between the plates.
The gangs were connected together to provide the needed maximum capacitance.
If your attempts to obtain a suitable
capacitor fail, there is always the possibility of making one. Full
construction details appear in DK1NB's magnetic loop design program (details
later).
Construction
The first step in assembling the loop is to
make the wooden cross that supports the aluminium element. This is done by
bolting a 1.5m horizontal cross piece to the 1.8m vertical section. A white
polyethylene chopping board is used for the antenna's base. The two
right-angled brackets are used to attach this to the vertical section.
The next step is to bend the three lengths
of aluminium so that they form a 1.5 metre square loop able to fit on the frame
when bolted together. As is visible in Figure Two, two pieces are "L"
shaped, while the other is bent into a shallow "U". Note that the two
L-shaped pieces are about 10cm apart at the top of the loop. These are
physically joined by the bakelite insulation block that is attached to the top
of the length of pine. The upper L-shaped pieces meet with the lower U-shaped
piece at points 'v' and 'w'. The overlap is about 40-50 millimetres. Make the
electrical connection at these points as good as possible. To achieve this,
sand the aluminium at the point of contact and use two or more small bolts to
hold the pieces together. Use special conductive paste if available.
The variable capacitor is mounted on a home
made metal bracket so that its shaft faces downwards. To the shaft is attached
a vernier reduction drive. Use either small brackets, fishing line or glue to
fasten the frame of the reduction drive to the 1.8 metre vertical section. Note
the thick, low-resistance conductors between the end of the loop and the tuning
capacitors. Braid from a length of coaxial cable was used in the prototype.
Make these connections short to minimise losses.
The loop is fed at the bottom. The braid of
the feedline connects to the centre of the lower horizontal element (see
diagram, point 'x'). The inner conductor connects to the loop at point 'y' via
a 900mm length of coaxial cable (inner and braid soldered together). At both
'x' and 'y', a small bolt, nut and eye terminal connector is used to make
connections to the aluminium element. The distance between 'x' and 'y' and the
length of the coaxial cable may both have to be varied for proper matching -
this is discussed later.
Adjustment
The object of the adjustment process is to
adjust the section between 'x' and 'y' until the antenna's feedpoint impedance
can be made to equal 50 ohms on the bands of interest.
The first step is to connect the antenna to
an HF receiver tuned to 7 MHz. Set the receiver's RF and AF gain controls to
near maximum and the antenna's capacitor to minimum capacitance (plates fully
unmeshed). Then gradually increase the capacitance. Not much will happen at first,
but the noise from the receiver should gradually start to increase. Further
adjustment of the capacitor will result in the received noise falling. Turn the
capacitor back to the position where the noise peaks. Depending on the value of
your capacitor, the plates should be around a quarter meshed at this point.
This test confirms that the antenna can be tuned to 7 MHz.
Repeat the process for 80 metres. This time,
the noise should peak when the capacitor is near maximum capacity. If it is not
possible to obtain a peak, try setting the receiver to a higher frequency (4 or
5 MHz) and tune for a peak. If a peak is obtained there, but not on 3.5 MHz, it
is likely that the variable capacitor's maximum capacitance is too low for
eighty metres. Possible remedies include substituting a larger capacitor,
connecting high voltage fixed capacitors in parallel with the variable
capacitor or making the loop larger.
Having confirmed that noise peaks can be
obtained on all frequencies of interest, it is now time to ensure that the
antenna's impedance is 50 ohms at these frequencies. This entails making
adjustment to the antenna's feed pont.
The use of a resistive antenna bridge is
recommended so that you can make antenna measurements without radiating a
signal. If all you have is a conventional SWR bridge, make adjustments during
the day to minimise the risk of interference to other stations.
Position the antenna near its final
operating position (which should be out of other people's reach). Set your
transceiver to about 3.580 MHz. Adjust the variable capacitor for maximum
received noise. Transmit a steady carrier and note the reflected power or SWR.
Adjust the transmitter up and down 40 or 50 kilohertz to find the precise
frequency where the SWR is lowest. Note the reading at this frequency. If you
are lucky, the reflected power should be nearly zero. Otherwise, adjust the
length and position of the 900mm lead joining the feedline to point 'y' and/or
the spacing between points 'x' and 'y'. You will find that there is some
interaction between these adjustments and the setting of the variable
capacitor. Every time a change has been made, adjust either the transmitting
frequency or the antenna's variable capacitor for the point where reflected
power is lowest. Repeat these procedures until reflected power is either zero
or close to it.
When making these adjustments, there is a
temptation to leave the transmitter keyed while making changes to the antenna
or adjusting the variable capacitor. This should not be done for two reasons.
The first is that the voltages at the top of the antenna element can be quite
high (hundreds or even thousands of volts) even with quite low transmitting
powers. The second is that the loop is detuned when people are near it. Thus
any adjustment made when you are near the loop will not be optimum when you
move away. This effect is particularly pronounced on higher frequencies, and
applies to metal objects as well as humans.
Once a length and position for the 900mm
coaxial cable has been found, along with an appropriate spacing between 'x' and
'y', all further adjustments can be done with the antenna's variable capacitor.
Operating the antenna is described in the next section.
Operation
The Q of this antenna is very high. This
means that it can only operate efficiently over a narrow frequency range (5-10
kHz typical). Almost every time you change frequency, you will have to change
the setting of the variable capacitor.
As mentioned before, this is done by peaking
the capacitor for maximum received noise at the desired operating frequency. If
the reflected power is high, make further adjustments until it is acceptable.
Again the use of a resistive-type bridge (rather than a conventional SWR meter)
is preferred because of the ability to tune up without causing interference.
Note that the loop is directional, with a
sharp null when the element is facing the direction of the incoming signal.
This makes its behaviour different to that of full-sized quad elements, where
the null is off the sides of the loop. This directivity can be useful when
nulling out interference. It is also useful to remember when other stations
report difficulty in hearing you - turning the loop may improve your signal.
Results
This loop has been used extensively on
eighty metres. Most contacts have been made with the antenna indoors. Though
performance is well down on a dipole, contacts into Western Australia and New
Zealand have been made with it. The power used was twenty watts. Lower powers
have been tried, but results have not been good.
Contests are always good events to test the
effectiveness of new antennas. During July 1997's hour-long 3.5 MHz
Australasian CW Sprint, twelve contacts were made with the loop. This was
despite the added handicap of having to retune the antenna with every
significant frequency shift.
As would be expected, the loop's
disadvantage when compared to full-sized antennas falls with increasing
frequency. On 7 MHz for instance, the theoretical difference between the loop
and a half-wave dipole is barely one s-point. Tests have confirmed the
effectiveness of the loop on 40 metres, though all contacts have so far been
within VK/ZL.
Improving the loop's efficiency
The antenna described is capable of good
results on 80, 40 and probably 30 metres. However, it is a compromise, designed
for low cost and easy construction with basic tools. Doing any of the following
will increase its efficiency and/or usefulness.
1. Use copper rather than aluminium. Copper
is more conductive (but more expensive) than aluminium. This means that a
version of this antenna using copper rather than the specified aluminium is
likely to be more efficient than the prototype. Copper water pipe (the thicker
the better) should be suitable.
2. Soldering the loop element directly to
the variable capacitor will also improve performance and long-term reliability,
especially if the antenna is used outdoors. The reason why this wasn't done in
the prototype was due to the difficulty in soldering to aluminium.
3. Use a single piece of metal for the
conductor to reduce resistive losses. Where this is not possible, either
solder/weld pieces together, or use conductive paste to minimise losses.
4. Make the loop a circle or octagon instead
of a square. Square loops are the easiest to make, but cover less area for a
given perimeter than other shapes. This lowers efficiency.
5. Make the antenna rotatable. The loop's
deep nulls can be used to advantage in nulling out interference from power
lines, TV sets and other stations.
6. Use a larger loop. Efficiency increases
rapidly with loop size. Even a 2 or 2.5 metre square loop should be noticeably
more efficient than the 1.5 metre antenna presented here. The use of magnetic
loop simulation software (see elsewhere) allows one to estimate the improvement
possible by making this and other changes suggested above.
7. Use more reduction on the variable
capacitor to make adjustment easier. The first prototype had only one vernier
drive on the capacitor's shaft. With this arrangement, getting the antenna
tuned to the desired frequency was tedious because the tuning is sharp. If you
routinely change frequency, a second drive is well worth the cost, particularly
if 40 and 30 metres are the main bands of interest.
To perform this modification, install the two
vernier drives in tandem, as shown in Figure Two. If the front drive contains a
0-100 dial, you may find that the knob is limited to three turns and the back
part restricted to 180 degree rotation. To overcome this, remove the knob,
unscrew the 0-100 dial, and remove the c-shaped bracket that is restricting
movement.
Information about magnetic loops
All information used in the construction of
the prototype came from the following Internet sites:-
http://ourworld.compuserve.com/homepages/csl/magloop.htm
http://www.gqrpclub.demon.co.uk/ants.htm
http://www.cdrom.com/simtel.net/msdos/hamradio.html
Hans Joachim Kramer, DK1NB has developed a
DOS computer program useful for those who design magnetic loops. Able to
calculate efficiencies and bandwidths, this freeware program also contains much
useful constructional advice (including pictures) to assist those who
experiment with magnetic loops. This excellent program (mloop31.zip) is
available from the last mentioned site on the list above.
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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.