Why is an Magnetic Loop antenna so special, this antenna is picking only the MAGNETIC part of the ELEKTRO MAGNETIC radio wave. The big advantage of this antenna is that the electric interference from the big city (streetlights, television's , cars etc...) have no influence on the received signal. With the loop you can hear other stations that you can't hear if you use a DIPOLE, with a dipole the stations are buried in the noise.
This is the first loop I build from a article in the QST from February 1996, it's 30 Inch-diameter, and it's designed by G2BZQ/WØ for 80 M.
The first one turn loop
that I built was made from 75 Ohm TV Coax and with a small explanation
in the RSGB handbook for radio amateurs. I used the outer screen from the coax
and the results of the loop where good.The next loop I
built is a octagon loop in 15mm copper tube with a circumference of
4.8 meter (16 feet).
The frequency range of this
loop goes from 14 MHz to 7 MHz and works fine. The
biggest problem is the tuning capacitor, if you transmit with a power
of 100 W you need a
capacitor with a voltage rating of 5000 Volt.
A capacitor which can handle
this voltage is hard to find over here and if you find one they are very
expensive.
The first capacitor I built
was a design from GW3JPT from a article in the RADIO
COMMUNICATION from February 1994. It is a split stator capacitor with a
capacitance of 140 pF and with a voltage rating of 6000 Volt.
The capacitor is remote
tuned with the use of a small BBQ spit motor.
The second capacitor I built
is my own design and it's a butterfly capacitor because the losses are lower
than a split stator.
The capacitance is 5-65 pF
and the voltage rating is 7200 volts. I used
it for the small loop with a
dia. of 800 mm (2.66 feet) and the frequency range of this loop is from 28 MHz
to 14 MHz. The Aluminum plates of 1 mm
for the capacitors are cut
with a JIG SAW.
Most asked Questions:
I`d like to talk a little
more on your setup. it seems like something whichI could get together if only
some more data was available. do you have anynotes etc still laying about since
its build ?
The theory for calculating the loop is very simple.The circumference of a
magnetic loop is 1/4 wave of the designfrequency.
Example for 14 MHz.
300 / 14 MHz = 21.428 m is 1 wave
21.428 / 4 = 5.357 m is 1/4 wave circumference
5.357 / 3.14 = 1.706 m diameter.
The recommendations are that
you can tune the loop from the design frequency to the frequency divided by 2 to
keep the effiecency acceptable.
14 MHz / 2 = 7 MHz
I made the small loop (800mm / 31.5 ") from soft copper tube on a role that
you can buy in a plumbershop and it's easy to make a nice circle if you draw on
the ground a circle with a rope and a piece of chalk.
For mounting the loop to the hardboard I used plastic clamps
that they use for mounting copper tube on the wall.
why is a butterfly
capacitor better?
For high voltages and
currents the use of Capacitors with wiper contacs is not recomended. That's why
they use capacitors in serie's.The pro for serie's capacitors is that the
voltage rating is doubled.The anti is that the value of capacitance is divided
by 2.
For the split stator
capacitor the 2 capacitors are connected in series
by the shaft (bleu) and the red spots on the first drawing are losses.
For the butterfly capacitor the 2 capacitors are directly connected
in series by the rotors and gives less losses.
Do you know of anyone
that has built a similar loop that
outperformed a garden variety dipole?
Compare antenna's is very difficult , sometimes I have for 60 % better signals
in RX and TX on the loops then on the dipole.
In Theorie is the performance of a magnetic loop - 0.4 dB lower then a dipole or
a vertical .
I have over here a homebrew trap dipole from 40-20-15-10m and the height
aboveground is only 7 m(23 ft), for a good performance on 40 m the dipolemust
have a height of 1/2 wave above ground ( 66ft). I don't have a radiation angle
on 40 m and it's only good for contacts in Europe and not good for DX, now the
1.5 m loop tuned to 7 MHz with a effiecency of 38 % (38 w ERP ) and a angle
radiation of about 20 degrees performs
better than the dipole because the vertical magnetic loop only 1 M above
ground as a angle radiation and the dipole don't.
Another advantage is that the reception on a loop is mutch better,
on 20m I have with the dipole S5 noise from the big city, if I switch to the
loop I have S1 noise and hear stations who are buried in the noise when
I use a dipole.
Coupling loop dimensions?
I find that the best way to feed the loop is with the shielded 1/5 Faraday loop
made from coax RG213 or RG8, I tried the gamma match bud I had problems to keep
the VSWR low on all Bands, the shielded loop gives on all bands VSWR 1.1 and
reduce more noise pick-up then the gamma match.
I found out that if you use a 1/5 Faraday loop, that the loop is to big, making
the loop smaller with 0.5 inch by the time in circumference and checking with a
field strength meter you can see that the radiated power increase.
The place off the feeding
loop is placed at the electrically neutral point on the loop and that is 180°
from the capacitor and I have the best results with the feeding loop close to
the ground and the capacitor far from the ground.
I was wondering if you worried about the resistance of the mechanical joints (copper pipe bolted to the capacitor) significantly reducing your radiation efficiency as I think the radiation of these antennas can get as low as .01 ohms
Soulder or weld the
capacitor plates is always the best, but I'm afraid if you make the spacers and
the plates in ALU that with welding everything is gona bend from the heat and I
know from practice ( I work in a maintenance workshop ) that welding ALU is
coarse. Another possibility is using all brass or copper and solder, there are
hams that using double PC board for the plates.
I made a QSO in phone with Florida, RPRT 5-5 and the other station used a
vertical antenna, with the small loop (800mm and theoretical effiecency 41 % on
14 Mc) vertical in the garden and the states side is through the house. I was
very happy with the results , so I think that a capacitor
maded with torqued compressed joints is good enough for using 100 W.
Have the dissimilar metal
joints weather well?
To keep the oxidation low on the dissimilar metals I used a thin coat of
vaseline after assemble the capacitor and with the tupper ware a like plastic
box it is good protect against all wheather conditions.
How to find the radiation angle of the antenna?
Can it be found practically?
Finding the radiaton of a
magnetic loop is very easy, with a TL-lichgt tube you can see it, with abt 10 w
power on the the loop with the TL-tube in the plain of the loop at right angle
to the circle you see the tube lightning, there where the the light is the
farest on the tube thats the radiation angle.
When you refer to washers, nuts and rods you use the term "M6".Please forgive my ignorance, but to what does "M6" refer? Does this mean 6mm?
M6 is (M=metrical) and 6 is
indeed 6 mm threaded rod and
you can compare the size with W1/4" (6.35mm) .
A very easy to build Piston Capacitor.
How to build your own Butterfly Capacitor.
The best material for the
front and the back is CLEAR PVC 3 or 5 mm thick as alternative you can use GREY
PVC or 2 sheets pcboard together with the copper removed .
The best material for the washers, nuts (M6) and threaded rod (M6) is brass or
stainless steel,( NON MAGNETIC MATERIALS for the losses).
For the spacing of the vanes you can use 2 washers M6= ( 6Kv) or a nut M6 =( 12 Kv) if you use aluminum plate 1 mm thick.
If you use a nut then the
best thing to do is remove the thread by drilling withØ 6.2 mm.
The effective area for the vanes is 11.7 cm² and with the formula
for 2 washers = (0.0885 x 11.7 cm²)/ 0.1 cm = 10.35 pF for 1 air gap.
for 1 nut = (0.0885 x 11.7
cm²)/0.2 cm = 5.17 pF for 1 air gap
Example:
If you you make a capacitor with 2 washers as spacing and you make
5 rotor vanes and 6 stator vanes then you have 10 air gaps.
10.35 pF x 10 = 103 pF + 10 pF stray capacitance = 113 pF / 2 = 56 pF
The final result is a capacitor with a value from 5 - 56 pF.
Last Update 03 December 1999. ON4CEQ Van Herck, Tony QTH: Anwerp( JO21FF ) BELGIUM.