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HB9ABX > TECHNIK 08.06.20 09:14l 121 Lines 6149 Bytes #999 (999) @ WW
BID : 86UDB0FHN02G
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Subj: Magnetic Loop Antenna
Path: DB0FFL<DB0FHN
Sent: 200608/0813z @:DB0FHN.#BAY.DEU.EU [JN59NK Nuernberg] obcm1.07b12 LT:999
From: HB9ABX @ DB0FHN.#BAY.DEU.EU (Felix)
To: TECHNIK @ WW
Reply-To: HB9ABX @ HB9EAS.CHE.EU
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Magnetic Loop Antenna
The magnetic loop antenna is an extremly efficient short wave antenna
for the small size it constitutes.
It consists of a loop radiator made of copper or aluminium tubing and
a tunable capacitor.
The size of the antenna is very small as compared to the size of a
traditional antenna as dipole, beam, quad or vertical. The diameter
of the loop is in the range of 1/10 to 1/100 of the wavelength.
The antenna works with the magnetic component of the EM field, which
extends to the both EM components on larger distance. For that reason
the antenna operates well close to ground and radiates a much
stronger signal than a dipole when both are close to ground.
Of coarse, a dipole mounted in its optimal hight radiates better than
a magnetic loop, but due its efficiency at low height the magnetic
loop is an excellent portable antenna or may be used well as indoor
antenne when external antennas are not permitted.
The capacitor of the antenna needs to be remotely adjustable to allow a
frequency tuning range from 1:2 or 1:3. When properly built, the SWR is below
1.1 on the tuned frequency over the full tuning range.
The bandwith is always very small and covers only a few KHZ. The high Q of the
antenna allows a selective receiption and suppresses effectively QRM from
nearby BC stations, as well as other QRM.
Here I am using 2 loop antennas, one for 3.5 to 10 MHz and one from 14 to 30
Mhz, both antennas with only 85cm diameter, below the roof.
The 14 to 30 MHZ antenne with 50 W output allows for regular worldwide contacts
with good results.
The antenna can easily be built as homebrew project if one can find or build a
suitable capacitor.
Below you will find a loop calculation program for your own design together
with detailed instrucions for magnetic loop antennas from 3.5 to 30 MHZ.
The following links work only, if you are connected via Internet
http://www.dl0hst.de/magnetlooprechner.htm
Magnetic Loop Antenna : Construction hints
----------------------------------------------------------
The following instructions should be observed for successful construction and
operation of magnetic loop antennas:
DANGER : IMPORTANT NOTES !
The radiated field is very concentrated and may produce health problems.
Therefore, one has to keep distance to antenna of at least 5 meters
if the power exceeds 10 watts.
Coupling to the loop is done mostly at the lower side of the loop and the
tuning capacitor is placed on top. Due to mechanical stability I installed the
capacitor with motor on the bottom and the coupling loop on the top without
having any deficiency in HF radiation.
There are several coupling loop constructions. The simplest one is by forming a
loop of installation wire (bare or isolated), or using a coax cable (type
RG58).
When using coax, cut the braid about 3 cm from the end and remove the
isolation from the center conductor over 1 cm length.
Form now a loop of the desired length with the end of the coax cable and solder
the free inner conductor to the braid at the beginning of the loop. Cut
carefully the coax isolation over the braid to obtain a small hole to allow
soldering.
The coupling loop has to be deformed slightly in order to reach best SWR.
The distance between main loop and coupling loop can vary from 0 to 2 cm to
obtain lowest SWR over the full tuning range.
The environment close to the loop influences this adustment.
With proper adjustment a SWR below 1.1 can be reached.
Main loop and coupling loop should not to be connected directly, as RF coupling
to the feeding coax may appear easily and can produce RF interference.
However, the radiator may be charged by static electricity and discharges by
producing QRM bursts. This can be eliminated by inserting a small coil between
center of main loop and the coax shield of the feeding coax.
If this connection is done without a coil, radio interference (RFI) may be
produced due to small misbalance.
Coil data: 40 turns of .3 to 0.5 mm enameld copper wire at 1cm diameter wound
over 2 cm length.
The main loop may be made of tubing (copper or aluminum) or thick coax cable.
If coax cable is used (RG213 or RG8 or similar) the inner conductor and the
braid (= shielding/ground) is soldered together at both ends. These ends are
then connected to the capacitor.
Very high current flows in the main loop. Therefore thick and short copper
wires are required to connect the capacitor.
The form of the main loop may be square, n-square or round.
The round form is most efficient as the losses are minimal. (Best ratio of
L:R).
Nearby environment affects the SWR strongly.
In free field, the body of the loop should be 2 loop diameters above ground.
Good are 5 loops diameter hight, higher elevation gives only small signal
difference.
If the loop is installed below the roof, then keep 10 to 30 cm space below roof
brick and avoid nearby lines and metallic constructions.
Of coarse, the roof above the loop may not be covered by large metallic foils.
It's important to observe that the feeding coax below the loop is kept in the
symetric center between the two half loops straight downward at the length of
one loop diameter.
By not observing this rule, RFI may be generated !
The loop capacitor needs to withstand high voltage and high currents.
Butterfly capacitors are a very good choice as they have no sliding contacts.
100 watt hf power produce about 4000 volts on the capacitor
(see program). Required distance between plates is ~ 1 mm per 1000 V.
A DC motor with strong reduction (1000:1 to 2000:1) serves to
control the capacitor. Suitable motors can be found in airplane
or car model shops.
(E.G: Robbe No.4103 with 2430:1 reduction).
I recommend to use pulsed DC current to control precisely DC motors.
By adjusting pulse ratio properly, small increments may be controlled
perfectly.
The remote control motor may be fed through the same coax
feeding the loop, hereby no separate control cable is required.
73, Felix, HB9ABX ( felix-abx at gmx.ch )
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