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HB9ABX > TECHNIK 03.06.20 08:04l 121 Lines 6792 Bytes #999 (999) @ WW
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Subj: mag loop ant-constr.hints
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Sent: 200603/0648z @: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 : 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). The diameter of the coupling loop is about 20% of the diameter of the
transmitting loop. One end of the coupling loop is soldered to the center
conductor of the coax, and the other end of the coupling loop is soldered to
the braid of the coax.
There are several types of feeding: either capacitive coupling or inductive
coupling. The easiest one being just a simple wire loop as you see here.
Here you see a capacitive coupling and here you see a gamma coupling. The
position of the coupling loop is inside the loop, exactly opposite to the
capacitor. At this point, the voltage of the transmit loop is zero.
The distance between coupling loop and transmit loop varies from 0 to 6 cm.
By changing this distance, the lowest SWR is adjusted. Fine adjustment can be
done by changing the form of the coupling loop, wider or smaller.
A good coupling loop is the symmetrical loop.
The environment close to the loop influences this adjustment.
With proper adjustment a SWR lower than 1.1 can be reached.
Main loop and coupling loop should not be connected directly, as RF coupling to
the feeding coax may appear easily and can produce RF interference (TVI/BCI),
therefore no direct connection between center of main loop and coupling loop.
However, the main loop may be charged by static electricity and discharges by
producing QRM bursts. This can be eliminated by inserting a small coil between
the center of the main loop and the coax braid 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 .2 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.
In tubes, the current flows only on the surface due to the skin effect,
therefore the use of foil is an interesting method.
Very efficient and lightwight loops can be built by using a thick plastic
support and placing copper foils over this structure. The foil can be placed in
narrow strips in the direction of the loop circumference, as placing the foil
in one step produces crumpling.
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.
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.
Note that the roof above the loop should not be a closed metallic
construction.
It's important to observe that the feeding coax below the loop is kept in the
symmetric center between the two half loops straight downward at the length of
one loop diameter.
By not observing this rule, RFI may be generated !
In any case, I recommend to insert a broadband current choke into the feeding
coax, about 1 m away from the transmitting loop, as stray currents on the coax
cannot be prevented completely.
The loop capacitor needs to withstand high voltages and high currents.
Butterfly capacitors are a very good choice as they have no sliding contacts.
Variable vacuum capacitors are an excellent choice.
100 watts RF power produce about 4000 volts on the capacitor . Required
distance between plates is ~ 1 mm per 1000 V.
See here: http://www.surplussales.com/variables/airvariables/airvar1.html
A DC motor with strong reduction (about 2000:1) serves to control the
capacitor. Suitable motors can be found in airplane or car model shops.
-----
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.
Surely, a full size 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 bandwidth is always very small and covers only a few KHz. The high Q of the
antenna allows a selective receiption and suppresses effectively QRM of nearby
BC stations, as well as other QRM.
Here I am using 2 loop antennas, one for 3.5 to 10 MHz and one for 14 to 30
Mhz, both antennas with only 85cm diameter, below the roof.
The 14 to 30 MHZ antenna with 50 W output allows for regular worldwide contacts
with good results.
The antenna can be built easily 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.
With best 73, Felix, HB9ABX (felix-abx @ gmx.ch)
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