"DX from your apartment" by PA0FVH"
A magnetic loop antenna is easy to construct and is suited for working DX.The magnetic loop antenna uses the magnetic component of the electromagnetic field and because of this it size is much smaller than the conventional electromagnetic antenna like the dipole. This makes this antenna ideal for apartment and or limited space environments. In contrast with vertical antennas the magloop does not need any radials while its low take of angle can compete with vertical antennas. The bandwidth of the magloop is very small on its resonance frequency which means that you have to tune the antenna a lot more than a conventional antenna. For apartment dwellers I recommend to use the digital modes of operation or CW to avoid RFI at your neighbors, which are many and nearby. If you go digital like PSK or RTTY the small bandwidth of the magloop is not a big problem anymore, because the digital modes frequencies are within a 10 to 20 kHz spectrum in our amateur bands. That's why little re-tuning is needed for working with the digital modes.
Tips to construct a magnetic loop antenna:
• Use both the web and the ARRL antenna book to get yourself familiar with magnetic loop antennas.
• Don't try to construct a 160 - 10 meters loop, but focus on 160-80, 40-20 or 20-10 meters.
• Use a magloop calculator, like the one from Glenn KI6GD to get information about efficiency, bandwidth, capacitor value and voltage.
• For maximum efficiency, with commercially available components, make the loop as an octagon, use copper pipe with an OD as big as you can get. In DIY stores copper pipe OD 22mm was the maximum I could get. As efficiency increases, bandwidth will decrease!
• The lower leg of the loop should be 50cm plus the diameter of the loop above the ground.
• Find a suited vacuum variable capacitor 5kV for 100Watts PEP transceiver output. Remember that a vacuum C has at least 10 turns for min max tuning; this avoids a huge gearbox on the DC motor to drive the capacitor for tuning. Try to get a vacuum C that suits your frequency range, this will make tuning more accurate than for instance a 1000pF capacitor. A stepper motor with a wide spaced air variable capacitor could be an alternative.
• Use a gamma match to drive the loop, in my opinion more robust than the faraday loop.
• An antenna analyzer is a useful tuning aid, however you could tune the loop on max noise.
• Instead of using a vacuum capacitor you could use a piece of coax cut for the capacitance needed. I have tested my loop is way on 20 meters.
Data of my loop:
- Copper pipe OD 22mm
- Diameter of the loop 118cm
- Vacuum C 4 - 100pF (10 turns) ex. military equipment
- DC motor with 1:150 gearbox
- Frequency range 30 up to 12 meters
- Efficiency 60 to 97%
- Gamma match
I have made contacts with New Zealand, Antarctica and the America's with this antenna from my apartment patio in digital modes with 40Watts. After using the magnetic loop I store the antenna inside. The last frequency used by the antenna is stored in the antenna analyzer, so next time you use the antenna you know its frequency (if you don't use the analyzer for other measurements in the meantime).
More gain and bandwidth needed?
A figure-8 magnetic loop antenna could be the answer. Read all about the experiments of Otto (PA9OK sk) with this double loop antenna on the website of PA0FRI. Preliminary measurements (performed on a version for 144MHz) showed a gain of 3dB for the figure-8 loop against a single magnetic loop antenna. The bandwidth of this type of antenna has increased too. The capacitance needed is about twice what is needed for a single loop.
Stepper motor control?
WinStepper is a program written by Lodewijk PA3BNX to control stepper motors. The program drives the stepper motor which is connected to the variable capacitor of the magnetic loop antenna to the disired frequency. For CAT controlled tranceivers a calibration table is included to lock the stepper. Tuning of a magnetic loop antenna hasn't been that simple with this program.
Pictures of the circuit diagram are included in the software. Please follow this link to download the program or the readme file.