Since long time was searching for a small but moderately efficient Ham antenna for limited space application. Magloop or Magnetic loop antenna is very suitable for this purpose. Let me state some basic concepts, which I presented during LARC Hamfest on 21-22 Dec 2019 at Hyderabad.
MAGNETIC LOOP ANTENNA
- Radio frequency waves
Generated due to oscillating electrical signal in an antenna in radio frequency
range. Consists of electrical and magnetic components at right angles to each
other, hence called electromagnetic wave.
2.Regular antenna from ¼ to full wave length.
Evolution from simple dipole to full wave loop
These antenna work by receiving electrical field, small current is trancduced in
antenna, Rx amplifies it and converts to audio/intelligence.
WHAT IS MAG LOOP ANT
3.Mag loop work differently, they tranceduce magnetic portion of the wave, like a
secondary coil in a transformer and are basically LC tuned circuit and are not
affected by electrical interference.
Can be practically easily home-brewed from low 3.5mHz to high 30 MHz.
- It is a compromise small antenna, performance is less then a full size wire
antenna on low HF bands.
- The circumference of magnetic-loop antenna as recommended should be less then
0.1 wave length, if more, it will become an electrical loop antenna.
However ARRL suggests circumference to be 0.08 wave length.
- There are equations and tables available to design this antenna, but
experimental results do not always agree.
- There are lot of controversies and opinions on exactly how these antenna work.
This presentation is based on my experiments and opinion.
Advantages of Magloop antenna
- Mag loops are quite – not affected by electrical interference.
2.They are small for given wave length, generally 0.1 lambda or smaller
circumference say 2m for 20meter band. Occupying less floor space.
- Can be mounted close to ground, typically 1 or 2 dia off the ground.
- It is a bidirectional beam antenna, having max radiation in plane of antenna.
1.It requires high voltage high current variable capacitor.
- Has very low radiation resistance so construction requires care to minimise loss
- MGL have small band width typically 1%.
To build one you requires
- High voltage plate type capcitor suitable for desired power output OR
Any other high voltage type like Trombone,sliding disc or coaxial cable.
How to tune
- It is a high Q LC circuit use antenna analyzer to get lowest SWR.
- Listen to peak volume in Rx.
- Large dia conductor will give larger band width.
- For dimensions and values Tables and calculators available on net.
5 .Calculations above give theoretical values. Actual construction affects the
antenna performance greatly. Do not hesitate to vary the parameters and see the
- Coaxial cable loop RG58 with broadcast variable capacitor and torroidal
secondary pick up winding for rX only for 40-20meter band.
- 3 mm dia wire loop 1meter dia, variable capacitor and wire pick up coil for 20
Worked ,quite ,low swr, small band width,good for RX and Qrpp 2watt due to
capacitor plate spacing less.
- Coaxial LMR 400 ,3.3 meter (10 feet) circumference loop, home brewed 2 section
wide spaced gang capacitor driven by 1 RPM DC motor,6mm dia copper pipe pick up
loop 1/5 of main loop circumference. Capacitor switching arrangement to work for
7.00 to 21 MHz.
Good SWR 1:1.1 to 1:1.6 for different bands, band width about 50 Kc. Loaded up to
25 watts and used with Ubitx.
- ½ inch copper pipe loop in construction.
- Manual tuning- required location of loop nearby.
- Remote tunning- Various methods possible
A. Receiver Volume – SWR not 1:1 for TX purpose
B. Field Strength – Continuous transmission required during tuning, may cause PA
overload at mismatched state, requires low output 1-2watt capacity,
modifications to Ubitx.
c. Memory locations – costly develop sketch etc.
D. AGC based tune – Pre tune to peak and fine tune with remote relay tunner with
TX on for minimum SWR.
Developed the above last system With Arduino based Slow Fast tuning With The help
of our Star programmer Om Dhiru VU3CER,by sensing AGC voltage and final relay up
down movement of DC motor manually if required. Tested during Field Day. Gave 1 s
unit less reception and 57 reports with Ubitx 10 watt on 40 meters.