Small Loop Antennas

  • MFJ\'s 1786 Loop
    MFJ\'s 1786 Loop
  • My 10-30 mhz Homebrew Loop        A work in progress
    My 10-30 mhz Homebrew Loop A work in progress
  • Note the off center feed and control cable routing necessitatied by the configuration of the R3 Capacitor
    Note the off center feed and control cable routing necessitatied by the configuration of the R3 Capacitor
  • R3 Capactor & Controller
    R3 Capactor & Controller
  • The R3 Variable Capacitor
    The R3 Variable Capacitor
  • The \
    The \

Low profile operating -- SMALL LOOP ANTENNAS


Magnetic loops are very effective small antennas. these 3 to 3 1/2+ foot antennas perform close to and in some cases (low mounting heights for one) better than even a small beam. The reason is that a magnetic field is much more concentrated than an electrical one, for example a small horizontal loop at 17 feet performs better (lower radiation angle) than a full size dipole at 35 feet, in fact better for DX than a beam at that height because of the much lower takeoff angle. If you place a beam 1 foot off the ground it will only radiate straight up while a vertical loop will still work DX stations quite well.


This smaller more intense magnetic field also has the advantage of greatly reducing TVI - RFI potential if the loop is more than 15 feet or so away from TV antennas, electronics ect. Another advantage of this magnetic field is the very low background noise heard on the loop because most man made noise is electrical fields. For example if you lived next to a Shopping Mall the loop would not hear all that lighting and power transformers. Also reducing interference is the loops "Hi Q" which means that it receives & transmits on a narrow band range compared to the "full size" antennas.

This effect is very pronounced on the lower bands that the loop will work. I have built several loops and along the way have learned quite a bit about these "little wonders". Usually I use the parts from one to make the next but my "ugly loop" is one that I keep in service.


While looking thru old QST's I read up on the Cushcraft R-3 vertical antenna. I made a mental note that the remote controlled variable capacitor looked like it would make an ideal loop component. I found this one on E-Bay for about $25, this was nice considering the most expensive part of the loop is the High Voltage Variable Capacitor and motor drive and controller.The only drawback to this capacitor was the metal mounting (which increases the stray capacitance) and the fact that it was a single stator type not the less lossy butterfly type.


To reduce the losses I directly connected (metal braid) the shaft on the capacitor to the loop itself and polished the capacitor plates to increase power handling. The range of this setup was around 30 thru 180 pf and it will handle 100 watts. I could of reduced the minimum value of the capacitor by eliminating the metal half (future project ?) of the mounting but instead used it as part of the loop. I wanted to test some of the current theories of loop design and this setup was ideal. I built up a PVC frame (easy change of dimensions) and used of all things roof flashing (very thin aluminum) for the loop itself. This cheap material made dimension testing easy, the final size of the loop is 42" in diameter by 1.6" wide. One thing that the loop programs (like mloop32) do not take into account is the fact that if the material is too wide it starts acting like a capacitor not just an inductor. This loop is completely assembled with mechanical (nuts & bolts) connections.


This is a big "no no" in loop construction which I used to make testing of different configurations easy and to test that "all connections must be welded" theory. On the air tests, field effects and bandwidth checks have shown that this loop is very efficient until you tune to 40 meters (around 7%), not that surprising since that band is below the theoretical range of a 42" loop. It sure is fun to stretch it that far and to make 40 meter contacts on such a small antenna. The loop is feed with an 8" diameter faraday loop not on the centerline (null) of the loop, this is because the capacitance circuit break is on the side of the capacitor assembly.


The pictures show how the control wires are routed "off center" but this is in the null of this loop, if you don't do this the wires will couple with the magnetic field and detune the antenna. I found very little performance change with the feed loop off center so I left it there for mechanical simplicity. I learned the hard way about the high voltage's (8,000 V +) in the loop when insulators in the path of RF flow started burning thru (bad) until redesigned. The R-3 controller does a good job of tuning the loop 19 thru 7 MHZ, its meter is a position feedback that lets you know where the capacitor is tuned. A loop like a commercial version like the MFJ "HI Q" Loop (below) or a home brew may have the unique properties that may help you out in your situation.


NOHC Article originally available at