My ten foot air-core receiving loop was originally designed and constructed for NDB DXing.
More recently, it has been pressed into service for LOWFER DX work. The loop has been
instrumental in achieving two notable low-frequency receptions - the first Trans-Pacific reception of ZL (New Zealand) amateur radio LF signals in North America as well as
establishing the present long-distance record for LOWFER reception in North America, the 1-watt signal of "NC" from North Carolina to
British Columbia, an overland distance of 2,360 miles. The loop is a good performer!
OVERVIEW Basically the loop is like most antennas, just a big resonant tuned circuit probe that is coupling into
the radio signal's electromagnetic field. Electrically, the loop looks like the tuned circuit shown here.
The 1-turn link couples signals out of the main loop to be fed to the receiver. The loop is tuned (signals peaked)
with the variable tuning capacitor.
The frequency tuning range of the loop basically depends on the number
of turns in the main coil and the size of the tuning capacitor. My ten-foot loop has 14 turns of
#20 plastic coated stranded copper wire. The tuning capacitor is a 400pf dual variable with both sections
connected in parallel. The tuning range of the loop is from 180kHz to 410kHz approximately. By adding a 250pf silver mica
fixed capacitor in parallel with the loop, tuning will drop to around 135kHz at the low end, perfect
for the new 136kHz band when it arrives. Conversely, by shorting out the first turn in the loop,
the lower end of the broadcast band can be tuned. Reducing the number of turns will raise
the overall frequency tuning range. For the BCB DXer, I would suggest reducing the number of turns to
8 or 9. Going with even fewer turns, the loop would make a great low-noise, directional 160m receiving antenna.
CONSTRUCTION- The loop frame is built from 2 x 3/4 Western Red Cedar. Redwood, Yellow Cedar or
even marine-grade plywood should serve equally well. Perhaps you have something similar, indigenous to
your area that could be procurred. The frame must be strong and weather resistant and of course the
lighter you can make it, the better. The frame must not be metallic! The four legs of the frame are
all 10 feet in length. At each crossover point, a half-lap joint has been cut to join the frame
together. Each joint has been screwed with a single brass wood screw and glued with two-part
exterior water-proof glue. The loop has been outdoors for about 4 years now and has survived
several dozen severe windstorms coming directly off the ocean. Part of its "survivability"
is due to the flexibility of its 2" plastic ABS support mast. This allows the mast to absorb
the brunt of the wind blasts.
There have been several storms where the loop has been bent-over at
a 30 degree angle for long periods yet it continues to survive, so far!
Attached to the end of each frame leg is a wooden "wire-comb" that is used to keep the
loop wires in alignment. The spacing between each wire will determine the length of the comb.
The wider the spacing, the better, but a point is quickly reached where the spacing makes the loop windings
too "floppy". Each turn of my loop is spaced .25 inches apart. The wooden combs are 1.5" X
5" and have been glued and dowelled into a groove cut in the end of each frame leg. They
could just as effectively be glued and screwed to the outside ends of all legs.
In order to effectively capture each coil winding, the combs were drilled with the correct number
of holes and then the top part of the comb was cut off, leaving the bottom half of
each hole to act as a groove to capture and hold the wire.
Once the loop was wound, a
heavy coating of the same two-part waterproof glue used earlier was liberally applied over the wires where
they lay on the combs, anchoring them firmly in place.
The pickup loop (fed to the receiver via 50 ohm
coaxial cable) is a 1-turn loop wound around the inside of the main loop. It is spaced approximately
6" inside the main winding.
Wider spacing will result in reduced signal pickup
as well as sharper tuning, necessitating frequent re-peaking. An advantage of
the single turn pickup loop is that the loop is completely electrically isolated from the
main station ground, often a source of induced noise pickup. In its present
configuration, it is only necessary to re-peak the tuning capacitor every 50KHz approximately.
When DXing the NDB band (200-530KHz) it can often take an hour or more to carefully tune
through the chosen 50KHz segment before re-tuning for the next range. If used for LOWFER
DX work, it is simply a matter of 'set and forget' tuning!
PERFORMANCE As outlined earlier, the ten-foot loop has proven to be a good
performer right from the start. Preamplifiers are often used with small homebrew loops to
compensate for their low level of signal pick-up. The large aperture of this loop provides
more than enough signal pick-up. No preamplification is used or required. Preamp-induced overload
or cross-mod problems caused by nearby "blowtorch" signals are non-existent. The limiting
factor of reception is always local or (more often than not) atmospheric noise, easily reached with
the large aperture.
In an electrically noisy location, the loop can often be used to best
advantage as a steerable "null" to eliminate or vastly reduce the noise source.
In quiet locations, the loop can be turned to enhance signal pick-up.
Maximum signal pick-up is in the plane of the loop. For more information on loop antennas, see the
links section below and ....good looping!
Steve (VE7SL)
LOOP LINKS
RECEIVING LOOP AERIALS FOR 1.8MHZ (VK5BR) - Plans for 160m
RX loop and preamp
A.M. LOOP ANTENNAS - Construction info for several AM BCB loops
LOOP ANTENNAS - Almost everything you ever wanted to know about loops!
THE LOW FREQUENCY RECEIVING LOOP - A 6' air - core loop for the LOWFER band
SUPERLOOP RECEIVING ANTENNA FOR 136KHz - A nice air - core loop from G3LDO
THINKING ABOUT IDEAL LOOPS - A detailed discussion of loop dynamics
DOUG'S RADIO-ELECTRONICS LOOP ANTENNA PAGE - Various loop characteristics
W8JI's LOOP PAGE - Good facts about shielded loops
160 / 80m COAXIAL RECEIVING LOOPS - Practical construction data
G4CNN's Loop Page - G4CNN's LF Loop Tips
YAHOO! 'LOOPS' DISCUSSION GROUP - Talk with others about all kinds of loops!
HARDCORE DX SITE - Contructing a 160/80m shielded loop.
