A considerable number of low band low-noise receive antenna designs exist and are in use by hams and SWL enthusiasts throughout the world. This page presents a number of them but this list by no means comprises all of the types/variants/sub-types. One of the more enjoyable aspects of amateur radio is experimenting with antennas, and for the topbander no greater challenge exists than developing a receive antenna system which can pull the weak ones out of the noise.
K9AY TERMINATED LOOP
Possessing all the qualities of the EWE but with a much smaller footprint, the Terminated Loop was introduced to readers of QST in September, 1997 on page 43. For those lacking even enough real estate to install one or more EWE antennas, the terminated loop should fill the bill and provide performance equal to or better than several other designs. I built and installed one of these antennas at our summer home in June, 2008 and it works! Imagine listening to a broadcast station on about 1600 and switching direction on the antenna.....the station disappears, to be replaced by one from the opposite direction! Initial results on both 80 and 160 meters are excellent so far. I have a 20 db preamp in the line to equalize signals with my Beverages, and signals on the K9AY are on a par generally with my "Boomerang Beverage" (described fully on another page).
The QST article is well written but here are some other sources of commentary on this excellent antenna:
A TERMINATED RECTANGULAR SMALL LOOP
In 2006, I was sent design information on a terminated loop by VE3DO and urged to try it. Very little information other than a diagram was actually provided and no information on a matching system-only the dimensions, height above ground and some information on the ground material beneath the loop he had experimented with. It looked to me to be a K9AY loop with a rectangular shape, and that is the way I treated it. In the summer of 2007 I constructed a loop whose dimensions conformed to those sent to me by VE3DO and fed it using a 9:1 transformer. The diagram below created by Jose, N1BAA, depicts what I installed at our summer home. The wire is continuous and is broken in the exact centre of the bottom horizontal run. The matching transformer and termination are connected as shown. NOTE: In June, 2008 I lowered the bottom wire to a position 6" above ground level and have noticed increased directivity. I employ a relay to switch direction, as in the K9AY loop.
I installed a 4 ft ground rod at the centre of the antenna. The antenna fires in the direction TOWARD the matching transformer (opposite to a Beverage and the same as the K9AY Loop). In my first installation I spaced the wire 18" above ground which is moist clay loam. The transformer uses a BN73-202 binocular core wound with 5 turns "secondary" winding and 2 turns "primary" winding. The secondary of the transformer connects between one side of the bottom horizontal wire and ground while the terminating resistor connects between the other side of the bottom horizontal wire and ground. The shield of the coax (which is 75 ohms for the turns ratio noted above) is NOT connected to the common ground of the transformer and resistor. This ground isolation contributes to the very low-noise characteristics of the antenna. In my case the coax is buried to within a foot of connection to the transformer. I installed a relay to switch the positions of the transformer HI winding and termination resistor, thereby switching the direction from which the antenna receives.
Initial results have been very favourable; the comparison antennas at the summer home have been a "boomerang" beverage and a 550 ft standard Beverage. The loop is quieter than either but requires about 20db of preamplification to raise the signal level to that of the Beverages. On many mornings when the Beverages are too noisy for comfortable copy the loop is often able to provide fully copyable signals. This is not to say that one should remove the Beverages and receive only with terminated loops, because there is evidence to suggest that the loop responds to signals arriving at a higher angle than do Beverages. This means that the loop will be inferior to a 1 wavelength Beverage at least some of the time, at least equal to the Beverages some of the time, and better than the Beverages some of the time.
K8GG has done an EZNEC analysis of the antenna with some interesting findings. First of all, he found that the most effective height above ground for the bottom horizontal wire is 8-12" and that the best terminating resistance is 510 ohms on 160M. With these values and the wires installed taut as per the diagram, he obtained the following:
Gain is -26dbi (a 1 wavelength Beverage has
a gain of about -8 dbi or less, so a preamp is required for this loop antenna)
Front-to-back ratio is approximately 20 db
Side lobes are down only about 4-5 db
Front lobe is approximately 150 degrees between
the 3db points on 160 meters
TOA (angle at which the antenna receives optimally)
is 33 degrees, somewhat higher than that of a Beverage, nominally 10-20
degrees, but response is good down
to about 10 degrees with less gain.
There is some evidence from both K8GG and W0OU to suggest that, while a ground is required (thus making the antenna "ground-dependent"), an extensive grounding system is not required, and a single ground rod of from 4-8 ft is sufficient.
In the fall of 2007 I installed another loop
at the home QTH. The comparison RX antennas are several 1000 ft Beverages
and a standard pennant oriented 320 degrees; the soil is moist to wet clay
loam. Results with this loop have been similar; on many days when
the Beverages can hear virtually nothing because of high noise the loop
often provides a copyable signal.
THE K6STI LOW NOISE ANTENNA
By the mid-90s with the demise of Cycle 22,
low band activities started on an upswing and large numbers of hams, denied
the pleasure of DX on 15 and 10 meters turned to the low bands. An
almost obsessive compulsion to develop/install receive antennas which would
hear that elusive DX swept the ham community. Hot on the heels of
the EWE came another low-noise, low band receive antenna, introduced by
K6STI in QST for September, 1995 p. 33 with construction details in another
article by W6KUT in the same issue on page 37. With a small footprint
and gain/noise reduction similar to that of the EWE, this might be another
good choice for the ham with limited real estate. In addition to
reference in the Topband Reflector Archives (above), here is another source
to explore for more information on the K6STI loop: K3KY
at ANGELFIRE
Many articles have been written on small receive loops, both shielded and non-shielded. Some of these designs are small enough to sit on a table and be rotated by hand. Several of the articles have been published in QST and anyone interested in pursuing construction of this type of antenna should search the archives of QST on the ARRL Web Page.
The late Doug Demaw, W1FB published, through ARRL, his very readable W1FB's Antenna Notebook, in which chapter 8 details two small loops.
ARRL publishes a series of antenna books which reprint articles previously published in QST. The Antenna Anthology includes an article on the "Scoop Loop", a physically small receive loop for the low bands (page 76).
Volume 1 of The Antenna Compendium has an article titled "A Crossed-Loop Goniometer DF Antenna for 160 Meters" on page 127.
Searching the web for "receive antennas", "low noise antennas" and such will yield a number of additional articles for consideration.