Practical_Antenna_Handbook_0071639586

C h a p t e r 1 4 : r e c e i v i n g A n t e n n a s f o r H i g h F r e q u e n c y 359
tential it needs to be kept away from other conductors. This constraint can substantially
increase the actual space required by the EWE.
Pennants, Flags, and the K9AY Loop
The appearance of the EWE inspired other experimentally inclined radio amateurs to
develop additional low-band receiving antennas for small spaces. Subsequent to publication
of the EWE, at least three new single-turn terminated loop designs came into
being. As is the case with the EWE, for those lacking the necessary space for a set of Beverage
antennas, these loops provide an alternative, although those fortunate enough to
have the space for both generally report hearing a little bit better with their Beverages.
Directivity of this family of loops is based on constructive versus destructive combining
of the E- and H-fields that constitute an arriving radio wave. An arriving E-field
in the plane of the loop induces a voltage in the loop just as it would in any other short
(relative to wavelength) wire. The associated H-field at right angles to the loop plane
induces a current in the loop that becomes a voltage across the terminating resistor.
When these fields arrive from one direction, the two voltages sum; when arriving from
the opposite direction, they subtract.
Figure 14.20 depicts the K9AY terminated loop. Exact shape of the loop is not critical,
but the bottom leg must come near earth for the termination and transformer
ground connections. The total length of wire in the loop is about 85 ft, and a single support
only 25 ft tall is required. The feedpoint impedance is similar to that of a typical
Beverage, so a 9:1 transformer is required. Like the EWE and unlike the Beverage, however,
the direction of maximum signal pickup is opposite the terminated end.
The pattern of the K9AY loop is unidirectional. By using relays to switch the termination
and feedpoint, one loop can be switched between two opposing directions. Two
loops, at right angles to each other and similarly switched, can provide four directions
from a single fixed mast.
Like the other members of the pennant and flag family, the K9AY loop is quite sensitive
to detuning from nearby conductors. As a result, some of its impressive space savings
are not always fully realizable if the user also has transmitting antennas and towers
nearby. Proper operation of the K9AY is fairly sensitive to the exact value of the termination
resistor and depends on both the operating frequency and the actual characteristics
of the ground beneath the antenna. Many implementations use a remotely
controlled termination resistor to optimize the depth of the null to the rear as the received
frequency is changed and to compensate
for seasonal variations in soil
Incoming
signal 25'
2'
15'
R T = 500 3:1 turns
characteristics.
In many operating scenarios, the value
of this antenna is likely to be more for its
null off the rear than for any forward directivity
it exhibits. But that’s quite appropriate
for much of the weak-signal work that
occurs on the 160- and 80-m amateur bands,
or when broadcast band DXing.
To receiver
Figure 14.20 K9AY loop.