Practical_Antenna_Handbook_0071639586

C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 151
In other words, by cutting the power to the original TX antenna in half, we have
reduced the received E-field strength from that TX antenna to 71 percent of its original
value. This is a consequence of power in a resistive load being proportional to the square
of the current.
But there are two currents at the receiving site, each resulting from intercepting the
radiated field from one of the two TX antennas, and their magnitudes are in phase, so
thanks to the principle of superposition we can add the two:
E = 2( 0.707)
kI = 1.414kI
(5.4)
RXnew TXorig TXorig
Thus, the new current at the receiver resulting from the same total power at the transmitting
site is 1.414 times the original current—or 3 dB larger!
What we have just created is an antenna array—more specifically, a two-element alldriven
phased array operated in its broadside mode. (In broadside mode, the element feed
currents are in phase and the direction(s) of maximum radiation are on a line at right
angles (hence, broadside) to an imaginary line—called the array axis, and indicated by the
vertical dotted line in Fig. 5.1B—connecting the elements.) In a sense, we have created
“something from nothing”. Through the simple expedient of erecting a second antenna
and splitting our allowed transmitter power equally between the two antennas, we
have accomplished an increase in received signal strength at a distant point—an increase
that would have required us to double our transmitter power (from, say, 100 to
200 W) if we had continued to use just the original antenna.
This seems magical and it is, in a way, but “there’s no free lunch”, as the saying
goes. In this case, as with all the arrays we will discuss, we have increased our radiated
field in some directions at the expense of the field or signal strength in others. It
may be helpful to visualize the radiation field of the original antenna as a spherical
balloon, and a given transmit power corresponds to blowing the balloon up to a certain
size. Now wrap your two hands around the middle of the balloon and squeeze.
Pushing in on part of it causes the skin of another part of the balloon to extend, even
though the total air within the balloon remains constant. Similarly, an array of antenna
elements modifies the radiation field of the original antenna, making it stronger
in certain directions while weakening it in others. In fact, if we were to move our receiving
antenna to a distant point anywhere on the array axis we would find the received
E-field to be zero or nearly so.
Let’s see why that is. The two transmitting verticals of Fig. 5.1B are spaced l/2
apart, but their feed currents are in phase. When the field from antenna A reaches antenna
B, it will have traveled a half-wavelength, and, hence, its waveform will be 180
degrees out of phase from its starting point at A and also 180 degrees out of phase with
the waveform of antenna B’s field (since we said at the beginning that the two TX feedpoint
currents were in phase at the base of each antenna). Because A and B are so close
together (compared to the distance to the receiving antenna), the magnitudes of the two
fields from the two TX antennas are virtually identical at the receiving antenna. As the
fields from antennas A and B propagate outward along an extension of the line connecting
A and B, they are essentially the same strength at every point on the line, but 180
degrees out of phase. That is to say, they almost completely cancel at all receiving points
on a line connecting the centers of the two antennas. The received signal strength from
this two-element phased array is, for all intents and purposes, zero anywhere along the
axis of the array. The array has “stolen” power from certain directions to increase the