Chuck, K2CG wrote:
I enjoy putting together and playing with simple wire antennas for field
operations. I have also been fortunate to enjoy moderate success with my
efforts.
However I am looking to refine my approach in an effort to make my systems
more efficient. My approach in the past has been to follow designs and
dimensions for established dipole's and doublets, then check my effort by
using an MFJ-259 to verify that they fall within acceptable limits for my
tuner and desired frequency range.
I am not looking to build an antenna that will give me 1:1.0 SWR but rather
try to change the feed-line impedance to produce a better load transfer.
Currently I am using a 135' doublet with 50' of 450 ohm ladder line, but I
am looking at trying an 88' Doublet and different types of balanced
transmission lines to vary the input impedance, by using 300 ohm, 450 ohm
ladder line or even trying some home built 650 ohm ladder line.
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In truth, there isn't much to it! That's why simple antennas aren't
belabored in the literature. It only gets complicated when you want passive
antenna systems that show a low SWR on many bands or which has a lot of
directivity, etc.
The first requirement for an efficient antenna is low reactance at the
connection to the transmitter. Low reactance is required for maximum power
transfer. Almost 200 years ago Heinrich Hertz discovered that a wire that is
electrically 1/2 wave long is naturally resonant; that is, it shows minimum
reactance to the signal applied. That's the shortest length of antenna that
is naturally or self resonant when operated independently (e.g. free
space).
Marconi had a huge problem as he started tinkering with antennas. To be
self-resonant at the frequencies he was using in his early work - perhaps 50
or 100 kHz and probably less - he'd need an antenna at least 40,000 feet
long. Even Gugliemo's mom must have balked at that idea! (He was still
living at home.) He tinkered and worked out that if he connected one end of
his antenna system to the earth, it would resonate at half the length of a
Hertzian antenna. Well, that did save 20,000 feet of wire!
(Now, I confess I'm telling tall tales here. It's not likely Heinrich or
Gugliemo really understood resonance or, as Gugliemo like to call it
'synchronicity'. But as we better understood the effects of reactance on an
antenna system in later years, what Heinrich and Gugliemo were doing by
trial and error became evident.)
When an antenna is longer than its natural resonant length, it'll have
inductive reactance. If it's shorter, it'll have capacitive reactance. If
it's not convenient to make the antenna the right length, we can add
suitable inductance or capacitance and bring the system to resonance. Our
antenna tuners will do that for us, if they have the required range to match
our antenna at the frequency we want. That's one of the two things a tuner
must do. We'll touch on the other in a moment.
With the right amount of inductive or capacitive reactance, you could, on
paper, have an antenna 1 inch long that would radiate as well as any wire
ever launched into space, except for one problem: resistance.
Here's why resistance is a problem.
A resonant antenna looks like a resistor to the transmitter. If it has zero
losses, that resistance is a fictional amount representing the RF that is
radiated as electromagnetic waves. Since we're dealing with alternating
currents here (at RF frequencies) we don't say resistance but impedance.
Since the antenna is resonant (no reactance) the impedance is equal to the
resistance. The value of that impedance varies according to the length of
the antenna. If the antenna is 1/2 wave long, the value of that impedance at
the end may be several thousand ohms. At the center it's about 75 ohms if
the antenna is in free space. (Getting it close to the earth were most of us
have to put them brings it down closer to 50 ohms). But, as you make the
antenna shorter than 1/2 wave the impedance drops. At 1/4 wave (the length
Marconi used) it's 35 ohms.
As you go shorter the impedance plummets like a lead garden gnome dropped
from that 90-foot tower you dream of owning.
Many short antennas, like mobile antennas, have an impedance of less than 1
ohm, sometimes down in the range of 0.1 or even 0.01 ohms! That alone isn't
a problem except for the fact that the antenna wire and the wire in any
inductors we use to make it resonant also have resistance. The power is
shared between the resistance of the wire and the radiation resistance. And
don't forget that the RF resistance of a wire is much greater than the DC
resistance because RF only flows along the skin of conductors. (That's why
some RF conductors are silver plated for best efficiency.) Also, your feed
line is part of that antenna system, so if it has a very high SWR it'll show
high ohmic losses at the points where the RF currents are high. Even open
wire line will do that under extreme conditions. I
