"Steve Lusardi" wrote in news:gaf992$2tl$01$1
@news.t-online.com:
Larry,
Why would this antenna's radiation not be coupled to ground, as it is
so
close and in parallel alignment with the grounded stay?
Steve
The grounded stay would re-radiate RF, probably out-of-phase partially
killing our signal, from induced currents caused by the transmitting
antenna. Every piece of metal on the boat will have induced currents,
especially near the antenna. They all contribute to screwing up the
total signal and radiation pattern.
Here is one wavelength of voltage on a wi
+ X
X X
X X
X X
0X X X
X X
X X
X X
- X
Make believe this is a sine wave which is impossible on my keyboard.
The length of a wave that travels at 300,000km/sec, the speed of light
depends on how fast we create the wave. 300/Mhz=meters of wavelength.
10 Mhz equals 30 meters (look on your old shortwave receiver for
wavelengths).
Now, if we fit this wave on a wire, such as a backstay wire, you can see
that 1/4 wavelength from 0 volts (your reference we call ground) is
either a very high + or - voltage at this instance. Because we are
holding one end of the wire at ground potential, the wave is forced to
stay so that one place on the wire is at zero volts (ground). The wave
continues to oscillate between + and - at the frequency the wave
oscillates at, so at the point 1/4 wavelength from the left end (ground)
of my rotten picture, the voltage swings from +HV to -HV at that
frequency.
At ground, however, the CURRENT wave caused by the transmitter's power
is at maximum (nearly across a short, a very low resistance). At
ground, the current swings from maximum + current to maximum - current.
1/4 wavelength away from this maximum current on this current sinewave
is a point that appears to stand still (we call them standing waves for
this reason.) The current waveform zero is 1/4 wavelength away from
this grounded point, exactly where the maximum voltage standing wave
occurs.
These conditions along any length of conductor repeat themselves every
1/2 wavelength no matter how long the wire is...out to the end.
At the grounded end (left end above), we have maximum CURRENT and
minimum VOLTAGE R=V/A, not exactly zero, but a very low number.
1/4 wavelength from that grounded point we have maximum voltage, but
almost no current, a very HIGH resistance AT THIS RF FREQUENCY.
If you were to plot R=V/A at all points along the wire, you'd get a
tangent function curve whos maximum is 1/4, 3/4, 1 1/4, 1 3/4, etc.
along this wire.
This is called the impedance at any point (Z) along the wire.
So, a solid piece of wire with RF applied to it has constantly changing
impedance along its length....from low to high to low to high.
As long as the wire is of infinite length, this is true. But that wire
is too long to drag under a bridge. We only need a wire long enough to
have a good place of high voltage (which is real easy to get on any
frequency) AND a wire long enough to have a good CURRENT place along it.
The current is harder to get on very short antennas that have an OPEN
END, instead of being grounded....the whip antenna on the stern, or the
insulated backstay with an insulator at the top.
When the antenna ends in an insulator (air in the whip), it's the OPEN
END that dictates how the wave fits on the wire. At the open end...it's
OPEN...high voltage..REALLY high voltage...and NO CURRENT as there is no
place for current to flow off the end of the wire (unless you're running
a few kilowatts like my ham mobile and it simply arcs into the air!) On
open ended antennas, we must fit the wave on the wire from this open
end, and work backwards towards the bottom where you hook the tuner.
A funny thing happens to either of these wires. If the wire is exactly
or nearly the right length to fit the wave onto, we call that a TUNED
antenna, and it will have a perfectly fitting standing wave on it, even
if it is only 1/4 wavelength long. This is called a RESONANT antenna.
When the wave fits onto the wire perfectly, only one wave shows up and
the antenna makes a HUGE RF radiation. No "tuner" is necessary. A 1/4
wavelength length of wire that's open on the top, insulated from ground,
displays a resistive load to our transmitter around 72 ohms against a
horizontal "earth", such as a large metal plate, the surface of the
ocean, or the 36 slightly longer than 1/4 wavelength wires radiating
away from your local AM radio station's 1/4 wavelength tower... The 1/4
wavelength vertical is the most used vertically polarized antenna on the
planet. That 23' whip is a resonant antenna around 10 Mhz, by the way.
The transmitters are designed to match the feed impedance of this
antenna, which with a sloping ground is around 52 ohms....just like the
coax cable.
Now, along comes your need to operate on 2, 4, 6, 8, 12, 16, 22 Mhz
marine bands.....NONE of which have a wave that fits just right on the
backstay.
If the wire is way off being "resonant" in length, the impedance
displayed to the transmitter becomes a complex circuit of either
resistance in series with capacitance, or a complex circuit of
resistance in series with inductance. Inductance and Capacitance STORE
energy as the wave increases, then RETURN this energy when the wave
decreases....in this case returning the power to the transmitter, not
radiating it towards that idiot you're trying to talk to. Every CBer
you meet will know that SWR is "bad". Standing Wave Ratio (SWR) is the
ratio of the wave the transmitter is sending DOWN the coax to the
antenna, in relation to the power being REFLECTED by the capacitance (if
the antenna is too short) or inductance (if the antenna is too long) of
the non-resonant random length wire....the backstay or 23' untuned whip
on your sailboat.
All is not lost! We put a box between the coax cable and the antenna
called a "tuner". Inside the box are a combination of inductors (coils)
and capacitors that a little computer capable of measuring what the
antenna is displaying can switch into the circuit in series and parallel
combinations until it finds a combination that "compensates" for the
capacitance or inductance of the antenna AT THIS FREQUENCY. You simply
press the TUNE button and the little computer takes a few seconds to
find the magic combination to RESONATE the antenna for this channel. It
stores what it finds, so when you come back to this frequency later, it
doesn't have to feel around for the match, it simply switches back to
what it used before then makes a small adjustment, if your mate is
leaning against the antenna detuning it, for instance.
What makes this grounded or ungrounded piece of wire RADIATE RF energy
is the COMBINATION of the VOLTAGE field (called E-field) from end to end
in parallel with the wire, and the CURRENT field (called M-field) AROUND
the wire perpendicular to it. BOTH these fields are required to RADIATE
RF energy. That's what we're trying to do by tuning the wire....get
both. There are many kinds of antennas that radiate all kinds of crazy
patterns of RF energy. Some are very directional, boat antennas are
omni-directional so it doesn't matter where you point the boat, the guy
can hear you.
Your wifi, your cellphone, your Bluetooth, all have antennas buried in
them. But at 800 or 1850 or 1900 or 2450 Mhz, their wavelengths are
VERY short, a few cm of a metal strip against the plastic case under
your hand. HF antennas at much lower frequencies must be longer, much
longer. Here's a proper HF transmitting antenna:
http://www.hawkins.pair.com/voanc/voanc14.jpg
It's about 400M high for reference, but we can't fit it on the boat!
It radiated to Europe for 50 years American propaganda, mostly for
Russians. The transmitter was from 250,000 to 1,000,000 watts. I've
been there when it was on the air. Your tooth fillings talk to you...
(c;
http://www.hawkins.pair.com/voanc/voanc03.jpg
250,000 watt transmitter that fed it....most impressive.
Sorry this went long....I used to teach electronics and still get
carried away....(c;