Next message: David W. Wisham: "lv-ab: Fw: Time to move"
In a message dated 05/05/2001 9:10:47 AM Eastern Daylight Time,
writes:
> but I was wondering if it's possible to use an
> isolated part of a wire backstay as antenna for the VHF, in the same way as
> with SSB antennae?!? What's inside these fiberglass VHF antennae anyway???
>
Transmitting antennas work best when they resonate with the transmitting
frequency. This means adjusting the length so that the electricity vibrates
in the antenna naturally at the same rate as the transmitter is vibrating.
This is just like a guitar string vibrating when in the presence of a tone of
the same frequency as the guitar string is tuned to. You can make the antenna
wire seem longer, to vibrate at a lower frequency, by adding inductance (a
coil of wire) in series with it, or make it seem shorter by adding
capacitance (a capacitor) in series with it. This is what an antenna tuner
does.
The "standard" antenna is a dipole, like the letter T with a wire connecting
each arm of the T to the transmitter to form a sort of a high frequency
electromagnet. When each arm is a quarter wavelength (one wavelength is the
distance a radio wave travels in one cycle considering that it travels at
330,000,000 meters/second) the thing resonates nicely. Turn the T on it's
side, stick one arm in the ground, and you have a vertical, or whip antenna
such as used for SSB on a backstay, a VHF, or a cell phone. This is why it
is so important to have a good ground, or "counterpoise" as it is sometimes
called, because it is actually half of the antenna.
Generally antennas such as a backstay, which wants to be a half of a dipole
antenna, is tuned to be a quarter of a wavelength with the help of an antenna
tuner.
As for what is inside VHF antennas, they could be simply a quarter wave wire,
but a "high gain" antenna I took apart once had sections of coaxial cable cut
into short pieces about 6 inches long and soldered center conductor to shield
to each other up the length of the antenna, which was about 8 feet long or
so. The object was to flatten and spread out the normally donut shaped
radiation pattern of the simple quarter wave wire so as to extend the range,
very much like a Fresnel lens does to light.
Norm
___________________________________________________________________________
|| The Live-Aboard List : send a "subscribe" or "unsubscribe" request ||
|| in body of message to: ||
