"Short Wave Sportfishing" wrote in message
...

On Tue, 15 Jan 2008 16:50:09 -0800 (PST), Tim
wrote:

OK, I picked this up on another board,a nd seeing that Eisboch, Tom,
Gene and Larry have had dealings with this stuff. I thought I'd
present it here. It has my curiosity up as well.

Wavelength is the length measurement from the beginning to the end of
one full cycle. Or think of it another way, the distance a wave at a
given frequency to travel from 0 degrees to 360 degrees.

Frequency is the number of wave periods passing a point in time and is
inversely proportional to wavelength - the higher the frequency of the
signal, the shorter the wavelength.


And by the way Tim .... the "waveform" need not be sinusoidal. It could be
a square wave, sawtooth, virtually anything.
For practical purposes to us, the wavelength is important in determining the
proper electrical length of an antenna for maximum power transfer and
resultant radiation. Ideally, the antenna would be "cut" (it's electrical
length) to the physical wavelength of the transmitting frequency, or an even
fraction of it, (i.e. 1/2 wavelength, 1/4 wavelength, 1/8, etc.)

A mismatch, meaning the antenna is electrically too short or too long,
causes out of phase "standing waves" to be set up in the antenna and/or
transmission line (coax) that is reflected back to the transmitting radio,
effectively decreasing the radio's transmitting power. The measurement of
reflected power to transmitted or "forward" power is what is referred to as
the "Standing Wave Ratio" or "SWR". In high power applications, a high
reflected standing wave can damage the radio and/or transmission line. I've
burned up transmission lines several times trying to apply high power RF
into a plasma contained in a vacuum chamber. Hate that when it happens.
It's unlikely that low power transmitters like a VHF marine radio would be
damaged (unless there was a direct short on the antenna or line) due to a
mismatch, but it *will* cut done on the effective radiated power. Your
marine radio, rated at 25 watts is actually transmitting much less,
especially if the antenna is not matched or "cut" properly.

Eisboch

On Wed, 16 Jan 2008 00:19:55 -0500, "Eisboch" wrote:


"Short Wave Sportfishing" wrote in message
.. .

On Tue, 15 Jan 2008 16:50:09 -0800 (PST), Tim
wrote:

OK, I picked this up on another board,a nd seeing that Eisboch, Tom,
Gene and Larry have had dealings with this stuff. I thought I'd
present it here. It has my curiosity up as well.

Wavelength is the length measurement from the beginning to the end of
one full cycle. Or think of it another way, the distance a wave at a
given frequency to travel from 0 degrees to 360 degrees.

Frequency is the number of wave periods passing a point in time and is
inversely proportional to wavelength - the higher the frequency of the
signal, the shorter the wavelength.

And by the way Tim .... the "waveform" need not be sinusoidal. It could be
a square wave, sawtooth, virtually anything.
For practical purposes to us, the wavelength is important in determining the
proper electrical length of an antenna for maximum power transfer and
resultant radiation. Ideally, the antenna would be "cut" (it's electrical
length) to the physical wavelength of the transmitting frequency, or an even
fraction of it, (i.e. 1/2 wavelength, 1/4 wavelength, 1/8, etc.)

A mismatch, meaning the antenna is electrically too short or too long,
causes out of phase "standing waves" to be set up in the antenna and/or
transmission line (coax) that is reflected back to the transmitting radio,
effectively decreasing the radio's transmitting power. The measurement of
reflected power to transmitted or "forward" power is what is referred to as
the "Standing Wave Ratio" or "SWR". In high power applications, a high
reflected standing wave can damage the radio and/or transmission line. I've
burned up transmission lines several times trying to apply high power RF
into a plasma contained in a vacuum chamber. Hate that when it happens.
It's unlikely that low power transmitters like a VHF marine radio would be
damaged (unless there was a direct short on the antenna or line) due to a
mismatch, but it *will* cut done on the effective radiated power. Your
marine radio, rated at 25 watts is actually transmitting much less,
especially if the antenna is not matched or "cut" properly.

Another term that Tim will probably run into is Effective Radiated
Power or ERP.

ERP is a measure of total radiated power - subtracting system losses
to the system gains resulting in ERP as measured in Decibels (dB). ERP
is nominally referenced to a resonant 1/2 wave dipole (or a center fed
antenna 1/2 wavelength of any given frequency - 1/4 wave length on
either side of the center feed point) in free space radiating in a
given direction. In theory, for every 3 dB of gain, the ERP,
theoretically, will double.

For example, in theory, if you have 6 dB of gain and 3dB of loss, the
ERP is 3 dB. The free space multiplier is 2.1. If you input 25 watts
to an antenna with 3 dB gain, the ERP will be a theoretical 50 watts.