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Author Topic:   Power Rating; Effect of Transmitter Power on Received Signal; More
handn posted 02-07-2006 08:42 AM ET (US)   Profile for handn   Send Email to handn  
Does the legal limit of 25 watts have exceptions?

Is there a performance advantage with higher transmit power?

Here in Mexico there are a few boaters using juiced up radios who use regular Shakespere antennas. There is distortion when the signal is received at close range possibly because the radios are of poor quality or because the antennas are not rated for the extra power.

jimh posted 02-07-2006 01:14 PM ET (US)     Profile for jimh  Send Email to jimh     
The transmitter power for recreational vessels using the VHF Marine Band is 25-watts in the United States.

The susceptibility of receivers to overload is a common problem in radios of poor design.

The effect of increase or decrease in transmitter power on the reception of that transmission is a proportional and linear increase or decrease in signal strength. To imagine otherwise would require some non-linear phenomenon in radio wave propagation. This would be difficult to imagine.

There is little potential for the transmit antenna to cause distortion in the signal which would affect reception of it. The modulation technique is frequency or phase modulation, and most antenna problems would only affect the amplitude of the signal, not its frequency or phase. If the antenna were damaged so that it was no longer efficiently transmitting, it would reduce the strength of the radiated signal, but it would be unlikely to induce distortion into it.

As mentioned above, many low-quality and poorly designed VHF Marine Band radios are unable to receive strong signals from nearby transmitters without being overloaded by them. The remedy is to purchase a better radio.

jimh posted 02-07-2006 08:24 PM ET (US)     Profile for jimh  Send Email to jimh     
[These questions were originally appended to another discussion on a different topic; they have been separated into their own discussion thread.]
Chuck Tribolet posted 02-07-2006 09:08 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
If the problem were on the transmit side, and the
transmit antenna couldn't handle the extra power without
distorting, it would be distorted at all ranges. Therefore
the problem is on the receive side.


handn posted 02-10-2006 10:23 AM ET (US)     Profile for handn  Send Email to handn     
I appreciate the explanation for the distortion.
It happened on my old boat with a budget radio and presumably a less than a perfect receiver. I havn't noticed any distortion with my Icom 602, except a broadcast full of static when the transmitting radio has a loose connection to the antenna.
Does the Coast Guard use higher power transmitters on their VHF radios?
As I understand jimh's explanation higher power increases the rating of the transmit power like the effect of a longer antenna?
Is there any real world advantage to a higher powered transmit signal?
Chuck Tribolet posted 02-10-2006 11:10 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The CG has high-gain (9dB at least) antennas mounted very
high. Dunno about their power.

VHF is more or less line of sight. You can do a little better,
but not tons better. If you have enough power to get to the
horizon, more won't really help. 25 watts seems to be enough,
esp. with an 8' 6dB gain antenna. 5 watts doesn't seem to
be enough.

So don't bother with the linear amp.


swist posted 02-10-2006 11:10 AM ET (US)     Profile for swist  Send Email to swist     
Don't forget that VHF is line of sight. You can triple your transmit power and it won't do you a lot of good on an inlet surrounded by elevated land and trees if you antenna is only 10' above sea level.

Yes the coast guard has more transmit power but that is secondary to the fact that their land antennas are on very tall towers, and they are up as high as possible on masts on their boats.

jimh posted 02-12-2006 01:32 PM ET (US)     Profile for jimh  Send Email to jimh     
There is no correlation between antenna length and transmitter power.

There seems to be great confusion here between the concept of a transmitter's power output (measured in watts) and the effective radiated power of a station produced by a combination of transmitter power and antenna gain. The effectiveness of radio transmitters is often increased through the use of antennas systems which have gain, that is, they produce an increased field strength compared to a standard or reference antenna.

The concept of an antenna having gain is only understandable in reference to the signal it creates as compared to another antenna. An antenna is said to produce gain when it creates a stronger field (signal) than a reference antenna. For most cases the reference antenna is a theoretical antenna which radiates equally well in all directions; this is called an isotropic antenna. Any practical antenna will typically have gain in reference to an isotropic antenna simply because it does not have a perfectly uniform radiation pattern and therefore concentrates more energy in a particular direction. All references to antenna gain are to the gain in the main lobe of the antenna. It is also a simple concept that the more gain an antenna has in its main lobe, the less signal it will be radiating in other directions or minor lobes.

It is important to understand that an antenna which has gain will only exhibit that gain in the direction of its major lobe. If the major lobe of the antenna is not pointed in the direction of the distant station with which communications is being attempted, the antenna will produce less signal at the distant station. The higher the gain of an antenna the more important it is to aim its major lobe in the desired direction.

A more reasonable method of comparing antenna gain is to use a simple physical antenna as the reference. The most common reference is to use a half-wave dipole antenna. Such an antenna would demonstrate gain compared to an isotropic antenna of about 2 dB.

Manufacturers of VHF Marine Band antennas often cite figures for gain, however they generally fail to mention the reference antenna. For example, it is often cited that a halfwave marine antenna has a gain of 3 dB. This must be in comparison to something even worse than an isotropic radiator, as a simple practical halfwave antenna is generally considered to only have a gain of 2 dB greater than isotropic.

The effective radiated power is generally considered to be the transmitter power, minus transmission line loss, multiplied by the antenna gain.

Antenna gain is usually given in decibels, and this must be reduced to the actual linear gain. For example, consider an antenna has a gain of 2 dB, which is how I would characterize the typical halfwave antenna

If the transmission line has a loss of -2 dB per 100 feet, and you have 15-feet of feedline, you will have a transmission line loss of

LOSS = 15/100 X -2 dB

LOSS = -0.3 dB

We can add these together to arrive at the total gain in the transmitter system:

TOTAL = -0.3 + 2

We have to convert this to a linear multiplier to discover the effective radiated power:

dB = 1.7
GAIN = 10^(1.7/10)
GAIN = 10^(0.17)
GAIN = 1.48

Assuming the transmitter has an output of 25 watts, the effective radiated power (ERP) will be

ERP = 25 X 1.48
ERP = 37 watts

The dimensions of a halfwave antenna are proportional to the frequency. For 156 MHz as used in the VHF Marine Band, a halfwave antenna has a length of

FEET = 468/F where F is in MHz


FEET = 468/156
FEET = 3

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