Antenna gain question
 

I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA

Liam Ness wrote:
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise.

Of course, it would.

They didn't seem
to be talking about more effieciently radiating the transmitters

I guess that's the point.

power, but actually increasing it above what is present at the antenna
port. Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

If anybody invented such a device, the Nobel Prize would be sure.

--
Pawel Stobinski SQ9NRY
Republic of Poland

Liam Ness wrote:
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise.

Of course, it would.

They didn't seem
to be talking about more effieciently radiating the transmitters

I guess that's the point.

power, but actually increasing it above what is present at the antenna
port. Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

If anybody invented such a device, the Nobel Prize would be sure.

--
Pawel Stobinski SQ9NRY
Republic of Poland

Nope -- you can not get out more power than you put in.
Antenna gain is essentially squishing energy into a narrower pattern.
Hence gain over the previous pattern or reference antenna in a given
direction and angle
Gain is referenced to a dipole or isotropic radiator

antenna gain: The ratio of the power required at the input of a loss-free
reference antenna to the power supplied to the input of the given antenna to
produce, in a given direction, the same field strength at the same distance.
Note 1: Antenna gain is usually expressed in dB. Note 2: Unless otherwise
specified, the gain refers to the direction of maximum radiation. The gain
may be considered for a specified polarization. Depending on the choice of
the reference antenna, a distinction is made between:


a.. absolute or isotropic gain (Gi), when the reference antenna is an
isotropic antenna isolated in space;

b.. gain relative to a half-wave dipole (Gd) when the reference antenna is
a half-wave dipole isolated in space and with an equatorial plane that
contains the given direction;
For example: A radio's transmitting power can be concentrated along the
horizon by use of a GAIN antenna. Although you may still be transmitting
with four watts of power, your EFFECTIVE RADIATED POWER (ERP) will be
greatly increased.
See URL: http://www.firestik.com/Tech_Docs/gain_erp.htm

Some more references
http://www.arrl.org/members-only/tis...df/8211035.pdf

http://www.marcspages.co.uk/tech/antgain.htm


--
73 From Key Largo
--------------------------
"Liam Ness" wrote in message
...
I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA

Nope -- you can not get out more power than you put in.
Antenna gain is essentially squishing energy into a narrower pattern.
Hence gain over the previous pattern or reference antenna in a given
direction and angle
Gain is referenced to a dipole or isotropic radiator

antenna gain: The ratio of the power required at the input of a loss-free
reference antenna to the power supplied to the input of the given antenna to
produce, in a given direction, the same field strength at the same distance.
Note 1: Antenna gain is usually expressed in dB. Note 2: Unless otherwise
specified, the gain refers to the direction of maximum radiation. The gain
may be considered for a specified polarization. Depending on the choice of
the reference antenna, a distinction is made between:


a.. absolute or isotropic gain (Gi), when the reference antenna is an
isotropic antenna isolated in space;

b.. gain relative to a half-wave dipole (Gd) when the reference antenna is
a half-wave dipole isolated in space and with an equatorial plane that
contains the given direction;
For example: A radio's transmitting power can be concentrated along the
horizon by use of a GAIN antenna. Although you may still be transmitting
with four watts of power, your EFFECTIVE RADIATED POWER (ERP) will be
greatly increased.
See URL: http://www.firestik.com/Tech_Docs/gain_erp.htm

Some more references
http://www.arrl.org/members-only/tis...df/8211035.pdf

http://www.marcspages.co.uk/tech/antgain.htm


--
73 From Key Largo
--------------------------
"Liam Ness" wrote in message
...
I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA

Hi,

Firstly, I am not familiar with Part 15 and any comments that follow
do not take into account system losses or inefficiencies.

It is generally NOT possible to increase the power output of a
transmitter just by changing the antenna connected to it. A transmitter
of 'X' watts connected to antenna 'Y' would produce exactly the same
radiated power if it were connected to a completely different antenna
'Z' instead. The exception to this is if changing the antenna causes the
transmitter to generate more power because of changes in its operating
conditions such as in the impedance match presented to it.

However every antenna, other than an isotropic radiator (a
hypothetical device that radiates equally in all directions), will have
gain in some direction at the expense of that in others. The product of
the gain and transmitter power is called the EiRP (equivalent isotropic
radiated power) and this is what is generally quoted as the limiting
power allowed.

So, if a particular antenna is stated as having a gain of 2 (3dB)
over an isotropic radiator then, along the axis of the antenna that has
the maximum gain, it will appear that the transmitter power has been
increased by 3dB (twice the power output) although the transmitter
itself has not in fact produced any more. This 'extra', of course, comes
from not radiating power in all the other directions.


Cheers - Joe

Hi,

Firstly, I am not familiar with Part 15 and any comments that follow
do not take into account system losses or inefficiencies.

It is generally NOT possible to increase the power output of a
transmitter just by changing the antenna connected to it. A transmitter
of 'X' watts connected to antenna 'Y' would produce exactly the same
radiated power if it were connected to a completely different antenna
'Z' instead. The exception to this is if changing the antenna causes the
transmitter to generate more power because of changes in its operating
conditions such as in the impedance match presented to it.

However every antenna, other than an isotropic radiator (a
hypothetical device that radiates equally in all directions), will have
gain in some direction at the expense of that in others. The product of
the gain and transmitter power is called the EiRP (equivalent isotropic
radiated power) and this is what is generally quoted as the limiting
power allowed.

So, if a particular antenna is stated as having a gain of 2 (3dB)
over an isotropic radiator then, along the axis of the antenna that has
the maximum gain, it will appear that the transmitter power has been
increased by 3dB (twice the power output) although the transmitter
itself has not in fact produced any more. This 'extra', of course, comes
from not radiating power in all the other directions.


Cheers - Joe

Antennas have directional properties.

It the total available power is concentrated in one direction then, as far
as the receiver is concerned, the APPARENT power of the transmitter has
increased.

But receivers in less-favoured directions from the transmitter will
experience an APPARENT reduction in the transmitter's output power.

Antennas have directional properties.

It the total available power is concentrated in one direction then, as far
as the receiver is concerned, the APPARENT power of the transmitter has
increased.

But receivers in less-favoured directions from the transmitter will
experience an APPARENT reduction in the transmitter's output power.

Liam,

You are absolutely right that the antenna cannot radiate more power
than you feed it. They are probably talking about "effective ratiated
power", which is closely related to antenna gain. You get more power
at the receivers, but only in the directions where radiation is
maximized. For example, a Yagi directs the power more in one
direction, and a "vertical collinear" directs it in a horizontal
plane. Each does so at the "expense" of power radiated in other
directions, but if you don't care about reaching receivers in those
other directions, it's not a problem. But be careful: do the rules
limit the maximum power, or the maximum ERP (effective radiated
power)? (The same thing works for receiving, too. You can use
antenna directivity to increase the received signal level, and to
reject signals or noise coming from directions you don't care about
receiving.)

Cheers,
Tom

Liam Ness wrote in message . ..
I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA