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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 |
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 |
Thank you to everyone who responded to my question. And thank you for making what I thought I had a handle on so much clearer to me. Liam Ness |
Thank you to everyone who responded to my question. And thank you for making what I thought I had a handle on so much clearer to me. Liam Ness |
Liam Ness wrote: 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. Yes, IF the present antenna presents a bad impedance match to the transmitter. Two factors come into play: 1) maximum power transfer occurs when the source (transmitter) and load (antenna) impedances match; and 2) the efficiency of the output stage of the transmitter may be compromised when connected to a mismatched load, resulting in less RF power produced. But I doubt whether that was the point. I think that all they were trying to say was that you would get more effective radiated power. And the claim of doubling the power raises suspicion. If memory serves me, part 15 radiations are severely limited. That may preclude 3db gain antennas. 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: 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. Yes, IF the present antenna presents a bad impedance match to the transmitter. Two factors come into play: 1) maximum power transfer occurs when the source (transmitter) and load (antenna) impedances match; and 2) the efficiency of the output stage of the transmitter may be compromised when connected to a mismatched load, resulting in less RF power produced. But I doubt whether that was the point. I think that all they were trying to say was that you would get more effective radiated power. And the claim of doubling the power raises suspicion. If memory serves me, part 15 radiations are severely limited. That may preclude 3db gain antennas. 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 |
You've gotten some good advice some others. I'll just add that most Part
15 devices are specified in terms of field strength at some distance from the antenna, depending on frequency, and not in terms of power or ERP. There might be some sections with other criteria, but if there are, field strength specification is by far the most common. The FCC does cut some slack in testing for home-built devices (not marketed, not constructed from a kit, and built in quantities of five or less for personal use), in section 15.23. My copy is nearly ten years old now, so I suggest checking a newer copy of Part 15. It's likely on the Web these days. Roy Lewallen, W7EL Liam Ness wrote: 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 |
You've gotten some good advice some others. I'll just add that most Part
15 devices are specified in terms of field strength at some distance from the antenna, depending on frequency, and not in terms of power or ERP. There might be some sections with other criteria, but if there are, field strength specification is by far the most common. The FCC does cut some slack in testing for home-built devices (not marketed, not constructed from a kit, and built in quantities of five or less for personal use), in section 15.23. My copy is nearly ten years old now, so I suggest checking a newer copy of Part 15. It's likely on the Web these days. Roy Lewallen, W7EL Liam Ness wrote: 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 |
In article , "Reg Edwards"
writes: 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. Quite true, Reg. To get even more basic for new folks to radio theory, assume the ideal isotropic antenna, one that radiates equally in all directions. It creates an EM field of the same density of RF energy per square area all around an ideal sphere enclosing the isotropic antenna. A half-wave dipole that is very high (elevated above ground) has an antenna pattern (of RF energy per square area) that is maximum perpendicular to the axis of the wires. That RF energy is minimum along the wire axes. An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic antenna and such gain is referred to as "2.4 dbi" with the little "i" indicating the reference to the isotropic. Because it is difficult to build a reference antenna that can perform like the ideal isotropic, many more complex antennas reference their gain to the half-wave dipole and those gains, in db, are labeled as "dbd" with the little "d" suffix refering to a dipole. FM and TV broadcast antennas are usually designed for antenna patterns that are almost omnidirectional in the horizontal plane and have very little RF energy at elevations above or below horizontal. The term "ERP" for Effective Radiated Power was first used with FM and TV broadcasting to indicate the basic power output of the transmitter multiplied by the antenna gain. For broadcast listeners they would "hear" a signal as if the station's power output was as strong as the ERP value. Directional antennas simply focus the RF energy in certain directions. What would be an equal value of RF in all directions with an ideal isotropic antenna now increases above ideal in the direction of maximum RF energy. That is the "gain." If one were to plot RF energy density per square whatever in a 3-D graph (almost always the case in antenna analysis programs or on antenna range receiver plotters), the "shape" formed is what everyone intuitively describes as the "pattern." [it could be called an "isopower" plot, I would guess] For an ideal isotropic antenna, the "pattern" is a sphere. For a good half-wave dipole very high above ground, the pattern looks like a torus or doughnut shape. A typical FM or TV antenna pattern looks like a fat pizza. A very high gain parabolic reflector radar antenna pattern looks sort of like a long breadstick. Complex wire antennas start looking like the outline of the contents of a spaghetti bowl. It's about 10:30 PM local time here and I'm getting hungry...bye...:-) Len Anderson retired (from regular hours) electronic engineer person |
In article , "Reg Edwards"
writes: 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. Quite true, Reg. To get even more basic for new folks to radio theory, assume the ideal isotropic antenna, one that radiates equally in all directions. It creates an EM field of the same density of RF energy per square area all around an ideal sphere enclosing the isotropic antenna. A half-wave dipole that is very high (elevated above ground) has an antenna pattern (of RF energy per square area) that is maximum perpendicular to the axis of the wires. That RF energy is minimum along the wire axes. An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic antenna and such gain is referred to as "2.4 dbi" with the little "i" indicating the reference to the isotropic. Because it is difficult to build a reference antenna that can perform like the ideal isotropic, many more complex antennas reference their gain to the half-wave dipole and those gains, in db, are labeled as "dbd" with the little "d" suffix refering to a dipole. FM and TV broadcast antennas are usually designed for antenna patterns that are almost omnidirectional in the horizontal plane and have very little RF energy at elevations above or below horizontal. The term "ERP" for Effective Radiated Power was first used with FM and TV broadcasting to indicate the basic power output of the transmitter multiplied by the antenna gain. For broadcast listeners they would "hear" a signal as if the station's power output was as strong as the ERP value. Directional antennas simply focus the RF energy in certain directions. What would be an equal value of RF in all directions with an ideal isotropic antenna now increases above ideal in the direction of maximum RF energy. That is the "gain." If one were to plot RF energy density per square whatever in a 3-D graph (almost always the case in antenna analysis programs or on antenna range receiver plotters), the "shape" formed is what everyone intuitively describes as the "pattern." [it could be called an "isopower" plot, I would guess] For an ideal isotropic antenna, the "pattern" is a sphere. For a good half-wave dipole very high above ground, the pattern looks like a torus or doughnut shape. A typical FM or TV antenna pattern looks like a fat pizza. A very high gain parabolic reflector radar antenna pattern looks sort of like a long breadstick. Complex wire antennas start looking like the outline of the contents of a spaghetti bowl. It's about 10:30 PM local time here and I'm getting hungry...bye...:-) Len Anderson retired (from regular hours) electronic engineer person |
On Wed, 08 Oct 2003 23:29:46 -0700, Roy Lewallen
wrote: You've gotten some good advice some others. I'll just add that most Part 15 devices are specified in terms of field strength at some distance from the antenna, depending on frequency, and not in terms of power or ERP. There might be some sections with other criteria, but if there are, field strength specification is by far the most common. The FCC does cut some slack in testing for home-built devices (not marketed, not constructed from a kit, and built in quantities of five or less for personal use), in section 15.23. My copy is nearly ten years old now, so I suggest checking a newer copy of Part 15. It's likely on the Web these days. Roy Lewallen, W7EL Yea, thanks Roy, you are right. I know that Part 15 limits are field strength based, and there isn't a direct corrolation with output power. I have been relying in the 15.23 home built portion, which provides some lee way to tinkerers. 15.23 acknowledges that home builders probably don't have test equipment, and instead it is directed at design. My stuff is super low power, at least that is what my spice program tells me, so I'm not really worried. If I do screw up, and an FCC representive contacts me (the first highly likely, the second much less so) I'll push section 15.23's applicability, stop using the transmitter and redesign per section 15.5. Even though I'm in a really rural, really mountainous location, I spent a good amount of time going over part 15 so I could justify that anything I do is done in a good faith effort to be in compliance. You are also right that part 15 is now web based. They just released a new version of the regulations in August and you can d/l it as a PDF from the FCC website. |
On Wed, 08 Oct 2003 23:29:46 -0700, Roy Lewallen
wrote: You've gotten some good advice some others. I'll just add that most Part 15 devices are specified in terms of field strength at some distance from the antenna, depending on frequency, and not in terms of power or ERP. There might be some sections with other criteria, but if there are, field strength specification is by far the most common. The FCC does cut some slack in testing for home-built devices (not marketed, not constructed from a kit, and built in quantities of five or less for personal use), in section 15.23. My copy is nearly ten years old now, so I suggest checking a newer copy of Part 15. It's likely on the Web these days. Roy Lewallen, W7EL Yea, thanks Roy, you are right. I know that Part 15 limits are field strength based, and there isn't a direct corrolation with output power. I have been relying in the 15.23 home built portion, which provides some lee way to tinkerers. 15.23 acknowledges that home builders probably don't have test equipment, and instead it is directed at design. My stuff is super low power, at least that is what my spice program tells me, so I'm not really worried. If I do screw up, and an FCC representive contacts me (the first highly likely, the second much less so) I'll push section 15.23's applicability, stop using the transmitter and redesign per section 15.5. Even though I'm in a really rural, really mountainous location, I spent a good amount of time going over part 15 so I could justify that anything I do is done in a good faith effort to be in compliance. You are also right that part 15 is now web based. They just released a new version of the regulations in August and you can d/l it as a PDF from the FCC website. |
I guess this all (well most anyway) makes sense. And while at first I
was glad that you all confirmed my thinking that you can't get more out than you put in (although you can get what you put in out more efficiently), Roy's part 15 comments are making rethink things. Because I'm field strenght limited, I concievable could be over limit in one direction. I don't think that I could accidentally rig an antenna like in Reg's example, but just for piece of mind could you give me an idea of how far from isotropic (or how close to ultradirectional) a vertical piece of wire would be at 1/4, 1/2 and not even close to being matched to wavelength? I'm not looking for calculations or anything, a simple very, not very or somewhere in between to isotropic would be fine. TIA On 09 Oct 2003 16:27:30 GMT, (Avery Fineman) wrote: In article , "Reg Edwards" writes: 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. Quite true, Reg. To get even more basic for new folks to radio theory, assume the ideal isotropic antenna, one that radiates equally in all directions. It creates an EM field of the same density of RF energy per square area all around an ideal sphere enclosing the isotropic antenna. A half-wave dipole that is very high (elevated above ground) has an antenna pattern (of RF energy per square area) that is maximum perpendicular to the axis of the wires. That RF energy is minimum along the wire axes. An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic antenna and such gain is referred to as "2.4 dbi" with the little "i" indicating the reference to the isotropic. Because it is difficult to build a reference antenna that can perform like the ideal isotropic, many more complex antennas reference their gain to the half-wave dipole and those gains, in db, are labeled as "dbd" with the little "d" suffix refering to a dipole. FM and TV broadcast antennas are usually designed for antenna patterns that are almost omnidirectional in the horizontal plane and have very little RF energy at elevations above or below horizontal. The term "ERP" for Effective Radiated Power was first used with FM and TV broadcasting to indicate the basic power output of the transmitter multiplied by the antenna gain. For broadcast listeners they would "hear" a signal as if the station's power output was as strong as the ERP value. Directional antennas simply focus the RF energy in certain directions. What would be an equal value of RF in all directions with an ideal isotropic antenna now increases above ideal in the direction of maximum RF energy. That is the "gain." If one were to plot RF energy density per square whatever in a 3-D graph (almost always the case in antenna analysis programs or on antenna range receiver plotters), the "shape" formed is what everyone intuitively describes as the "pattern." [it could be called an "isopower" plot, I would guess] For an ideal isotropic antenna, the "pattern" is a sphere. For a good half-wave dipole very high above ground, the pattern looks like a torus or doughnut shape. A typical FM or TV antenna pattern looks like a fat pizza. A very high gain parabolic reflector radar antenna pattern looks sort of like a long breadstick. Complex wire antennas start looking like the outline of the contents of a spaghetti bowl. It's about 10:30 PM local time here and I'm getting hungry...bye...:-) Len Anderson retired (from regular hours) electronic engineer person |
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