View Full Version : antenna reciprocity
AC0FP
05-29-2008, 08:58 PM
There has been a lot of discussion involving antennas on the forms the last couple of days. I am posting a question with multiple choice answers.
We have two identical IC-718's (common rig). One is using a three element 20 meter beam and the other is using a G5RV type dipole and the G5RV height above ground is adjusted so the TOA (take off angle) is the same as the three element beam. The two radios are in QSO at a 1000 mile distance and the S-Meter reading on the IC-718 using the G5RV is S9 +10.
Question: What is the S-meter reading of the radio using the 3 element beam. Choose the best answer.
1.) It is also S9 +10
2.) There is no way to estimate this answer.
3.) A dipole is a crap antenna, therefore S9.
4.) Beams "RULE" the G5RV signal is down at S2.
There has been a lot of discussion involving antennas on the forms the last couple of days. I am posting a question with multiple choice answers.
We have two identical IC-718's (common rig). One is using a three element 20 meter beam and the other is using a G5RV type dipole and the G5RV height above ground is adjusted so the TOA (take off angle) is the same as the three element beam. The two radios are in QSO at a 1000 mile distance and the S-Meter reading on the IC-718 using the G5RV is S9 +10.
Question: What is the S-meter reading of the radio using the 3 element beam. Choose the best answer.
1.) It is also S9 +10
2.) There is no way to estimate this answer.
3.) A dipole is a crap antenna, therefore S9.
4.) Beams "RULE" the G5RV signal is down at S2.
See my answer in your other post.
It contains the only limits that would void answer 1.
There are too many variables left out of the scenario. What is the signal's heading relative to the pattern of the G5RV and Yagi ? What is the height of the yagi, is it aimed at the incoming signal or at a random heading. What feedline is used and how long for the antennas (we know part of the G5RV is a piece of ladderline) ??????????????
I would not put up a G5RV antenna because I am not that interested in 20 M !
wa4brl
05-30-2008, 01:12 AM
Not enough information. Define the beam.
Many thousands of short-boom tri-band have beams have been sold that actually produce less erp in their main lobe than a dipole will.
All antennas are compomises. Some more than others. How much of a compromise depends upon ones needs and how well any antenna meets them.
ke7tfn
05-30-2008, 01:18 AM
1000 miles north?
I am going to say the S-Meter reading will be what it is and no poll is going to change the facts.
Not enough information. Define the beam.
Many thousands of short-boom tri-band have beams have been sold that actually produce less erp in their main lobe than a dipole will.
All antennas are compomises. Some more than others. How much of a compromise depends upon ones needs and how well any antenna meets them.
Wrong.
He gave all the information needed except for three small potential errors:
1.) Lossy networks are not bilaterial.
2.) We have to define the radios both deliver the same power into each antenna system
3.) The receivers have the same S reading at the same signal power and the receiver is impedance matched to the same impedance as the transmitter.
If those three cases are met or are "close enough", number 1 is the correct answer.
It will NOT:
a.) matter what the antenna gain is in either antenna.
b.) matter what direction any antenna is pointed.
Any gain applied to one end of the system or any loss works equally all through the system. So if ONE S meter reads S9 +10 dB they BOTH will read S9 +10 so long as the three conditions I outlined, that both transmitters deliver the same power into the feedline and both receivers are matched to the transmitter and antenna impedance and both read the same signal level for the same power from the antenna.
This is why we cannot compare transmitted reports to received reports to tell antenna system efficiency.
So he does have a valid test, within the limits of power delivered into the antenna system at each end being equal and the receivers being equal and the radios being matched to the feedlines impedance.
This does not mean the S/N ratio will be the same at each end, it just means the meters will read the same. If we placed a 10dB attenuator in series with one antenna in this test, BOTH receivers would drop by exactly 10dB.
That's how it works.
73 Tom
The results are the same if the three element beam is replaced with a 20 element beam, the beam is rotated any direction, or the beam is replaced with a hunk of wire and a tuner.
If ONE S meter reads 10 over 9, BOTH will read 10 over 9.
This assumes the S meters are identical in receiver input port sensitivity, transmitter power into the feedline identical, and each receiver and each transmitter is matched to the load.
k5tee
05-30-2008, 06:10 PM
Wrong.
He gave all the information needed except for three small potential errors:
1.) Lossy networks are not bilaterial.
2.) We have to define the radios both deliver the same power into each antenna system
3.) The receivers have the same S reading at the same signal power and the receiver is impedance matched to the same impedance as the transmitter.
If those three cases are met or are "close enough", number 1 is the correct answer.
It will NOT:
a.) matter what the antenna gain is in either antenna.
b.) matter what direction any antenna is pointed.
Any gain applied to one end of the system or any loss works equally all through the system. So if ONE S meter reads S9 +10 dB they BOTH will read S9 +10 so long as the three conditions I outlined, that both transmitters deliver the same power into the feedline and both receivers are matched to the transmitter and antenna impedance and both read the same signal level for the same power from the antenna.
This is why we cannot compare transmitted reports to received reports to tell antenna system efficiency.
So he does have a valid test, within the limits of power delivered into the antenna system at each end being equal and the receivers being equal and the radios being matched to the feedlines impedance.
This does not mean the S/N ratio will be the same at each end, it just means the meters will read the same. If we placed a 10dB attenuator in series with one antenna in this test, BOTH receivers would drop by exactly 10dB.
That's how it works.
73 Tom
There has been a lot of discussion involving antennas on the forms the last couple of days. I am posting a question with multiple choice answers.
We have two identical IC-718's (common rig). One is using a three element 20 meter beam and the other is using a G5RV type dipole and the G5RV height above ground is adjusted so the TOA (take off angle) is the same as the three element beam. The two radios are in QSO at a 1000 mile distance and the S-Meter reading on the IC-718 using the G5RV is S9 +10.
Question: What is the S-meter reading of the radio using the 3 element beam. Choose the best answer.
1.) It is also S9 +10
2.) There is no way to estimate this answer.
3.) A dipole is a crap antenna, therefore S9.
4.) Beams "RULE" the G5RV signal is down at S2.
One thing left out is what frequency they are using. The OP only states that they are using a 20 Meter Beam and a G5RV, but does not state that they are using 20 meters for this experiment. They could be trying 80 Meters or 10 Meters or even 2 Meters, we do not know. So the answer is not enough information.
Tom
One thing left out is what frequency they are using. The OP only states that they are using a 20 Meter Beam and a G5RV, but does not state that they are using 20 meters for this experiment. They could be trying 80 Meters or 10 Meters or even 2 Meters, we do not know. So the answer is not enough information.
Tom
Nope Tom. Doesn't matter a bit.
Doesn't matter what the gain is or the pattern or direction or anything.
If we have two transmitters with the same power into the load, and two receivers with meters that have the same reference power from a source for the same reference reading, say S-9 is 50uV at 50 ohms and the antennas and receivers and transmitters are matched to 50 ohms, the meters will always read EXACTLY the same.
It's really a very clear question with a clear answer, but the responses clearly show a weak area in our understanding as a group.
Think of it this way. Anything you do at the transmitter on either end to reduce the signal....turn the antenna, add loss to the feeder, etc....also reduces the receive the same amount.
Since he quantified the signal at one end to be S9 +10 and since he specified power to be the same, that means the S meters at each end are the same.
Receiver----loss-----transmitter or transmitter---loss----receiver
If the S meters have the same sensitivity and one reads S 9 and the power is the same, they BOTH read S9 because the net loss is the same either direction.
So by saying the power is the same, and the receive signals are the same, it defines the answers.
There are a few slight exceptions. Lossy networks do not have to be bilaterial and the receiver or transmitter might have different optimum impedances compared to the antennas and transmission lines, but if we say the impedances are matched in both directions then the answer is both ends have the same S meter reading.
If we increase the gain of one antenna 10dB, both the receive and transmit changes the same number of dB. That means the signal at the far end goes up 10dB, and the receiver goes up ten dB on the gain end also.
We can rotate antennas until we turn blue and the change is the same at BOTH ends of the path.
Now this does not mean the S/N ratio is the same at each end, or that the S/N changes in step with a gain change, it just means the absolute level of the transmitted signal changes the same as the absolute level of the receiver.
So twice as many answered this wrong, even though it is a simple network problem, as answered correctly.
73 Tom
CT2JUT
05-30-2008, 09:11 PM
This poll can be clarified with the Antenna ERP (Effective Radiated Power) subject.
There are many factors than can improve or not the signal.
It depends seriously from the cable type and lenght feeding the antenna, and many other factors.
So iīm sure that there isnīt a general rule for this comparision.
w6vps
05-30-2008, 10:40 PM
Antenna Reciprocity ?
I understand the text of the question well enough...but does this title really make sense?
Seems to me the word was grabbed out of thin air.
Reciprocity
1: the quality or state of being reciprocal : mutual dependence, action, or influence
Regardless..the post makes for interesting thought and reading.
This poll can be clarified with the Antenna ERP (Effective Radiated Power) subject.
There are many factors than can improve or not the signal.
It depends seriously from the cable type and lenght feeding the antenna, and many other factors.
So iīm sure that there isnīt a general rule for this comparision.
Not true. There is an absolute rule. It is well known.
You have this system:
A-----TOTAL LOSS OR GAIN OF ANTENNA SYSTEMS-------B
If A is 100 watts and B is a receiver with an S meter, it does not matter a single bit if you reverse the system. So if A is a transceiver and B is an identical transceiver in every single way, it doesn't matter what we do to the total loss or gain the meters on each end will read the same level.
Now there is an advanced thing that can happen since loss does not need to be bilaterial (like a very lossy matching network), but for the purposes of this exercise it is so small it can be ignored. It has nothing to do with the antennas, only impedance matching errors. So if we say the systems are matched (perhaps through good low loss tuners or just antenna design) then the system is truely bilaterial.
This is why the antenna does not matter, the feedline does not matter, the direction of the antenna does not matter.
It is a simple basic problem with a simple answer.
73 Tom
Here is a statement that may help explain the concept.
I have an antenna with zero dB gain, and replace it with one having 10dB gain in the direction of a reference antenna.
1.) It is like the transmitter got 10dB louder, so the distant meter will be (if a perfect meter) ten dB higher.
2.) With no change on the far end in transmitter power, the receiver will now read 10dB higher.
Logically we can conclude that if the transceivers are perfectly identical in every way at each end, no matter what we do with any antenna or feedline the change in signal level will be exactly the same at each end. So if the radios are indentical, the signal reading on each meter will be the same no matter what it is. If one meter is 30 over, the other has to be 30 over. If one is S2, the other is S2 if we look only at the signal and not local noise.
Signal to noise can change in random ways, but NOT signal level.
73 Tom
CT2JUT
05-31-2008, 12:38 AM
Not true. There is an absolute rule. It is well known.
You have this system:
A-----TOTAL LOSS OR GAIN OF ANTENNA SYSTEMS-------B
If A is 100 watts and B is a receiver with an S meter, it does not matter a single bit if you reverse the system. So if A is a transceiver and B is an identical transceiver in every single way, it doesn't matter what we do to the total loss or gain the meters on each end will read the same level.
Now there is an advanced thing that can happen since loss does not need to be bilaterial (like a very lossy matching network), but for the purposes of this exercise it is so small it can be ignored. It has nothing to do with the antennas, only impedance matching errors. So if we say the systems are matched (perhaps through good low loss tuners or just antenna design) then the system is truely bilaterial.
This is why the antenna does not matter, the feedline does not matter, the direction of the antenna does not matter.
It is a simple basic problem with a simple answer.
73 Tom
My statement was regarding output power from the antenna.
Being like this, iīm sure that i wrote is correct.
Maybe i didnīt understand the question correctly.
So please ignore my post, if this is not the point/subject.
Anyway, i donīt see how you can create a general rule for s-meter readings in this situation, even well fundamented.
Keep posting! I like these questions! :)
AC0FP
05-31-2008, 01:01 AM
Antenna Reciprocity ?
I understand the text of the question well enough...but does this title really make sense?
Seems to me the word was grabbed out of thin air.
Reciprocity
1: the quality or state of being reciprocal : mutual dependence, action, or influence
Regardless..the post makes for interesting thought and reading.
Antenna reciprocity is a fundamental RF antenna theory. It merely states that an antenna has the same gain regardless of whether it is transmitting a signal or receiving a signal. This is fundamental to the question.
I'm glad you like the post and find it interesting. It was hoping that the post might stimulate thought about antennas and their effect on performance on each end of a QSO, particularity for those who can't put up the big Stepper beam.
73,
Frank:)
P.S: W8JI has done a great job in explaining the theory.
As to the question, there are two "straw dogs" included. The mileage and TOA are also irrelevant to the answer to the question.
W4INF
05-31-2008, 01:26 AM
Please stand by, I feel an IEEE about to drop a turd in your punchbowl and tell you an S-Meter reading has no bearing on signal strength, therefore you cannot evaluate an antenna's performance what so ever, however they will be happy to model it for you and state that is the way it WILL perform (Ignoring real world results and common (read: "poor man's") methods of evaluation.)
Be patient, the HRO gods will soon be dispatched.
Otherwise, I have nothing constructive to add to the post so sorry for the digital QRM.
Cheers,
Andrew
wa4brl
05-31-2008, 01:33 AM
Wrong.
He gave all the information needed except for three small potential errors:
1.) Lossy networks are not bilaterial.
2.) We have to define the radios both deliver the same power into each antenna system
3.) The receivers have the same S reading at the same signal power and the receiver is impedance matched to the same impedance as the transmitter.
If those three cases are met or are "close enough", number 1 is the correct answer.
It will NOT:
a.) matter what the antenna gain is in either antenna.
b.) matter what direction any antenna is pointed.
Any gain applied to one end of the system or any loss works equally all through the system. So if ONE S meter reads S9 +10 dB they BOTH will read S9 +10 so long as the three conditions I outlined, that both transmitters deliver the same power into the feedline and both receivers are matched to the transmitter and antenna impedance and both read the same signal level for the same power from the antenna.
This is why we cannot compare transmitted reports to received reports to tell antenna system efficiency.
So he does have a valid test, within the limits of power delivered into the antenna system at each end being equal and the receivers being equal and the radios being matched to the feedlines impedance.
This does not mean the S/N ratio will be the same at each end, it just means the meters will read the same. If we placed a 10dB attenuator in series with one antenna in this test, BOTH receivers would drop by exactly 10dB.
That's how it works.
73 Tom
I was completely puzzled by your response, so I went back to reread the original post. I discovered that I had, in fact, misread it. Upon undrstanding the hypothetical situation, I must agree that you are correct, sir. The system gain does not change, so the S-metter readings will indeed be equal.
(I had read it as both rigs being in the same shack, switching A/B style between them while in QSO with another station. Another senior moment, perhaps.)
w6vps
05-31-2008, 04:10 AM
Antenna reciprocity is a fundamental RF antenna theory. It merely states that an antenna has the same gain regardless of whether it is transmitting a signal or receiving a signal. This is fundamental to the question.
I'm glad you like the post and find it interesting. It was hoping that the post might stimulate thought about antennas and their effect on performance on each end of a QSO, particularity for those who can't put up the big Stepper beam.
73,
Frank:)
P.S: W8JI has done a great job in explaining the theory.
As to the question, there are two "straw dogs" included. The mileage and TOA are also irrelevant to the answer to the question.
Frank:
I stand corrected...many thanks...I learned a lot on this thread
Paul ;)
ve2nsm
05-31-2008, 04:24 AM
I will not repeat everything, but I totally agree with w8ji.
Of course, there are some who will look down on this post with disdain because it talks about S-meters.
Everybody knows that a real ham gives signal strength reports by ear ;)
Please stand by, I feel an IEEE about to drop a turd in your punchbowl and tell you an S-Meter reading has no bearing on signal strength, therefore you cannot evaluate an antenna's performance what so ever, however they will be happy to model it for you and state that is the way it WILL perform (Ignoring real world results and common (read: "poor man's") methods of evaluation.)
Be patient, the HRO gods will soon be dispatched.
Otherwise, I have nothing constructive to add to the post so sorry for the digital QRM.
Cheers,
Andrew
Andrew,
If you understood this, you would see why your test where you compared the sent report to the received report was meaningless for determining antenna efficiency or performance.
The ONLY reliable way to tell a difference (short of a good model) in gain or efficiency (for the same pattern) between two antennas is to switch between a known reference antenna and the antenna under test. Emotions won't change that. :-)
Not doing this is what allows so many people to develop a fantasy about their favorite antenna, like "my loop is quieter than a dipole" or "my superfield 2000 is a great antenna".
73 Tom
ve2nsm
05-31-2008, 03:38 PM
Not doing this is what allows so many people to develop a fantasy about their favorite antenna, like "my loop is quieter than a dipole" or "my superfield 2000 is a great antenna".
Or: A beta (hairpin) match has more gain than a gamma match because it connects on both sides of the element.
Yes, I heard that, and he was convinced about it.
K1VSK
05-31-2008, 08:57 PM
Andrew,
If you understood this, you would see why your test where you compared the sent report to the received report was meaningless for determining antenna efficiency or performance.
The ONLY reliable way to tell a difference (short of a good model) in gain or efficiency (for the same pattern) between two antennas is to switch between a known reference antenna and the antenna under test. Emotions won't change that. :-)
Not doing this is what allows so many people to develop a fantasy about their favorite antenna, like "my loop is quieter than a dipole" or "my superfield 2000 is a great antenna".
73 Tom
Finally someone restated what was apparent 3 pages ago - this comparison is meaningless but it is interesting to see the self-proclaimed "experts" vivid imaginations.
It doesn't make sense to move a technical thread to talks and opinions. Some moderator must have lost his compass.
If recriprocity is a hard and fast rule of physics, why do I sometimes get a 599 report from someone I can barely hear, supposedly running the same power and similar antenna, and on other days I get a 449 report from someone who is
s8 and perfectly clear with the equivalent station to mine ???
ve2nsm
06-01-2008, 07:51 PM
If recriprocity is a hard and fast rule of physics, why do I sometimes get a 599 report from someone I can barely hear, supposedly running the same power and similar antenna, and on other days I get a 449 report from someone who is
s8 and perfectly clear with the equivalent station to mine ???
Because some people think the S-meter is an evil thing, that you should not use it, so they give the report by how well they "hear" you, which is of course the job of the readability report ;)
If they hear you loud and have no noise, they will give you a S9 even if you're S3, some other times if they have a S9 noise floor, they will give you S3 even if you're S9+10... go figure :mad:
Remember, signal strength meter is an evil thing, a real ham should be able to give a signal strength reading by ear, despite the noise floor, QRM, AGC and DSPs. As a matter of fact, a REAL ham should know exactly how many ĩV are present at his antenna connector at all times... with his eyes closed.
S meters? we don't need so stinking S meters !!! ;):D
AC0FP
06-03-2008, 06:15 AM
If recriprocity is a hard and fast rule of physics, why do I sometimes get a 599 report from someone I can barely hear, supposedly running the same power and similar antenna, and on other days I get a 449 report from someone who is
s8 and perfectly clear with the equivalent station to mine ???
This is the question this thread originally was intended to answer, however, since it is now in "Talk and Opinions" you can only receive "opinions".
:rolleyes:
Opinion to follow, but not today.