Home |
Search |
Today's Posts |
#101
|
|||
|
|||
Parallel coax
In message , John S
writes On 9/27/2015 1:42 PM, John S wrote: On 9/27/2015 1:20 PM, Wayne wrote: "rickman" wrote in message ... On 9/27/2015 10:41 AM, kg7fu wrote: Matching the antenna won't make the Return Loss go away but it will make the transmitter happy. Can you explain this? I thought matching the antenna would *exactly* make the return loss go away because it would eliminate the mismatch. Not wanting to put words in his mouth.... I read that to mean that the high SWR between the ATU and the antenna would remain, but the transmitter would be happy with the SWR on the transmitter/ATU coax. Rick is correct. If the antenna (load) is matched to the line, there is no return loss, hence no SWR. The ATU will be adjusted (hopefully) to make the transmitter operate properly with the impedance as seen at the transmitter end of the line. I apologize. My statement that "there is no return loss" above is technically incorrect. The return loss with matched conditions is maximum. It will peg your meter. Are you both getting this right? With a perfect match, the return loss is infinite (ie there is absolutely no reflection). Your SWR meter will read 1:1. With a total mismatch, the return loss is zero (ie the reflection is as strong as the forward signal). Your SWR meter will be pegged hard over at FSD. -- Ian |
#102
|
|||
|
|||
Parallel coax
In message , John S
writes On 9/29/2015 1:45 PM, Wayne wrote: We always used positive numbers and viewed RL as the difference in dB between the forward and reflected power. That makes perfect sense. You could never have a negative dB. It might be instructive for us all to have a quick look at this information (especially the last sentence!): http://www.microwaves101.com/encyclopedias/vswr "Thus in its correct form, return loss will usually be a positive number. If it's not, you can usually blame measurement error. The exception to the rule is something with negative resistance, which implies that it is an active device (external DC power is converted to RF) and it is potentially unstable (it could oscillate). Not something you have to worry about if you are just looking at coax cables! However, many engineers often omit the minus sign and talk about "-9.5 dB return loss" for example. People that find it necessary to correct engineers who do this have underwear that is too tight." . -- Ian |
#103
|
|||
|
|||
Parallel coax
On 9/29/2015 5:05 PM, Ian Jackson wrote:
In message , John S writes On 9/27/2015 1:42 PM, John S wrote: On 9/27/2015 1:20 PM, Wayne wrote: "rickman" wrote in message ... On 9/27/2015 10:41 AM, kg7fu wrote: Matching the antenna won't make the Return Loss go away but it will make the transmitter happy. Can you explain this? I thought matching the antenna would *exactly* make the return loss go away because it would eliminate the mismatch. Not wanting to put words in his mouth.... I read that to mean that the high SWR between the ATU and the antenna would remain, but the transmitter would be happy with the SWR on the transmitter/ATU coax. Rick is correct. If the antenna (load) is matched to the line, there is no return loss, hence no SWR. The ATU will be adjusted (hopefully) to make the transmitter operate properly with the impedance as seen at the transmitter end of the line. I apologize. My statement that "there is no return loss" above is technically incorrect. The return loss with matched conditions is maximum. It will peg your meter. Are you both getting this right? With a perfect match, the return loss is infinite (ie there is absolutely no reflection). Your SWR meter will read 1:1. With a total mismatch, the return loss is zero (ie the reflection is as strong as the forward signal). Your SWR meter will be pegged hard over at FSD. You are correct, of course. I was thinking of a dB meter that reads from 0 to some large value (say, 100) full scale. It is not an SWR meter. I have probably just confused the whole thing. Sorry. |
#104
|
|||
|
|||
Parallel coax
On 9/29/2015 8:19 AM, Jerry Stuckle wrote:
On 9/29/2015 3:47 AM, Ian Jackson wrote: In message , Jerry Stuckle writes On 9/28/2015 7:12 PM, John S wrote: On 9/28/2015 1:51 PM, Jerry Stuckle wrote: On 9/28/2015 1:42 PM, rickman wrote: You said return loss increases with lower SWR. It does not. It does. Sorry, a lower SWR does not increase the amount of loss. Of course it doesn't. No one said it did. It does the opposite, ie a lower SWR gives less loss on the feeder. Please cite a reliable reference that says it does. Even the table Rick cited shows a negative value for return SWR. What is this 'Return SWR'? I'm not familiar with it. Sorry, writing too quickly. I meant return loss. Do you mean Return Loss Ratio (RLR)? This is a simple, easily measurable, and meaningful statement of how strong the returning reflected signal is compared with the outgoing forward signal. The reflected signal is a weaker version of the forward signal. It's expressed as a loss, an attenuation, or relatively how much down the level of the reflection is. You can express this as a numerical ratio - the reflection coefficient (rho) - or (as often more convenient) rho in dB. As others have suggested, what is apparently a negative sign in the chart is presumably more artistic licence than scientific accuracy. If you lose $10, you don't say that you lost 'minus $10'. Similarly, when you lose 10dB of signal, you don't say you lost 'minus 10dB'. Which is greater - 10db or -30db? Which is greater: -20dB or +20dB? |
#105
|
|||
|
|||
Parallel coax
In message , Jeff writes
On 29/09/2015 14:31, Jerry Stuckle wrote: On 9/29/2015 4:40 AM, Jeff wrote: Physicists and engineers do not mix gain and loss. Gain is always shown as a positive number and loss as a negative number. For instance - a system shows a gain and loss of +3, +5, +2, +1. What is the total gain or loss of the system? Of course they do, particularly when dealing with a quantity that is defined as a LOSS. I have never heard any engineer when asked the question 'what is that attenuator' reply minus 3 dB. It is always 3dB. It is always called a 3dB attenuator, not a minus 3dB attenuator. Jeff Yes, and the power out is never +3db relative to the input. It is always -3db. Which is often referred to as 3dB loss (or 3dB down), ie a positive quantity. And Jerry still hasn't answered my question about him losing -$10. -- Ian |
#106
|
|||
|
|||
Parallel coax
On 9/30/2015 10:12 AM, Ian Jackson wrote:
In message , Jeff writes On 29/09/2015 14:31, Jerry Stuckle wrote: On 9/29/2015 4:40 AM, Jeff wrote: Physicists and engineers do not mix gain and loss. Gain is always shown as a positive number and loss as a negative number. For instance - a system shows a gain and loss of +3, +5, +2, +1. What is the total gain or loss of the system? Of course they do, particularly when dealing with a quantity that is defined as a LOSS. I have never heard any engineer when asked the question 'what is that attenuator' reply minus 3 dB. It is always 3dB. It is always called a 3dB attenuator, not a minus 3dB attenuator. Jeff Yes, and the power out is never +3db relative to the input. It is always -3db. Which is often referred to as 3dB loss (or 3dB down), ie a positive quantity. And Jerry still hasn't answered my question about him losing -$10. Ever since he was presented with "reliable sources" he has been silent. I was beginning to think that he would accept no source at all if it didn't agree with him. However, his silence now is a plus for him. |
#107
|
|||
|
|||
Parallel coax
"Ian Jackson" wrote in message ... In message , John S writes On 9/29/2015 1:45 PM, Wayne wrote: We always used positive numbers and viewed RL as the difference in dB between the forward and reflected power. That makes perfect sense. You could never have a negative dB. It might be instructive for us all to have a quick look at this information (especially the last sentence!): http://www.microwaves101.com/encyclopedias/vswr "Thus in its correct form, return loss will usually be a positive number. If it's not, you can usually blame measurement error. The exception to the rule is something with negative resistance, which implies that it is an active device (external DC power is converted to RF) and it is potentially unstable (it could oscillate). Not something you have to worry about if you are just looking at coax cables! However, many engineers often omit the minus sign and talk about "-9.5 dB return loss" for example. People that find it necessary to correct engineers who do this have underwear that is too tight." LOL. Yes I have run into people like that. Years ago, I worked with a young engineer who would ask me the following question when I said "SWR". He would ask "Do you mean VSWR?" After a few of his questions, I constructed a nomograph that would convert SWR to VSWR for him. And I even wrote an equation for him. SWR=10(VSWR)/10 hope this doesn't start a new thread |
#108
|
|||
|
|||
Parallel coax
On 9/30/2015 12:57 PM, John S wrote:
On 9/30/2015 10:12 AM, Ian Jackson wrote: In message , Jeff writes On 29/09/2015 14:31, Jerry Stuckle wrote: On 9/29/2015 4:40 AM, Jeff wrote: Physicists and engineers do not mix gain and loss. Gain is always shown as a positive number and loss as a negative number. For instance - a system shows a gain and loss of +3, +5, +2, +1. What is the total gain or loss of the system? Of course they do, particularly when dealing with a quantity that is defined as a LOSS. I have never heard any engineer when asked the question 'what is that attenuator' reply minus 3 dB. It is always 3dB. It is always called a 3dB attenuator, not a minus 3dB attenuator. Jeff Yes, and the power out is never +3db relative to the input. It is always -3db. Which is often referred to as 3dB loss (or 3dB down), ie a positive quantity. And Jerry still hasn't answered my question about him losing -$10. Ever since he was presented with "reliable sources" he has been silent. I was beginning to think that he would accept no source at all if it didn't agree with him. However, his silence now is a plus for him. And no need to go on about the more dramatic aspects of this conversation. If everyone is happy with the information indicating that return loss is conventionally a positive dB value, let's move on. While I may enjoy showing the "truth" to someone who is being obstinate, I don't wish to make anyone feel like I'm rubbing their nose in it. -- Rick |
#109
|
|||
|
|||
Parallel coax
In message , rickman
writes On 9/30/2015 12:57 PM, John S wrote: On 9/30/2015 10:12 AM, Ian Jackson wrote: In message , Jeff writes On 29/09/2015 14:31, Jerry Stuckle wrote: On 9/29/2015 4:40 AM, Jeff wrote: Physicists and engineers do not mix gain and loss. Gain is always shown as a positive number and loss as a negative number. For instance - a system shows a gain and loss of +3, +5, +2, +1. What is the total gain or loss of the system? Of course they do, particularly when dealing with a quantity that is defined as a LOSS. I have never heard any engineer when asked the question 'what is that attenuator' reply minus 3 dB. It is always 3dB. It is always called a 3dB attenuator, not a minus 3dB attenuator. Jeff Yes, and the power out is never +3db relative to the input. It is always -3db. Which is often referred to as 3dB loss (or 3dB down), ie a positive quantity. And Jerry still hasn't answered my question about him losing -$10. Ever since he was presented with "reliable sources" he has been silent. I was beginning to think that he would accept no source at all if it didn't agree with him. However, his silence now is a plus for him. And no need to go on about the more dramatic aspects of this conversation. If everyone is happy with the information indicating that return loss is conventionally a positive dB value, let's move on. While I may enjoy showing the "truth" to someone who is being obstinate, I don't wish to make anyone feel like I'm rubbing their nose in it. So let's get back to the original question. Was it ever really answered? I think it was made slightly more complicated by the fact that the antenna feedpoint impedance was not purely resistive, but was actually around 20-j130 (at 14 MHz), Was there any advantage in having the coax paralleled (both for 20 ohms resistive, and for 20-j130)? -- Ian |
#110
|
|||
|
|||
Parallel coax
"Ian Jackson" wrote in message ... In message , rickman writes On 9/30/2015 12:57 PM, John S wrote: On 9/30/2015 10:12 AM, Ian Jackson wrote: In message , Jeff writes On 29/09/2015 14:31, Jerry Stuckle wrote: On 9/29/2015 4:40 AM, Jeff wrote: So let's get back to the original question. Was it ever really answered? I think it was made slightly more complicated by the fact that the antenna feedpoint impedance was not purely resistive, but was actually around 20-j130 (at 14 MHz), Was there any advantage in having the coax paralleled (both for 20 ohms resistive, and for 20-j130)? Expanding on the original question.... Antenna feedpoint approximately 20-j130 The ATU drives the antenna through about 15 feet of coax. Assuming that the ATU provides a +j130 conjugate match, does that leave the coax with a SWR of 50/20= 2.5:1? If so, then I will not bother with considering 2 parallel coax. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
OT Parallel to USB Cable | CB | |||
Parallel Lines | Antenna | |||
varicaps in parallel | Homebrew | |||
varicaps in parallel | Homebrew | |||
Parallel runs of coax to antenna | Antenna |