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Are pi networks THAT INefficient?
Ralph Hanna, W8QUR, in a brief article "Pi Networks" on page 108 of
the December, 1965, issue of 73 MAGAZINE, after discussing power- supply filters and high- and low-pass TV filters, wrote: (Paraphrasing) "The most popular of all pi networks is the output circuit of a transmitter ... with which the output of almost any transmitter can be matched to almost any antenna ... another advantage is the reduction of harmonics.... (Actual quote) "The big disadvantage of this system is the low efficiency. It is not possible to run more than 50% efficiency and it tends to be more like 30%. Other methods of feeding the antenna will result in efficiencies of as high as 65% to 70%." Is that "low efficiency" of 30-50% really true? --Myron, W0PBV. -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license |
Are pi networks THAT INefficient?
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Are pi networks THAT INefficient?
Myron, W0PBV wrote:
"Is that low efficiency of 30-50% really true?" No. The pi network would not be so popular were that true. The efficiency of a Class B or Class C amplifier is higher than that and the network by itself is very low loss. RF amplifiers typically have efficiencies well above 50% because much of their source resistance is of the lossless variety. Search the internet for: "pi network antenna tuner". One entry near the top of the list is from Collins for its 180S-1 Antenna Tuner. It is basically a 1000 watt "pi" network for matching various antenna impedances to a 50 ohm coaxial transmission line in the range of 3-30 MHz. In most cases it is used as an "L" network, but when the "L" network cannot match the desired antenna, the complete "pi" circuit is used. The vacuum variable capacitor employed in the output circuit can be connected either in series or shunt with the antenna. The 180S-1 is useful for tuning trailing wires on large aircraft. Ralph Hanna, W8QUR had it wrong when saying "The big disadvantage of this system is the low efficiency." Best regards, Richard Harrison, KB5WZI |
Are pi networks THAT INefficient?
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Are pi networks THAT INefficient?
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Are pi networks THAT INefficient?
"Richard Harrison" wrote in message ... Myron, W0PBV wrote: "Is that low efficiency of 30-50% really true?" No. The pi network would not be so popular were that true. The efficiency of a Class B or Class C amplifier is higher than that and the network by itself is very low loss. RF amplifiers typically have efficiencies well above 50% because much of their source resistance is of the lossless variety. Best regards, Richard Harrison, KB5WZI If the efficiency or loses were in 50% range, we would be "ungluing" the components in the PI network at the 2 kW power levels. Can you picture 1 kW being "lost" in the coil and capacitors? Typical matching network or tuners in the transceivers have loss about 10% when power output is measured with tuner in or out while maintaining same input. Amps with decent copper and quality caps should be less than 10% in loses. Yuri, K3BU |
Are pi networks THAT INefficient?
On Wed, 23 May 2007 23:33:45 -0400, "Jimmie D"
wrote: Giving W8QUR the benifit of the doubt is like saying a 4 cylinder Hummer's efficiency would be improved if you removed the chassis. 73's Richard Clark, KB7QHC |
Are pi networks THAT INefficient?
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Are pi networks THAT INefficient?
Cecil Moore wrote:
wrote: Is that "low efficiency" of 30-50% really true? 50 ohm solid-state transceivers have a fixed filter on their outputs which is often a multi-stage pi- network. That fact alone should answer the question. Doesn't the impedance transformation ratio have an effect on the efficiency of a pi network? Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Are pi networks THAT INefficient?
Chuck wrote:
Doesn't the impedance transformation ratio have an effect on the efficiency of a pi network? The question is not: Can a particular pi-network plus load be 30-50% efficient? Certainly, it can. The question is: Are all pi-networks 30-50% efficient? The answer is "No!". -- 73, Cecil http://www.w5dxp.com |
Are pi networks THAT INefficient?
Cecil Moore wrote:
Chuck wrote: Doesn't the impedance transformation ratio have an effect on the efficiency of a pi network? The question is not: Can a particular pi-network plus load be 30-50% efficient? Certainly, it can. No disagreement, Cecil. My question wasn't intended as a response to your post. Sorry about that. The question is: Are all pi-networks 30-50% efficient? The answer is "No!". If that is the question, then no disagreement there either. We don't know all of what W8QUR said, and none of what he meant, but there are contexts in which each of his three assertions could be correct. I guess the one thing he doesn't seem to have said is that all pi-networks are 30-50% efficient. ;-) 73, Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Are pi networks THAT INefficient?
Chuck wrote:
No disagreement, Cecil. My question wasn't intended as a response to your post. Sorry about that. Didn't mean to sound grouchy - it was before my first cup of coffee. We don't know all of what W8QUR said, and none of what he meant, but there are contexts in which each of his three assertions could be correct. I guess the one thing he doesn't seem to have said is that all pi-networks are 30-50% efficient. ;-) We do know what seemed to be inferred about the inference of what W8QUR said. :-) -- 73, Cecil, w5dxp.com |
Are pi networks THAT INefficient?
"Jimmie D" wrote in message ... "Owen Duffy" wrote in message ... wrote in : Ralph Hanna, W8QUR, in a brief article "Pi Networks" on page 108 of the December, 1965, issue of 73 MAGAZINE, after discussing power- supply filters and high- and low-pass TV filters, wrote: (Paraphrasing) "The most popular of all pi networks is the output circuit of a transmitter ... with which the output of almost any transmitter can be matched to almost any antenna ... another advantage is the reduction of harmonics.... (Actual quote) "The big disadvantage of this system is the low efficiency. It is not possible to run more than 50% efficiency and it tends to be more like 30%. Other methods of feeding the antenna will result in efficiencies of as high as 65% to 70%." Is that "low efficiency" of 30-50% really true? Myron, The temptation is to see that the second paragraph is about Pi networks, though it doesn't actually use the term. It does refer to a "system" and goes on to discuss efficiency in the context of "feeding the antenna". There is no doubt that practical Pi networks in transmitters operate at efficiencies much greater than 50%, and the design efficiency is a trade- off with harmonic suppression (for the low pass configuration in a typical PA). If the term "system" is to include more than just the Pi network, then lower system efficiciency will prevail, but without a clear definition of the "system", it is not possible to comment on the reasonableness. For example, if a Pi coupled transmitter feeds a full wave dipole via a substantial length of coax, system efficiency might well be much less than 10%. Does he include DC to RF conversion loss in his view of system efficiency? Owen Giving W8QUR the benifit of the doubt I thought he may be including feedline losses which could be from 1 to 2 db for coax compared to balanced line used with a balanced output network. I think something may be lost in the paraphrasing and this is probably a comparison of balanced to unbalanced systems rather than a comparison of Pi-net to other types of tuner networks. Jimmie In the 1960's it was common to refer to efficiency as relating to the entire system converting AC or DC power into RF out of the antenna. Total system efficiencies of 30-50% would have been normal taking into account the losses involved in running valve heaters, HT valve supplies and final stage cooling fans. The reference to Pi matching output circuits is a bit of a red herring. These were probably the most commonly used system used to match transmitters that were required to operate on a wide range of frequencies because they were so effective at this task, and relatively cheap to manufacture. Other more efficient matching methods could be used for fixed frequency valve transmitters. Even up to the 1980's, many shipboard emergency transmitters were valve based. Radio Officers were required to make regular checks and efficiency calculations to ensure that the emergency transmitters and receivers could operate for a minimum specified period from a bank of emergency batteries. Even with several hundred amp hours of battery capacity, only around 16 hours of full power (100watts) operation was the maximum that could be expected. Mike G0ULI |
Are pi networks THAT INefficient?
On May 23, 1:08 pm, Wes Stewart wrote:
On 23 May 2007 09:37:00 -0500, wrote: Ralph Hanna, W8QUR, in a brief article "Pi Networks" on page 108 of the December, 1965, issue of 73 MAGAZINE, after discussing power- supply filters and high- and low-pass TV filters, wrote: (Paraphrasing) "The most popular of all pi networks is the output circuit of a transmitter ... with which the output of almost any transmitter can be matched to almost any antenna ... another advantage is the reduction of harmonics.... (Actual quote) "The big disadvantage of this system is the low efficiency. It is not possible to run more than 50% efficiency and it tends to be more like 30%. Other methods of feeding the antenna will result in efficiencies of as high as 65% to 70%." Is that "low efficiency" of 30-50% really true? As others have stated, No. Clearly at that time the author was talking about a vacuum tube transmitter where the pi-network was used to transform the load impedance (usually 50 ohm) up to the load that the tube(s) want to see. The usual implementation was the low-pass form of shunt C(s), series L, although this isn't the only option. The network can be thought of as two L-networks back-to-back with a "virtual" impedance common to the midpoint. The usual design sets a overall network Q (the sum of the two L-network Q's) at something between 10 and 12 for harmonic suppression reasons. The loss in the network is usually considered to be only in the inductor, (although this isn't totally correct) because inductors generally have lower unload Qs than the air or vacuum variable capacitors that are typically used. The network efficiency using this assumption is then: eff = 1 - (Ql/Qu) So for example if the inductor Q = 200 (a reasonable value) and the network Q is set to 12 then the efficiency is 94%, a long way from what the author claims. At higher frequencies with tubes with high output capacitance it may be necessary to design for a higher loaded Q than we would like. In this case, the efficiency will reduce as is often the case with amplifiers on 10-meters for example. All of this stuff in any ARRL Handbook and can be worked out by the reader. I haven't thought terribly deeply about this, but it occurs to me you're caught between a rock and a hard place any time you are stuck with a tube whose output capacitance represents a low reactance at the operating frequency, and which wants to see a high load impedance. However you resonate that capacitance, you end up with a high Q. It is convenient that the Q of coils goes up as the frequency increases, and for practical tubes at VHF/UHF, you can use transmission lines that are physically large enough to have very high Qu. In fact, it's not just the tube capacitance that gives you grief--it's the ratio of the reactance and the desired load resistance. And for a pure pi network, it's also the ratio between the resistance you're matching: if you want to present a 5000 ohm load to a tube and transform that to 50 ohms, the Q of the pi will be at least 10, at which point the network has degenerated into a simple L with no output capacitance. If you need to get from 10k ohms to 10 ohms, then the loaded Q is 31.6 minimum. But if you add just one more inductor forming a cascade of two L networks each performing a 31.6:1 impedance transformation (for the 10k to 10 ohm example), the Ql of each will be about 5.6. The capacitance at the plate end becomes much smaller, though, so this method is only practical at lower frequencies. The comparison between the "minimum Q" pi degenerated into a single L network and the cascade of two L networks is interesting: the -3dB bandwidth of the single L is about 6%, versus 26% for the cascade of two; but the harmonic attenuation is better for the cascade: at the second harmonic, it's 42dB versus 33.5, and at the third, 59dB versus 42dB. Loss with Q=100 coils is also better for the cascade, about .48dB versus .72, although if you use the same volume for the single coil case as you do for the two coil network, the loss is pretty similar since the larger coil has higher Qu. You can carry this even further and cascade more L sections to get a flatter wide passband, better harmonic suppression, and reasonably low loss. Cheers, Tom |
Are pi networks THAT INefficient?
On 24 May 2007 14:46:38 -0700, K7ITM wrote:
[all my good stuff snipped] I haven't thought terribly deeply about this, but it occurs to me you're caught between a rock and a hard place any time you are stuck with a tube whose output capacitance represents a low reactance at the operating frequency, and which wants to see a high load impedance. However you resonate that capacitance, you end up with a high Q. It is convenient that the Q of coils goes up as the frequency increases, and for practical tubes at VHF/UHF, you can use transmission lines that are physically large enough to have very high Qu. In fact, it's not just the tube capacitance that gives you grief--it's the ratio of the reactance and the desired load resistance. And for a pure pi network, it's also the ratio between the resistance you're matching: if you want to present a 5000 ohm load to a tube and transform that to 50 ohms, the Q of the pi will be at least 10, at which point the network has degenerated into a simple L with no output capacitance. If you need to get from 10k ohms to 10 ohms, then the loaded Q is 31.6 minimum. But if you add just one more inductor forming a cascade of two L networks each performing a 31.6:1 impedance transformation (for the 10k to 10 ohm example), the Ql of each will be about 5.6. The capacitance at the plate end becomes much smaller, though, so this method is only practical at lower frequencies. The comparison between the "minimum Q" pi degenerated into a single L network and the cascade of two L networks is interesting: the -3dB bandwidth of the single L is about 6%, versus 26% for the cascade of two; but the harmonic attenuation is better for the cascade: at the second harmonic, it's 42dB versus 33.5, and at the third, 59dB versus 42dB. Loss with Q=100 coils is also better for the cascade, about .48dB versus .72, although if you use the same volume for the single coil case as you do for the two coil network, the loss is pretty similar since the larger coil has higher Qu. You can carry this even further and cascade more L sections to get a flatter wide passband, better harmonic suppression, and reasonably low loss. Yep. A number of years ago in this group our departed friend, Reg, made a comment more or less saying that the fewer (non-ideal) reactances were in the matching network, the lower the losses were. I offered an example that proved this wrong. I'm extremely strapped for time but I think the thread has something to do with L-networks if anyone cares to search for it. Wes |
Are pi networks THAT INefficient?
"Wes Stewart" wrote in message
... On 24 May 2007 14:46:38 -0700, K7ITM wrote: (snip) A number of years ago in this group our departed friend, Reg, made a comment more or less saying that the fewer (non-ideal) reactances were in the matching network, the lower the losses were. I offered an example that proved this wrong. I'm extremely strapped for time but I think the thread has something to do with L-networks if anyone cares to search for it. Wes http://tinyurl.com/2vn4sa |
Are pi networks THAT INefficient?
"Richard Clark" wrote in message ... On Wed, 23 May 2007 23:33:45 -0400, "Jimmie D" wrote: Giving W8QUR the benifit of the doubt is like saying a 4 cylinder Hummer's efficiency would be improved if you removed the chassis. 73's Richard Clark, KB7QHC You either have to give the benifit of the doubt to W8QUR or the origonal poster W8QUR had benifit of editors that reviewed his work and decided it was correct. Saying that a Pi-net is as inefficent as the OP interpretted W8QUR as saying is a huge blunder that I doubt anyone who has the credentials to author a magazine article would make. The OP only assumed he was talking about Pi-networks and admitted W8QUR did not directly refer to the network as this even though in 1965 it was still called a Pi-net. It is also uncommon to call a pi-net a "system". While you cant be sure unless you have the origonal article in its entirity I will put my money on W8QUR in this case. Jimmie |
Are pi networks THAT INefficient?
On Sat, 26 May 2007 11:15:06 -0400, "Jimmie D"
wrote: You either have to give the benifit of the doubt to W8QUR or the origonal poster Hi Jimmie, This is absurd on the face of it. Nearly every poster has performed nothing more than "tea leaves reading" to massage inferences into actualities. Look at the subject line heading every post and respond to THAT. I've see nothing in three days that has diverged from my initial response. To mold supposed quotes into a new text that conforms to conventional thinking does not confer nobility on the source. Using loose references is, however, the staff of life in this forum. 73's Richard Clark, KB7QHC |
Are pi networks THAT INefficient?
"Richard Clark" wrote in message ... On Sat, 26 May 2007 11:15:06 -0400, "Jimmie D" wrote: You either have to give the benifit of the doubt to W8QUR or the origonal poster Hi Jimmie, This is absurd on the face of it. Nearly every poster has performed nothing more than "tea leaves reading" to massage inferences into actualities. Look at the subject line heading every post and respond to THAT. I've see nothing in three days that has diverged from my initial response. Nor Have I, As a matter of fact I hadnt read your post until just now. I feel your explanation has credability but we will not know until the OP further explains his paraphrase. To mold supposed quotes into a new text that conforms to conventional thinking does not confer nobility on the source. Using loose references is, however, the staff of life in this forum. I dont doubt the nobility of the source but I do doubt its accuracy. Primarily because of the change of language from "Pi-network" paraphrased part to "system" in the quoted part. I think it would be very unusual to refer to a pi-net as a system While comparing a balanced system to a single-ended sytem with 'system' meaning tuner plus transmission line would be more logical. This assumption would also fit the W8QRU quote. While I admit this is best guess I expect the OP to let me know if I am wrong, not you, unless of course you have a copy of the related article. Either the OP or W8QRU made a mistake. At this time I am slightly in favor of W8QRU. This is why I was taught many years ago in Mrs. Mary Ruth Smiths English class that when paraphrasing someone it is a good idea to retain certain key words. Sincerely Jimmie |
Are pi networks THAT INefficient?
Jimmie D wrote:
"Richard Clark" wrote in message ... On Wed, 23 May 2007 23:33:45 -0400, "Jimmie D" wrote: Giving W8QUR the benifit of the doubt is like saying a 4 cylinder Hummer's efficiency would be improved if you removed the chassis. 73's Richard Clark, KB7QHC You either have to give the benifit of the doubt to W8QUR or the origonal poster W8QUR had benifit of editors that reviewed his work and decided it was correct. Hmmm.. but the editors at QST and similar magazines actually don't do that much technical review. Sometimes there are typos that don't get caught, as well. There are several instances of incorrect or misleading data in a QST article in the last few years (and, I suspect, if one took the time to go back and look in decades gone by, you'd find errors there as well). Sometimes it gets corrected in a subsequent issue, sometimes not. QST isn't a peer reviewed technical journal. Lots of good ideas, but it's always wise to look at some background info too. Jim, W6RMK |
Are pi networks THAT INefficient?
"Jim Lux" wrote in message ... Jimmie D wrote: "Richard Clark" wrote in message ... On Wed, 23 May 2007 23:33:45 -0400, "Jimmie D" wrote: Giving W8QUR the benifit of the doubt is like saying a 4 cylinder Hummer's efficiency would be improved if you removed the chassis. 73's Richard Clark, KB7QHC You either have to give the benifit of the doubt to W8QUR or the origonal poster W8QUR had benifit of editors that reviewed his work and decided it was correct. Hmmm.. but the editors at QST and similar magazines actually don't do that much technical review. Sometimes there are typos that don't get caught, as well. There are several instances of incorrect or misleading data in a QST article in the last few years (and, I suspect, if one took the time to go back and look in decades gone by, you'd find errors there as well). Sometimes it gets corrected in a subsequent issue, sometimes not. QST isn't a peer reviewed technical journal. Lots of good ideas, but it's always wise to look at some background info too. Jim, W6RMK True enough, all it would take to settle this is for someone to have a copy of the article in question and to post it. I have trouble with the OPs equating pi-net with system. Just doesnt sound right to me. I welcome anyone sending me a copy of the article in question. Jimmie |
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