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On Sun, 5 Sep 2004 11:51:57 -0500, "Richard Fry"
wrote: appears to be Hi OM, Another punt. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote
"Richard Fry" wrote: You assume That is called a punt. __________ Neither of us can write that our conclusion about that spec was based on fact. Neither of us knows. RF |
"Richard Harrison" wrote in message
... Brian Reay wrote: "---dying to hear at what frequencies directional couplers suddenly begin to exist." It isn`t sudden. They sure work at audio frequencies. In telephones, they are used to prevent the user`s voice from overpowering the distant party`s voice in the user`s ear. They are called hybrids. Hybrids are also used to couple a 2-wire circuit which simultaneously carries both directions of transmission with a 4-wire circuit consisting of a transmit pair and a receive pair. Best regards, Richard Harrison, KB5WZI Thank you for that Richard but I think you missed the British sense of humour (or humor) ;-) -- Brian Reay www.g8osn.org.uk www.amateurradiotraining.org.uk FP#898 |
Richard Fry wrote:
"At least there appears to be an acknowledgement that some RF amplifiers do not have a source impedance that is the conjugate of their load impedance." Those may be anomalous. I recommend King, Mimno, and Wing to anyone desiring the complete story on the conjugate matches. To the extent that the amplifier is designed for a performance on demand that stresses it to its maximum safe dissipation, an amplifier of the Class C variety is designed for a perfectly matched load. It`s the economical thing to do. You supply the tube with about all the volts it can safely take. Then you supply it with just enough load impedance to limit its current to all it can take under the heaviest loading it well encounter. That would be when it is conjugately matched to a 50-ohm load, the usual cable impedance specification. The tank circuit is mostly a harmonic filter providing a very high impedance to the fundamental frequency and shorting out the harmonics. Best regards, Richard Harrison, KB5WZI |
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On Sun, 5 Sep 2004 12:21:21 -0500, "Richard Fry"
wrote: "Richard Clark" wrote "Richard Fry" wrote: You assume That is called a punt. __________ Neither of us can write that our conclusion about that spec was based on fact. Neither of us knows. Hi OM, This is called indicting your own authority - far more desperate than a punt. So,to this point you have failed to offer a technical discussion (Xeroxed work of others accomplishments offered in its place is rather banal), no personal experience at the bench to support your thesis, you condemn your own authority, and you complain of my attitude. Of those, perhaps the last is accurate as I find your responses to my technical comments (supported by others you claimed would not rise to a common camp) are met with risible content. Even in the comedic foray you are seriously mismatched. 73's Richard Clark, KB7QHC |
Richard Fry wrote:
"I also vaguely note some inference of peduliar intermodulation products that would be produced by a transmitter with 50 Ohm output characteristics---" First transmitting plant I worked in back in 1949 was that of KPRC / KXYZ. These each used an RCA 5-C. Transmitters were 250-watt exciters driving 5 KW water cooled linear final amplifiers. The interesting thing about this plant was that the 950 KHz and 1320 KHz transmitters both fed a common main antenna. Each station had its own directional tower on the side. The way to avoid intermodulation is to keep the foreign signals out of the electronics so they don`t mix. Well designed and adjusted pass/reject fikters in the transmission circuits of KPRC and KXYZ saw to that. Best regards, Richard Harrison, KB5WZI |
"Wes Stewart" wrote in message ... On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards" wrote: |What's a directional coupler? |What do they look like? |Don't bother answering those questions. | |Why do the arguers, when caught in a tight corner, always escape to UHF for |help from directional couplers? | |There are NO directional couplers at HF. They are as scarce as real swr |meters. So they cannot be used in futile attempts to explain what really |happens at HF. | |You're next move will be to drag in scattering-matrices. Why not. I have used an HP3577 network analyzer with an S-parameter test set that was specified to work over the frequency range of 100 Hz to 200 Mhz. I guess the guys at HP didn't realize that you can't do this. ================================= Wes, Why do USA citizens invariably introduce the type numbers of their favourite, indeed worshipped articles when they have not the slightest bearing on an argument. To base one's position on a lifeless piece of hardware rather than logic is surely unsafe. What on Earth is an HP3577? In the whole of my career I have never heard of the number 3577. Is it a prime? Traditionally, in the UK, the letters HP on the side of a savoury sauce bottle stand for "Houses of Parliament". Unless a 3577 can unambiguously measure the swr on a non-existent transmission line I'm afraid its presence will serve only to further agravate the argument. Which I'm sure is the last thing you would want. ;o) Can it? --- Reg, G4FGQ |
"Richard Harrison" wrote
Richard Fry wrote: "I also vaguely note some inference of peduliar intermodulation products that would be produced by a transmitter with 50 Ohm output characteristics---" _________ How do find it justified to assign a literal quote to me that I did not write in the first place? RF |
"Richard Clark" wrote
authority - far more desperate than a punt. ...So,to this point you have failed to offer ... personal experience at the bench to support your thesis... Incorrect. See my post earlier today about reflection measurements I made of TV transmit antenna systems. That's better than the bench. It's real life. The thesis has been proven. Even in the comedic foray you are seriously mismatched. An arena in which I don't (and don't wish to) compete, however. RF |
Gentlemen, excuse me butting in.
I suspect all these learned articles to be invalid because the authors incorrectly assume the source impedance to be constant and the load impedance to be the variable in any analyses or sets of measurements. Whereas the internal or source impedance is actually a function of the load (and many other factors). I have not read these papers or articles but base my comments on what I have gleaned from newsgroup conversations over the years. In brief, how can you have a conjugate match with the source impedance hopping about trying to follow the load? ;o) ---- Reg, G4FGQ =================================== You assume he refers to the source impedance of/at output of the amplifier. More likely he is following convention and stating the load impedance that the amplifier was designed to work into. The source impedance of most transmitters is not published even today. If it was, probably we wouldn't be having all of this confusion about it, and its effects. |
"Richard Harrison" wrote
The way to avoid intermodulation is to keep the foreign signals out of the electronics so they don`t mix. Well designed and adjusted pass/reject fikters in the transmission circuits of KPRC and KXYZ saw to that. _____________ Thanks. You report more evidence that the source impedance did not match the load impedance of these txs. If they did, each of these txs would absorb the coupled signal of the other -- and neither of them would generate mixing products. RF |
On Sun, 5 Sep 2004 14:13:36 -0500, "Richard Fry"
wrote: The thesis has been proven. Hi OM, It's always fun to find in the heat of discussion an opponent who impeaches his own witnesses. This is like how Reggie abandons Lords Kelvinator and Plushbottom to their graves when a troll is so much more entertaining with that glass of wine. Such are the vagaries of esteem so lightly held by gossamer minds. It has been quite amusing. :-) 73's Richard Clark, KB7QHC |
Richard Fry wrote:
"How do (you) find it justified to assign a literal quote to me that I did not write in the first place?" A mistake? Sorry if I misquoted Richard Fry on peculair intermodulation products. The power hungry RCA 5 C`s with their linear amplifiers were later replaced with RCA BTA5F`s which had Class C final amplifiers. Despite a phony ceremonial switchover to the new equipment in which pads were pulled from the audio lines to the new transmitters, the stations really sounded the same before and after the equipment change. Best regards, Richard Harrison, KB5WZI |
Reg, G4FGQ wrote:
"In brief, how can you have a conjugate match with the source impedance hopping about trying to follow the load?" I think you take control of the process and tune for maximum smoke. Best regards, Richard Harrison, KB5WZI |
On Sun, 5 Sep 2004 07:49:06 -0500, "Richard Fry" wrote:
snip Walter Maxwell wrote The last sentence in the paragraph above is incorrect. This shows that the writer of the quote is in the unbelievably large group that still believes incorrectly that half of the tx power would be lost if if it were conjugately matched. But we all know that efficiencies greater than 80% is achieved by Class C amps, and greater than 60% is achieved by Class B amps when the source impedance of the tx is 50 ohms resistive and the load impedance is also 50 ohms resistive. _______________ To Walter Maxwell: 1. You may be interested in reading Mendenhall's complete paper, which I will email to you. The lab measurements reported in it used two, operating, high-power FM broadcast transmitters -- and support his statements about amplifier source impedance and its consequences. 2. I will ask again, if transmitters have a 50 ohm source impedance, what accounts for the fact that TV ghosts are produced by an antenna system reflection having a sufficient delay time? Calculations and measured data show that the energy that produced the ghost originated by re-reflection off the TV transmitter output stage of far-end reflections in the antenna system. If the tx source impedance was 50 ohms, it would absorb the far-end reflection, which would be incapable of producing a ghost image. Further, if the tx source impedance was 50 ohms, then the RF intermodulation measured and reported in Mendenhall's paper -- and verified in real-world installations by the radiated interference those IM products produced -- would not occur. RF Thank you Rick, I've received the Mendenhall paper, which I'll review and comment later. However, before reading it I have one comment. On the condition that the tx has tubes (and I assume it does) with some sort of LC output coupling network, then if the source impedance of the tx is 50 ohms it will not absorb the far-end reflection, because the source impedance of this type of tx is not absortive. The source impedance of a resonant tank circuit is a resistance determined by the voltage-current ratio in the tank--high resistance at the tank input and low resistance at the output. The resistance being proportional to the load line there is no dissipating resistor involved. The only dissipative resistance in the system is the cathode-plate resistance, which is separate from the output resistance. And contrary to what I've skimmed in the Mendenhall paper, the output circuit of the tx is linear, not non-linear as Mendenhall says, because the energy storage of the tank isolates the non-linear input from the linear output. Remember, the tx output is a nearly perfect sine wave. I'll not comment further on this point, Rich, until I've reviewed the Mendenhall paper. |
"Walter Maxwell"
Thank you Rick, I've received the Mendenhall paper, which I'll review and comment later. (etc) ______________ Thanks for your very civil response. I was on Geoff Mendenhall's staff for some years before I retired, and will contact him to ask for his response to whatever of your points seem appropriate for that. He's a very busy person these days and may not have the time to get involved, but there's no harm in trying. RF |
On Sun, 5 Sep 2004 14:53:47 -0500, "Richard Fry"
wrote: |"Richard Harrison" wrote | The way to avoid intermodulation is to keep the foreign signals | out of the electronics so they don`t mix. Well designed and | adjusted pass/reject fikters in the transmission circuits of | KPRC and KXYZ saw to that. |_____________ | |Thanks. You report more evidence that the source impedance did not match the |load impedance of these txs. |If they did, each of these txs would absorb |the coupled signal of the other -- and neither of them would generate mixing |products. Why would they? One is tuned to 950 KHz, the other to 1320 KHz, a good part of an octave difference. This is the same fuzzy logic that says that you can measure the output Z of an amplifier at one frequency by injecting another signal at a different frequency. If it was this easy, the bother of load pull techniques wouldn't have become popular and the norm. |
On Sun, 5 Sep 2004 19:24:02 +0000 (UTC), "Reg Edwards"
wrote: |Gentlemen, excuse me butting in. Not at all, after all, you started this mess. | |I suspect all these learned articles to be invalid because the authors |incorrectly assume the source impedance to be constant and the load |impedance to be the variable in any analyses or sets of measurements. | |Whereas the internal or source impedance is actually a function of the load |(and many other factors). | |I have not read these papers or articles but base my comments on what I have |gleaned from newsgroup conversations over the years. What a hoot. You constantly battle old wives and then say that you base your comments on newsgroup conversation! | |In brief, how can you have a conjugate match with the source impedance |hopping about trying to follow the load? ;o) Iteration. |
Richard Fry wrote:
"Roy Lewallen" wrote: Sorry, it still isn't clear. What, then, is "system SWR"? How do you define it? System SWR is the net SWR of a component assembly present at its input terminals. "Antenna system SWR" then is comprised of the net SWR of everything in the RF path from the output of the SWR meter to and including the antenna. In a transmitter, the antenna system begins electrically at the output of the SWR meter -- physically close to the output connector of the tx. You've still lost me. Let's say the "component assembly" is a half wavelength of 75 ohm transmission line terminated with a 75 ohm resistor. What is its "net SWR"? How about a half wavelength of 75 ohm line terminated with 50 ohms? Or a plain 75 ohm resistor? You surely have an equation you use to calculate "system SWR" or "net SWR" -- can you share it with us? This is getting more complicated rather than simpler. We now have "true SWR", "antenna system SWR", and "net SWR". Quite a step from the ratio of maximum to minimum voltages on a transmission line. . . . Roy Lewallen, W7EL |
Richard Fry wrote:
. . . The source impedance of most transmitters is not published even today. If it was, probably we wouldn't be having all of this confusion about it, and its effects. Who's confused? It has no effect. Roy Lewallen, W7EL |
"Bob Nielsen" wrote in message ... (Jumping in after this argument has gone on for a long time) This has evolved somewhat into an "apples and oranges" discussion. What Richard Fry appears to be concerned with is the reflection of a video pulse by the load (antenna) and its re-reflection (at a later time, dependent on the length and Vp of the transmission line) by the source (transmitter) and subsequent radiation (as a ghost image--the effect repeats, ad infinitum). While this is certainly a real phenomenon, its effect is a function of both the mismatch at the load (S11) and the mismatch looking from the transmission line back toward the source (S22). It does NOT influence the VSWR and could, obviously, be significantly reduced by the use of a circulator at either end of the transmission line (which may not be practical at the power levels of televison transmitters, to use his example.) 73, Bob Nielsen, N7XY To shed more light on this, when I tried to find information in the literature about transmitter output impedance, I ran across two cases where reflections from the transmitter are bad. One was the case of multiple transmitting antennas on one tower; a particular antenna could pick up another transmitter, and then re radiate it with some delay. The second was the case of a wide band spread spectrum transmitter, where it is not possible to have a good antenna match on all frequencies. Tam/WB2TT |
On Sun, 5 Sep 2004 20:25:31 -0400, "Tam/WB2TT"
wrote: when I tried to find information in the literature about transmitter output impedance, I ran across two cases where reflections from the transmitter are bad. Hi All, It doesn't take much research into the matter of matching and Harris Transmitters to find from their Digital AM stations: The newest Harris AM Transmitters of 10 KW and higher (up to 1 megawatt and more) are totally solid state and use a completely different modulation system called Digital AM Modulation. This scheme is another Harris Patented system, using a large quantity of plug-in modules, each one generates RF at a different modulation level; in order for this to function, any incoming audio (if analog) is broken down into digitized data, then used to drive the various modules at the varying levels needed to have an amplitude modulated output. It sounds pretty simple, in fact, each module has a torroid (coil) which is the load for that module; all those torroids are lined up and an iron pipe is run through these torroids so that the combined output of all the modules is coupled into this pipe. One end of the pipe is at ground, the other end is the RF output of the transmitter. There is an output network to match impedance of the pipe, which is probably only a few Ohms, to the required output impedance which is usually 50 Ohms, although sometimes high power transmitters are set up to provide 75 Ohms, once in a while even 300 Ohms, to drive an open wire type transmission line system. Hardly a wit difference between Geoff's first FM amplifier design that was documented for FCC type acceptance and now when it comes to matching - all very commonplace and not even a hair out of place in the conventional wisdom found in Terman (one of his references). As for the specification for output impedance gone missing :-) MW-10B SPECIFICATIONS RF OUTPUT IMPEDANCE: 50 ohms, unbalanced. Other output impedances available on special order. Harris Platinum Z FM transmitter 100 ohm output impedance (unbalanced) HARRIS SW-50 A RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR Well, instead of reciting their complete catalogue, it is easier to simply say I could not find any product that did not specify an output impedance, much less that others were available on special order. One wonders if it doesn't matter, do they just change the spec page and the sticker above the connector? ;-) 73's Richard Clark, KB7QHC |
On Sun, 5 Sep 2004 10:44:04 -0500, "Richard Fry" wrote:
snip When this test shows a 5% pulse return 2 µs after the incident pulse time (for example), then the same pulse passed through the tx also shows nearly exactly the same reflection % and time separation -- assuming there is enough RF delay in the system for the reflection to be resolved in the demodulated waveform. As the directional coupler driving the normal demodulator at the TV station is looking at forward power only, it is clear that the reflection from the far end of the antenna system has been re-reflected from the TV tx output stage, and NOT absorbed by it in its "conjugate impedance." RF Rich, I've got to tell you that you've just proved that the output circuitry of the tx doesn't absorb the reflection from the antenna, but instead, re-reflects it, because the output source resistance is not dissipative, only the cathode to plate resistance is dissipative. And the output source resistance is not dissipative or absorptive if the impedance match is 'conjugate'. Contrary to what Mendenhall said that the tx would absorb half the power if the source resistance is 50 ohms, there is nothing special about the conjugate relationship that would produce this effect. The tx will re-reflect whatever the match relationship is between the tx and its load. Walt |
On Sun, 5 Sep 2004 17:01:14 -0500, "Richard Fry" wrote:
"Walter Maxwell" Thank you Rick, I've received the Mendenhall paper, which I'll review and comment later. (etc) ______________ Thanks for your very civil response. I was on Geoff Mendenhall's staff for some years before I retired, and will contact him to ask for his response to whatever of your points seem appropriate for that. He's a very busy person these days and may not have the time to get involved, but there's no harm in trying. RF Hi Rich, it's interesting that you were on Geoff's staff. On the paper you sent me, the location of the company appears as Quincy, IL. When I bought the tx for new BC stn WCEN in 1948 it was from Gates Radio in Quincy. Is it possible that Geoff's business is a spinoff from Gates? It's also interesting that you were also with RCA BC Div. I was with the RCA Labs in Princeton, and working with me in the same lab room in 1960 was Don Peterson, who was running an experiment suggested by Dr. George Brown. The experiment was in developing the use of TDR for finding discontinuities in RF feed lines for TV. With the success of his experiments we could pinpoint the location of a discontinuity that produced a reflection causing no greater than 1.03 SWR. He developed a kit for use in the field for locating ghost problems in the lines connecting the tx to the antenna. Were you aware of Don's work in this area? Walt |
On Sun, 5 Sep 2004 18:30:53 +0000 (UTC), "Reg Edwards"
wrote: | |"Wes Stewart" wrote in message .. . | On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards" | wrote: | | |What's a directional coupler? | |What do they look like? | |Don't bother answering those questions. | | | |Why do the arguers, when caught in a tight corner, always escape to UHF |for | |help from directional couplers? | | | |There are NO directional couplers at HF. They are as scarce as real swr | |meters. So they cannot be used in futile attempts to explain what really | |happens at HF. | | | |You're next move will be to drag in scattering-matrices. | | Why not. I have used an HP3577 network analyzer with an S-parameter | test set that was specified to work over the frequency range of 100 Hz | to 200 Mhz. | | I guess the guys at HP didn't realize that you can't do this. | |================================= | |Wes, | |Why do USA citizens invariably introduce the type numbers of their |favourite, indeed worshipped articles when they have not the slightest |bearing on an argument. To base one's position on a lifeless piece of |hardware rather than logic is surely unsafe. Well, I cannot speak for all US citizens and *invariably* is a little too all encompassing for me. But I suppose it's similar to declaring so unequivocally and "logically" that there are NO directional couplers at HF. When presented with a real-life instrument that includes these non-existant couplers that you refer to, you wish to attack and belittle the presenter. If you don't know what an HP 3577 is, you might ask in a civil manner, in which case I would say that it is a vector network analyzer, covering the frequency range of 5 Hz to 200 MHz that was widely used in this country as well as around the world. An accessory to the instrument is an S-parameter test set that offered the ability to measure full two-port parameters over the frequency range of 100 Hz to 200 MHz, which last time I checked, includes the hf spectrum. |What on Earth is an HP3577? See above. |In the whole of my career I have never heard of |the number 3577. Clearly, you have lived a sheltered professional life. |Is it a prime? Traditionally, in the UK, the letters HP |on the side of a savoury sauce bottle stand for "Houses of Parliament" Perhaps, but me thinks that the sauce that you are more familiar with says something different. | |Unless a 3577 can unambiguously measure the swr on a non-existent |transmission line I'm afraid its presence will serve only to further |agravate the argument. Surprising to you perhaps, yes it can. |
On Sun, 5 Sep 2004 19:24:02 +0000 (UTC), "Reg Edwards"
wrote: snip In brief, how can you have a conjugate match with the source impedance hopping about trying to follow the load? ;o) ---- Reg, G4FGQ Reg, I have a myriad of measurements which prove that when the pi-network is adjusted to deliver max power for any reasonable drive level into any reasonable value of load, the source impedance is the conjugate of the load impedance. When adjusting the load control of the network for max output the output resistance equals the resistance of the load. And when the plate tuning is adjusted for max out the reactance injected into the network equals the reactance opposite to that in the load. To be practical, be assurred that the tuning and loading controls are alternately adjusted continuously until they converge and the max power is delivered. Having performed these measurements many, many times, I assure you that the resulting data isn't a series of coincidences. So instead of the source impedance 'hopping about trying to follow the load', when the network is correctly adjusted, the source impedance 'finds' the load and stays put until the load is changed and the network readjusted to accomodate the new load. Walt |
On Mon, 06 Sep 2004 01:23:04 GMT, Richard Clark wrote:
On Sun, 5 Sep 2004 20:25:31 -0400, "Tam/WB2TT" wrote: when I tried to find information in the literature about transmitter output impedance, I ran across two cases where reflections from the transmitter are bad. Hi All, It doesn't take much research into the matter of matching and Harris Transmitters to find from their Digital AM stations: The newest Harris AM Transmitters of 10 KW and higher (up to 1 megawatt and more) are totally solid state and use a completely different modulation system called Digital AM Modulation. This scheme is another Harris Patented system, using a large quantity of plug-in modules, each one generates RF at a different modulation level; in order for this to function, any incoming audio (if analog) is broken down into digitized data, then used to drive the various modules at the varying levels needed to have an amplitude modulated output. It sounds pretty simple, in fact, each module has a torroid (coil) which is the load for that module; all those torroids are lined up and an iron pipe is run through these torroids so that the combined output of all the modules is coupled into this pipe. One end of the pipe is at ground, the other end is the RF output of the transmitter. There is an output network to match impedance of the pipe, which is probably only a few Ohms, to the required output impedance which is usually 50 Ohms, although sometimes high power transmitters are set up to provide 75 Ohms, once in a while even 300 Ohms, to drive an open wire type transmission line system. Hardly a wit difference between Geoff's first FM amplifier design that was documented for FCC type acceptance and now when it comes to matching - all very commonplace and not even a hair out of place in the conventional wisdom found in Terman (one of his references). snip Well, instead of reciting their complete catalogue, it is easier to simply say I could not find any product that did not specify an output impedance, much less that others were available on special order. One wonders if it doesn't matter, do they just change the spec page and the sticker above the connector? ;-) 73's Richard Clark, KB7QHC To Rich Fry, Rich, I just now saw Richard Clark's post above. Until reading it I was totally unaware that high-power BC transmitters used solid-state final amps. Consequently, my comments were all directed toward tx's using tubes and LC networks to isolate the non-linear input from the linear output. I'm not at all familiar with the circuitryused in solid-state amps, please interpret my comments accordingly. Walt |
On Sun, 05 Sep 2004 19:06:03 -0700, Wes Stewart wrote:
| On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards" | wrote: |snip | |There are NO directional couplers at HF. They are as scarce as real swr | |meters. So they cannot be used in futile attempts to explain what really | |happens at HF. | | | |You're next move will be to drag in scattering-matrices. | | Why not. I have used an HP3577 network analyzer with an S-parameter | test set that was specified to work over the frequency range of 100 Hz | to 200 Mhz. | | I guess the guys at HP didn't realize that you can't do this. | |================================= | |Wes, | |Why do USA citizens invariably introduce the type numbers of their |favourite, indeed worshipped articles when they have not the slightest |bearing on an argument. To base one's position on a lifeless piece of |hardware rather than logic is surely unsafe. Well, I cannot speak for all US citizens and *invariably* is a little too all encompassing for me. But I suppose it's similar to declaring so unequivocally and "logically" that there are NO directional couplers at HF. When presented with a real-life instrument that includes these non-existant couplers that you refer to, you wish to attack and belittle the presenter. If you don't know what an HP 3577 is, you might ask in a civil manner, in which case I would say that it is a vector network analyzer, covering the frequency range of 5 Hz to 200 MHz that was widely used in this country as well as around the world. An accessory to the instrument is an S-parameter test set that offered the ability to measure full two-port parameters over the frequency range of 100 Hz to 200 MHz, which last time I checked, includes the hf spectrum. |What on Earth is an HP3577? See above. |In the whole of my career I have never heard of |the number 3577. Clearly, you have lived a sheltered professional life. |Is it a prime? Traditionally, in the UK, the letters HP |on the side of a savoury sauce bottle stand for "Houses of Parliament" Perhaps, but me thinks that the sauce that you are more familiar with says something different. | |Unless a 3577 can unambiguously measure the swr on a non-existent |transmission line I'm afraid its presence will serve only to further |agravate the argument. Surprising to you perhaps, yes it can. And Reg, in conjunction with the HP-8405 Vector Voltmeter, the HP-778D dual directional coupler can measure SWR from 1 MHz to 1 GHz with no transmission line whatever. This combination is the workhorse of my RF Lab. Walt |
Hi Walter,
In MY kitchen I have two workhorses, comprising the - Autek Research, RF Analyst, Model RF-1, Frequency range 1.1 - 36 Mhz and the - MFJ, HF/VHF SWR Analyser Model 259-B, Frequency range 1.8 - 170 MHz. Both were made in the USA. I bought the 2nd one in the hope it might be better than the first. It wasn't. Nevertheless, between them they are the best things since sliced bread. The last measurements made using them were of soil impedance vs frequency, in the kitchen sink, on samples from my backyard. And the accuracy of any instrument depends on how it is used rather than on what the manufacturer says about it. ---- Reg, G4FGQ ================================ "Walter Maxwell" wrote And Reg, in conjunction with the HP-8405 Vector Voltmeter, the HP-778D dual directional coupler can measure SWR from 1 MHz to 1 GHz with no transmission line whatever. This combination is the workhorse of my RF Lab. Walt |
Walter, W2DU wrote:
"Is it possible that Geoff`s business is a spinoff grom Gates?" Harris Intertype Corporation, predecessor of Harris Electronics, axquired Quincy, IL`s Gates Radio in the 1950`s, I believe. Then Harris really hit the acquisition trail. It acquired R.F. Communications, a 2-way radio manufacturer. It also acquired a telephone and SCADA equipment manufacturer, which had a government equipment and semiconductor division in Florida. It acquired Farinon Electric a microwave manufacturer in California. There were other acquisitions, Lanier which made office equipment such as duplicators. It acquired a hardware manufacturer which made 19-inch equipment racks among its products. I don`t about other shuffles in Harris` portfolio of companies, but the companies it bought were among the best at what they did at the time of acquisition. Best regards, Richard Harrison, KB5WZI |
In brief, how can you have a conjugate match with the source impedance hopping about trying to follow the load? ;o) ---- Reg, G4FGQ Reg, I have a myriad of measurements which prove that when the pi-network is adjusted to deliver max power for any reasonable drive level into any reasonable value of load, the source impedance is the conjugate of the load impedance. When adjusting the load control of the network for max output the output resistance equals the resistance of the load. And when the plate tuning is adjusted for max out the reactance injected into the network equals the reactance opposite to that in the load. To be practical, be assurred that the tuning and loading controls are alternately adjusted continuously until they converge and the max power is delivered. Having performed these measurements many, many times, I assure you that the resulting data isn't a series of coincidences. So instead of the source impedance 'hopping about trying to follow the load', when the network is correctly adjusted, the source impedance 'finds' the load and stays put until the load is changed and the network readjusted to accomodate the new load. Walt ================================ Walt, forgive me for stating the obvious, but when operating under non-linear conditions, as tubes and transistors are, I think you'll agree that when the load resistance is changed, the tubes operating conditions also change, and so does the tube's internal resistance. I don't know exactly how you have conducted your measurements. But I suggest that when you change the load you automatically force the internal resistance to change with it. But in any case, whichever way the source changes, the basic idea of a conjugate match to an unstable source is itself not very sound. The best way of determining source impedance is to mathematically model and calculate it. It need not be accurate. It is necessary only to show that it does change. ---- Reg, G4FGQ |
"Walter Maxwell" wrote If you question my statements above see the data from my measurements using professional grade instruments in either QEX, May/Jun 2001, Chapter 19 in Reflections II, ============================ The accuracy of measurements depends on who uses the instruments rather than on what the manufacturer says in his sales catalogue. I'd much prefer just to take your word for it, Walt. The manufacturer's type number is superfluous - it sounds like a gratuitous advert. ---- Reg, G4FGQ |
Walter, W2DU wrote:
"A conjugate match can be obtained for any loading you choose." No doubt. A conjugate match permits maximum power transfer. No other load can extract as much output as a conjugate match. Were I designing a broadcast transmitter for a 50-ohm or a 75-ohm load, I would want my money`s worth from the output devices I`ve selscted, consistent with a long useful duration. I`d design for a conjugate match at the rated load and include overload protection for an output short circuit or near short. For solid-state I`d provide overvoltage protection.in addition to overcurrent protection. Harris solid-state medium wave transmitters have a number of amplifiers supplying the output and a faulty amplifier can be easily removed from service while the remainder continue operation almost as if nothing had happened, if I read their internet pages correctly. Best regards, Richard Harrison, KB5WZI |
Reg Edwards, W4FGQ wrote:
"---when the load resistance is changed, the tube`s operating conditions also change, and so does the tube`s internal resistance." I would think that change in operating conditions result from readjustment of the amplifier, not from changing the load on it. Most of the output impedance of a Class C power amplifier tube does not come from the saturation resistance when switched on, but comes from the duration of the period in which it is switched off. Until you readjust drive, the on and off periods and even the saturation resistance of the tube remain unchanged. Best regards, Richard Harrison, KB5WZI |
"Richard Clark" wrote about the specs for Harris Broadcast Txs:
As for the specification for output impedance gone missing :-) MW-10B SPECIFICATIONS RF OUTPUT IMPEDANCE: 50 ohms, unbalanced. Other output impedances available on special order. Harris Platinum Z FM transmitter 100 ohm output impedance (unbalanced) HARRIS SW-50 A RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR Well, instead of reciting their complete catalogue, it is easier to simply say I could not find any product that did not specify an output impedance, much less that others were available on special order. _____________ I won't try to soften this: your conclusion above doesn't just _appear_ to be wrong, it IS wrong. 100% wrong. The impedance published by Harris is the expected load impedance, not the tx source impedance -- which is the convention used by all OEMs of broadcast txs (at least). How do I know? I was the author of all of the brochures and technical data sheets for Harris' entire FM product line for the ten years before I retired. That any published value of impedance applied to the load impedance expected was/is universal across the product lines: AM/FM/TV. Your post above showing 100 ohms for a Platinum Z FM tx also is wrong. It is designed for a 50 ohm load. Here is a link to an on-line brochure for the current 10kW model: http://www.broadcast.harris.com/prod.../HAR173569.pdf Observe the statement at the bottom of the right column on the last page thereof, which I quote below for your convenience: "All specifications referenced to any single output frequency (87.5-108MHz), nominal rated output power, and 50 ohm, isolated, non-reactive load." No other RF impedance spec is given in the very detailed list of specs contained in these brochures. Kindly recant your accusations. RF |
"Walter Maxwell" wrote to R. Fry:
I'm not at all familiar with the circuitry used in solid-state amps, please interpret my comments accordingly. __________ Understand, and will keep that in mind. However I won't be reading/responding to anything for the next few days, as I'll be taking a short "R&R" trip. RF |
"Walter Maxwell"
When I bought the tx for new BC stn WCEN in 1948 it was from Gates Radio in Quincy. Is it possible that Geoff's business is a spinoff from Gates? Yes, Harris-Intertype (later just "Harris Corp") bought Gates Radio, lock, stock and barrel -- not a spinoff. Don Peterson, developing the use of TDR for finding discontinuities in RF feed lines for TV. He developed a kit for use in the field for locating ghost problems in the lines connecting the tx to the antenna. Were you aware of Don's work in this area? Vaguely. My primary contact and mentor was Dr Matti Siukola, lead engineer at RCA's antenna lab and test range at Gibbsboro, NJ. Under his direction I was responsible for updating and improving the VHF and UHF RF pulse test sets used by RCA Field Service. RF |
Hi Richard,
I don't have anything to do with broadcasting, but when I first ran across their DX line of AM transmitters, it really blew my mind. It seems though, that it is really hard to find detailed info on them. Do you have a link that describes what happens in some detail. The best I have been able to come up with was anecdotal data let to from the WLW web site. There must be some article that was published in some trade magazine. My questions a The 50 KW transmitter supposedly has 60 some modules of about 1KW each. How does this jive with being able to get the 200KW PEP that is required for 100% upward modulation? Does the combiner add voltage or power? Seems like it would be voltage, but then the impedance a module sees would depend on how many are active at a given time. What is the digital sampling rate? Seems to me they need a high power bandpass filter to get rid of the sampling frequency after the combiner. BTW, I think a lot of people are going to confuse this with Digital AM. Tam/WB2TT "Richard Fry" wrote in message ... "Richard Clark" wrote about the specs for Harris Broadcast Txs: As for the specification for output impedance gone missing :-) MW-10B SPECIFICATIONS RF OUTPUT IMPEDANCE: 50 ohms, unbalanced. Other output impedances available on special order. Harris Platinum Z FM transmitter 100 ohm output impedance (unbalanced) HARRIS SW-50 A RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR Well, instead of reciting their complete catalogue, it is easier to simply say I could not find any product that did not specify an output impedance, much less that others were available on special order. _____________ I won't try to soften this: your conclusion above doesn't just _appear_ to be wrong, it IS wrong. 100% wrong. The impedance published by Harris is the expected load impedance, not the tx source impedance -- which is the convention used by all OEMs of broadcast txs (at least). How do I know? I was the author of all of the brochures and technical data sheets for Harris' entire FM product line for the ten years before I retired. That any published value of impedance applied to the load impedance expected was/is universal across the product lines: AM/FM/TV. Your post above showing 100 ohms for a Platinum Z FM tx also is wrong. It is designed for a 50 ohm load. Here is a link to an on-line brochure for the current 10kW model: http://www.broadcast.harris.com/prod.../HAR173569.pdf Observe the statement at the bottom of the right column on the last page thereof, which I quote below for your convenience: "All specifications referenced to any single output frequency (87.5-108MHz), nominal rated output power, and 50 ohm, isolated, non-reactive load." No other RF impedance spec is given in the very detailed list of specs contained in these brochures. Kindly recant your accusations. RF |
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