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F-connectors
Hi Group,
Many thanks for the input with the open and shorted transmission lines. On another subject, F-connectors are used for just about everything video, with RG-59 being 75 ohms is the F-connector designed for 75-ohms or can I use this type of connector with 50 ohm systems? de KJ4UO |
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You would have trouble physically doing it. Remember, you need coax with a
skinny solid center conductor. There is such RG58, but the shield diameter is too small. It might be possible to use 8X if you cut off part of the strands that make up the center conductor, and tin the remaining ones to make a rigid conductor. Tam/WB2TT "PDRUNEN" wrote in message ... Hi Group, Many thanks for the input with the open and shorted transmission lines. On another subject, F-connectors are used for just about everything video, with RG-59 being 75 ohms is the F-connector designed for 75-ohms or can I use this type of connector with 50 ohm systems? de KJ4UO |
"PDRUNEN" wrote in message
... Hi Group, Many thanks for the input with the open and shorted transmission lines. On another subject, F-connectors are used for just about everything video, with RG-59 being 75 ohms is the F-connector designed for 75-ohms or can I use this type of connector with 50 ohm systems? de KJ4UO Hi, the F-connector is 75 ohm, and I have had to mate it to PL-259 to connect Belden 9913 before. This required the awkward addition of an F-to-BNC then BNC-to-SO-239 connector. The line loss added to this arrangement was probably significant, but the reason was to adapt an inexpensive log periodic beam antenna. Although manufacturer claimed transmit capable, and I did test this on vhf-marine bands, the mismatch of connectors should be used for receive-only. Jack Painter Virginia Beach, Va |
Provided the mechanical connection is sound, you can use any coaxial
connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. |
"Reg Edwards" wrote
Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. _________________ ??? Using connectors that don't maintain the characteristic impedance of the transmission lines they connect _will_ produce undesired effects. The effects may be negligible to amateur radio operators used to operating transmitters into rather high mismatches, but they would never be tolerated in most professional operations, including high-power broadcast systems. Years ago a common impedance for rigid transmission line used in broadcast systems was 51.5 ohms. Later the more common value was/is 50 ohms. While a mechanical adapter was available to allow connecting a 51.5 ohm inner conductor to 50 ohm inner conductor (SWR= 1.03), better installations installed an RF transformer section at these interfaces. A 1.1 SWR at the input of a TV transmit antenna using ~500 or more feet of transmission line will produce a visible "ghost" in the transmitted picture, as seen by a careful observer. At 1.25 SWR it can be seen, and will objectionable to almost everyone. RF Visit http://rfry.org for FM broadcast RF system papers. |
To save the trouble of calculating it I'll take a guess. A connector less
than 1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an SWR of 1.03:1 or anything anywhere near to it at frequencies less than 1000 MHz. A 1" long connector WILL produce an SWR of 1.03:1 around 3 GHz but no worse. If you can reliably measure it. What matters is the ratio of connector length to wavelength along the line. For the same reason, at HF, bringing the two wires of an open wire line close together for the purpose of drawing them through a single small hole in the wall, will not produce any noticeable effect on line performance. ---- Reg. ================================= "Richard Fry" wrote in message ... "Reg Edwards" wrote Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. _________________ ??? Using connectors that don't maintain the characteristic impedance of the transmission lines they connect _will_ produce undesired effects. The effects may be negligible to amateur radio operators used to operating transmitters into rather high mismatches, but they would never be tolerated in most professional operations, including high-power broadcast systems. Years ago a common impedance for rigid transmission line used in broadcast systems was 51.5 ohms. Later the more common value was/is 50 ohms. While a mechanical adapter was available to allow connecting a 51.5 ohm inner conductor to 50 ohm inner conductor (SWR= 1.03), better installations installed an RF transformer section at these interfaces. A 1.1 SWR at the input of a TV transmit antenna using ~500 or more feet of transmission line will produce a visible "ghost" in the transmitted picture, as seen by a careful observer. At 1.25 SWR it can be seen, and will objectionable to almost everyone. RF Visit http://rfry.org for FM broadcast RF system papers. |
Connector salesmen (and no doubt ladies) have the habit of exaggerating the
importance and magitude of SWR ON THE LINE associated with the precision of connector manufacture. The habit transfers itself into magazine articles without any supporting practical experiments. |
Right-- but WRONG! Wouldn't be too concerned about the IMPEDENCE mismatch
at this short distance, but, in THIS case, as the center conductor of the coax in the CENTER PIN of the CONNECTOR, might have problem with connection, or SPREADING the female center connector, so as to not make good connection with the proper coax in the future! Jim NN7K "Richard Fry" wrote in message ... "Reg Edwards" wrote Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. _________________ ??? Using connectors that don't maintain the characteristic impedance of the transmission lines they connect _will_ produce undesired effects. The effects may be negligible to amateur radio operators used to operating transmitters into rather high mismatches, but they would never be tolerated in most professional operations, including high-power broadcast systems. Years ago a common impedance for rigid transmission line used in broadcast systems was 51.5 ohms. Later the more common value was/is 50 ohms. While a mechanical adapter was available to allow connecting a 51.5 ohm inner conductor to 50 ohm inner conductor (SWR= 1.03), better installations installed an RF transformer section at these interfaces. A 1.1 SWR at the input of a TV transmit antenna using ~500 or more feet of transmission line will produce a visible "ghost" in the transmitted picture, as seen by a careful observer. At 1.25 SWR it can be seen, and will objectionable to almost everyone. RF Visit http://rfry.org for FM broadcast RF system papers. |
"Reg Edwards" wrote
A 1" long connector WILL produce an SWR of 1.03:1 around 3 GHz but no worse. If you can reliably measure it. AND Connector salesmen (and no doubt ladies) have the habit of exaggerating the importance and magitude of SWR ON THE LINE associated with the precision of connector manufacture. The habit transfers itself into magazine articles without any supporting practical experiments. ______________________ REG: Please consider this. Most TV transmit antenna systems have an adjustable RF transformer installed at the antenna input connector. This transformer consists of 4 or 5 brass "pins" of about 5/8" diameter spread evenly across a 90 degree section of rigid transmission line. These pins can be inserted radially into the space between the outer and inner conductors of that line section. When the pins are withdrawn fully, they have no affect on the natural impedance of that line section, but can produce a spatially discrete SWR as function of their insertion distance into the line. I, personally, and many other broadcast engineers have been involved in the adjustment of such RF transformers to optimize the match between the main transmission line and the antenna input [including its elbow complex(es)], at the frequencies used in commercial VHF/UHF television -- which start at 54 MHz. This requires (1) purchase and installation of the transformer, (2) deployment of a tower crew to adjust it, and (3) use of a qualified field engineer in the tx bldg with the appropriate test equipment and field experience to direct the adjustment of that variable transformer. Obviously, these processes are not inexpensive, and would not be undertaken if there was no reason. The reason: to optimize the match between the main line and the antenna, and thus to transmit the "cleanest" video. This PRACTICAL experience illustrates that an impedance change even in a 54-60MHz TV channel occurring within a physical space of less than one inch can produce important and commercially supportable system benefits, despite your statements quoted above. I invite you to post the contrary result(s) of your own "practical experiments," and/or those of others. RF Visit http://rfry.org for FM broadcast RF system papers. |
Richard, I have no reason to doubt anything you say.
I have no reason to doubt what I have said. But why imply there's serious disagreement when there is none? From the tone of your comments I correctly guessed you were a 'magazine' author. Are you also a spare-time connector salesman? ;o) ;o) ---- Reg, G4FGQ |
Richard, WE just look at PL259's from somewhat different viewpoints and see
different magnitudes of the factors involved, with different objectives, even different costs. ---- Reg, G4FGQ |
"Richard Fry" wrote in message ... "Reg Edwards" wrote Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. _________________ ??? Using connectors that don't maintain the characteristic impedance of the transmission lines they connect _will_ produce undesired effects. The effects may be negligible to amateur radio operators used to operating transmitters into rather high mismatches, but they would never be tolerated in most professional operations, including high-power broadcast systems. Connectors available off the shelf are not perfect, especially the pl259 type that are usually used by many. While I don't doubt the problems to TV signals, this is an amateur group type discussion. Many antennas are nowhere near the 50 ohms that the cable is. Most any connector will not cause any more problems than the coax/antenna mismatch will in all but the most demanding systems (maybe EME or some satellite work). |
"Reg Edwards" wrote
Richard, I have no reason to doubt anything you say. I have no reason to doubt what I have said. But why imply there's serious disagreement when there is none? _______________________ Clips from our previous posts (below) - I don't know what constitutes a "serious disagreement" to you, Reg, but I think most readers would say that we have opposite conclusions about this topic. If you have no reason to doubt what I wrote about this, how can you continue to support what you wrote? Our statements are mutually exclusive. YOU: "Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. A connector less than 1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an SWR of 1.03:1 or anything anywhere near to it at frequencies less than 1000 MHz." ME: "This PRACTICAL experience illustrates that an impedance change even in a 54-60MHz TV channel occurring within a physical space of less than one inch can produce important and commercially supportable system benefits, despite your statements quoted above." - RF |
God knows how you thought that one up! I think you know you're wrong.
"Reg Edwards" wrote in message ... Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. |
Richard,
A radio amateur, by an easy mistake, uses a 75-ohm plug and socket in a 50-ohm coaxial transmission system. The total length of the plug plus socket is 1" As a result of the mismatch what is the SWR produced on the 50-ohm line at 2 MHz. At 30 MHz? At 150 MHz? Is the amateur, or anyone else, likely to be aware of any difference in performance? No need to make measurements. If you are unable to make a simple calculation and answer the question then you are not qualified to continue the discussion. ---- Reg, G4FGQ |
God knows how you thought that one up! I think you know you're wrong.
================================= I can see you are not accustomed to thinking in terms of magnitudes and numbers. A QUOTATION: "When you can measure what you are speaking about and express it in numbers you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science." : William Thomson, Lord Kelvin, 1824-1907. ---- Reg, G4FGQ |
"Reg Edwards" wrote
No need to make measurements. If you are unable to make a simple calculation and answer the question then you are not qualified to continue the discussion. __________________ How convenient. Why _would_ you want to make measurements, and prove yourself wrong? |
On Mon, 17 May 2004 06:36:43 -0500, "Richard Fry"
wrote: If you have no reason to doubt what I wrote about this, how can you continue to support what you wrote? Our statements are mutually exclusive. Ah Richard!! Tell me, please, this is the FIRST time you have read our master fence sitter (in the American historical context otherwise known as a Mugwump: someone who has his mug on one side of the fence and his wump on the other). 73's Richard Clark, KB7QHC |
Reg,
You sent the 2nd post below within ten minutes of the 1st one below. Exactly WHAT is your position on this subject? - RF _______________________ "Reg Edwards" wrote first: No need to make measurements. If you are unable to make a simple calculation and answer the question then you are not qualified to continue the discussion. AND THEN, quoting Lord Kelvin: "When you can measure what you are speaking about and express it in numbers you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science." |
"Richard Clark" wrote
Tell me, please, this is the FIRST time you have read our master fence sitter (in the American historical context otherwise known as a Mugwump: someone who has his mug on one side of the fence and his wump on the other). ________________ No, which is the prime reason I chose to confront him. - RF |
On Mon, 17 May 2004 15:02:19 -0500, "Richard Fry"
wrote: "Reg Edwards" wrote No need to make measurements. If you are unable to make a simple calculation and answer the question then you are not qualified to continue the discussion. __________________ How convenient. Why _would_ you want to make measurements, and prove yourself wrong? Ah Richard! Confrontation? Feeble at best when I doubt you could even get his recipe for RF Mud. ;-) Poor Reggie can quote Dead White Scientists like Lord Kelvinator with high dudgeon and still ignore the tenet of the plagiarized message. 73's Richard Clark, KB7QHC |
Reg Edwards wrote:
Richard, A radio amateur, by an easy mistake, uses a 75-ohm plug and socket in a 50-ohm coaxial transmission system. The total length of the plug plus socket is 1" As a result of the mismatch what is the SWR produced on the 50-ohm line at 2 MHz. At 30 MHz? At 150 MHz? Is the amateur, or anyone else, likely to be aware of any difference in performance? The practical answer for amateurs is somewhere between the two extreme positions that Richard and Reg are taking. Richard quotes a case where even very small impedance bumps do matter; but it's in full-quality TV broadcasting, not amateur radio. Reg, on the other hand, wants to dumb it down too far. There *are* cases in amateur radio where small impedance bumps are at least noticeable. At 2MHz or 30MHz, the effect is so small that no amateur would notice it. Even using professional test equipment, you'd be hard-pressed to measure the effect of a single connector of the wrong impedance. At 150(144)MHz, even a single connector is noticeable... but that's not the problem. The real problem is that if people believe a simple slogan like "connector impedances don't matter", they will probably go ahead and use *several* mismatched connectors, at various places along the line. Then they start to find bewildering problems at 144MHz and above, such as indicated SWR and power output values that vary according to the length of the coax jumpers that they use. It still may not matter in terms of the contacts they can make, but they are completely unable to understand what is happening - and that *does* matter! (What is happening, by the way, is that the lengths of the line sections between the mismatched connectors will determine how the small reflections from each one combine together. If you're lucky with the line lengths, they may tend to cancel; if you're not, they may tend to add... and usually it's somewhere in between.) No need to make measurements. In this particular case, that's true. When the impedance bump is small, it is easier and more accurate to calculate the effect than to measure it. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Let's do what nobody has ever done before, not even in the ARRL handbooks or
in Terman, and get an idea of the magnitudes involved. Examine two cases over a range of frequencies. Case (1). In a 50-ohm system, use of a poor connector having an impedance deviating 10 percent from its nominal value of 50 ohms. Case (2). Making the mistake of using a 75-ohm connector in a 50-ohm system. In both cases the connector, plug and socket, is 1" (25.4mm) long. We first calculate the input impedance of a 75-ohm transmission line, 1" long, terminated with 50 ohms. Zin will not be very much different from 50 ohms. We then calculate the SWR on a 50-ohm line which is terminated by the afore-mentioned input impedance. RESULTS of calculation MHz SWR Case 1 SWR Case 2 ------ ----------------- ---------------- 2 1.0002 1.0009 30 1.0028 1.0146 150 1.014 1.073 300 1.029 1.145 1000 1.105 1.524 It is seen that results do not become significant to a radio amateur, and almost everybody else, until he has made the serious mistake of using the wrong impedance connector, and the frequency has risen to 1000 MHz for which he hasn't an SWR meter anyway. Below 300 MHz the results are submerged well beneath the uncertainty of an SWR meter. Now we can take a balanced view of the situation. ---- Reg, G4FGQ |
"Reg Edwards" wrote in message ... Let's do what nobody has ever done before, not even in the ARRL handbooks or in Terman, and get an idea of the magnitudes involved. Examine two cases over a range of frequencies. Case (1). In a 50-ohm system, use of a poor connector having an impedance deviating 10 percent from its nominal value of 50 ohms. Case (2). Making the mistake of using a 75-ohm connector in a 50-ohm system. In both cases the connector, plug and socket, is 1" (25.4mm) long. We first calculate the input impedance of a 75-ohm transmission line, 1" long, terminated with 50 ohms. Zin will not be very much different from 50 ohms. We then calculate the SWR on a 50-ohm line which is terminated by the afore-mentioned input impedance. RESULTS of calculation MHz SWR Case 1 SWR Case 2 ------ ----------------- ---------------- 2 1.0002 1.0009 30 1.0028 1.0146 150 1.014 1.073 300 1.029 1.145 1000 1.105 1.524 It is seen that results do not become significant to a radio amateur, and almost everybody else, until he has made the serious mistake of using the wrong impedance connector, and the frequency has risen to 1000 MHz for which he hasn't an SWR meter anyway. Below 300 MHz the results are submerged well beneath the uncertainty of an SWR meter. Now we can take a balanced view of the situation. ---- Reg, G4FGQ Reg, I suspect your 1 inch length is overly pessimistic. Clearly, for an F connector, it is more like 1/2 inch. Lastly, if the load at the end is not 50.0, then any small deviation in the feedline could just as well improve things, as make it worse; this is probably not true for pulses or video. Think of an antenna tuner. It does nothing to the SWR on the main piece of line. Tam/WB2TT |
Reg,
Your math example does not illustrate the reality that the impedance change produced by a ~5/8" OD brass pin inserted radially into the dielectric space of coaxial transmission line can match that line into an adjacent termination of fairly high SWR (1.3:1 or so), at frequencies as low as 54MHz. As I wrote earlier, this technique is widely and successfully used to match the main transmission line of broadcast TV and FM stations to the net input impedance of their antenna (including its input elbows). - RF |
Your math example does not illustrate the reality that the impedance change produced by a ~5/8" OD brass pin inserted radially into the dielectric space of coaxial transmission line can match that line into an adjacent termination of fairly high SWR (1.3:1 or so), at frequencies as low as 54MHz. ============================= It's not intended to. But I'm sure you are right. Try not to worry about it. |
Wait a minute here... Seems to me that Reg is talking about two
opposing steps separated by a very short distance, and Richard is talking about a single step from one impedance to another. Clearly, Richard's case results in uncancelled echos related to the ratio of the impedances at the step. In fact, Reg's example results in significant uncancelled echos if the steps are separated by enough distance: worst at odd multiples of pi/2 electrical degrees. When the connector is only perhaps a few electrical degrees long, the steps nearly cancel. How long is 1", in electrical degrees? Well, at 1000MHz and a v.f. of perhaps 0.7, longer than one might have anticipated: almost 45 degrees long. So in fact at 1000MHz, a 1" section of 51.5 ohm connector might introduce a bit more than 1.03:1 SWR. But didn't we start out talking about a much lower frequency, and a much larger impedance difference? Another example I ran was 4 electrical degrees of connector...perhaps a bit less than an inch at 150MHz...where it's 75 ohms in a 50 ohm system. The swr for that came out about 1.06:1. The TV broadcast engineer should worry about things like that. The typical ham doesn't have equipment calibrated accurately enough that s/he should be worried about it, and it's unlikely to make any substantive difference anyway in typical ham work. Cheers, Tom "Richard Fry" wrote in message ... "Reg Edwards" wrote Richard, I have no reason to doubt anything you say. I have no reason to doubt what I have said. But why imply there's serious disagreement when there is none? _______________________ Clips from our previous posts (below) - I don't know what constitutes a "serious disagreement" to you, Reg, but I think most readers would say that we have opposite conclusions about this topic. If you have no reason to doubt what I wrote about this, how can you continue to support what you wrote? Our statements are mutually exclusive. YOU: "Provided the mechanical connection is sound, you can use any coaxial connectors you like, regardless of nominal impedance, at frequencies less than about 300 MHz without any observed ill effects. A connector less than 1" long of impedance 51.5 ohms in a 50 ohm system will NOT produce an SWR of 1.03:1 or anything anywhere near to it at frequencies less than 1000 MHz." ME: "This PRACTICAL experience illustrates that an impedance change even in a 54-60MHz TV channel occurring within a physical space of less than one inch can produce important and commercially supportable system benefits, despite your statements quoted above." - RF |
I won't "worry about it," as long as you refrain from posting absolute
statements about this that are demonstrably untrue. - RF "Reg Edwards" wrote in message ... Your math example does not illustrate the reality that the impedance change produced by a ~5/8" OD brass pin inserted radially into the dielectric space of coaxial transmission line can match that line into an adjacent termination of fairly high SWR (1.3:1 or so), at frequencies as low as 54MHz. ============================= It's not intended to. But I'm sure you are right. Try not to worry about it. |
Below is my response to an off-list e-mail I received on this topic, which
to some readers may add additional perspective on this topic. RF Visit http://rfry.org for FM broadcast RF system papers. _____________________________ Thanks for your comments. The reason I chimed in originally was because of Reg's absolute statement that preserving system Z throughout the transmission line and its connectors was unimportant below 1GHz (later 300MHz?). He said further that an impedance change across a 1" length of transmission line was irrelevant in that same spectrum. That is demonstrably untrue, as I pointed out by a real-world example taken from the broadcast industry. The net terminating Z of a TV/FM transmit antenna with its input elbows usually is not exactly 50 ohms, even though that is the impedance value that the hardware was designed to provide. Manufacturing, assembly and installation issues can and do change it. The main transmission line normally is closer to 50 ohms across the relevant bandwidth than the antenna/elbow termination connected to the far end of that line. The Z-matching hardware I described is effective at optimizing this far-end match, it _does_ improve the quality of the radiated signal, and it minimizes the stress on the main transmission line and the transmitter. And it does so by changing the impedance in a 1" or less length of transmission line adjacent to the antenna input, regardless of Reg's beliefs. Hams operating on the HF bands may not wish or even need to consider this, as I mentioned in an earlier post to this thread. But that is not license for Reg or anyone else to write that such disregard is universally justified. - RF |
A QUOTATION:
"When you can measure what you are speaking about and express it in numbers you know something about it. But when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. It may be the beginning of knowledge but you have scarcely in your thoughts advanced to the state of science." : William Thomson, Lord Kelvin, 1824-1907. Think in terms of magnitudes and frequency of occurrence of the effects. One 1" brass slug in a TV transmitting station, even if the subject of a learned paper, is surely of less consequence than millions of amateur and other SWR meters. Beware of unnecessarily frightening amateurs by inadvertently exaggerating the importance of SWR due to ordinary variation in connector plugs and sockets. ---- Reg, G4FGQ |
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