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Shorting out a transmission line
I recall a story from many years ago - possibly an urban myth -
where some guy stuck a pin through a ham's coax feeder and thereby took him off air/blew up his rig etc. Given that RF shorts are a totally different kettle of fish from DC shorts, I'm just wondering how feasible from a technical perspective this reported act of sabotage is. I'm no expert on transmission lines, but it strikes me that the efficacy of such a stunt depends to a great extent on the point in the line where the pin is inserted as related to the wavelength of the transmitted signal. We all know short and open stubs are used as matching elements at the higher frequencies, so it's implicit that just sticking a pin in anywhere isn't necessarily going to adversely affect the efficiency of an antenna system, unless one hits a node at the frequency of operation. What I mean is, IOW, you won't successfully short out coax at RF unless you stick the pin in at an appropriate point. Of course, I might be full of crap on this one as antennas have never been my strong point. Can anyone enlighten me? btw: this is for academic discussion only! I've no beef against any amateur and have been one myself for over 20 years. -- "What is now proved was once only imagin'd" - William Blake |
Shorting out a transmission line
Inside the coax cable are two conductors carrying current, the inside of
the shield and the outside of the center conductor. The current on one of those conductors travels to the antenna, and an equal current returns on the other conductor. At the point where you insert the pin, the current has two possible paths: it can continue down the cable as it normally does, or it can return to the other conductor via the pin. The fraction which goes each way is determined by the impedance of each path. A pin is electrically very short at frequencies at which the coax can be effectively used, so it has negligible reactance. Assuming that it's making good contact with both the shield and center conductor -- which it might not be -- the resistance will also be small. So it makes a good RF short circuit. Therefore a large fraction of the current will return via the pin rather than going on down the cable. So the first effect will be that it will greatly reduce the amount of power which reaches the antenna to be radiated. What will happen to the transmitter? That depends on the transmitter and where the pin is inserted. If the cable didn't have any loss and the pin had zero resistance, the transmitter would see a pure reactance. That is, what it would see would look like a pure L or C, with the value and sign depending on the pin's position relative to the transmitter. In practice, the pin will have some resistance and the cable will have some loss, so the transmitter will also see some amount of resistance, the amount again depending on the pin position, as well as the cable loss and pin resistance. I suspect that most modern 100 watt-class solid state transceivers would probably just shut down their output stage and not be permanently damaged, but I'd rather not experiment with my own rig. The result might be more spectacular with a tube type linear with pi network output. But again, it would depend on the design of the transmitter and the particular impedance it sees. Roy Lewallen, W7EL Paul Burridge wrote: I recall a story from many years ago - possibly an urban myth - where some guy stuck a pin through a ham's coax feeder and thereby took him off air/blew up his rig etc. Given that RF shorts are a totally different kettle of fish from DC shorts, I'm just wondering how feasible from a technical perspective this reported act of sabotage is. I'm no expert on transmission lines, but it strikes me that the efficacy of such a stunt depends to a great extent on the point in the line where the pin is inserted as related to the wavelength of the transmitted signal. We all know short and open stubs are used as matching elements at the higher frequencies, so it's implicit that just sticking a pin in anywhere isn't necessarily going to adversely affect the efficiency of an antenna system, unless one hits a node at the frequency of operation. What I mean is, IOW, you won't successfully short out coax at RF unless you stick the pin in at an appropriate point. Of course, I might be full of crap on this one as antennas have never been my strong point. Can anyone enlighten me? btw: this is for academic discussion only! I've no beef against any amateur and have been one myself for over 20 years. |
Shorting out a transmission line
In article ,
Paul Burridge k wrote: I recall a story from many years ago - possibly an urban myth - where some guy stuck a pin through a ham's coax feeder and thereby took him off air/blew up his rig etc. Given that RF shorts are a totally different kettle of fish from DC shorts, I'm just wondering how feasible from a technical perspective this reported act of sabotage is. "Pinning" a coax has a long history in the mythos of RF... I've heard stories about it for years, usually involving somebody pinning the coax of an obnoxious CB operator. I'm no expert on transmission lines, but it strikes me that the efficacy of such a stunt depends to a great extent on the point in the line where the pin is inserted as related to the wavelength of the transmitted signal. Well, an effective short at point along the coax is going to cause a complete reflection at that point, and a very high SWR on the line. This may appear to the transmitter as a short, as an open, or as an intermediate resistance with a boatload of reactance, depending on the distance from the transmitter to the short. A well-designed modern transmitter/amplifier may survive this sort of nasty load well enough, through e.g. voltage and current sensing circuitry which feed back to the bias or ALC circuit, and reduce the power to avoid overcurrent or overvoltage damage, and/or through the use of internally-ballasted RF finals transistors with a big safety margin. A cheap amplifer (such as many of the "multiple pill" not-so-"linear" amplifiers I see being sold to the CB-cowboy market) could very easily leak out all of its Magic Blue Smoke quite quickly, working into this sort of load. We all know short and open stubs are used as matching elements at the higher frequencies, so it's implicit that just sticking a pin in anywhere isn't necessarily going to adversely affect the efficiency of an antenna system, unless one hits a node at the frequency of operation. Not so, I believe. Remember, what you're doing is creating a trivially-short, shorted "stub" across the line. The pin itself will present a low-R, low-Z impedance - most of the power flowing up the line from the transmitter will go into this impedance, and very little will flow up the remainder of the line to the antenna. Radiated power will drop very sharply, and the transmitter/amp is likely to indicate its distress in one way or another. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Shorting out a transmission line
I leave only this. At VHF and up it's common to use a shorted 1/4 wave section for second harmonic suppression at the output. Very effective and dirt cheap. The finals are not the least bit bothered. If a short appeared near a 1/4 wave node at operating frequency it might go unnoticed. Allison KB!GMX |
Shorting out a transmission line
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Shorting out a transmission line
On Sat, 24 Dec 2005 15:40:25 -0800, Roy Lewallen
wrote: wrote: I leave only this. At VHF and up it's common to use a shorted 1/4 wave section for second harmonic suppression at the output. Very effective and dirt cheap. The finals are not the least bit bothered. If a short appeared near a 1/4 wave node at operating frequency it might go unnoticed. I'm afraid it wouldn't go unnoticed. The transmitter would see an open circuit, instead of the proper load of typically 50 ohms. The effect on the transmitter would be the same as disconnecting the feedline at the transmitter. Roy Lewallen, W7EL I did use the word "might" rather than will. Actually it depends on the real life characteristics of the short. If it were a perfect short (in theory) yes. But if there is any varience from that it's going to be harder to predict. Likely it world look more like a higher impedence, but not completely. In all likelyhood the parameter that needs to be know more than any one its frequency. At 432 it's impact would be very different than say 7.2mhz. Allison |
Shorting out a transmission line
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Shorting out a transmission line
I leave only this.
At VHF and up it's common to use a shorted 1/4 wave section for second harmonic suppression at the output. Very effective and dirt cheap. The finals are not the least bit bothered. True, but that's not the situation we're dealing with here. If you place a shorted quarter-wave section directly at the transmitter's terminals, in parallel with the antenna feedline, then the transmitter "sees" the impedance of the feedline (its usual load) in parallel with the impedance of the shorted stub (very high). The net impedance is that of the load (the admittance of the shorted stub is nearly zero) and the transmitter does not "notice" the presence of the shorted stub. That's not the situation which occurs if the feedline itself is shorted 1/4 waveline towards the load. In that situation, the *only* thing that the transmitter will see is the shorted quarter-wavelength "stub" between itself and the short. The impedance at the point of the short is nearly zero - it's the impedance of the short itself, in parallel with the impedance of the antenna as seen when looking up the remainder of the feedline. No matter what the antenna's impedance is, the very low impedance of the short itself is going to dominate the parallel combination. The resulting near-zero-ohm combination will be transformed, by the quarter-wavelength distance back to the transmitter, so that it appears as an open circuit to the transmitter. The transmitter cannot, in effect, "see past the short circuit" to the antenna itself. The same is true no matter how far up the feedline from the transmitter the short/pin happens to be. At the point of the short, the impedance is going to be nearly zero, and this near-zero impedance will be transformed to some other value on the same very-high-SWR circle (neglecting consideration of feedline loss, of course). No matter where you pin the coax, the transmitter is going to be unhappy. If a short appeared near a 1/4 wave node at operating frequency it might go unnoticed. Different situation, I'm afraid. If you have an antenna analyzer, try it out for yourself. Take an arbitrary-length section of RG58 with a 50-ohm load at one end and a BNC at the other. Run it into a BNC "T". Out the other leg of the T, run an adjustable length of RG-58 to the antenna analyzer. You ought to measure 50 ohms in this situation. Now, stick a short directly across the third branch of the T connector ("pinning" the coax, so to speak), and see what your analyzer tells you. It may read high-Z, or low-Z, or intermediate-Z with a lot of reactance... but it'll be a high indicated SWR, and it won't be anywhere near 50+j0. Then, disconnect the antenna from the "T". The impedance and indicated SWR won't change significantly. Try changing the length of the RG58 between the "T" and the analyzer. You'll get a different Z value with the short in place (whether the antenna is or is not attached) but it'll still have a really high SWR, no matter what coax length you choose. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Shorting out a transmission line
On Sat, 24 Dec 2005 20:44:34 -0500, Gary Schafer
wrote: I did use the word "might" rather than will. Actually it depends on the real life characteristics of the short. If it were a perfect short (in theory) yes. But if there is any varience from that it's going to be harder to predict. Likely it world look more like a higher impedence, but not completely. In all likelyhood the parameter that needs to be know more than any one its frequency. At 432 it's impact would be very different than say 7.2mhz. Allison A short across the transmission line will have much the same effect at 432 as it will at 7 mhz. What you are thinking about is a shorted stub attached to the transmission line or output of the transmitter. A shorted 1/4 wave length stub at the operating frequency placed across the transmitter output will present a high impedance at the operating frequency and will not be noticed by the transmitter. But at the second harmonic of the stub it will be a 1/2 wave shorted stub which will present a short at the output of the transmitter at the 2nd harmonic frequency. If it were a "perfect" short yes. For real life the short have real impedence between center conductor and shield. As frequency goes up a .2" peice of wire accumulates enough real resistance and reactance to be a factor at high VHF and uhf. My favorite filter for 2m is found on the ARRL.com TIS site. it's made with series and parallel shorted sections operating as tapped resonant circuits. The stubs are only something like 2" and for 2m thats about 13" short of 1/4 wave. Just shows what happens when a transmission line stops being simply that. The shorted stub would still allow energy to flow to the antenna normally. But shorting the transmission line would not no matter where it was. Yes and No. See above. Allison Kb!gmx |
Shorting out a transmission line
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Shorting out a transmission line
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Shorting out a transmission line
On Sun, 25 Dec 2005 12:44:12 -0500, Gary Schafer
wrote: Try it and you will be surprised. 73 Gary K4FMX I have at 2400mhz too! Down around DC (sub 30mhz) your "pin" is a short and the effects are mostly (though measurable) a shorted coax with all the effects as expected. As you get up there in frequency the "short" as described doesn't behave as it did at DC. The problem is similar to another thread concerning real world components where the discussion finally recognized that like other real world components a short is not always what it may look like. Allison |
Shorting out a transmission line
In article ,
Paul Burridge k wrote: I have at 2400mhz too! Down around DC (sub 30mhz) your "pin" is a short and the effects are mostly (though measurable) a shorted coax with all the effects as expected. As you get up there in frequency the "short" as described doesn't behave as it did at DC. The problem is similar to another thread concerning real world components where the discussion finally recognized that like other real world components a short is not always what it may look like. Sigh... I can't differentiate between you and that other chap on this issue. You both cite perfectly legitimate grounds and come to entirely separate conclusions. You can't both be right, but neither of you seem to be wrong! Can we focus down on *one* issue to avoid disappearing up our own backsides: as far as the tx is concerned, is the portion of the feed line beyond the pin relevant at all or does it effectively cease to exist, as would be the case at VLF/DC? It's really a question of the problem domain - that is, what are the frequencies involved, and what are the sizes of the feedline and the "width" of the shorting bar/pin? Allison and I have been talking about rather different sets of test conditions. I think we're actually in "violent agreement" about what actually goes on. At upper-UHF and microwave frequencies, I agree that Allison is correct. The length of the pin is a significant fraction of a wavelength, and it thus does not behave as a true short circuit - rather, it's an inductor of significant value shunted across the transmission line. At these frequencies, in this problem domain, you have to consider the shunt combination of two non-zero impedances. "Shorting" the feedline with this 'straight pin' inductor will probably have a significant effect on the impedance seen by the transmitter, but it won't be as simple as I had portrayed. At HF (and, I think, VHF up through the 2-meter frequency range) a shorting pin of perhaps 1/4" in length is a negligible fraction of a wavelength long. Whatever small amount of inductance it introduces will have a reactive impedance whose magnitude is far below that of the 50-ohm load, and its very high admittance will swamp the lower admittance of the load. Hence, the load impedance will be of negligible importance in deciding what the transmitter "sees" at these frequencies, under these conditions... the transmitter "sees" only the impedance of the short itself, transformed by however much line is between transmitter and short. If I can find a scrap BNC male connector, and make a shorting-plug out of it, I'll run the coax-and-T experiment I suggested, and post some actual numbers for the systems's behavior at those frequencies I can coerce out of my MFJ-269. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Shorting out a transmission line
If I can find a scrap BNC male connector, and make a shorting-plug out
of it, I'll run the coax-and-T experiment I suggested, and post some actual numbers for the systems's behavior at those frequencies I can coerce out of my MFJ-269. OK, I ran the test, and the results are pretty much as I had anticipated. Although the test arrangement and gear isn't lab-standard, I think it's good enough to confirm the basic principles. Test equipment: MFJ-259 antenna analyzer, with an N-to-BNC adapter. Transmission line and load: 12' length of RG-58 coax, with a 50-ohm Ethernet terminator attached to the end. Shorting insertion: an additional 6' length of RG-58, with a couple of BNC "T" adapters, which can be inserted between the analyzer and the T-line-and-load. If the shorting plug is inserted it'll be 6' from the analyzer and 12' from the load. Short circuit: a male BNC plug, with a short inserted between center pin and shell as far down inside the shell as possible. First test: check quality of T-line and load, using only the 12' section of RG-58 between analyzer and load. Note that the Imag(load) numbers do not indicate the sign of the reactance... the MFJ won't do that, alas. Frequency Real(load) Imag(load) Indicated SWR 2.5 52 4 1.0 5 58 6 1.2 10 57 7 1.2 15 49 1 1.0 20 56 6 1.1 50 54 2 1.1 144 51 4 1.0 166 59 8 1.2 440 1.4 Impression: either the RG-58 or the Ethernet terminator isn't exactly 50 ohms (not unexpected) or the MFJ's calibration isn't perfect (likewise) but the figures are good enough to let us draw some reasonable conclusions. Second test: insert the 6' RG-58 and its T connectors between the analyzer and the 12' section. See how much this additional length of line, and the parasitics of the T connectors, affect the load and SWR. Frequency Real(load) Imag(load) Indicated SWR 2.5 54 4 1.1 5 59 5 1.2 10 50 10 1.2 15 49 1 1.0 20 51 8 1.1 50 48 5 1.1 144 50 4 1.0 166 40 2 1.2 440 1.1 Impression: the measured values change a bit, but the SWRs are close or identical (save for the 440 measurement). The additional length of line, and the parasitics from the T connectors, are shifting things around a bit (especially at 440)... not unexpected. Third test: connect the shorting plug to one of the T connectors. See what "pinning the line" does to the analyzer's view of the load. Frequency Real(load) Imag(load) Indicated SWR 2.5 0 6 31 5 0 12 31 10 1 26 31 15 1 43 31 20 2 70 31 50 0 44 28.7 144 7 33 15.4 166 166 353 13.0 440 5 Impression: yeah, that looks like a short circuit, transformed via a feedline. Nothing close to a 50-ohm-resistive load shows up at any of the test frequencies. Fourth test: disconnect the 50-ohm load from the end of the 12' line, creating an abrupt change in the load impedance. See what this does to the figures from the third test - how much of the antenna load change gets back "around" the short? Frequency Real(load) Imag(load) Indicated SWR 2.5 0 6 31 5 0 12 31 10 1 25 31 15 1 43 31 20 2 70 31 50 6 44 29.0 144 6 30 18.5 166 133 317 14.1 440 5 Impression: changing the antenna load from 50-ohms-resistive to near-infinite made a slight change in the impedance seen by the analyzer, but not much at all at any frequency. The presence of the short circuit 6 feet from the analuzer is still dominating the load that the analyzer "sees". Special-distance test: with the short, and the 50-ohm load both in place, sweep the analyzer frequency around the ranges at which the analyzer-to-short distance is around 1/4 and 1/2 wavelength. See if we can "see past" the short under these conditions. Measurements: with the short circuit in place, and a 50-ohm load at the end of the 12' line, an impedance peak (Z1500 ohms) is noted when sweeping between 32.14 MHz and 32.94 MHz. An impedance minimum (Real(load) of 2-3 ohms, Imag(load) of 0 ohms) is noted between 65.7 MHz and 65.85 MHz. The indicated SWR remains high (22 or above) at all frequencies between the impedance maximum and impedance minimum... there's no point at which the load resembles anything like "50 ohms resistive, little reactive component". Removing the 50-ohm load, and thus open-circuiting the antenna feedline, has no significant effect on the impedance as seen at the quarter-wavelength maximum, at the half-wavelength minimum, or at points in between. Impression: we cannot "see past" the short circuit, no matter whether it's a quarter-wavelength from the transmitter, a half-wavelength, or some distance in between these two. The analyzer "sees" only what would be expected for a short circuit, transformed by 6' of coax. Short-circuit quality test: measure impedance of the shorted BNC plug. At 16 MHz and below, the MFJ-269 shows it as 0+0j. Above 16 MHz, some reactance shows up... 2 ohms at 28 MHz, 3 ohms at 42 MHz, 4 ohms and 55 MHz, 8 ohms at 112 MHz, 10 ohms at 144 MHz, 12 ohms at 169 MHz. The inductance of the shorted plug itself, and/or the length of the N-to-BNC adapter on the analyzer, is having some effect. SWR remains unreadably high: 31 up through VHF and 5 on 440. It's not the highest-quality "short circuit" in the world, for certain, but at HF and VHF it's close enough for our purposes here. So... what do I conclude from this? I conclude that at HF and VHF frequencies, if you accidentally short your transmitter-to-antenna coaxial feedline (with a pin, nail, or a loose strand of coax braid which "gets loose" inside a connector), you're going to present your transmitter with an unrealistic load (high, low, or nastily reactive) and that the power flow up the feedline is going to stop at the short circuit. Since no "short circuit" in the real world is going to have 0+0j impedance, this isn't *absolutely* true, but you can probably consider it to be *practically* true and correct at these frequencies, with this sort of shorting. I further conclude that with respect to the above, there's no 'magic' about shorts which happen to occur at a quarter-wavelength distance from the transmitter. In this situation, the transmitter will 'see' something which behaves like a quarter-wavelength shorted stub... the load further up the transmission line at the antenna remains 'invisible' to the transmitter. At high-UHF and SHF/microwave frequencies, where a direct "short" is likely to be a significant fraction of a wavelength, then the additional reactance of the "short" will certainly affect how the undesired connection affects what the transmitter "sees". It may hurt, or help (helping to "tune out" reactance from the antenna itself), or may be completely invisible. Allison is entirely correct on this point, under these particular conditions. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Shorting out a transmission line
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Shorting out a transmission line
On Mon, 26 Dec 2005 08:04:02 -0600, "Richard Fry"
wrote: wrote At VHF and up it's common to use a shorted 1/4 wave section for second harmonic suppression at the output. Very effective and dirt cheap. The finals are not the least bit bothered. ___________ Yes, and in typical configuration it is an electrical 1/4-wave coaxial section connected in parallel with both conductors of the main transmission line. It does not terminate the main transmission line, so this application/example is not very relevant to the "pin through the coax" question of the OP. And it would not result in a dead short at the carrier frequency, no matter where it is located in the output system. Some FM broadcast antennas also include them to supply a DC short from inner to outer conductors of the antenna coax to provide some protection from lightning. A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast band. The need there is to add a deep notch at stations whose 2nd harmonic falls in the broadcast band. This stub is used to add to the attenuation of the already compliant 2nd harmonic level coming out of the tx, but which, without the stub can be heard on broadcast receivers within a short distance from the broadcast antenna site. WJR (760 kHz) is one station using this technique. That just strikes me as plain stoopid. At MW, such filtering would be far better achieved by lumped elements. A quarter wave stub at such frequencies appears impractical, unwieldy and rather expensive! -- "What is now proved was once only imagin'd" - William Blake |
Shorting out a transmission line
wrote
At VHF and up it's common to use a shorted 1/4 wave section for second harmonic suppression at the output. Very effective and dirt cheap. The finals are not the least bit bothered. ___________ Yes, and in typical configuration it is an electrical 1/4-wave coaxial section connected in parallel with both conductors of the main transmission line. It does not terminate the main transmission line, so this application/example is not very relevant to the "pin through the coax" question of the OP. And it would not result in a dead short at the carrier frequency, no matter where it is located in the output system. Some FM broadcast antennas also include them to supply a DC short from inner to outer conductors of the antenna coax to provide some protection from lightning. A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast band. The need there is to add a deep notch at stations whose 2nd harmonic falls in the broadcast band. This stub is used to add to the attenuation of the already compliant 2nd harmonic level coming out of the tx, but which, without the stub can be heard on broadcast receivers within a short distance from the broadcast antenna site. WJR (760 kHz) is one station using this technique. RF (WJR engr in mid-1960s) |
Shorting out a transmission line
"Paul Burridge" wrote
A 1/4-wave shorted stub is used at frequencies as low as the MW broadcast band. The need there is to add a deep notch at stations whose 2nd harmonic falls in the broadcast band. This stub is used to add to the attenuation of the already compliant 2nd harmonic level coming out of the tx, but which, without the stub can be heard on broadcast receivers within a short distance from the broadcast antenna site. WJR (760 kHz) is one station using this technique. (RF quote) That just strikes me as plain stoopid. At MW, such filtering would be far better achieved by lumped elements. A quarter wave stub at such frequencies appears impractical, unwieldy and rather expensive! ____________ It also provides a low-impedance and fairly wideband path to ground for the insulated, series-fed tower used by most broadcast stations -- which drains off any static charges that may collect on the tower, and so reduces the probability of lightning strikes. Lumped elements are less effective at this. RF |
Shorting out a transmission line
"Paul Burridge" k wrote in message ... That just strikes me as plain stoopid. At MW, such filtering would be far better achieved by lumped elements. A quarter wave stub at such frequencies appears impractical, unwieldy and rather expensive! -- Even at 50 KW? 73, Steve, K,9.D;C'I P.S. I suspect it is air line, no? |
Shorting out a transmission line
I believe this is a good test and conclusion. I can only add two small
things. 1- the impedance gets further from 50 ohms, the 269 MFJ ( Mine has this characteristic) shows more and more error. Try paralleling four 50 ohm loads (with your Tees) or measuring a good 200 ohm load. The 269 shows considerable imaginary at 4:1 real (:-( I think I have heard that the 259 is better. 2- The constant reference to the "pin" being much less than a 1/4 wave is not the correct focus. It is the impedance (primarily inductance in this case) which is the factor of interest relative to the frequency. What's that old rule of thumb? One nano Henry per inch?? One nano Henry per 1/10 inch... Been off that bench too long. 73, Steve, K9DCI "Dave Platt" wrote in message ... If I can find a scrap BNC male connector, and make a shorting-plug out of it, I'll run the coax-and-T experiment I suggested, and post some actual numbers for the systems's behavior at those frequencies I can coerce out of my MFJ-269. OK, I ran the test, and the results are pretty much as I had anticipated. Although the test arrangement and gear isn't lab-standard, I think it's good enough to confirm the basic principles. Test equipment: MFJ-259 antenna analyzer, with an N-to-BNC adapter. Transmission line and load: 12' length of RG-58 coax, with a 50-ohm Ethernet terminator attached to the end. Shorting insertion: an additional 6' length of RG-58, with a couple of BNC "T" adapters, which can be inserted between the analyzer and the T-line-and-load. If the shorting plug is inserted it'll be 6' from the analyzer and 12' from the load. Short circuit: a male BNC plug, with a short inserted between center pin and shell as far down inside the shell as possible. First test: check quality of T-line and load, using only the 12' section of RG-58 between analyzer and load. Note that the Imag(load) numbers do not indicate the sign of the reactance... the MFJ won't do that, alas. Frequency Real(load) Imag(load) Indicated SWR 2.5 52 4 1.0 5 58 6 1.2 10 57 7 1.2 15 49 1 1.0 20 56 6 1.1 50 54 2 1.1 144 51 4 1.0 166 59 8 1.2 440 1.4 Impression: either the RG-58 or the Ethernet terminator isn't exactly 50 ohms (not unexpected) or the MFJ's calibration isn't perfect (likewise) but the figures are good enough to let us draw some reasonable conclusions. Second test: insert the 6' RG-58 and its T connectors between the analyzer and the 12' section. See how much this additional length of line, and the parasitics of the T connectors, affect the load and SWR. Frequency Real(load) Imag(load) Indicated SWR 2.5 54 4 1.1 5 59 5 1.2 10 50 10 1.2 15 49 1 1.0 20 51 8 1.1 50 48 5 1.1 144 50 4 1.0 166 40 2 1.2 440 1.1 Impression: the measured values change a bit, but the SWRs are close or identical (save for the 440 measurement). The additional length of line, and the parasitics from the T connectors, are shifting things around a bit (especially at 440)... not unexpected. Third test: connect the shorting plug to one of the T connectors. See what "pinning the line" does to the analyzer's view of the load. Frequency Real(load) Imag(load) Indicated SWR 2.5 0 6 31 5 0 12 31 10 1 26 31 15 1 43 31 20 2 70 31 50 0 44 28.7 144 7 33 15.4 166 166 353 13.0 440 5 Impression: yeah, that looks like a short circuit, transformed via a feedline. Nothing close to a 50-ohm-resistive load shows up at any of the test frequencies. Fourth test: disconnect the 50-ohm load from the end of the 12' line, creating an abrupt change in the load impedance. See what this does to the figures from the third test - how much of the antenna load change gets back "around" the short? Frequency Real(load) Imag(load) Indicated SWR 2.5 0 6 31 5 0 12 31 10 1 25 31 15 1 43 31 20 2 70 31 50 6 44 29.0 144 6 30 18.5 166 133 317 14.1 440 5 Impression: changing the antenna load from 50-ohms-resistive to near-infinite made a slight change in the impedance seen by the analyzer, but not much at all at any frequency. The presence of the short circuit 6 feet from the analuzer is still dominating the load that the analyzer "sees". Special-distance test: with the short, and the 50-ohm load both in place, sweep the analyzer frequency around the ranges at which the analyzer-to-short distance is around 1/4 and 1/2 wavelength. See if we can "see past" the short under these conditions. Measurements: with the short circuit in place, and a 50-ohm load at the end of the 12' line, an impedance peak (Z1500 ohms) is noted when sweeping between 32.14 MHz and 32.94 MHz. An impedance minimum (Real(load) of 2-3 ohms, Imag(load) of 0 ohms) is noted between 65.7 MHz and 65.85 MHz. The indicated SWR remains high (22 or above) at all frequencies between the impedance maximum and impedance minimum... there's no point at which the load resembles anything like "50 ohms resistive, little reactive component". Removing the 50-ohm load, and thus open-circuiting the antenna feedline, has no significant effect on the impedance as seen at the quarter-wavelength maximum, at the half-wavelength minimum, or at points in between. Impression: we cannot "see past" the short circuit, no matter whether it's a quarter-wavelength from the transmitter, a half-wavelength, or some distance in between these two. The analyzer "sees" only what would be expected for a short circuit, transformed by 6' of coax. Short-circuit quality test: measure impedance of the shorted BNC plug. At 16 MHz and below, the MFJ-269 shows it as 0+0j. Above 16 MHz, some reactance shows up... 2 ohms at 28 MHz, 3 ohms at 42 MHz, 4 ohms and 55 MHz, 8 ohms at 112 MHz, 10 ohms at 144 MHz, 12 ohms at 169 MHz. The inductance of the shorted plug itself, and/or the length of the N-to-BNC adapter on the analyzer, is having some effect. SWR remains unreadably high: 31 up through VHF and 5 on 440. It's not the highest-quality "short circuit" in the world, for certain, but at HF and VHF it's close enough for our purposes here. So... what do I conclude from this? I conclude that at HF and VHF frequencies, if you accidentally short your transmitter-to-antenna coaxial feedline (with a pin, nail, or a loose strand of coax braid which "gets loose" inside a connector), you're going to present your transmitter with an unrealistic load (high, low, or nastily reactive) and that the power flow up the feedline is going to stop at the short circuit. Since no "short circuit" in the real world is going to have 0+0j impedance, this isn't *absolutely* true, but you can probably consider it to be *practically* true and correct at these frequencies, with this sort of shorting. I further conclude that with respect to the above, there's no 'magic' about shorts which happen to occur at a quarter-wavelength distance from the transmitter. In this situation, the transmitter will 'see' something which behaves like a quarter-wavelength shorted stub... the load further up the transmission line at the antenna remains 'invisible' to the transmitter. At high-UHF and SHF/microwave frequencies, where a direct "short" is likely to be a significant fraction of a wavelength, then the additional reactance of the "short" will certainly affect how the undesired connection affects what the transmitter "sees". It may hurt, or help (helping to "tune out" reactance from the antenna itself), or may be completely invisible. Allison is entirely correct on this point, under these particular conditions. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Shorting out a transmission line
Even at 50 KW?
73, Steve, K,9.D;C'I P.S. I suspect it is air line, no? _____________ Yes. In the case of WJR, it was a length of 1-5/8" OD air-insulated rigid line (20 foot standard lengths plus a custom length). |
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