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#21
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Coax + Ladder Line
On Jun 13, 12:05*am, Jim Lux wrote:
Ham applications in any case are kind of an odd thing, efficiency-wise, since the limit is on RF power at the transmitter output connector. Jim, That's not the case in the UK. The licence conditions specify ".... power supplied to the antenna by a transmitter ...." Steve G3TXQ |
#22
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Coax + Ladder Line
On Jun 13, 6:54*am, steveeh131047 wrote:
On Jun 13, 12:05*am, Jim Lux wrote: Ham applications in any case are kind of an odd thing, efficiency-wise, since the limit is on RF power at the transmitter output connector. Jim, That's not the case in the UK. The licence conditions specify ".... power supplied to the antenna by a transmitter ...." Steve G3TXQ How interesting, so in both VK and UK (mixing abbreviations).. the "reference plane" for the measurement is potentially "after" the transmission line. The 5kW Active Antenna Tuner lives! Here in the US, I'm sure the rule is interpreted the way it is because of the heritage of "1kW DC power to the final stage" rule, which in turn came from pre-ham radio rules for marine transmitters using sparkgaps, etc... It was simple change from measuring plate current/ voltage to hooking a power meter at the output connector. I wonder if anyone ever measured RF power on a non-50 ohm transmitter in an enforcement action? My father and grandfather both had stories, apocryphal perhaps, about clever hams having very large "exciters" that fed through a relatively low power "final stage"... using the final tube as a coupling capacitor, perhaps, with unity gain. |
#23
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Coax + Ladder Line
On Jun 12, 5:43*pm, Owen Duffy wrote:
wrote : ... Even with a 80/1 mismatch, ladder line has low loss. 30m of Wireman 551 with a load of 5+j0 has VSWR=80 at the load end, VSWR=28 * at the source end, and the transmission loss is 4.6dB... 65% of the input power is converted to heat. Speaks to your meaning of "low loss". Mythical properties are ascribed to ladder line, some of attributable to ARRL publications. Owen I might have been a bit carried away with the degree of mismatch and loss, but I still think running the ladder/window line the whole way to the tuner will still be the best in most cases for "all band" use. If the major portion of the feed line will still be ladder line, even with the coax lead in, he will still deal with the same line losses for the majority of the run with such a severe mismatch. It won't be that bad most of the time. With the coax and choke addition, even more loss is added to the system. And if he saw an 80:1 mismatch, just 15 feet of RG-8 should cost you half your power at 14 mhz. That's more loss than 15 extra feet of the ladder line which at 80:1, should be about 1 db loss or so. I just find the addition of the coax and choke as totally unnecessary unless there is no way at all to feed the ladder/window line through to the shack. Sure, there are losses with the balun used in most tuners, but I consider that a price to pay for a multi band antenna. Myself, I don't use many all band antennas.. Most of my dipoles are single band, and coax fed. :/ Here is one online calculator which seems to match your numbers fairly close if set for 14 mhz. They include the 551 line as one of the choices. I notice "generic" 450 line has less loss.. I don't know what the difference is between generic and 551.. Maybe 551 has more plastic, and less open window.. I notice generic 600 ohm ladder line will show about 2.2 db loss with an 80:1 mismatch. I'll agree.. Maybe not low loss, but it's quite usable considering an ugly 80:1 mismatch. http://www.qsl.net/co8tw/Coax%20Calculator.htm |
#24
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Coax + Ladder Line
Kash J. Rangan wrote:
Is it OK to place a 4:1 balun just outside of the window for the transition? Given the impedance seen by the balun, is a 4:1 transformation what you want? If the impedance seen by the balun is 10 ohms, for instance, do you think taking the impedance down to 2.5 ohms is an improvement? -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#25
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Coax + Ladder Line
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#26
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Coax + Ladder Line
"Kash J. Rangan" wrote in
: Is it advisable to use a combination of Coax and a ladder line to feed a simple horizontal multiband dipole antenna? In my new QTH it would be very easy for me to get a short run of coax out of my shack window but I would like to use a low loss ladder line for the main run up to the antenna feed point. Is it OK to place a 4:1 balun just outside of the window for the transition? If your antenna is balanced, it would probably be better to simply split the ladder line onto the center conducters of two identical short coaxes and then run ladder line inside to your tuner. If you MUST ground the shields that's OK and you can use lightning arrestors on both coaxes. But remember, no lighning arrestor is as good for protecting equipment as a foot or two of air. Disconnecting during thunderstorms is solid policy! -- Dave Oldridge+ ICQ 454777283 |
#27
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Coax + Ladder Line
"Dave Oldridge" wrote in message 0... "Kash J. Rangan" wrote in : Is it advisable to use a combination of Coax and a ladder line to feed a simple horizontal multiband dipole antenna? In my new QTH it would be very easy for me to get a short run of coax out of my shack window but I would like to use a low loss ladder line for the main run up to the antenna feed point. Is it OK to place a 4:1 balun just outside of the window for the transition? If your antenna is balanced, it would probably be better to simply split the ladder line onto the center conducters of two identical short coaxes and then run ladder line inside to your tuner. If you MUST ground the shields that's OK and you can use lightning arrestors on both coaxes. But remember, no lighning arrestor is as good for protecting equipment as a foot or two of air. Disconnecting during thunderstorms is solid policy! -- Dave Oldridge+ ICQ 454777283 Sounds a little backwards. The Ladder Line should transition to coax (balanced as you pointed out) inside to hopefully reduce RF in the shack. An arrestor outside in addition to disconnecting from equipment will avoid arcing in the shack at least. You have to be there to even remember to disconnect. |
#29
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lightning protection Coax + Ladder Line
Dave Oldridge wrote:
"Kash J. Rangan" wrote in If your antenna is balanced, it would probably be better to simply split the ladder line onto the center conducters of two identical short coaxes and then run ladder line inside to your tuner. If you MUST ground the shields that's OK and you can use lightning arrestors on both coaxes. But remember, no lighning arrestor is as good for protecting equipment as a foot or two of air. Disconnecting during thunderstorms is solid policy! If you have a direct hit, a one foot air gap isn't necessarily going to do you much good, unless the antenna end of the gap is on the ground surface. (i.e. the wire going from where the coax ends to your lightning dissipation ground has some non-zero inductance/resistance) If you're worried about induced voltages from adjacent strikes, then a good transient suppressor will help, but almost all suppressors have "let through" voltage that is above the damage threshold for, say, a FET front end. Depends on what your equipment sensitivity is. Shorting the input of the radio and tying it to chassis ground.. that WILL protect the radio. |
#30
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Coax + Ladder Line
"Jim Lux" wrote in message ... Mike Coslo wrote: wrote: Sure, there are losses with the balun used in most tuners, but I consider that a price to pay for a multi band antenna. Chiming in late on this one... Mythical attributes aside, I believe the real reason that ladder line has become more popular is: 1. Almost all rigs are All HF band these days. 2. Many people have just one antenna they can put up at any given time. Whether it is space considerations, maintaining a low profile or keeping the XYL happy I remember the discussions with my wife when I wanted to put up the HF vertical. "Why do you need two antennas?" "Are you going to transmit two signals at the same time?". Twasn't easy, and a lot of Hams might lose that argument. So assuming it is a doublet, and I wanted to run 80 to 10 meters, I'd put up much wire as I could, not mess with coax, just run ladder line to a window panel, then into the house, into a tuner that can handle balanced line. And pay attention to the lengths that might make for trouble in matching. Or, put up as much wire as you can, put an autotuner at the feedpoint, and run coax back to the shack. probably a six of one, half dozen of the other, depending on what you already have, what kind of support structures you have (if you are putting up an inverted V, then supporting the weight of the tuner isn't an issue..), etc. for most people the feedline won't be that long so just put up the inverted v, wind a coax choke at the feed point (or not) and just run it back to the shack and use the tuner in the rig. |
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