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#11
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Ladder line Vs. Coax
On May 28, 9:16 am, Sonny Hood wrote:
I have an efficiency question concerning feed lines. My present system is RG-8X to my 75 meter inverted vee which is about 85 feet away from the shack. I propose to replace some 88 feet of coax with 300 ohm window ladder line that is inserted into the coax run with 4:1 baluns to match the coax on each end. Also at the feed point of the antenna switch from a voltage balun to a current balun (ferrite chock type). By my calculations with a 98 watt generator I will increase the power to the load by about 11-20 watts and with a 985 watt generator, 117-210 more watts will reach the load. Figuring theoretical total system (A) against total System (B) or by just the difference in the 88 feet of ladder line versus coax. What do you think the increase will be? If you switch from coax to ladder line and baluns or tuners, you are going to lose a bit of efficiency I bet. I always have when I've compared the two on that band. You are converted from a pure and simple system which is a 50 ohm radio, feeding 50 ohm line, which BTW at 75m is very low loss, into an appx 50 ohm load. This "pure" system is probably 95%+ efficient on the 75m band as long as the coax is not junk. In my opinion there is no better method of feeding a single band dipole. Anything else is pretty much a step down as far as I'm concerned. The *only* reason I would ever run that way is if I wanted multi band use using ladder line and a tuner. But in my case, I prefer multiple dipoles fed with a single coax. The decrease in loss switching from coax to ladder line *on 75m* is so small as to be nothing. But then you have baluns and tuners. This is what is going to spoil the food. You will actually end up with a net loss at the end of the day. Maybe be a small loss, but I bet it will be measurable on a meter if you used an A/B switch. It was in my cases. Then you have the moisture issues, etc.. Do what you want, I prefer coax when feeding dipoles. I usually use 213 on HF here at the house. But at 3 mhz, even RG-58 is fairly low loss. The mini 8 you are using should be a bit better than rg-58. I think it's slightly larger and using foam if I remember right. |
#13
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Ladder line Vs. Coax
Sonny Hood wrote in
: I have an efficiency question concerning feed lines. My present system is RG-8X to my 75 meter inverted vee which is about 85 feet away from the shack. I propose to replace some 88 feet of coax with 300 ohm window ladder line that is inserted into the coax run with 4:1 baluns to match the coax on each end. Also at the feed point of the antenna switch from a voltage balun to a current balun (ferrite chock type). By my calculations with a 98 watt generator I will increase the power to the load by about 11-20 watts and with a 985 watt generator, 117-210 more watts will reach the load. Figuring theoretical total system (A) against total System (B) or by just the difference in the 88 feet of ladder line versus coax. What do you think the increase will be? Sonny, I have resisted the temptation to tell you that the question is wrong rather than to answer the question... however... You are trying to compare two configurations that are not optimal. An optimal coax configuration would have a balun, usually at the antena feed point so your coax loss budget is short changed a balun. I assume from the fact that you have proposed coax that this is a one band antenna. You haven't made it clear whether you can use an ATU to transform the load to suit the tx, I will assume you can. Why do you need two 4:1 baluns / transformers? The dipole is already an approximately balanced load and loss on open wire line with a VSWR of 4:1 is still quite low. You could just attach the feedline directly to the antenna, and use a 1:1 balun at the ATU. Alternatively, you could lengthen the feedline a little so that it was an electrical half wave and dispense with the ATU (still need the balun). In the case of the latter (an electrical half wave of open wire line, 1:1 balun at the tx) you will achieve the lowest loss, but it is just a few tenths of a db better than the balun/RG8X config if you work them through. The open wire line might not be as easy to route into and around the shack, and depending on construction, more affected by water. Owen |
#14
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Ladder line Vs. Coax
On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote:
Thanks Guys, I got some good replies and it cause me to reconsider the change. I find that the best and most simple change will be to remove the RG-8X and it's high attenuation and install all JT-2015 (RG-8 Type) from R&L Electronics. Also to put a good 1:1 balun at the feed point. I have a 160M windom and a G5RV and will dedicate this inverted vee antenna to 75M. This will be not used on 40M due to the high resistance on the second harmonic with the current balun. Yes the added baluns for the ladder line would increase the insertion loss and yes the wet ladder line is a poor conductor. And the cost will be spent for a more efficiency designed system. K4WYS |
#15
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Ladder line Vs. Coax
Sonny Hood wrote:
I have an efficiency question concerning feed lines. My present system is RG-8X to my 75 meter inverted vee which is about 85 feet away from the shack. I propose to replace some 88 feet of coax with 300 ohm window ladder line that is inserted into the coax run with 4:1 baluns to match the coax on each end. Also at the feed point of the antenna switch from a voltage balun to a current balun (ferrite chock type). By my calculations with a 98 watt generator I will increase the power to the load by about 11-20 watts and with a 985 watt generator, 117-210 more watts will reach the load. Figuring theoretical total system (A) against total System (B) or by just the difference in the 88 feet of ladder line versus coax. What do you think the increase will be? There is so little loss at 3.5 mHz for RG8X you would have a hard time telling any difference. |
#16
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Ladder line Vs. Coax
Sonny Hood wrote in
: I have an efficiency question concerning feed lines. My present system is RG-8X to my 75 meter inverted vee which is about 85 feet away from the shack. I propose to replace some 88 feet of coax with 300 ohm window ladder line that is inserted into the coax run with 4:1 baluns to match the coax on each end. Also at the feed point of the antenna switch from a voltage balun to a current balun (ferrite chock type). By my calculations with a 98 watt generator I will increase the power to the load by about 11-20 watts and with a 985 watt generator, 117-210 more watts will reach the load. Figuring theoretical total system (A) against total System (B) or by just the difference in the 88 feet of ladder line versus coax. What do you think the increase will be? In one short word: imperceptible. Using twinlead or ladder line is a good idea if you're feeding an antenna that has a high SWR at the feedpoint on some of the frequencies you're going to use it with, but at 75 meters, RG8X vs. ladder line will only make a small difference if the SWR is at all close to 1. Less than half a decibel, or a tenth of an S-unit! Sure, it might make enough difference on very weak paths, but most of the time nobody would be able to tell the difference on a direct on-air A-B test! And even that advantage almost disappears when the ladder line is wet. Like I said, the picture is quite different at high SWR. -- Dave Oldridge+ ICQ 1800667 |
#17
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Ladder line Vs. Coax
Sonny Hood wrote in
: On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote: Thanks Guys, I got some good replies and it cause me to reconsider the change. I find that the best and most simple change will be to remove the RG-8X and it's high attenuation and install all JT-2015 (RG-8 Type) from R&L Electronics. Sonny, run the numbers at http://www.vk1od.net/tl/tllc.php before you spend the money. BTW, it will make NO difference on receive since S/N ratio on 80m is almost entirely determined by band noise and small change in TL loss will make no significant difference. Change in TL loss will affect the strength of your transmitted signals. Also to put a good 1:1 balun at the feed point. Good move. Owen |
#18
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Ladder line Vs. Coax
"Sonny Hood" wrote in message ... On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote: Thanks Guys, I got some good replies and it cause me to reconsider the change. I find that the best and most simple change will be to remove the RG-8X and it's high attenuation and install all JT-2015 (RG-8 Type) from R&L Electronics. Also to put a good 1:1 balun at the feed point. I have a 160M windom and a G5RV and will dedicate this inverted vee antenna to 75M. This will be not used on 40M due to the high resistance on the second harmonic with the current balun. Yes the added baluns for the ladder line would increase the insertion loss and yes the wet ladder line is a poor conductor. And the cost will be spent for a more efficiency designed system. K4WYS YOu have had many answers to your question, but have drawn the wrong conclusion. If you replace the rg8x with some of the best low loss coax (lmr400 for my example) you go from about .7 db to .2 db of loss on 80 meters. That is .5 db less loss. You will almost need an electron microscope to see the s-meter move . Also 80 meters has so much noise on it most of the time, you will not be able to hear the difference. I think it was one db that was suppose to be the minimal detected difference in hearing when the db scale was first put into use. It may seem that if you have a 100 watt output transmitter you will go from about 85 to 95 watts in a matched system. This may seem to be a lot , but on the receving end it will not be noticed. On maybe 20 meters and higher, especially 2 meters and up it does make good use of the money to go to a higher grade of coax for weak signals. Spend your money on something more productive. |
#19
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Ladder line Vs. Coax
Owen Duffy wrote:
(Dieter Kiel) wrote in news:1ihnz3n.wxm0z7nvxaivN% : http://www.w8ji.com/vswr_reactive_power.htm I assume Dieter that this is your recommendation of the article. That article uses the term 'reactive power' in a non-conventional way, though the term is a well known one (ie has a conventional meaning). Conventional use is that the term 'apparent power' is applied to the product of RMS voltage and current flowing into a two terminal load, and the units are VoltAmps (VA), not Watts as used in the article. Reactive power is the reactive component of apparent power, and expressed in units of 'VoltAmpsReactive' (VAR). 'Real power' is the real component of apparent power and expressed in units of Watts (W). The relationship is that apparentpower = (realpower^2 + reactive power^2)^0.5 . This is all basic lumped component AC circuit theory, and holds at RF. True, but we still aren't there. It's very misleading to quote "VAR powers" in the kilowatt range, because the only power available to melt the feedline is 100W from the transmitter. There is no magnification of real power. The high value of "VAR power" is a theoretical result of the large RF currents in the system. These result from an antenna feedpoint impedance that has a very low resistive part and is almost entirely reactive. The large RF currents are a genuine physical phenomenon, as also are the high voltages a quarter-wavelength back along the feedline (if the feedline is long enough, of course)... but if there were no losses in the feedline, these would have no further effect. In spite of the wild values of impedance, current, voltage, VSWR etc, if there were no losses in the feedline then all of the RF power would still reach the antenna. In a real feedline, the effect of the high currents is to divert almost all of the available RF power away from the antenna and into the feedline's own resistive losses - skin-effect losses in the copper conductor, and dielectric losses in the plastic. Both of these result in heating and softening of the plastic, which makes the dielectric loss even higher. This tends to divert even more of the available power into the weak spots, where the plastic finally melts. But there is still only 100W available to do the damage. No argument about the final conclusion - it ain't gonna work - but I don't care for the explanation. There's no problem with "VAR power" for anyone who already has a firm grip on the concepts, but it is not a good way to explain those concepts to a newcomer. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#20
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Ladder line Vs. Coax
Ian White GM3SEK wrote:
.... a very good posting with the exception (IMO) of the choice of the word "magnification". There is no magnification of real power. There is indeed no net increase in real average power. However, "magnification" seems to be a poor choice of words since its definition contains such words as "apparent", "seems", "exaggerate", "overstate", ... "magnify - to cause to *seem* greater" certainly describes a feedline with 100W source power and 200W forward power. But there is still only 100W available to do the damage. True, but a 100W laser can do a lot of damage. :-) And a *magnifying* glass can start a fire. -- 73, Cecil http://www.w5dxp.com |
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