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#21
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"Reg Edwards" wrote in message ... "Cecil Moore" says - There's no such thing as a 100% efficient tuner. ==================================== Cecil, not a very surprising statement. There's nothing which is 100% efficient. What is the typical efficiency of a typical tuner with a typical antenna. Does it differ significantly from 100% ? Yes or no? ---- Reg. I did some measurments on a MFJ tuner into a dummy load. While I am not sure if the losses would be more or less than to an acutal antenna, I would assume it should be the best case . The losses were around 5 to 10 watts starting with 100 watts input depending on the band. I think 80 meters was the worse from 80 to 10 meters. This is a small tumer rated around 200 or 300 watts and has a built in wattmeter. This may account for a small aditional loss. I did not use this meter but did use 2 Bird meters , one on the input and one on the output to keep the input power constant at 100 watts. |
#22
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Can you give us a magnitude, for example for a g5rv on 3.8 MHz fed with 50
feet of 50 ohm coax. Is it .5 or 5 dB? Or we can be more specific and say we want to match a 50 Ohm source to R 5000 -J720 using an MFJ989C. What loss is to be expected? Granted there are cases of attempting to load a short (with respect to frequency) antenna that has extremely low values of R can cause large losses. I would think that a general statement could be made about antennas that are 1/2 wavelength or longer at the operating frequency and the magnitude of losses one could expect from the typical tuner. Is there no way to quantify the losses, no rule of thumb, should we all throw our tuners away because they may have unpredictable horrendous losses? What steps should be taken by the average amateur to obviate these losses. I am at a loss to understand how bad the losses are that you and Cecil refer to. Can you lead me from the darkness Sir!. "Roy Lewallen" wrote in message ... Fred W4JLE wrote: Can you give us a typical loss for a tuner matching a G5RV? No, sorry, I can't, and I don't believe there is such a thing. It's possible of course to find the input impedance of any wire antenna like the G5RV at each of the frequencies of interest. But transmission line lengths and impedances vary, which transforms the impedance seen by the tuner. Those impedance changes can be dramatic, and can result in widely varying tuner efficiency. On top of that, you have the variations in tuner topology and construction which makes a "typical" tuner also an elusive beast. I'd be very suspicious of any "typical" tuner loss figure, and wouldn't expect to see it in practice unless the conditions are spelled out really well and my setup was very similar. You see, I'm not a true Guru. A real one wouldn't waffle like this, but would give you a positive answer, and you'd very probably never have reason to doubt it. Roy Lewallen, W7EL |
#23
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"Fred W4JLE" wrote in message ... Can you give us a magnitude, for example for a g5rv on 3.8 MHz fed with 50 feet of 50 ohm coax. Is it .5 or 5 dB? .................................................. ....... ................................. QST has on several occasions tested antenna tuners. The most recent one I found was from last year. The worst case I saw was something like 40% loss when matching an impedance of 6.25 Ohms on, I think, 40 meters. It is worth noting that the 2KW tuners will have less loss than the 300W jobs. Also, the loss depends on the impedance it sees, rather than the SWR. So, for an 8:1 SWR, the loss at 6.25 Ohms will be a lot worse than at 400 Ohms. Among other things, the loss will then depend on the length of the feedline. The high power tuners had a loss of less than 10% over most of the range. Tam/WB2TT |
#24
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Fred W4JLE wrote:
Can you give us a magnitude, for example for a g5rv on 3.8 MHz fed with 50 feet of 50 ohm coax. Is it .5 or 5 dB? Sorry again, I can't help you much. There are a number of tuner topologies in use (L, pi, high and lowpass tee, LCC, "ultimate transmatch" to name a few), and the loss of one compared to another can be quite different depending on the load impedance, even if all use the same inductor(s). The information you gave is enough to calculate the impedance seen by the tuner, though, assuming your G5RV is 102 feet long and not one of the countless variations. Assuming a height of 40 feet over average ground, the impedance at 3.8 MHz seen at the input end of 50 feet of 50 ohm coax should be about 2.5 + j23 ohms (SWR ~ 27:1) if the coax is RG-8, or about 4.1 + j22 ohms (SWR ~ 17:1) if the coax is RG-58. (The SWR at the load will be greater than 60:1. You should maybe be worrying more about feedline loss than tuner loss.) I didn't take into account the effect of feedline radiation, which I'll assume is suppressed with a balun. Or we can be more specific and say we want to match a 50 Ohm source to R 5000 -J720 using an MFJ989C. What loss is to be expected? Using the Z you specified or the Z I calculated, all that's necessary then is to obtain a schematic of the tuner and information about the physical construction of the inductor(s), calculate what settings will achieve a match, and find out which of these settings you're using. Then calculate the coil currents and from that the loss. I don't have a schematic of the tuner nor a physical tuner -- I haven't used one for 30 years or more except for a small homebrew tuner for correcting small mismatches to keep my QRP Field Day rig happy. So I don't have a clue. Perhaps someone else who has the tuner, the math ability, the patience, and the time will be willing to help you. Granted there are cases of attempting to load a short (with respect to frequency) antenna that has extremely low values of R can cause large losses. I would think that a general statement could be made about antennas that are 1/2 wavelength or longer at the operating frequency and the magnitude of losses one could expect from the typical tuner. I think you can say that if the SWR is no greater than 2:1 or maybe 3:1, the loss in a properly designed tuner should be pretty low. Outside that range, like your 17 - 27 SWR example, I wouldn't speculate. Is there no way to quantify the losses, no rule of thumb, should we all throw our tuners away because they may have unpredictable horrendous losses? Hm, are those the only two options? Why not use the tuner, make contacts, and be happy? And if the tuner has a horrendous loss, why will throwing it away help? You'll still be better off with it than without it. What steps should be taken by the average amateur to obviate these losses. Adjust the feedline length for each band to present a moderate impedance to the tuner. Cecil has posted quite a bit about this in the past. And use a tuner with the largest coil(s) possible. You could set up jumpers so you can change the topology of the tuner to pi, L, tee, etc. On each band, try each configuration and, among those which effect a match, choose the one that gives you the greatest field strength from your antenna or the highest current to it. Of course, you could do what I do -- I settle for other than all-band operation, and make my antennas resonant or nearly resonant. When matching is required, I do it at the feedpoint with components I've characterized -- I know the antenna Z and the inductor Qs, so I can calculate the loss. But most people don't want to do this, and happily put up with whatever losses they get in exchange. Or you can get something like the B&W terminated folded dipole, and put the losses at the antenna instead of the tuner. I am at a loss to understand how bad the losses are that you and Cecil refer to. Can you lead me from the darkness Sir!. Nope, sorry, I can't. What's the approximate value of resistors? I can't answer that one, either. Roy Lewallen, W7EL |
#25
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For efficiency of the L-network tuner on different bands, and losses
in transmission lines and other data, download program DIPOLE3 from website below. The program covers dipole + balanced line + balun + coax line + tuner L and C settings. With one hit of a key the antenna system can be modelled as a G5RV. Download program DIPOLE3 in a few seconds. It is also easy to find the resonant frequencies of the G5RV. ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
#26
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On Mon, 6 Jun 2005 15:06:13 -0400, "Fred W4JLE"
wrote: Let's put this an easier way, how many of you have a low pass filter in line. Most manufacturers claim a .25dB or less insertion loss. These filters consist of a boat load of coils and caps. A tuner normally has 2 varible caps and a single silver plated coil. While any device indeed has loss, here we are picking fly crap out of pepper. Clearly you didn't read the links or you wouldn't be asking all of the questions. |
#27
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Is there no way to quantify the losses, no rule of thumb, should we
all throw our tuners away because they may have unpredictable horrendous losses? In general I do....I don't use a tuner for any of my everyday antennas... What steps should be taken by the average amateur to obviate these losses. Quite simple really. Use the very minimum inductance needed to get a usable match. I once did tests to determine the appx amount of tuner loss when tuning properly vs improperly. When using an excess of inductance to match, I saw tuner losses approach 20%. IE: 20w out of 100w. When carefully tuning, using the very least inductance, this loss could be reduced to a value nearly unreadable. BUT!!! This was on a resonant 50 ohm system. You could expect the losses of an extreme mismatch to be higher, even if you tuned very carefully. The losses and coil warming when feeding a 1/2 size dipole with a T net tuner are well known. Thats how I got a free MFJ 989c when a friend of mine couldn't keep from frying the plastic coil forms. He bought a nye viking, and gave me the 989c. Which I've used since with no problems at all....It wasn't really the tuners fault...He was abusing it trying to feed a 1/2 size dipole, and then running a TL-922 flat out on top of that... MK |
#28
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#29
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My 2 cents on the subject of tuner loss.
A tuner will Always introduce more loss to the system. If the total power output of the transmitter is not improved(ie, at 2:1 SWR), then using the tuner to reduce SWR to 1:1 is counter productive, isn't it? In other words, getting the SWR down to 1:1 from anything = 2:1, will always cause a reduction in total power out. Pat W0OPW |
#30
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