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#1
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My 132 foot dipole is fed by lengths of 450 Ohm line of lengths depending on
frequency. The first part acts as a matching section to achieve 50 Ohms. This feeds an electrical 1/2 wavelength 450 Ohm section the 50 ohm impeadence is seen by the rig, while maintaning a 9:1 swr on the 1/2 wave section. 450/50=9:1 "Wes Stewart" wrote in message news On Tue, 7 Jun 2005 12:19:23 -0400, "Fred W4JLE" wrote: All my feedlines have a 9:1 SWR by design. Really, and how is that? |
#2
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On Tue, 7 Jun 2005 15:01:23 -0400, "Fred W4JLE"
wrote: My 132 foot dipole is fed by lengths of 450 Ohm line of lengths depending on frequency. The first part acts as a matching section to achieve 50 Ohms. This feeds an electrical 1/2 wavelength 450 Ohm section the 50 ohm impeadence is seen by the rig, while maintaning a 9:1 swr on the 1/2 wave section. 450/50=9:1 Because you see 50 ohm (more or less) at the input of your "450 ohm" (more like 400 ohm) line with an arbitrary impedance at the other end, you incorrectly conclude the 450 ohm, variable length line is operating at 9:1. It appears that you've copied Cecil's design (ingenious BTW) but even he admits to anywhere from 6:1 to 13:1 on his line. For discussion purposes let's say that a 132 foot dipole, 50' above average ground operated on 3.5 MHz has a feedpoint Z of about 61 -j63. That's about SWR = 7.5 using 450 ohm as a normalizing factor. Except for the effects of loss, the SWR *everywhere* on the line is 7.5:1. You don't get to say that 1/2 wavelength of it is operating at 9:1 and some other part is operating at something else. Change the frequency to 7 MHz and the SWR is ~10:1 and so forth. |
#3
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Nonsense Wes, take the case of an antenna that is of sufficient length,
height, or what ever so that at the antenna is exactly R 50 J0. I feed it with an electrical 1/2 wave length of exactly 450 Ohm feedline. The 50 Ohms from my antenna is repeated at the other end of the feedline. An SWR bridge calibrated for 50 Ohms and my rig see 1:1. Would you deny that the SWR on the feedline is 9:1? What is my SWR? Can I say I have 1:1 at the ends and 9:1 anywhere on the feedline? I do get to say it! Now if you want to argue that it will never be exactly 9:1 because the feedline is never exactly 450 Ohms, or the antenna has a reactive portion, you will have to change the subject. No generalization is valid, including mine that I have a 9:1 by design. But I suspect you already new that and decided to increase the quantity of fly crap in the pepper. "Wes Stewart" wrote in message ... On Tue, 7 Jun 2005 15:01:23 -0400, "Fred W4JLE" wrote: You don't get to say that 1/2 wavelength of it is operating at 9:1 and some other part is operating at something else. |
#4
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On Tue, 7 Jun 2005 18:02:43 -0400, "Fred W4JLE"
wrote: You started with this (mis)statement: All my feedlines have a 9:1 SWR by design. I challenged this crap and showed that, using your own example, this was not the case. You called my challenge "nonsense" and then go on to agree with my original point: No generalization is valid, including mine that I have a 9:1 by design. Simply amazing. |
#5
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You win Wes, pick up the marbles.
"Wes Stewart" wrote in message news On Tue, 7 Jun 2005 18:02:43 -0400, "Fred W4JLE" wrote: You started with this (mis)statement: All my feedlines have a 9:1 SWR by design. I challenged this crap and showed that, using your own example, this was not the case. You called my challenge "nonsense" and then go on to agree with my original point: No generalization is valid, including mine that I have a 9:1 by design. Simply amazing. |
#6
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Pardon me while I learn from this discussion.
The 450 ohm antenna is designed to be tuned by using exactly 1/2 electrical wave lengths to match both the transmitter and antenna. This should mean that any differing feedline should work equally as well (except for the losses) if I am correct.... On Tue, 7 Jun 2005 18:02:43 -0400, "Fred W4JLE" wrote: I feed it with an electrical 1/2 wave length of exactly 450 Ohm feedline. The 50 Ohms from my antenna is repeated at the other end of the feedline. An SWR bridge calibrated for 50 Ohms and my rig see 1:1. -- 73 for now Buck N4PGW |
#7
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On Wed, 08 Jun 2005 12:52:01 -0400, Buck wrote:
Pardon me while I learn from this discussion. Hold on Buck. So far there has been very little "learning" to be done from this discussion. I dislike getting personal but I strongly suggest that you reject anything Fred has put forth. Some of what he says is correct, but for the most part you are being led astray. Since you don't yet have the skills to separate the good from the BS, the safest thing to do is ignore it all. For example he has suggested that an all-band antenna can be nothing more than a 130' dipole fed through 100' of 450 ohm line to which you connect a 50 ohm coax and "your rig will be happy." Let's examine this premise, shall we. If you don't already have it, download the free version of EZNEC. www.eznec.com If you have MS Excel do the following. If you don't skip down below the dotted line. Download the program XLZIZL.xls at: http://www.qsl.net/ac6la/xlzizl.html Using EZNEC set up a frequency sweep, for example 3.5 to 4.0 MHz in 50 KHz steps and check Microsmith Files as an output and give a file name ("80meter" for example) "Build" the 130' long antenna in the wires menu, add a source in the middle and do the frequency sweep. (I used a height of 50' and 12 AWG wire) Open xlzizl.xls and select the "ZIZL" worksheet if it isn't already active. "Press" the "Clear all freq..." button (near cell A25). Press the "Read file for Freq, R, X" button (near cell D25). Navigate to the file location where you specified the Microsmith files be located. The default is in the Smith subdirectory in the EZW directory. Select the file "80meter.gam" and open it. The calculated data will populate the Frequency, R at load and X at load cells. Press the "Refresh and show Smith" button (near cell I28). The Smith chart will open and display the calculated impedance data for the 130' antenna operated from 3.5 to 4.0 MHz. Now we are going to add a transmission line. Select the "ZIZL" worksheet again and press the "Set via Dialog" button (near cell A34). The "Network definition" window will open with "Element position 1" selected by default. Change it to "5". Under "Element type" select Transmission line. Under "Qualifier" scroll the dropdown menu to either "Generic 450 ohm window" or if you more more realistic numbers you can select one of the Wireman Ladder line types. (Ignore "wet" ones.) Mouse down to the "Element Value" area an type in "100" for the length and press the "Set This Element" button and then close the Network definition window. Press the Refresh and show Smith Chart button and you will now see two traces, the original "load" data and the feedpoint data at the input to the 100' transmission line. If you know anything about Smith charts you will immediately notice that the match is worse at the input of the line than it was at the antenna. So much for the "magic" 100' length that Fred proposes. Going back to the ZIZL sheet and looking in the "results" area you can see that at 3.5 MHz the SWR is 36:1. Without the "magic" 100' of ladderline, the SWR is 3:1. You can repeat this exercise at different frequency ranges and see just how awful this idea is. Let's continue... Assume that your 50-ohm coax is 50' long. Go back to the Network "Set via Dialog" button. Accept the "1" default for Element Position and again select Transmission Line for Element Type. For Qualifier, let's use Belden 9258 (RG8X), although you can use what you use. Set the length to 50' and press "Set this Element." Close the window. Press the Refresh and show Smith button and you will now see three traces on the chart: the load, the impedance at the inut of the 450 ohm line and the impedance at the input of the 50 ohm line. Return to the ZIZL sheet and under results note that the SWR at 3.5 MHz is down to "only" 17:1. This is what your rig is going to see. Also note that the network loss (the line loss) is over 3 dB. Now just for giggles, let's take out the 100' of ladderline and make the whole 150' run out of RG8X. You can just highlight the cells "G34 through G36" and delete them and then select cell C36 and type in 150 and tab out. Press "F9" and see the new results. The 3.5 MHz SWR is now 2.4:1 and the total network (line) loss is 1.2 dB. So replacing the "low loss" ladderline with "lossy" coax improved the match and lowered the loss. I don't know how much of this "magic" I can stand. ************************************************** ***************** If you don't have Excel there is a more labor intensive method that is just as accurate. Go to http://www.qsl.net/ac6la/tldetails.html and download the program and open it. Select a transmission line type, "Generic 450 ohm Window" for example. The line parameters will populate the boxes to the right. Under Set Frequency, type in 3.5. Run EZNEC one frequency at a time, beginning with 3.5 MHz. Look at the source data and note the Impedance R and X values. Pay attention to the sign of X. Copy these values to "R" and "X" in the TLdetails program. Remember the sign of X. For example I used R = 61, X = -64. In the "results" area under "At Input" you can see the R and X values at the input and note that the SWR in the 450 ohm line is about 7:1 and in the 50 ohm feeder Fred would have you connecting at this point the SWR is as before ~36:1. If you want to "add" the 50 ohm line, copy down the R and X at the input (132, -470) and enter them in the the R and X boxes above. Change the line type to Belden 9258 and the length to 50 feet and as above, the SWR at the input is ~17:1. To summarize: There are ample free tools to work these problems out without relying on bafflegab. You don't have to take my word or anyone else's; work the problem yourself and learn something while doing it. |
#8
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On Wed, 08 Jun 2005 12:23:50 -0700, Wes Stewart
wrote: [snip} Under "Qualifier" scroll the dropdown menu to either "Generic 450 ohm window" or if you more more realistic numbers you can select one of the Wireman Ladder line types. (Ignore "wet" ones.) This should of course read, "...if you want more realistic..." |
#9
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On Wed, 08 Jun 2005 14:16:06 -0700, Wes Stewart
wrote: On Wed, 08 Jun 2005 12:23:50 -0700, Wes Stewart wrote: [snip} Under "Qualifier" scroll the dropdown menu to either "Generic 450 ohm window" or if you more more realistic numbers you can select one of the Wireman Ladder line types. (Ignore "wet" ones.) This should of course read, "...if you want more realistic..." I am not sure what I just did or saw. I had several colored graphs charted and one gray. The gray didn't seem to move around, but the colored ones sure did. I am afraid I didn't see what you were trying to show me. I believe the colors were the different feed lines, capacitors, etc used in the feedline shown. I tried removing them and only leaving the feedline I chose to use. I don't know if I did something wrong or not, but the gray line appeared to be the antenna and it never appeared to move. The feedline markers moved radically. I haven't thought a lot about the losses in ladder-type feedline, but I do understand how the losses in coax can reduce High SWR by reducing both the outgoing and incoming reflected signals in the coax. This may make the antenna desirable to the rig, but it doesn't do any good for the operator who is trying to reach or copy that weak signal. It may well be that the Twin-lead has the same effect. I often read that one of the advantages of the twin-lead is that it can handle higher SWR without the higher losses of coax. I seem to have been seeing a lot about the technology of the twin-lead tuning technology, but then I can't be sure it wasn't from the same people here on this forum. I'll continue to look into it to see what I might learn. I was able to get past whatever block I had on learning to use EZNEC, so at least some good came from this discussion. Hopefully, I'll learn more and can model this antenna and others I read about. Thanks very much for taking the time to address this with me. 73, Buck N4PGW -- 73 for now Buck N4PGW |
#10
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Did you miss the tuner part? The "magic antenna" is the same all band sold
by a number of folks, i.e. Van Gordon. .. "Wes Stewart" wrote in message ... On Wed, 08 Jun 2005 12:52:01 -0400, Buck wrote: Pardon me while I learn from this discussion. Hold on Buck. So far there has been very little "learning" to be done from this discussion. I dislike getting personal but I strongly suggest that you reject anything Fred has put forth. Some of what he says is correct, but for the most part you are being led astray. Since you don't yet have the skills to separate the good from the BS, the safest thing to do is ignore it all. |
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