Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
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. |
#2
|
|||
|
|||
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. |
#3
|
|||
|
|||
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. |
#4
|
|||
|
|||
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. |
#5
|
|||
|
|||
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 |
#6
|
|||
|
|||
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. |
#7
|
|||
|
|||
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..." |
#8
|
|||
|
|||
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 |
#9
|
|||
|
|||
Buck wrote:
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. The "twin-lead tuning technology" is based on the laws of physics, not the approval of the gurus on this newsgroup. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#10
|
|||
|
|||
On Tue, 14 Jun 2005 13:13:01 -0400, Buck wrote:
[snip] 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 guess you're looking at the Smith chart in XLZIZL. What you are seeing is correct. The gray trace is the load impedance and absent changes in the antenna it will be fixed. If you look right to the legend you will see what each of the colors represents. So if you've modeled something in EZNEC and read the resulting data file into ZLZIZL.xls and did a "Refresh and show Smith chart" you will see the data plotted as the "Load" Let's do a little (bit long) experiment that will clarify this (I hope) and also demonstrate some of what happens in a matching network. 1. Open EZNEC and open the file "BYdipole.ez". 2. Go to "Setups" (at the top of the window) and select "Frequency Sweep." 3. When the pane opens, select "On" and type in 14.35 for the "Stop Frequency" and .05 for the "Frequency Step." 4. Under output. check the box "Microsmith Files" and for file name type "g5rv" and hit Ok. 5. Go to the "Wires" menu and change the Y value for End 1 to -51. Change the Y value for End 2 to 51. 6. Do a "Save As" and type "g5rv". 7. Perform a Freq Swp. (Bottom left button). 8. Open XLZIZL, clear any old data or networks and read the g5rv.gam file. 9. Press "Refresh and show Smith chart." 10. The gray trace is the plotted impedance of the feedpoint of the 102' dipole over the 20-meter band. 11. Now at this point Varney (G5RV) recommended a "matching section" of 34' of open wire line, followed by a run of 72 ohm line to the shack. (If you want to see how bad this idea was, you can experiment with adding the lines in the Network dialog) 12. We are going to do it the "modern" way, by using ladderline and a tuner. 13. In the Network definition dialog, select element position 5 and set the type to "Transmission Line", select "Wireman 554" and set the length to 50'. Set the element and close the pane. 14. Press the "Refresh and show Smith chart." The red trace shows the impedance at the input end of the 50' transmission line. Since the desired target is the center of the chart, we can seen that this was a negative "improvement." Note: I didn't contrive this feeder length, it was just a round number that seemed like an average situational value that follows the "conventional wisdom" that says, "Use a length long enough to reach from the antenna to the rig." 15. This is what we're stuck with if we decide to use ladderline and a tuner, so let's muddle on. 16. In the ZIZL worksheet at row 38 there are some boxes for Component Q. Enter 500 for Qu(C), 250 for Qu(L) and 14 for Q ref freq. 17. Now let's explore a slick feature of this program and design a matching network. Highlight the cells F106 and F107. 18. Look at cell I43 and press "Copy Selected R,X to T-Pi-L". 19. Focus will transfer to the T-Pi-L worksheet and a query window may open asking whether you want to copy the Q values from the ZIZL sheet. Answer Yes. You will no doubt get a second advisory pane that says that frequencies differ. Answer "Ok". Cell H10 will be selected. Type 14.2 into it and press Enter or Tab to input the data. 20. We are now presented with four built-in matching network solutions. The most used topography for commercial tuners these days is the "High Pass T." (Option 1) So we will use it. You can press "Schematic" and see the configuration. Press the "Copy to FT/Solver" button. (Cell B24) 21. The last action took the precalculated values and moved them to the Fine Tune / Solver Area. For now, we won't explore the attributes of this feature. Press the "Copy to ZIZL 1-3" button. (Cell K26) 22. Focus will return to the ZIZL sheet and a query pane will open. Answer "No." If you look at the network definition table area you can note that there are now three components in addition to the transmission line in evidence. 23. Press "Refresh and show Smith chart." 24. Focus returns to the Smith chart where a wealth of information is presented. In the upper right corner there is a box that shows exactly what the feed system consists of. The "Plot Segments" box shows the legend for each trace and what the effect of each network component is. The informational box to the left will show the parameters of any data point on any trace by just double clicking the point. Of course, the trace for Plot Segment 1, the input series C, is what we are after. All but the 14.0 MHz point fall inside a 2:1 SWR circle. Let's see if we can fix that. 25. Press the "Tune / Set" button (lower right corner). Long years of experience tell me to tune the input capacitor first, so that's what we'll do, but this may not always be the case. In the pane that opens, select, "1:Series C", which if all of the calculations are the same, should equal 51.2 pF. Mouse down a bit and find the +-1% option and select it. We are going to "tune" the input capacitor in 1% steps. Click the "Increase" up-arrow three times. Voila! The SWR is less than 2:1 over the whole 20-meter band with one set of adjustments. With each click you could see the action of "tuning" this component. Note that none of the other traces moved at all. All we changed was the match between the input and the junction of the input C and the shunt L. The match from the junction to the antenna was unaffected. Because this is getting a bit long-winded and off-topic, I'm going to stop here and pick this up in a new thread. 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. Correct. It may well be that the Twin-lead has the same effect. It does. 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. Some of the people here know what they are talking about... others I'm not so sure. I'm unlocking the tool box, so you can figure it out yourself. 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. I'm sure you will. Thanks very much for taking the time to address this with me. No problem. The guys that really deserve the thanks are the geniuses 1: who developed the NEC engine, 2: Roy, W7EL, (among others) who put a pretty face on it, and 3: Dan Maguire, AC6LA, who did the Excel stuff. And the absolutely amazing thing is, they give away their work! Is this a great country or what? Regards, Wes N7WS |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
160 Meter Band Balanced Coaxial Receiving Loop Antrenna by KN4LF | Shortwave | |||
The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF} | Antenna | |||
The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF} | Shortwave | |||
80 meter multi turn loop antenna | Antenna | |||
Should I run a Sky-wire loop? | Dx |