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#11
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EZNEC says that a 66 ft dipole used on 3.8 MHz, fed with 450 ohm
ladder-line, will have an SWR of greater than 100:1. This can lead to all sorts of undesirable effects including an almost impossible to match impedance at the tuner. A practical rule of thumb might be in order, e.g. mine = no more than 20:1 SWR on the ladder-line. Fact is Cecil I never pay any attention to VSWR, just complex numbers. Anyway your comments made me re-analyze the problem, and I realize I made an error in the transmission line and antenna tuner analysis. I thought the transmission line loss I came up with was a bit low. The final numbers are shown below. See if you agree with me; then explain where I went wrong, and why the match will not work. 66 ft dipole, 30 ft high, #14 AWG: Input Z = 11.3 - j961 ohms. 50 ft of 600 ohm line: Input Z = 5.48 + j189.85 ohms, loss = 1.95 dB Matching network: Shunt C = 296 pF, Series L = 22.8 uH. (Obviously half the L for each side of a balanced.line.) Max line voltage: 1.5 kW in = 3 kV. Tuner loss: 0.44 dB. Incidentally have you ever looked at a typical airport NDB site? For example a 45 ft monopole on 350 kHz has an input impedance of 0.2 - j7054, which is a VSWR of 4.9 million : 1 -- whatever that means -- virtually touching the edge of the Smith Chart. I have even seen 5 mile approach NDBs with 30 ft monopoles. Marine installations for 400 to 500 kHz operation frequently had electrically very small inverted "L" antennas. Of course 5 S/m sure helped, but the losses in the tuners must have been significant. 73, Frank |
#12
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My second choice would be to go with the 66 ft. total length with high-Q loading coils in the center of each 33 ft. leg. -- 73, Cecil http://www.qsl.net/w5dxp Did that for a number of years on 80 CW ........worked fine ..... 73 and God Bless .....KI3R Tom Popovic Belle Vernon PA. |
#13
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"chuck" wrote in message ink.net... I guess my question wasn't clear, Max. Even though your space is limited, the type of antenna you install should be informed if possible by the type of operating you intend to do. If DX is very important to you (and here, DX could mean one or two thousand miles) you might want to work in some vertically polarized radiation, such as with an inverted L or a T (vertical with capacitive element on top) or a sloper. How tall are those maple trees? It may even be possible to load the R7 on 80 with a switched loading coil at the base. Wouldn't win any efficiency awards I'm sure. But thinking about it is pretty cheap and even fun. Chuck The maple trees are pretty tall, getting up in there high enough would be a challenge. I may try 66' per side and see how high in the tree I have to go to get the angle out to the corners of the lot. Would a direct feed with 50 ohm coax be suitable with a choke balun? Mainly interested in local comms out to 3 or 400 miles. Max |
#14
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On Mon, 15 Aug 2005 16:48:54 -0400, "MD" wrote:
"chuck" wrote in message news Can you tell us what you want to do with it? I have an R7 up for 40-10. Two maple trees on the property, one in the front yard and one in the back but not far enough apart for an 80m dipole. The trees runs east west over the house. I would prefer to run the dipole north south, using the backyard maple for a support. The lot at this point is 66' wide. One other thing to mention I don't think I can run ladder line or open wire feeder into the house. I am pretty much limited to coax. Max, is that to mean you want to feed the 66' dipole with 50 ohm coax (all the way from the rig to the centre of the dipole)? Assuming the feedpoint impedance that others have modelled is about right, and that was 11-j961 at 3.8MHz... the losses in 20m (60') of RG213 would be around 18dB. That is, less than 2% of the power into the cable reaches the feedpoint. That isn't the end of the story, there will be tuner losses... but the good news is that partly as a result of the very high coax losses, the impedance presented to the tuner will be well within the range of loads that can be transformed at fairly low loss. So... less than 2% of the transmitter power reaching the feedpoint... is that acceptable? BTW, if you considered a shorter line, say half the length at 10m (or 30'), the losses would be around 13dB (note considerably more than half of the losses for 20m of line - the loss/unit-length is not constant along the line). It turns out that in practice, it is very hard to get most of the transmitter power to the feedpoint of a dipole when the dipole length gets much below 35% of the wavelength. Using lossy feedline (like coax) exacerbates the problem. Owen -- |
#15
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You can try bending the ends to fit in the space you have...something
like "[ " ought to work (top view--fed at the center, dipole). Or, if you want to make loading coils, you can find the values you need in the ARRL Antenna Book in the chapter on limited space antennas. Scott N0EDV MD wrote: I haven't got the room for a good 80m antenna. My vertical covers 40 so no trouble there. I am looking for ideas on a short 80m that will give half decent results. I have about 66' available straight or can make some kind of inverted vee ??? |
#16
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Lots of good information here guys, I appreciate all the help. Currently I
am using a long wire elevated at the ghastly height of 15' using the eaves of the house as a counterpoise. I can work stations out to ~ 400-500 kms, maybe that is the best I can manage. Once again, thanks for all the help. de VE3TMT |
#17
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"MD" wrote in message
... Lots of good information here guys, I appreciate all the help. Currently I am using a long wire elevated at the ghastly height of 15' using the eaves of the house as a counterpoise. I can work stations out to ~ 400-500 kms, maybe that is the best I can manage. Once again, thanks for all the help. de VE3TMT I failed to notice you did not want to use open wire line. If you use inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5 dB in the loading coils. The input impedance is 28 ohms, and the losses on 60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is unimportant since at 30 ft high the radiation is essentially omni-directional. 73, Frank |
#18
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"Frank" wrote in message news:X2lMe.197169$tt5.130324@edtnps90... I failed to notice you did not want to use open wire line. If you use inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5 dB in the loading coils. The input impedance is 28 ohms, and the losses on 60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is unimportant since at 30 ft high the radiation is essentially omni-directional. 73, Frank Hi Frank, Unfortunately using open-wire is not an option, as I have to feed the feedline in through the exterior brick wall of the house. I have 2 feedlines running out now. One to the R7 and the other for experimenting. I am going to try your suggestions with the loading coils, as soon as I get back from vacation!! |
#19
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MD wrote:
"Frank" wrote in message news:X2lMe.197169$tt5.130324@edtnps90... I failed to notice you did not want to use open wire line. If you use inductive loading, for example: nominal 46 uH, Q = 200, at 15 ft from each end of a 66 ft dipole. The radiation efficiency is 71%, i.e you loose 1.5 dB in the loading coils. The input impedance is 28 ohms, and the losses on 60 ft of RG213 will only be 0.24 dB. Orientation of the dipole is unimportant since at 30 ft high the radiation is essentially omni-directional. 73, Frank Hi Frank, Unfortunately using open-wire is not an option, as I have to feed the feedline in through the exterior brick wall of the house. I have 2 feedlines running out now. One to the R7 and the other for experimenting. I am going to try your suggestions with the loading coils, as soon as I get back from vacation!! Can you get something through a window? If the window can be opened, you can place plexiglas or wood in the window, and then cut the appropriate openings for ladder line. - Mike KB3EIA - |
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