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".J.S..." wrote in message . .. "John Passaneau" skrev i en meddelelse ... Hi: There is a large amount of interaction between the dipoles as they are tightly coupled to each other. The shorter or higher frequency are affected more by changes to the lower frequency i.e. longer dipoles than the lower frequency dipoles are by changes to the higher frequency or shorter dipoles. Strange.. I can understand that hanging a long wire next to a short will affect more than the other way round. But if they are both up there, I would have thought trimming the long wire would not change its affect on the short wire much. So to keep hair pulling to a minimum start with the lowest frequency dipole, tune that for the lowest SWR or for resonance and then do the next lower one. Ok, thats the order of trimming, but do you install one wire at a time or all of them at once ? /JS Yes put up all the wires at the same time. There will be an effect on the lower frequency antennas by the higher frequency ones and putting up all the wire will get you closer on the first try than putting them up one at time and tuning them will. I've used this type of antenna for years and they work quite well. The interaction between the dipoles can be minimized by putting the dipoles at right angles to each other. That is say, run the 80m dipole north/south and the 40m dipole east/west. That minimizes the coupling and the dipoles act almost like single band antennas. In my system I have a 160/80/40/30 meter dipole antenna. It has the 80/40 wires running parallel, spaced about 12" apart and at right angles to that is a loaded (shortened) dipole for 160m and the loading coils in the 160m antenna act like traps and make a 30m dipole. It's all feed with one coax. The 80m dipole acts just about the same as if it was the only dipole up there, but the 40m dipole is strongly affected by the 80m wire. The most noticeable effect is the SWR bandwidth is a bit smaller than I would expect from a single 40m dipole and the tuning is a bit more sensitive.By that I mean it takes a smaller change in length of the 40m wire to move the resonate point some KHz's than it would with a single band dipole. All that means is you have to be careful with how much you cut off and take it in baby steps, not big chunks. I first up the 80/40 antennas and a year later I decided to get on 160 so I added the 160 wires. When I added the 160/30m wires at right angles to the 80/40m ones there was almost no noticeable effect on the 80/40m dipoles. You can download the EZNEC demo antenna modeling program and see how the antennas interact your self. One thing you will notice is that when your operating on 40m the 80m is also working a little bit too. Anyway I have DXCC on 80 and 40 and working on it on 160 and 30m which are the last bands I need DXCC on. Good luck -- John Passaneau State College Pa. |
#2
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"John Passaneau" skrev i en meddelelse ... So to keep hair pulling to a minimum start with the lowest frequency dipole, tune that for the lowest SWR or for resonance and then do the next lower one. Ok, thats the order of trimming, but do you install one wire at a time or all of them at once ? Yes put up all the wires at the same time. There will be an effect on the lower frequency antennas by the higher frequency ones and putting up all the wire will get you closer on the first try than putting them up one at time and tuning them will. I've used this type of antenna for years and they work quite well. The interaction between the dipoles can be minimized by putting the dipoles at right angles to each other. That is say, run the 80m dipole north/south and the 40m dipole east/west. That minimizes the coupling and the dipoles act almost like single band antennas. In my system I have a 160/80/40/30 meter dipole antenna. Thats exactly what I am trying make on top of a flat-roof building. The building is 56x12 m so I cant quite get anything at a right angle but I will try to get as much angle as possible. Also it is not long enough for the 160m but the idea is to go diagonal and then across at the ends like a 'Z'. It has the 80/40 wires running parallel, spaced about 12" apart and at right angles to that is a loaded (shortened) dipole for 160m and the loading coils in the 160m antenna act like traps and make a 30m dipole. It's all feed with one coax. The 80m dipole acts just about the same as if it was the only dipole up there, but the 40m dipole is strongly affected by the 80m wire. The most noticeable effect is the SWR bandwidth is a bit smaller than I would expect from a single 40m dipole and the tuning is a bit more sensitive.By that I mean it takes a smaller change in length of the 40m wire to move the resonate point some KHz's than it would with a single band dipole. All that means is you have to be careful with how much you cut off and take it in baby steps, not big chunks. I first up the 80/40 antennas and a year later I decided to get on 160 so I added the 160 wires. When I added the 160/30m wires at right angles to the 80/40m ones there was almost no noticeable effect on the 80/40m dipoles. You can download the EZNEC demo antenna modeling program and see how the antennas interact your self. One thing you will notice is that when your operating on 40m the 80m is also working a little bit too. Anyway I have DXCC on 80 and 40 and working on it on 160 and 30m which are the last bands I need DXCC on. Good luck Thx and thx for the info /JS |
#3
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There will be an effect on the
lower frequency antennas by the higher frequency ones and putting up all the wire will get you closer on the first try than putting them up one at time and tuning them will. .................................. Myself, I think starting with the low band, and building up, is the best practical method. You can still do them all at once, but I'd tune each in the ballpark first, starting low,before I moved to the next higher band. Note your later statement... " The 80m dipole acts just about the same as if it was the only dipole up there, but the 40m dipole is strongly affected by the 80m wire." It's almost always the higher band that will have problems, as this example shows. It will be very rare for say a 40 to mess up a 80 ant... But quite common the other way around.. So being thats the case, I always start low, and work up through the bands...You can always fine tune after it's all done...I usually do in fact.... Like you say, if the dipoles are at right angles, there is almost no interaction...In fact, I've had a leg fall down before, and not effect the match of the other bands. I always spread them apart with the widest spacing I can...IE: right angles if two bands... Stacking them like that example on the web page is the worst way to go about it as far as coupling problems. But....Once you get one tuned, they will work...I just prefer spread apart... Also it is not long enough for the 160m but the idea is to go diagonal and then across at the ends like a 'Z'. .................................................. ... That's what I'm using here. I'm on a city lot, so 80m is the largest dipole I can put up in a straight line. I have a 160m "Z" dipole. It's a better compromise compared a loaded dipole to fit 120 ft. I have the 160, 80 and 40 on one feedline. I have extra insulators to turn the 160 into 20m if I want....I do that sometimes in the summer... MK |
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