Need some help,
I just set up a base Station in my new place. Do to restrictions I am using a dipole. My set up is this : dipole ( Bought off EBAY) strung up horizontal off the back porch. The ends are tied off with string to a metal hook. I have a Astatic 18' coax I bought running from that through the window to the radio. Here is the question, my SWR is 3 across the board. I am using the built in meter on the radio (Cobra 2000 gtl ). Anybody got ideas how to lower the SWR's ?
Thanks ,
LOU
DIPOLE SWR HELP
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'Doc
Lou,
There could be several reasons for a high SWR, can't really say for sure which may be the reason for what you are seeing without more information.
I would assume that your dipole was made for 11 meters, the CB band, right? If so, then depending on exactly what kind of dipole it is, it's length should tell you if it's at least ball-park close to being 'right'. So what's the right length? Divide 468 by 27.185 Mhz and the answer is in feet. It will be slightly longer than necessary, but that's much better than it being too short. Your dipole, if it's just a plain old wire dipole, ought to be within a couple of inches of that figure. That's one thing to check.
Another aspect of all antennas, not just dipoles, is that they 'like' to be as far from stuff as they can get, especially metals stuff. Stringing one up on your porch isn't totally outrageous, it can be 'compensated' for to some extent. It ought'a work. Not the best in the world, but so what.
I'm not familiar with 'Astatic' coax, so, I assume it is 50 ohms stuff, I hope. That 18 feet thingy isn't absolutely necessary, but it also shouldn't hurt anything. Making the coax come away from a dipole at a 90 degree angle for as far as possible before making any bends is a good idea. Keeps things sort of 'equal' to some extent (terrible way to say that, but it gets the idea across). that's not a 'biggy', so don't worry about it a lot either way. Just something to keep in mind.
Of all of that, I think the first thing to check is the length of that dipole. And make sure all the connections are right, no shorts, there should be an 'open' circuit between the shell and center pin of the radio end of the coax. If any measurable resistance, somethings not right, with an ordinary dipole. I honestly don't know where to go from there. See what things look like, measure stuff, then maybe someone would have a good idea of what's wrong.
- 'Doc
There could be several reasons for a high SWR, can't really say for sure which may be the reason for what you are seeing without more information.
I would assume that your dipole was made for 11 meters, the CB band, right? If so, then depending on exactly what kind of dipole it is, it's length should tell you if it's at least ball-park close to being 'right'. So what's the right length? Divide 468 by 27.185 Mhz and the answer is in feet. It will be slightly longer than necessary, but that's much better than it being too short. Your dipole, if it's just a plain old wire dipole, ought to be within a couple of inches of that figure. That's one thing to check.
Another aspect of all antennas, not just dipoles, is that they 'like' to be as far from stuff as they can get, especially metals stuff. Stringing one up on your porch isn't totally outrageous, it can be 'compensated' for to some extent. It ought'a work. Not the best in the world, but so what.
I'm not familiar with 'Astatic' coax, so, I assume it is 50 ohms stuff, I hope. That 18 feet thingy isn't absolutely necessary, but it also shouldn't hurt anything. Making the coax come away from a dipole at a 90 degree angle for as far as possible before making any bends is a good idea. Keeps things sort of 'equal' to some extent (terrible way to say that, but it gets the idea across). that's not a 'biggy', so don't worry about it a lot either way. Just something to keep in mind.
Of all of that, I think the first thing to check is the length of that dipole. And make sure all the connections are right, no shorts, there should be an 'open' circuit between the shell and center pin of the radio end of the coax. If any measurable resistance, somethings not right, with an ordinary dipole. I honestly don't know where to go from there. See what things look like, measure stuff, then maybe someone would have a good idea of what's wrong.
- 'Doc
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Big Lou
Doc , thanks for the reply, Ok , i played around with angles a little and if I use an inverted V, no good, if I drop one side at a inverted angle like this \ and leave the other side horizontal it reads about 2 at ch. 20 , 1.5 at ch 40 and 2.5 at ch 1. As far as you stating it likes to be away from stuff. Well it is hanging from the porch roof which is directly under a rain gutter, and the coax lays over the same gutter on its way through the window. I am kinda at a loss for different mounting options due to my living situation , the only other option is maybe a vertical hang ,but it would be hanging off a brick chimney up close to it to disguise it.
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'Doc
Well, from what you've said, it sort of sounds like the thing may be interacting with that rain gutter, or just might be a little too long. I wouldn't make any bets on that too long thingy, but there's a fairly simple way to find out. Don't cut anything, but just fold the ends back on themselves to shorten it equally on both ends. How much? Beats me. Try 3 or 4 inches on each end, see what happens. If it helps, try an inch or two more, see what happens. You see where that's going so try whatever you think may help, it just might make it usable. Just don't do anything that you can't back out of later, you know?
- 'Doc
- 'Doc
