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#1
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Vertical problem
Hi. Have a new Hustler 6 band vertical. Installed the radials and also buried 75 foot of RG213 coax between shack and antenna. I used a 6 foot peace of coax connected to adjust the antenna with a MFJ-259. Every band was tuned to the middle and SWR was lower than 1.7:1 at the edges. After the adjustments, I reconnected the buried coax to the antenna and on all bands everything had shift up by any where from 200 to 400 khz. Now the rig is seeing greater than 2:1 SWR.
Should I retune the antenna with the MFJ 259 in the shack? Thanks Vern M0WQR |
#2
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Vertical problem
826, My first thought is that the feed line has become 'part' of the antenna and has changed the antenna's characteristics accordingly. My second though, if that 6 foot of feed line was connected directly to the antenna when measurements were taken, is that you being that close to the antenna changed it's characteristics. So, what to do? How about doing that checking from the end of the feed line that will normally be used? Make any antenna adjustments accordingly. Don't want that feed line to be 'part' of the antenna? Well, how about using a feed line to do the checking that's of an appropriate length to sort of 'cancel' it's self out, to do the checking? Doing all that checking will also give you some idea as to the accuracy of your measuring device, and the characteristic values of the feed lines, etc, sort of. That's the 'long' version. The 'short' version is that you're probably doing something wrong, re-do it. - 'Doc Not much of an answer, huh? |
#3
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Vertical problem
wrote in message ... 826, My first thought is that the feed line has become 'part' of the antenna and has changed the antenna's characteristics accordingly. My second though, if that 6 foot of feed line was connected directly to the antenna when measurements were taken, is that you being that close to the antenna changed it's characteristics. So, what to do? How about doing that checking from the end of the feed line that will normally be used? Make any antenna adjustments accordingly. Don't want that feed line to be 'part' of the antenna? Well, how about using a feed line to do the checking that's of an appropriate length to sort of 'cancel' it's self out, to do the checking? Doing all that checking will also give you some idea as to the accuracy of your measuring device, and the characteristic values of the feed lines, etc, sort of. That's the 'long' version. The 'short' version is that you're probably doing something wrong, re-do it. - 'Doc Not much of an answer, huh? Surprise! It is turning out that coax isn't what you thought it was, is it? Lots of folks think that coax is some miracle device that absolutely contains all of the RF until the RF reaches (poetic license please) the termination point. In truth, no such creature exists in any form of transmission line. What you are trying to do is to make the transmission line the worst possible radiator and the antenna the best possible radiator. There are gazillions of possibilities somewhere betwen those two potentials. Run the full length of transmission line into your shack and then tune the antenna based upon the data gathered in the shack. Yes, it is a PITA, but it is the only way to do it, if you want the best tuning. Every antenna system is just that, a system. It cannot be tuned piecemeal to obtain the best end result. Welcome to the world of radio. Oh, it isn't so bad once you get used to it. Now you have an excuse to invite a ham over to help you by reading the analyzer while you are outside making adjustments to the antenna. Wait until you discover, if you haven't already, that adjusting one part of the antenna will cause you to readjust another part of the antenna, probably the one that you adjusted before the last part that you adjusted. Hang in there. It is doable. Good luck with your project. Jack |
#4
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Vertical problem
On Fri, 25 Jul 2008 18:44:46 +0100, "826" wrote:
Hi. Have a new Hustler 6 band vertical. Installed the radials and also buried 75 foot of RG213 coax between shack and antenna. I used a 6 foot peace of coax connected to adjust the antenna with a MFJ-259. Every band was tuned to the middle and SWR was lower than 1.7:1 at the edges. After the adjustments, I reconnected the buried coax to the antenna and on all bands everything had shift up by any where from 200 to 400 khz. Now the rig is seeing greater than 2:1 SWR. Should I retune the antenna with the MFJ 259 in the shack? Thanks Vern M0WQR I have a 5BTV with buried radials. Works and tunes like crap without the radials. Tune the antenna with the 259 connected to it with the shortest piece of coax you can work with, like 1 foot. If you're a purist - which I advise - lie down so you're as much out of the antenna field as possible when taking readings. Or back way off and read with binoculars. (Just don't let your body affect the antenna tuning.) When that's done, coil the main coax or use a common mode choke where the coax connects to the antenna to prevent the coax shield from acting like a radial. Forget that nonsense of adjusting SWR by adjusting coax length unless you're prepared to use a different length for every band. The MFJ 259 reads SWR with respect to it's own 50-ohm nominal impedance so if you can adjust to a very low SWR (and you can with a 5BTV or 6BTV), it should read the same at the radio end of a longer 50-ohm coax. |
#5
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Vertical problem
Hi, Thanks for all your input. Hustler recommended using 5-6 foot of coax in
there manual. This I did follow and the antenna tuned very nice. When I connected the 75 ft buried coax it threw me because the resonance frequency on each band went higher. I do not have a balun at the base of the antenna. I thought that after connecting the buried coax the frequency would go down because of the capacitance of the longer coax. After rethinking this I can see that adding a balun may help. Thanks for your help and will let you know how things turn out. Vern M0WQR "Jim Higgins" wrote in message ... On Fri, 25 Jul 2008 18:44:46 +0100, "826" wrote: Hi. Have a new Hustler 6 band vertical. Installed the radials and also buried 75 foot of RG213 coax between shack and antenna. I used a 6 foot peace of coax connected to adjust the antenna with a MFJ-259. Every band was tuned to the middle and SWR was lower than 1.7:1 at the edges. After the adjustments, I reconnected the buried coax to the antenna and on all bands everything had shift up by any where from 200 to 400 khz. Now the rig is seeing greater than 2:1 SWR. Should I retune the antenna with the MFJ 259 in the shack? Thanks Vern M0WQR I have a 5BTV with buried radials. Works and tunes like crap without the radials. Tune the antenna with the 259 connected to it with the shortest piece of coax you can work with, like 1 foot. If you're a purist - which I advise - lie down so you're as much out of the antenna field as possible when taking readings. Or back way off and read with binoculars. (Just don't let your body affect the antenna tuning.) When that's done, coil the main coax or use a common mode choke where the coax connects to the antenna to prevent the coax shield from acting like a radial. Forget that nonsense of adjusting SWR by adjusting coax length unless you're prepared to use a different length for every band. The MFJ 259 reads SWR with respect to it's own 50-ohm nominal impedance so if you can adjust to a very low SWR (and you can with a 5BTV or 6BTV), it should read the same at the radio end of a longer 50-ohm coax. |
#6
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Vertical problem
Funny, I've seen no one recommend addition of a 1:1 choke balun near the antenna feedpoint. If RF is flowing back down the shield of his feedline, that surely could cause some incorrect readings on SWR in the shack, wouldn't it? Ed K7AAT |
#7
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Vertical problem
On 27 Jul 2008 23:26:04 GMT, Ed wrote:
Funny, I've seen no one recommend addition of a 1:1 choke balun near the antenna feedpoint. If RF is flowing back down the shield of his feedline, that surely could cause some incorrect readings on SWR in the shack, wouldn't it? And, how well would rf flow back down the shield of "buried 75 foot of RG213..." ? :-) Jonesy -- Marvin L Jones | jonz | W3DHJ | linux 38.24N 104.55W | @ config.com | Jonesy | OS/2 *** Killfiling google posts: http://jonz.net/ng.htm |
#8
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Vertical problem
Funny, I've seen no one recommend addition of a 1:1 choke balun near the antenna feedpoint. If RF is flowing back down the shield of his feedline, that surely could cause some incorrect readings on SWR in the shack, wouldn't it? And, how well would rf flow back down the shield of "buried 75 foot of RG213..." ? :-) About as well as it flows up the center conductor, possibly. You don't think that just because the cable is buried that RF flow on the shield conductor is prevented, do you? Ed |
#9
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Vertical problem
Ed wrote:
... About as well as it flows up the center conductor, possibly. You don't think that just because the cable is buried that RF flow on the shield conductor is prevented, do you? Ed I sure can imagine one heck of a "capacitive load" on that outer conductor to ground! What, thousands/tens-of-thousands of pf? Regards, JS |
#10
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Vertical problem
On 28 Jul 2008 04:03:29 GMT, Ed
wrote: About as well as it flows up the center conductor, possibly. You don't think that just because the cable is buried that RF flow on the shield conductor is prevented, do you? Hi Ed, I think Jonesy is fairly safe in his presumption. The classic work performed by Brown, Lewis, and Epstein pretty conclusively reported the nearly complete attenuation of direct driven RF currents in ground radials at 3MHz. 75 feet of buried shield would seem to be quite snubbed by the proximity of earth over much of the range of frequencies reliably operated from a multi-band HF vertical. Of course, adding a choke can't hurt. 73's Richard Clark, KB7QHC |
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