Home |
Search |
Today's Posts |
#11
|
|||
|
|||
Vertical problem
Richard Clark wrote:
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 What Richard neglected to specify is that the work of B, L, & E shows that there won't be significant current on the *outside* of the shield because of the burial. The current on the *inside* won't be affected. What's unknown is whether there was significant current on the outside during the measurement with the shorter cable. If so, the cable and probably the person doing the measuring became part of the antenna and would therefore affect its impedance. Roy Lewallen, W7EL |
#12
|
|||
|
|||
Vertical problem
On Sun, 27 Jul 2008 21:49:05 +0100, "826" wrote:
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. I more than just suspect the length recommendation has a lot to do with the effect of your body closer to the antenna and nothing to do with the varying length of the coax as a matching network. 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 You're welcome. Yes, I would be interested in hearing how it turns out for you. Works well for me. BTW, my coax lays on top of the ground. Shouldn't matter with the choke at the feed point. 73, Jim KB3PU "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. |
#13
|
|||
|
|||
Vertical problem
...
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 I was not aware of the depth, length, and other specifics of the buried coax installation as I jumped into the thread a bit late. Sorry. Here where I'm located we have nothing but sand, which doesn't really provide much of a ground. I forget other people have real dirt! :^) Ed |
#14
|
|||
|
|||
Vertical problem
Hi,
Here is some more info. The depth of the coax is 6". Its length is 75 foot of RG-213. The ground here is below sea level (3 ft). It was reclaimed from salt water by wind driven pumps in the 14th and 15th century. The conductivity of the ground is great for antennas. But not sure about buried feed lines. I tested the antenna with the MFP-259 and 6 ft of cable on the ground. Yes the first thing I checked was if the reading changed if I backed off from the instrument and antenna. I have tried buried coax one other time with a 2m vertical and thought the coax was bad because it was used before. It also gave bad reading but can remember the details. Do remember running another peace of coax back to the shack overhead and everything was OK. Now I wondering if it is something to do with the installation. The coax is new and inside of a garden hose for protection. I did check the hose to make sure it was dry before it was used. I know that the antenna is adjusted correctly and have taken an FT-817 and SWR meter to the antenna and it also indicated good readings. Now something tells me not to tune the antenna with the instruments locate in the shack because it will not be curing the problem. It will just hid it until I start running some power. Then I would think something would start to get hot in the field. Like the traps. Because the antenna no longer wants my power because its no longer tuned to the ham bands. Regards Vern "Ed" wrote in message 92.196... ... 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 I was not aware of the depth, length, and other specifics of the buried coax installation as I jumped into the thread a bit late. Sorry. Here where I'm located we have nothing but sand, which doesn't really provide much of a ground. I forget other people have real dirt! :^) Ed |
#15
|
|||
|
|||
Vertical problem
826 wrote:
Hi, Here is some more info. The depth of the coax is 6". Its length is 75 foot of RG-213. The ground here is below sea level (3 ft). It was reclaimed from salt water by wind driven pumps in the 14th and 15th century. The conductivity of the ground is great for antennas. But not sure about buried feed lines. ... I, like Richard C., would like to encourage you to the use of a 1:1 current-choke/balun, either of a toroid core of proper material--or even a ferrite rod, beads, etc. -- and installed at the antenna feed point or both ends of the coax (xmitter also) ... I would have to install software and check out a couple of things. But, I suspect, and especially at ~28+ mhz, that capacitive load on the outer conductor can't help look like anything other than/near a direct short. Even the rf down the inside of the braid/shield must be tempted to that path if it nears or is less than ~50 ohms (well, some possible/noticeable effect/affect.) The coax I have buried, I always encased in ~1 inch PVC--possibly an overkill ... but hey, it was cheap when I bought it--I ran the UNUN (unbalanced-to-unbalanced current balun), just don't remember if I had to, or not ... Just my two cents worth--feel free to disregard if someone ventures better/more-accurate data or proofs ... but then, you already knew that. ;-) Regards, JS |
#16
|
|||
|
|||
Vertical problem
One other thing comes to mind: When you measured the antenna through the
short piece of coax, was the shield of the long buried piece connected to the antenna's ground system? If not, you might try that. It would act as an additional radial, which would have some affect on the antenna's impedance. Roy Lewallen, W7EL |
#17
|
|||
|
|||
Vertical problem
On Mon, 28 Jul 2008 19:55:16 +0100, "826" wrote:
Now something tells me not to tune the antenna with the instruments locate in the shack because it will not be curing the problem. It will just hid it until I start running some power. Then I would think something would start to get hot in the field. Like the traps. Hi Vern, You DO want to tune the antenna with instruments located in the shack. You are tuning an "antenna system" and the complete system should appear to be resonate to both the instruments AND the transmitter. More power (or less power) should have nothing to do with the state of tune - unless you have an intermittent. If you have an intermittant, this is a failure, not a mis-adjustment. From other indications you have shared, your problem is a comparatively subtle error, not a major failure. 73's Richard Clark, KB7QHC |
#18
|
|||
|
|||
Vertical problem
"826" wrote in
: 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? Vern, An oft asked question, and there are usually emphatic answers readily proffered. You might find my article at http://www.vk1od.net/VSWR/displacement.htm relevant to understanding what goes on 'within' the transmission line, and the 'Traps for players' heading offers reasons why behaviour that is inconsistent with transmission line behaviour may be observed. In your case, one of the things you appear to have done is the change the thing that was measured. You adjusted the antenna with the shield of the main feedline disconnected. This is discussed under the heading 'Disturbing the thing being measured'. Owen |
#19
|
|||
|
|||
Vertical problem
Owen Duffy wrote:
"826" wrote in : 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? Vern, An oft asked question, and there are usually emphatic answers readily proffered. You might find my article at http://www.vk1od.net/VSWR/displacement.htm relevant to understanding what goes on 'within' the transmission line, and the 'Traps for players' heading offers reasons why behaviour that is inconsistent with transmission line behaviour may be observed. In your case, one of the things you appear to have done is the change the thing that was measured. You adjusted the antenna with the shield of the main feedline disconnected. This is discussed under the heading 'Disturbing the thing being measured'. Owen But, if the choke/balun/unun suggested is implemented, shouldn't one expect the transmission lines influence to measurements be expected to drop to near zero effect/affect?--i.e., re-routing/lengthening/etc. that xmission line (coax), opposed to measurements taken? Or, in effect, your measurement(s) with a 1 ft. length of xmission line will remain very stable if that length were lengthened to one-hundred feet?; Given that the choke/balun is of proper construction to offer HIGH impedance to the freq(s) in question on the outer braid/shield? I certain have grown to expect this ... have I just been lucky? Regards, JS |
#20
|
|||
|
|||
Vertical problem
"Owen Duffy" wrote in message ... "826" wrote in : 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? Vern, An oft asked question, and there are usually emphatic answers readily proffered. You might find my article at http://www.vk1od.net/VSWR/displacement.htm relevant to understanding what goes on 'within' the transmission line, and the 'Traps for players' heading offers reasons why behaviour that is inconsistent with transmission line behaviour may be observed. In your case, one of the things you appear to have done is the change the thing that was measured. You adjusted the antenna with the shield of the main feedline disconnected. This is discussed under the heading 'Disturbing the thing being measured'. Owen Hi, Sorry I didn't rely sooner. The weather hasn't been too good hear. This morning I disconnected the coax from the shack and measured the antenna with the six foot length of coax. It looked OK. I then connected just the shield of the long coax going to the shack and still measuring with the short coax it shifted just like measured before in the shack. I have then made up a choke balun and installed it at the base of the antenna and it improved everything by about 60%. I will now make another one for the shack end. I think everything is under control now. Thanks to all for heading me in the right direction. Vern M0WQR |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Yagi in Proximity of Vertical/SNR problem..answers??? | Antenna | |||
Vertical antenna problem | Antenna | |||
Odd problem with 6/2/70 triband vertical | Antenna | |||
Dual Band vertical (2m/70 cm) problem / details | Antenna | |||
Hustler 6btv Vertical Tech Problem | Antenna |