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#1
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On Apr 10, 1:07*pm, "Rollie" wrote:
Maybe the coax ground is insufficent and a seperate ground from mother earth is required.....Maybe "Art Unwin" wrote in message ... I made a helical end fed antenna that is inside a cone shaped reflector The reflector is made from 1/2" mesh steel with an aluminum foil liner and connected to the braid of the feed coax. No baluns are used, just direct connections. I was surprised to hear signals from the rear! I thought that a dish reflector prevented such signals getting to the receiver. So what can be wrong with the reflector or can signals get reflected back from the frontal area? Antenna is at a 40 foot height Any ideas as to what the fault could be? Regards Art I have no experience with dishes thus the question Note, the helical antenna does not protrude beyond the dish envelope. Art Rollie I checked. The coax ground and the reflector is grounded at the same place at the top of the tower. All horizontal coax is buried. Seems like the dish reflector does not get a lot of attention from ham operators ! Art |
#2
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Art Unwin wrote:
On Apr 10, 1:07Â*pm, "Rollie" wrote: Maybe the coax ground is insufficent and a seperate ground from mother earth is required.....Maybe "Art Unwin" wrote in message ... I made a helical end fed antenna that is inside a cone shaped reflector The reflector is made from 1/2" mesh steel with an aluminum foil liner and connected to the braid of the feed coax. No baluns are used, just direct connections. I was surprised to hear signals from the rear! I thought that a dish reflector prevented such signals getting to the receiver. So what can be wrong with the reflector or can signals get reflected back from the frontal area? Antenna is at a 40 foot height Any ideas as to what the fault could be? Regards Art I have no experience with dishes thus the question Note, the helical antenna does not protrude beyond the dish envelope. Art Rollie I checked. The coax ground and the reflector is grounded at the same place at the top of the tower. All horizontal coax is buried. Seems like the dish reflector does not get a lot of attention from ham operators ! Art Hmmm, I guess this guy just talks to himself. http://www.signalone.com/kb2ah/KB2AHantennas.html -- Jim Pennino Remove .spam.sux to reply. |
#3
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#5
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![]() "Art Unwin" wrote in message ... Seems like the dish reflector does not get a lot of attention from ham operators ! certainly not by 160m ham operators! welcome back art, needed something to brighten up a dreary saturday morning! |
#6
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On Apr 10, 2:14*pm, Art Unwin wrote:
The coax ground and the reflector is grounded at the same place at the top of the tower. All horizontal coax is buried. ___________ An r-f ground does not exist at the top of your tower, or any tower. Unless some means is provided to prevent r-f current flow on the outside of the coax and on the tower structure, they will radiate/receive r-f energy. This probably accounts for most of the pattern effects that you didn't expect to have (regardless of the real pattern that your cone and helix generates). RF |
#7
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Richard Fry wrote:
Unless some means is provided to prevent r-f current flow on the outside of the coax and on the tower structure, they will radiate/receive r-f energy. Thus making the vertical antenna longer than 5/8WL. Using the top of the tower for a ground simply makes the tower part of the antenna system turning the entire array into an off-center-fed vertical dipole with the bottom end grounded. For instance, a 1/4WL 20m monopole mounted on top of a 60 foot tower using the tower as the coax shield ground has a take-off-angle of 57 degrees. The highest RF current is near the middle of the tower. :-( To make matters even worse: I had a similar problem with drooping 1/4WL radials DC insulated from the tower. The drooping radials coupled RF into the tower and turned it into a radiator which screwed, oops, I mean skewed the radiation pattern upwards. It took me a long time to figure out why my horizontal dipole was magnitudes better than my 1/4WL vertical on top of the 1.25WL tall tower which was grounded at the bottom and floating at the top. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
#8
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On Apr 19, 8:09*am, Cecil Moore wrote:
Richard Fry wrote: Unless some means is provided to prevent r-f current flow on the outside of the coax and on the tower structure, they will radiate/receive r-f energy. Thus making the vertical antenna longer than 5/8WL. Using the top of the tower for a ground simply makes the tower part of the antenna system turning the entire array into an off-center-fed vertical dipole with the bottom end grounded. For instance, a 1/4WL 20m monopole mounted on top of a 60 foot tower using the tower as the coax shield ground has a take-off-angle of 57 degrees. The highest RF current is near the middle of the tower. :-( To make matters even worse: I had a similar problem with drooping 1/4WL radials DC insulated from the tower. The drooping radials coupled RF into the tower and turned it into a radiator which screwed, oops, I mean skewed the radiation pattern upwards. It took me a long time to figure out why my horizontal dipole was magnitudes better than my 1/4WL vertical on top of the 1.25WL tall tower which was grounded at the bottom and floating at the top. -- 73, Cecil, IEEE, OOTC, *http://www.w5dxp.com Thank you all for those points raised. I added the ground to the dish because I was getting a lot of static one night, I have not had any since but I need time to compare. The grounding line is a heavy silver coated braid connected to each section and to ground. My coax drops to ground and then goes underground for a 100 feet or so and grounded again when it resurfaces. Regards Art |
#9
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On Apr 19, 8:09*am, Cecil Moore wrote:
To make matters even worse: I had a similar problem with drooping 1/4WL radials DC insulated from the tower. The drooping radials coupled RF into the tower and turned it into a radiator which screwed, oops, I mean skewed the radiation pattern upwards. ________________ Some designs use drooping radials to reduce the vertical angle of the peak radiation launched by the monopole section. But that is a conclusion made for an infinite distance, with consideration of the propagation environment on the intrinsic pattern launched by the monopole, and the height of the monopole + its elevated radials above the earth. The link below leads to paste-up of NEC screens showing the performance of a monopole driven against four 1/4-wave, essentially horizontal radials. The entire system is isolated from earth ground. The driving impedance, the elevation pattern shape, and the peak gain are close to "textbook" values for a 1/4-wave monopole driven against a perfect ground plane. A form of this design is being used with good success in the AM broadcast industry -- where using a conventional, buried-radial ground system is impractical due to rocky terrain. The groundwave performance of these systems shows that their intrinsic gain is maximum in the horizontal plane, and very close to the theoretical value of 5.15 dBi. http://i62.photobucket.com/albums/h8...dRadials_1.jpg RF |
#10
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On Thu, 9 Apr 2009 20:05:20 -0700 (PDT), Art Unwin
wrote: I made a helical end fed antenna that is inside a cone shaped reflector The reflector is made from 1/2" mesh steel with an aluminum foil liner and connected to the braid of the feed coax. No baluns are used, just direct connections. I was surprised to hear signals from the rear! I thought that a dish reflector prevented such signals getting to the receiver. So what can be wrong with the reflector or can signals get reflected back from the frontal area? Antenna is at a 40 foot height Any ideas as to what the fault could be? Regards Art I have no experience with dishes thus the question Note, the helical antenna does not protrude beyond the dish envelope. Art How do you know? The "rear" signals may come from the front side actually, having been reflected by your neighbours house, or distant mountains, or anything in between. w. |
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