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Old April 10th 09, 08:14 PM posted to rec.radio.amateur.antenna
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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
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Old April 10th 09, 08:30 PM posted to rec.radio.amateur.antenna
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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

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Old April 11th 09, 12:38 PM posted to rec.radio.amateur.antenna
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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!



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Old April 19th 09, 12:06 PM posted to rec.radio.amateur.antenna
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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
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Old April 19th 09, 02:09 PM posted to rec.radio.amateur.antenna
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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
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Old April 19th 09, 02:49 PM posted to rec.radio.amateur.antenna
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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
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Old April 19th 09, 04:09 PM posted to rec.radio.amateur.antenna
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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
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Old April 11th 09, 09:58 AM posted to rec.radio.amateur.antenna
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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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