On Apr 20, 8:43*pm, Art Unwin wrote:
On Apr 20, 7:28*pm, Jim Lux wrote:



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

What's the relative size of "reflector" and helix? *(i.e. is the
reflector in the near field of the helix, in which case, you could
easily have waves propagating along the surface of the reflector)

The helix is four foot long and a foot diameter. The base *of the
reflector is 1.5 feet
with a 45 degree angle. I have had the helix 0.5 feet shorter and 0.5
feet longer with similar results.On re examination of the antenn I now
see that the ground lead of the radiator is connected to the inside of
the reflector at a half way point and the coax
ground is connected at the base of the reflector. I think I will
change that ground connection to a common point.
Regards
Art

Changing the ground point did not clear up the reception from the
rear!
Have made a smaller antenna ( not for top band) and mounted on a
framework on the ground. Same thing happening but band does seem
squirrily!
Have put a tilt mechanism on it and I am working on putting a rotator
on it so that I can get a better feel on things. I was going to do
this anyway as I want to see what this arrangement has on TOA. If
radiation is a matter of charged particles then penetration of dish
would change the direction of gain.....food for thought
The group can now go back to the subject of change
Regards
Art