I agree with Tom completely... When you have elevated (or insulated)
radials you should never tie the coax to a "DC" ground at the
feedpoint. I discovered this by accident doing mobile experiments over
salt water years ago. The degree of lost efficiency will depend on
several things. (conductivity of the ground itself, efficiency of the
radial system, the number and height of the elevated radials, etc.)
Conceptually you can look at it this way. The elevated radial system
will be hot with RF to some degree. You want them to couple
electrically only to the driven element. If you ground the coax shield
at the antenna the radials will be also be coupling through space to
ground in search of a return path through that ground at the base of
the tower. This will reduce the current flow in the driven element as
return currents will now be divided between the driven element and the
radial-ground path. The portion of energy traveling between the ground
and radials will be wasted since it won't contribute to useful
radiation. If you have other reasons for wanting a DC ground I might
suggest doing that through an RF choke. Generally, buried radials
should be bare wire (copper hopefully) and above ground radials should
be well insulated if they're laying on top of the ground. Elevated
radials can be bare as long as they're not touching ground. I hope I
explained that properly. I operated extensively with a floating radial
system with very good results.
73,
Dennis
N0SP
>
>>In transmitting systems elevated radials should never have a ground path.
>>Grounding elevated radials reduces efficiency.
>
>>If the house has a lot of noise and you receive on the transmitting
>> antenna
>>you might not want to directly ground the antenna ground system to the
>>house. This is especially true with a small radial system.
>
>>73 Tom
>
> My elevated radials float, and are connected directly to the shield of the
> feed line. (7/8 inch Heliax.)
>
> When the other end of the Heliax is connected to the output of my amp, it
> is
> "grounded" to the "house" central grounding point because the chassis of
> the
> amp is strapped to it. Are you saying that the amp and transceiver should
> be
> isolated from "house" ground also? If so, just how much efficiency is
> gained?
>
> It has always been common practice to DC ground the radials of a ground
> plane antenna via the tower and its ground rod system, and by strapping
> the
> shield of the coax to the tower ground system at the base of the tower.
> Are
> you saying this method has less efficiency? If so how much less?
>
> (((73)))
> Phil Clements, K5PC
>
>
>
>
>
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