[Antennaware] re vertical with metal roof as groundplane.
Guy Olinger, K2AV
olinger at bellsouth.net
Sun Mar 1 14:02:30 PST 2009
----- Original Message -----
From: "George Warnock" <george_warnock at hotmail.com>
>>the radials will be following the pitch of the roof... is this a
The task of the radials in your design is to behave as a non-radiating
non-lossy current sink, a place to store the opposite charge carried on the
feedline shield. The radials are opposing in order to cancel out induction
and radiation on their part to the largest degree reasonable.
Laying a tuned radial (what you are doing) on or close to the roof will
force undesired current IN the roof, less as you separate. While not a
killer, that is not part of the task of the radial. Simple physical reality
forces some degree of closeness. You separate radial and roof until it
becomes difficult or mechanically unsimple. Judgement call by the one on the
A 40/80 vertical is certainly feasible, though the dimensions you give
suggest 40m. I thought you had specified a multiband antenna previously,
which was one of the assumptions in my postings. On 80m, the end of two 66
foot radials can be bent and pulled away parallel to the ground, forming a
half-swastika as seen from over head. I have seen coax center conductor
laid along a wooden fence top rail to complete a radial in this fashion.
Feasibility on 80 is mainly construction feasibility, practical height,
weight, guying, etc, etc. A simple quarter wave wire on the low end of 80
is 66 feet. If you can buy, borrow or build the multiband radiator and get
it survivably in place, the location particulars you have described are not
a detriment, if anything an improvement over what is usually available and
done at the ground on 80m verticals.
If you were talking about only a 40m vertical, then there is no reason not
to use a 4 quarter wave radials over the roof and get the likely 35 ohms up
to 50 using a coax-based series matching transformer which will work very
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