I guess I havent been getting thru to a couple on here possibly since I
wasnt explaining it well enough.
Lets start by saying that I never said maximum FEED current and the antennas
point of maximum radiation current coincided at all times. They are not the
same.
Even a half wave radiator has a current component at the base in the real
world since it is being fed between the radiator and a real ground via the
matching device. This is where some confuse radiation resistance with feed
point impedance. These dont show when working strictly from some textbooks.
At that prior QTH, about 30 years ago, with the ground screen I had a pair
of 1/4 wave 80M cage verticals that were also fed as 1/2 waves on 40M so Im
rather cognizant of how to feed them. Tests showed that they performed much
better when tied into the 80M radials of 60 at 70' long than to just a pair
of 8' ground rods. I found that much earlier the 1/2 wave 2M antenna on my
66 Corvette worked a heck of a lot better when I ran "radials" to the
furthest points of both frame rails and a bit later some Scotch copper tape
attached to the underside of the rear storage compartment which helped even
more. So much for the "no ground needed" claims of the manufacturer", the
design engineer must have spent too much time with calculus and zero on an
antenna range.
By 1985 I was ready for something better and put up a 4el 40M KLM at 140'.
Carl
KM1H
----- Original Message -----
From: "Charles Moizeau" <w2sh@msn.com>
To: "Topband" <topband@contesting.com>; <zr@jeremy.mv.com>
Sent: Monday, September 12, 2011 6:56 AM
Subject: Re: Topband: Fence "ground screen" instead of wire radials?
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> In my most recent posting on this subject I said:
> "...I'm puzzled by how the feed current at the base of any antenna,
> regardless of its length, will always be at a maximum at its base. A
> half-wavelength antenna presenting a few thousand ohms of input impedance
> at its base would, I think, have difficulty accepting "maximum FEED
> current" there..." Perhaps it was because the hour was late, but I
> completely disregarded the universal use of matching devices at a
> feedpoint in order to overcome feed difficulties. Apologies for the
> omission. In my case, with an inverted L antenna having a total length of
> 170 feet, just a series capacitor at the base feedpoint with a value of
> about 255 pF gives me an SWR of 1.15 to 1. However, that figure is
> measured at the shack, about 100 feet away. Also, I transitioned the
> subject matter too swiftly from feedpoint impedance to radiation
> resistance. The two are different and should not be confused with each
> other. 73, Charles, W2SH
>
>
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