At 04:38 PM 2007-04-23, Fred Mott AB8AH wrote:
>I am planning to install a large loop antenna for 160/80/40 meters on my
>155? Rohn 45G tower. I have been reading all the articles and information
>that I have seen. I plan on using ½ inch vinyl coated hardline rather than
>wire. I have read all I could find on electrostatically shielded loops. I
>seem to think that the hardline would work with its outer aluminum conductor
>being the shield and the inter-conductor copper wire as the loop. The
>shield would be open at the top of the tower. The loop would be close to a
>diamond shape in a vertical plane perpendicular to the earth. I would
>assume that the velocity factor of the hardline .81 would change the formula
>from 1005/f to 814/f. The hardline from the loop will intersect the tower
>at approximately 10 to 15 feet and would go straight down through a PVC tube
>into the basement of my Orchard Storage Building. At that point the coax
>shields would be bonded together to a bus bar that connects to the tower?s
>grounding system and the inter conductors would connect to a Ten Tec 253
>antenna tuner.
I've never seen a large (~1 wl) "shielded" loop
in use, although I'm sure it's been done. The
shield of the hardline in this case IS the
antenna and NOT an electrostatic shield (despite
widespread folklore -- please see W8JI's
excellent information as mentioned by AB7E). As
such, the formula for loop resonance (at one
wavelength circumference) will be close to 1005/f
except for some minor effect of the insulation on
the OUTSIDE of the hardline. (Note that you will
thus have to distort the diamond quite a bit to
fit it on your tower if you maintain a 1 wavelength circumference on 160m.)
The pattern of a one-wavelength bottom-fed or
top-fed loop has far-field nulls at very low
angles in the plane of the loop. (As opposed to
the nulls perpendicular to the loop plane with
small loops.) The balance of the loop isn't
nearly as critical with large loops. It is
likely that ground variations and the presence of
surrounding objects would unbalance it no matter
how impeccable the symmetry of the hardline shield.
On 160m this antenna will be essentially a
cloud-warmer and best for short to medium length
paths. And as KC1DI pointed out, the patterns on
80m and 40m will be "interesting" and should be
modelled to determine if the patterns meet your needs.
Unless you get free CATV hardline, just use #12
copper wire. It's costly enough as it is!
73, Terry N6RY
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