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G8MNY > TECH 25.11.14 11:16l 60 Lines 2984 Bytes #999 (0) @ WW
BID : 16159_GB7CIP
Read: GUEST
Subj: 23cm Flat Plate Aerial
Path: IZ3LSV<IV3SCP<SR1BSZ<GB7CIP
Sent: 141125/0947Z @:GB7CIP.#32.GBR.EURO #:16159 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Published in BATC'S CQTV 11/97, Updated for Packet Jan 03)
This is a simplified design for /P aerials yet offers about a +12dBd gain over
the whole 23/24cm band or it can be centred on 23 or 24cm (*) ends of the band.
A detailed design can be found in UHF Compendium, but I have found simpler
constructions work just as well, & on tests are better than some of the ATV
long yagis on offer!
The high gain is explained if one considers each full wave length loop to have
the gain of 2 dipoles = +3dBd, thus 2 loops = +6dBd so doubling again then 4
loops = +9dBd, & with a perfect reflector you get +12dBd, which is 14.4dBi. In
practice there are losses but well constructed will give +10dBd at least.
FIGURE "8" ELEMENTS
Two elements are construction from mains 2.5mm Cable with the inner insulation
left on. Each compound element is bent into a figure of "8" from 2 wave lengths
2x 24.5cms (* 23/24) long (2x49cms for both & leads), & equates to 4 dipoles
(8 for both elements). The start & stop of the two 8s are at the top & bottom
respectively, & the "8" centre cross over points are left insulated.
........................
: Stalk Mounting : Reflector 8" x 14" nominal made from ally sheet, foil
: v : on wood, or galvanised mesh with 1 cm squares.
: ----- :
: ! ! :
: ! ! : Point x is the wire crossover point, actually not
: ==x== : bent in, just alongside crossover. Wires can be glued
: ! ! : or tied together for neatness.
: ! ! :
: ==B== : The Balun is 6cms of scrap coax alongside the feeder.
: ! ! : -------------
: ! ! : B -@! dead coax ! @=Solder
: ==x== : ! ============@------------------
: ! ! : \[ Feeder
: ! ! : B ---------------------------------
: ----- : The feeder inner is connected to the dead coax braid
: ^ : BB are the 50 ohm balanced feed points that connect
: Stalk Mounting : either side of both of the 8s. The 2 coax balun can
........................ Glued into 2 holes in the reflector.
Stalk mountings can be made from UR67 inner insulation 1" long screwed into
from the reflector side & slotted & heat moulded to capture the wire.
SWR can be adjusted by the distance the 8's are from the reflector. As shown
aerial is horizontally polarised. The Polar diagram is 90deg coverage but very
tightly pointed at the horizon (stacked array gain).
Higher gain can be had by stacking & baying 2, 4 or more aerials, provided they
are all mounted flat to each other & the reflectors (one sheet?) do not
overlap. Identical feeder lengths & matching the aerial array then becomes a
problem.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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