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G8MNY > TECH 07.09.03 12:39l 56 Lines 2317 Bytes #999 (0) @ WW
BID : 14410_GB7CIP
Read: DF5NK DB0FHN GUEST OE7FMI
Subj: 2m Fullwave Dipole
Path: DB0FHN<DB0RGB<DB0MRW<OK0PPL<DB0RES<ON0AR<7M3TJZ<IK1ZNW<GB7CRV<GB7CIP
Sent: 030907/0937Z @:GB7CIP.#32.GBR.EU #:14410 [Caterham] $:14410_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
I have a design for a fullwave dipole (+6dBi). It is only suited for loft
environments as it is made from 2.5 bare copper wire hanging from the roof
apex, & giving vertical polarization. The feeder must be taken away
horizontally to the roof & then down so not interfere with the radiation
pattern.
LOFT APEX
!
!
Half wave !
Dipole !
! LINES
!
! T S
-------------- COAX FEED
-------------- =============\
! T \ S / \
! \______/ !
! !
!
!
Half wave !
Dipole !
!
The half waves are both 1M long, & the 2 matching lines are 1" apart, & are
just over a 1/4 wave long. There is a plastic insulator at the dipole feed
point (6000 Ohms). Point S is a sliding "Chock block" connector with a shorting
wire link. Point T is another connector but with the 200ohm balanced feed
connections.
The 50ohm unbalanced coax feed is made into a balanced 200ohm, with the
addition of half wave of coax (x velocity factor 0.8 or 0.66). All the coax
braids are joined to each other but nothing else. The feeder inner is connected
to the start of the half wave 4:1 balun, & the load (chock block line taps)
connected across either end of the half wave phase reversing balun coax.
When the line tapping point (impedance) & shorting point (tuning) are adjusted
to give a balanced 200ohms the coax will see a good 50ohm load, eg 100ohms one
side in parallel with a 100ohms seen down the half wave of balun coax.
The advantage of this design is it can always give 1:1 SWR even if the lengths
are not quite right, or the aerial is being de-tuned by the roof, & being a
symmetrical centre feed system all the power should go out to the horizon.
The design can easily be made into an outdoor version with solid tube for the
elements & TV aerial booms for the matching lines with the shorted end clamped
to a pole. Other band versions are also quite easy to construct.
/QSL
73 De John, G8MNY @ GB7CIP
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