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G8MNY  > TECHNI   11.05.22 11:40z 75 Lines 3228 Bytes #23 (0) @ WW
BID : 21373_GB7CIP
Subj: 2m Fullwave Dipole
Sent: 220511/0723Z @:GB7CIP.#32.GBR.EURO #:21373 [Caterham Surrey GBR]

By G8MNY                                     (Updated Mar 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have a design for a fullwave dipole. It is only suited for loft environments
as it is made from 2.5mm bare copper wire hanging from the roof apex, & giving
vertical polarisation. 

             LOFT APEX
    Half wave      
    Dipole              TROMBONE
                      MATCHING LINE
                      T        S
            chock      COAX FEED
            block      =============\
                      T \      S     /               \
                             4:1 balun                
    Half wave      

The half waves are both 1M long, & the 2 matching lines are 2cm apart, & are
just over a 1/4 wave long. There is a plastic insulator (Chock block) at the
dipole feed point (6000). Point S is a sliding "Chock block" connector with a 
shorting wire link. Point T is another connector but with the 200 balanced
feed connections.

The 50 unbalanced coax feed is made into a balanced 200, 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.

          T )=========================  RIG Coax
  200       )ͻ  Wave          50
Balanced  T )ͼ length
           All Braids Joined

When the line tapping point (impedance) & shorting point (tuning) are adjusted
to give a balanced 200 the coax will see a good 50 load, e.g. 100 one side
in parallel with a 100 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 feeder must be taken away horizontally to the roof & then down so not
interfere with the very tight horizon radiation pattern.
    ___           ___                                 _---_
  /~   ~\       /~   ~\                             /~     ~\
 |        \../        |                           |         |
          .X.             -> Horizon                 o     
 |        /  \        |     +6dBi                 |   OMNI  |
  \_   _/       \_   _/                             \_     _/
    ~~~           ~~~                                 ~---~
Gain to the horizon is substantially more than slim jim or 5/8 wave aerials.

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.

Why don't U sent an interesting Bul?

73 De John, G8MNY @ GB7CIP

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