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G8MNY  > TECH     12.05.22 09:25z 112 Lines 4871 Bytes #27 (0) @ WW
BID : 21457_GB7CIP
Subj: Band2 6el Broadband Yagi
Path: SP7YDD<SR1BSZ<IW0QNL<ON0AR<GB7CIP
Sent: 220512/0924Z @:GB7CIP.#32.GBR.EURO #:21457 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                (Corrected Feb 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
On looking at an old large well made home-made 6el broadcast Yagi beam (scrap)
as a starting point, I decided to re-model it using the MMANA-GAL program
(CD in 2010 RSGB yearbook & online). It showed it up to be rubbish as it was!

SUSY RADIO community radio station I put on, is too far away to hear at my home
so I needed a very good aerial. I wanted it to work well over the whole 20MHz
of UK's Band2 87.5-108MHz, with good Front to Back ratio & have some useful
gain.

Using the MMANA-GAL optimisation band options over frequencies near the edge of
Band2, I ended up with this, quite acceptable compromise in 20mm ally tube for
the elements, & boom was 40mm dia & 2.54m long!

 2x
849     4x
 mm    732
        ڿmm  2x
           647            2x
           mm            614        2x
                          mm       583             2x
                                    mm            522
                                                   mm
                                                   
                                                   
                                                   
 0Ĵ
   326  186      713          550          758     
    mm  mm        mm           mm           mm     
                                                   
                                                   
                                    
                          
           
        
        
     98mm dia
     folded ends.

Freq  Gain  F/B   R   X
 88   7.9  14.5   97  -4                    _-~~~-_
 90   7.7  12.8  146  41                   /       \
 92   7.6  11.7  198  71          Typical          
 94   7.7  11.1  252  86           polar   \       /
 96   7.8  11.0  308  82          diagram   \_   _/
 98   8.0  11.0  355  52           across     ><
100   8.3  11.3  375  -2           whole     (   )
102   8.6  11.7  346 -64            band.     -.-
104   9.0  12.4  263 -89
106   9.6  13.6  165 -49
108  10.1  15.6   87  47

AERIAL Z
The impedance of this folded dipole feed point is approx. 300 balanced over
most of the band. This is not normal for a folded dipole in this configuration,
as it increases the impedance by 4 & the reflector & director usually reduces
it by 4, resulting in the usual 75. But this design is optimised for bandwidth
so we need to do extra matching, to get to 75. I used a tiny centre tapped VHF
2 hole ferrite bead as the balun, with 2x 2 turns on the middle core if it, all
in the connection box.

SWR 300                                    .===========================.
                                          (( Folded            Dipole  ))
  3\                                      '=========(0)   (0)========='
    \                        /                        ())))
  2  \                      /             TRANSFORMER   ____
      '-.______         _.-'                4:1 BALUN o)coax
  1            ~~~~~~~~~                                 ~~~~~
   87.5  92.5  97.5  102.5 107.5MHz

R                                     X 
400                __                 200
               .-~'  ~-.                 
300         .-~         \             100      _...._
         .-~             \                 .'~      '.
200   .-~                 '.            0.'             \         /
   .-~                      .                            '._ _.'
100                          .       -100                    ~
   87.5  92.5  97.5  102.5 107.5MHz       87.5  92.5  97.5  102.5 107.5MHz

GAIN dBd                               F/B dB
 10                          /         15_                        _
                            /             -.                  _.-'
  9                       .'         12.5   '.           _.-'
                      _.-'                     '-..__..-'~
  8~-.           _.'~                 10
      '---....-'                        
  7                                   7.5
   87.5  92.5  97.5  102.5 107.5MHz       87.5  92.5  97.5  102.5 107.5MHz

Z MEASUREMENTS
Using a MFJ analyser, the match to 75 is pretty good over most of the band as
predicted by the model.

IN USE
It works well, 75 SWR was quite flat as above. But now regret I did not
designed it to be a spot frequency narrow band 1MHz wide aerial for SUSY RADIO
Rx, as that would give 2-3dB more gain & > 22dB F/B ratio. The back of the beam
QRM is the main problem I have!


See My Tech Bul "Band2 6el Narrowband Yagi" version!


Why don't U send out an interesting bul?

73 De John, G8MNY @ GB7CIP


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