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G8MNY > TECH 15.07.14 21:32l 111 Lines 4833 Bytes #999 (0) @ WW
BID : 2949_GB7CIP
Read: GUEST
Subj: Band 2 6el Broadband Yagi
Path: IZ3LSV<ED1ZAC<GB7CIP
Sent: 140715/0737Z @:GB7CIP.#32.GBR.EU #:2949 [Caterham Surrey GBR] $:2949_GB7C
From: G8MNY@GB7CIP.#32.GBR.EU
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 to be rubbish as it was!
The 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 Band 2 87.5-108MHz, with good Front to Back ratio & have some useful gain.
Using the MMANA-GAL optimisation options over frequencies near the edge of the
band, 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
³mm 732
³ Ú¿mm 2x
³ ³³ 647 2x
³ ³³ ³mm 614 2x
³ ³³ ³ ³mm 583 2x
³ ³³ ³ ³ ³mm 522
³ ³³ ³ ³ ³ ³mm
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
ÃÄÄÄÄÄÄÄÅ0ÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³ 326 ³³186³ 713 ³ 550 ³ 758 ³
³ mm ³³mm ³ mm ³ mm ³ mm ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³
³ ³³ ³ ³
³ ³³ ³
³ ³³
³ ÀÙ
³ 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 Þ Ý
94 7.7 11.1 252 86 polar \ /
96 7.8 11.0 308 82 diagram \_ _/
98 8.0 11.0 355 52 across >ú<
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. 300ê balanced over
most of the band. This is not normal for a folded dipole, as it increases the
impedance by 4 & the reflector & director usually reduces it by 4, resulting in
the usual 75ê. But this design is optimised for bandwidth, so we need to do
extra matching, to get to 75ê. I used a small 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.
IN GRAPHICS
SWR 300ê .==========================.
³ ((Folded Dipole ))
3´\ ³ '========(0) (0)========='
³ \ / Ã())))Ù
2´ \ / TRANSFORMER ³ ³____
³ '-.______ _.-' 4:1 BALUN ÀÄo)coax
1Å ~~~~~~~~~ ~~~~~
87.5 92.75 97.5 102.5 107.5MHz
R ê X ê
400´ __ 200´
³ .-~' ~-. ³
300´ .-~ \ 100´ _...._
³ .-~ \ ³ .ú'~ 'ú.
200´ .-~ '. 0´.' \ /
³.-~ . ³ 'ú._ _.ú'
100´ . -100´ ~
87.5 92.75 97.5 102.5 107.5MHz 87.5 92.75 97.5 102.5 107.5MHz
GAIN dBd F/B dB
10´ / 15´_ _
³ / ³ -. _.-'
9´ .' 12.5´ 'ú. _.-'
³ _.-' ³ '-..__..-'~
8´~-. _.ú'~ 10´
³ '---....-ú' ³
7´ 7.5´
87.5 92.75 97.5 102.5 107.5MHz 87.5 92.75 97.5 102.5 107.5MHz
Z MEASUREMENTS
Using a MFJ analyser, the match to 75ê is pretty good over most of the band as
predicted by the model.
IN USE
It works well, 75ê SWR was quite flat as above. But now regret I did not
designed it to be a spot frequency narrow band 1MHz wide aerial, 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 "Band 2 6el Narrowband Yagi" version!
Why don't U send out an interesting bul?
73 De John, G8MNY @ GB7CIP
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