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G8MNY > TECH 12.02.13 09:16l 111 Lines 4892 Bytes #999 (0) @ WW
BID : 11837_GB7CIP
Read: GUEST
Subj: Band 2 6el Broadband Yagi
Path: IZ3LSV<I0OJJ<GB7CIP
Sent: 130211/1335Z @:GB7CIP.#32.GBR.EU #:11837 [Caterham Surrey GBR] $:11837_GB
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 do 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 J
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 as a folded dipole increases the impedance
by 4 & the reflector & director usually reduces it by 4, resulting the usual
75ê. But this design is optimised for bandwidth, so we to do extra matching, to
get 75ê. I used a small centre tapped VHF 2 hole ferrite bread as the balun,
with 2x 2 turns on thr 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 an MFJ analyser, the match to 75ê is pretty good over most of the band as
predicted by the model. I hope when I put it up on a rotator it works as
predicted too.
IN USE
This 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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