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G8MNY > TECH 30.09.24 07:52l 112 Lines 4881 Bytes #999 (0) @ WW
BID : 17020_GB7CIP
Read: GUEST
Subj: Band2 6el Broadband Yagi
Path: IZ3LSV<DB0ERF<DK0WUE<DK0WUE<VK5RSV<PI8ZTM<I0OJJ<GB7CIP
Sent: 240930/0648Z @:GB7CIP.#32.GBR.EURO #:17020 [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
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 in this configuration,
as it increases the impedance by 4 & the reflector & director usually reduces
it by 4, resulting in the usual 75ohm. But this design is optimised for
bandwidth so we need to do extra matching, to get to 75ohm. 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 300ohm .===========================.
³ (( Folded Dipole ))
3´\ ³ '=========(0) (0)========='
³ \ / Ã())))Ù
2´ \ / TRANSFORMER ³ ³____
³ '-.______ _.-' 4:1 BALUN ÀÄo)coax
1Å ~~~~~~~~~ ~~~~~
87.5 92.5 97.5 102.5 107.5MHz
Rohms Xohms
400´ __ 200´
³ .-~' ~-. ³
300´ .-~ \ 100´ _...._
³ .-~ \ ³ .ú'~ 'ú.
200´ .-~ '. 0´.' \ /
³.-~ . ³ 'ú._ _.ú'
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´_ _
³ / ³ -. _.-'
9´ .' 12.5´ 'ú. _.-'
³ _.-' ³ '-..__..-'~
8´~-. _.ú'~ 10´
³ '---....-ú' ³
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 75ohm is pretty good over most of the band
as predicted by the model.
IN USE
It works well, 75ohm 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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