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G8MNY > TECH 17.05.12 00:17l 78 Lines 3104 Bytes #999 (0) @ WW
BID : 40574_GB7CIP
Read: GUEST
Subj: Band 2 6el Broadband Yagi
Path: IZ3LSV<I0OJJ<GB7CIP
Sent: 120516/2204Z @:GB7CIP.#32.GBR.EU #:40574 [Caterham Surrey GBR] $:40574_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Sep 11)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
On looking at an large old 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!
Unlike simple narrow band aerial designs, I wanted this design to be broadband
enough for 87.5-108MHz, with good front to back ratio & have some useful gain.
Using the optimisation 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 & 2.54m long!
2x
849 4x
³cm 732
³ Ú¿cm 2x
³ ³³ 647 2x
³ ³³ ³cm 614 2x
³ ³³ ³ ³cm 583 2x
³ ³³ ³ ³ ³cm 522
³ ³³ ³ ³ ³ ³cm
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 cm ³³cm ³ cm ³ cm ³ cm 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
This table shows the impedance of this folded dipole feed point is approx. 300
Ohms 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 75ohm. I used a small centre tapped VHF 2 hole
ferrite bread as the balun, with 2x 2 turns on it all in the connection box.
.========================.
((Folded Dipole ))
'========() ()=========='
\__/³
bead(oo)³_____________________
ctÀ(À)_____75ê Coax______
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.
Why don't U send out an interesting bul?
73 De John, G8MNY @ GB7CIP
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