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PA2SNK > TECH 28.09.12 21:20l 93 Lines 3568 Bytes #999 (0) @ WW
BID : 6158_CB9SNK
Read: GAST
Subj: 3m FM yagi homebrew by g8mny
Path: DBO595<DOK346<BBS645<NL3CAT<NL3PRC<CB9SNK
Sent: 120927/2011Z @:CB9SNK.FRL.NLD.EU #:6158 [6234] SNEEK $:6158_CB9SNK
From: PA2SNK@CB9SNK.FRL.NLD.EU
To : TECH@WW
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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
cm 732
cm 2x
647 2x
cm 614 2x
cm 583 2x
cm 522
cm
ď0ďďď
326 186 713 550 758
cm cm cm cm cm
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
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
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 75. 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 ))
'========() ()=========='
\__/
bead(oo)_____________________
ct()_____75 Coax______
Z MEASUREMENTS
Using an MFJ analyser, the match to 75 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's Danny
Ŀ Ŀ ¿ Ŀ Ŀ ڿ NLD net bbs voor de regio ZW-Friesland.
ô Ŵ ¿ ô 24 uur per dag QRV op Ch.24 1200 Baud.
CB: PA2SNK@CB9SNK.FRL.NLD.EU
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