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G8MNY > TECH 12.09.04 17:27l 74 Lines 3225 Bytes #999 (0) @ WW
BID : 10475_GB7CIP
Read: GUEST OE7FMI
Subj: Making a Phasing Harness
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0GOS<ON0AR<ON0AR<IW2ESA<IZ0AWG<IK1ZNW<
GB7CRV<GB7CIP
Sent: 040912/1353Z @:GB7CIP.#32.GBR.EU #:10475 [Caterham] $:10475_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (updated Mar 04)
Calculate 1/4 wave lengths of 75ê coax using the coax's velocity factor & the
wave length for your frequency of interest. None of this is too critical unless
above 1GHz, where cable Z change if you bend them & cutting to length is more
of an art, so the use of a solid power splitter is more common.
Decide which harness design you want:-
Join Tee == to aerial tee ========#== to aerial
50ê =======<ðððððððð[ 2x 50ê OR 50ê =======[ 2x75ê 2x 50ê
37ê == to aerial ========#== to aerial
(2x75ê)
eg both matching lines in a single feed plug (or socket) & an aerial plug (or
socket) with a aerial T connector. Or use the matching lines as the T with 3
plugs (or sockets). The latter is mechanically less strong, & waterproofing
may be more difficult. Alternatively you could splice in the matching sections,
but again waterproofing can be a problem.
Use good quality 75ê cable with heavy braid. You should find that 2 coaxes can
be made to fit N connectors with some difficulty. Use silicon rubber or grease
to seal/fill connectors once tested.
Testing can be done with a sensitive SWR meter a 1W rig & 2x small 47ê
resistors (with minimum leads) mounted one the ends of the lines to simulate
the 2 aerials. This should prove the cut lengths are OK before you terminate
the aerial ends.
The Aerials must be identical types (no early/later multibeam type mix). And
mounted so that the connections feed the same side of the driven aerial element
to be in phase. If the aerials are mirror images (one up side down) and the
element cannot be rotated, then add 1/2 wave length (x Velocity factor) to one
of the 50ê aerial feeders. The two aerial feed coaxes between the splitter &
aerials must be identical lengths & type.
SPACING
Aerial spacing, is variable, the manufacturers data may help, but a good guide
is about half the boom length for yagies & 2/3 boom length for QLY & ZLs. This
rule of thumb, makes sure the aerials capture areas (as if they were dish
aerials) do not overlap.
Gain
+3dB Ä´ _..------
+2dB Ä´ ,/''\._ _./'
+1dB Ä´ ./'\ / '
0dB _³_.,/' '-.-'
ÄÅÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄ> Spacing
Touching Too Close Half Boom
Close Optimum Length
Closer stacking or baying, gives variable results as all the aerial elements
will couple, & this will result in a different polar diagram & gains less than
optimum. But closely mounted aerials with high gains are possible, if difficult
to get the distance just right for one spot frequency!
Another point often forgotten is that both aerials should see the same RF field
to work. If one aerial is much closer to the ground or buildings it can cancel
any advantage as Rx signal power can actually be lost in the lower aerial!
ÃÅÅÅÅÅ <- - - - - - - - Even Dx
³ _ <- - - - RF Field
ÃÅÅÅÅÅ -> / \ <- - -
³ ³ ³
ÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄ
Why Don't U send an interesting bul?
/QSL
73 De John, G8MNY @ GB7CIP
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