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G8MNY > TECH 14.07.03 18:15l 112 Lines 4643 Bytes #999 (0) @ WW
BID : 5616_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: A Doppler DF Aerial Design
Path: DB0FHN<DB0RGB<DB0MRW<OK0PKL<OK0PPL<RZ6HXA<SP7MGD<7M3TJZ<EA5RKV<VK3TE<
GB7YFS<GB7CIP
Sent: 030714/1015Z @:GB7CIP.#32.GBR.EU #:5616 [Caterham] $:5616_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
THE DOPPLER DF PRINCIPLE
By switching between 2 identical aerials at a fast audio rate, any amplitude or
phase difference seen between the 2 aerial signals will show up on the Rx as a
Tone in AM/SSB/FM mode. The only time no tone will be detected is when both
aerials have identical signals which in an even RF field only occurs when the
aerials are equal distance from the source. This happens however strong or
overloading the signal in the Rx is!
ADDING A ZL AERIAL SYSTEM
The drawback to a simple 2 aerial doppler system is that it cannot give the
direction "SENSE" of the source just the line it is on, as three are 2 idential
positions of tone nulls in 360deg. So conversion of the 2 aerials into a simple
ZL type phased beam is needed as well, as this will give a Cardioid polar
response with a sharp null of the back of the beam. But this will become
unusable for DFing with a very strong signal.
USING BOTH METHODS
But if using both systems together, you get a poor correlation of heading
between them then the RF field being sampled is poor. This is becuse the aerial
is turned through 90 deg between the 2 measurement methods & is seeing an
uneven RF field, so the 2 results cannot be trusted. Repeating the tests just a
meter or so away normally solve this.
AERIAL LAYOUT
DIPOLE A ------- Greater than 1/4 wave ------- DIPOLE B
! !
! choke ferrite choke !
! balun ring balun !
! | /\ring wooden /\ DOPPLER /\ /\ | !
=@==================== & PHASING ===================@=
! \/ UR43 boom \/ CIRCUIT \/ UR43 \/ !
! COAX ring <!!> COAX ring !
! !! ring !
! coat hanger UR43 !! !
wire element COAX !!
!!
To Rig.
For 2M, the 2 101cm long coat hanger wire dipole aerials (A & B) are mounted
84cm apart on a wooden boom. The are connected to coax with coax choke baluns &
small ferrite rings reduce the effect of the unbalanced coax on the measured
RF field.
CIRCUIT
DIODE SW A
A ÄÄÂÄÄÄÄÂÄÄÄÄ´>ÃÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄTO RX
ANT --- L ³ --- L
/_\ ³ ÚÄ´<ÃÄÙ /_\ ³
GNDÄÄÄ´ 4K7 ³ ³ GND
--- ÀÄ ³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ ÄÄÄ¿
/_\ ³ SWITCH C ³ ³
³ DIODE ³ ³ ³
B ³ SW B ³ 3T ³ ³
ANT ÄÄÁÄÂÄÄÄÄÄÁÄ´ÃÄÂÄÄ´>ÃÄÄÄÂÄLÄ´ ³
L 1n L ³ ³ ³
³ ³ 18p=== === ³
4K7 4K7 GNDÄÄÁÄÄÄÙ18p ³
³ ³ DELAY ³
³ ³ ³
³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄ¿
³ 47K ³ ³ ³
³ _³_ ³ ³ ³
³ \ / ³ o o ZL
³ ÄÂÄ ³ MODE
³ ÃÄÄij>ÃÄÄGND ÀÄÄo oÄÄÄÄ+9V
³ u1 === \ \ CENTRE OFF
ÃÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo o
³ ³
³ 4ÚÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ ÚÁÄÁ¿8 6K8 DOPPLER
4u7=== ³ ÃÄÄÄÄ´ NULL TONE
+³ ³LM ³7 100 MODE
3ÀÄÄ´555ÃÄÄÄÄ´
³ ³6 ³
³ ÃÄÄÄÄ´
ÀÄÂÄÙ2 ³
1³ === u1
GNDÄÄÁÄÄÄÄÄÄÙ
HOW THE CIRCUIT WORKS
The 2 aerial inputs each see a diode switch (A & B) using 1N4148 type diodes.
The Ls are several turns on ferrite cores or beads, but if in series with 4K7
then they can be left out. Construction is on a PCB ground plain either by
knife etching circuitry or ugly.
Aerial switch C is used to join the 2 aerials together but aerial B via a
lumped delay line (approx the same as the A-B gap) L is about 3 turns & Cs used
for this are set up to cause a signal null in one direction.
In ZL mode diode switch B is off & switch C is on, passing the combined signal.
When the centre off mode switch is in doppler mode, the control of the A & B
diode switches is from a 555 square wave astable running at about 1KHz. With a
diode pump circuit to completely bias off the C switch path.
In the off position there is no though path!
It is possable to Tx up to 2W into the unit, but only a small amount of power
passes through to the aerial!
/QSL
73 de John, G8MNY @ GB7CIP
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