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G8MNY > TECH 18.03.04 00:06l 115 Lines 5079 Bytes #999 (0) @ WW
BID : 48711_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Doppler DF Aerial Design
Path: DB0FHN<DB0FOR<DB0SIF<DB0NHM<DB0ERF<DB0FBB<DB0GOS<DB0ACC<DB0EA<DB0RES<
ON0AR<GB7CIP
Sent: 040317/1140Z @:GB7CIP.#32.GBR.EU #:48711 [Caterham] $:48711_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jan 04)
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!
AERIAL LAYOUT
DIPOLE A <--------- Greater than 1/4 wave ---------> DIPOLE B
³ ³
³ choke ferrite choke ³
³ balun ring balun ³
³ | /\ring wooden /\ DOPPLER /\ /\ | ³
=@==================== & PHASING ===================@=
³ \/ UR43 boom \/ CIRCUIT \/ UR43 \/ ³
³ COAX ring <!!> COAX ring ³
³ !! ring ³
³ coat hanger UR43 !! ³
wire element COAX !!
!!
To Rig.
For 2M, the 2 (A & B) dipoles consist of 101cm long coat hanger wire dipole (2x
52cm per ¬ wave) aerials mounted into holes drilled close together each of the
wooden boom 84cm apart. The are connected to coax with coiled coax choke baluns
that is a slightly shorter than a 1/4 wave of coax tightly coiled up as close
as possible to the aerial feed point, & tapped the boom making sure the coil at
right angles to the dipole & does not couple to it! Also fit small ferrite
rings to reduce the effect of the unbalanced coax on the measured RF field.
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 there are 2
identical 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
Cardiod 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 because the
aerial is turned through 90 deg between the 2 measurement methods & you are
seeing uneven RF field, so the 2 results cannot be trusted. Repeating the tests
just a meter or so away normally solve this.
THE CIRCUIT
DIODE SW A
A ÄÄÂÄÄÄÄÂÄÄÄÄ´>ÃÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄTO RX
ANT --- L ³ --- L
/_\ ³ ÚÄ´<ÃÄÙ /_\ ³
GNDÄÄÄ´ 4K7 ³ ³ GND
--- ÀÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄ¿
/_\ 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 ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÂÄÄ¿
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 4ÚÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ ÚÁÄÄÄÁ¿8 6K8 DOPPLER
4u7=== ³ ÃÄÄÄÄ´ NULL TONE
+³ ³ LM ³7 100 MODE
3ÀÄÄ´ 555 ÃÄÄÄÄ´
³ ³6 ³
³ ÃÄÄÄÄ´
ÀÄÄÂÄÄÙ2 ³
1³ === u1
GNDÄÄÄÁÄÄÄÄÄÄÄÙ
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 (or use a length of coax approx the same as the A-B gap) L is
about 3 turns 6mm dia & 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 possible to Tx up to 2W into the unit without damage, but only a small
amount of power passes through to the aerial!
/QSL
73 de John, G8MNY @ GB7CIP
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