| |
G8MNY > TECH 04.03.09 01:00l 141 Lines 5804 Bytes #999 (0) @ WW
BID : 19627_GB7CIP
Read: GUEST
Subj: Doppler DF Aerial Design
Path: IZ3LSV<IK2XDE<F5GOV<F4BWT<ON0BEL<DB0RES<ON0AR<GB7CIP
Sent: 090303/2315Z @:GB7CIP.#32.GBR.EU #:19627 [Caterham] $:19627_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Jun 08)
(8 Bit ASCII Graphics use code page 437 or 850)
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.
Rx Ant1 * Tx * Rx Ant 1
| * \
Rx Ant2 * * Rx Ant 2
Identical Difference in
Aerial signals phase of aerials
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 1/4 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 360 degrees. 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.
Null
/~\/~\ Cardioid
³ ³ Polar
\ / Diagram
~--~
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 (distorted). This is
because the aerial is turned through 90 degrees between the 2 measurement
methods & you are seeing uneven RF field, so the 2 results cannot be trusted.
Repeating the tests just a few metres or so away normally solve this. (away
from overhead lines)
A label like this may be helpfull..
A Doppler /³\ B
* /_ ZL Tone ³ *
\ Null Nulls \³/
THE CIRCUIT
DIODE SW A
A Ant ÄÄÂÄÄÄÄÂÄÄÄÄ´>ÃÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄCOAX TO RX
Coax _³_ L1 ³ _³_ L ³
³ /_\ ³ ÚÄ´<ÃÄÙ /_\ ÃÄÄÄÙ
GNDÃÄÄÄ´ 4k7 ³ ³ GND
³ _³_ ÀÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄ¿
³ /_\DIODE ³ SWITCH C ³ ³
B Ant ³ SW B ³ 3T ³ ³
Coax ÄÄÁÄÂÄÄÄÄÄÁÄ´ÃÄÂÄÄ´>ÃÄÄÄÂÄLÄ´ ³
L2 1n L3 ³ ³ ³
³ ³ 18p=== === ³
4K7 4k7 GNDÄÄÁÄÄÄÙ18p ³
³ ³ DELAY ³
³ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÂÄÄ¿
³ 47k ³ ³ ³ ZL MODE
³ _³_ ³ o o (null carrier)
³ \_/ ³
³ ÃÄÄij>ÃÄÄGND ÀÄÄo oÄÄÄÄ+9V
³ u1 === \ \ (center off)
ÃÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo o
³ 4ÚÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ ÚÁÄÄÄÁ¿8 6k8 DOPPLER MODE
4u7=== ³ ÃÄÄÄÄ´ (null tone)
+³ ³ LM ³7 100ê
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
(L1-3) 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 with aerial B via a
lumped delay line (or use a length of coax approx the same as the A-B gap) this
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 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!
IN USE
This aerial, FT290, & Map, have won a few of DF fox hunts, but has occasionally
failed to find fox too, if the RF field is too distorted at some of the reading
sites!
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
