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G8MNY > TECHNI 10.06.26 11:05l 133 Lines 5765 Bytes #999 (0) @ WW
BID : 62042_GB7CIP
Read: OE5RCO GAST
Subj: 4 Aerial Doppler RDF
Path: DB0FFL<OE2XZR<OK1TOR<OK0NAG<OK0NBR<OK2PEN<GB7CIP
Sent: 260610/0848Z @:GB7CIP.#32.GBR.EURO #:62042 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
THE DOPPLER RDF PRINCIPLE
By moving a Rx aerial towards the Tx (rotating it around a center), the Rx
frequency increases, & when going away it decreases. So on a FM Rx a tone will
be heard at the frequency who's phase compared to the aerial rotation position
is the direction.
+ Freq ->
Tx
No change v ROTATNG ^ *
- Freq <- plan view
To mimic a spinning aerial a least 4 identical aerials are needed. And each of
these needs to be faded in & out in pseudo 1/2 sinewave manner. This assumes
the 4 aerials don't interact!
EARLY KIT RDF
I have a 1981 commercial USA kit by Doppler Systems Arizona, model 3003, it
uses a crystal timebase to clock 4 phases of an identical waveform stored in
Proms.
.-.
Ý1Þ They give the 4 phases of
Þ Ý smooth gain changing bias
.-~-./ 0ø \.-~-, for gate 2 of the 4 dual
. .-. gate mosfet preamps mimicing
Ý Ý2Þ a spinning aerial rotating
Þ Þ Ý 300 times per second.
\.-~-./+90ø \.-~-.
.-. .-.
Ý Þ Ý3Þ
Þ Ý Þ Ý
\.-~-./+180ø\.-~-.
.-. .-.
Ý Þ Ý4Þ
Þ Ý Þ Ý
-./ \.-~-./+270ø\
SCHEMATIC
Ant 1 Ant 2
³ ³
ÚÄÁÄÄÄÄÄÁÄ¿ 6 way ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³Four GainÃ==========´Waveform ³ Crystal³ Phase ³Calibration³Heading Deg &³
³Modulated³ ³Generator³Timebase³Comparitor³ Delay ³Polar Display³
³ Preamps Ã=======¿ ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÂÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÂÄÄÄÄÄÄÙ
ÀÄÂÄÄÄÄÄÂÄÙ coax ³ ³300Hz ³Kill
³ ³ ÚÁÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÁÄÄÄÄÄ¿ No 300Hz Squelch ³Display
Ant 4 Ant 3 ³ScannerÃÄÄÄÄÄÄÄÄÄÄÄÄ´ AF Signal ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ Rx ³ 300Hz + AF ³ConditioningÃÄOverload LED
ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÙ
AERIALS
These must be identical with same length feeder etc. I used 4 magmounts with
short 1m leads to BNC plugs. Into these I can plug in aerials, telescopic ones
give 100-432MHz, for lower frequencies "coat hanger" wire can be cut and put
into PL259 plugs. It is important they are wired up in order correctly!
------------- ------------- -------------
| ' ' | | . . | | |
| [] | | [] | | [] :: |
| . . | | ' ' | | |
------------- ------------- ------------- ú = Magmount
50MHz spacing 144MHz 432MHz [] = Preamp
The aerial spacing needs to be about 1/4 wave for the doppler tone to develope,
the distance and the shape of the vehicle roof all affect the bearing
calibration!
RX SETUP
I use the simular sized AOR2002 Scanner for the Rx, modified with a Pre Vol AF
output (see below). The aerial arrangement with external preamp is optional,
but less cables. My preamp came unmounted outside the unit, so I put it in a
diecast with magnetic underside and 4 BNCs, but I also modified it with 4 step
up broad band RF transformers for more gain and terminated them on the PCB with
470R instead of the 47Rs and added 2 clipping diodes as protection in case I
Tx! With the preamp output coil removed, it now gives useful gain 50-432MHz.
AF MODIFICATIONS
The original circuit had very little pre signal conditioning, only a few dBs
down at 200Hz & 1kHz, so any modulation kills the system. So I desigend a very
narrow wayne filter on a couple of the existing initial opamps in circuit...
ÚÄÄÄÄÄÄÄ100KÄ¿ dB ³
10n ³ ³\ ³ 0 ´ .ú.
InÄÄ´ÃÄ100KÄÂÁÄÄÄÄ´- \ ³ ³ Ý Þ
³ ³ >ÂÄÁÄÂÄ -10´ Þ Ý
u1 === ÚÄ´+ / ³ o ³ | |
³ ³ ³/ ³ Test -20´ / \
ÃÄÄÄ)ÄÄÄÄ´ÃÄÙ Point ³ / \
220R ³ u1 -30´ .' '.
³ ³ ³ .' '.
100R ³ -40´ _ .ú' 'ú. _
Preset ³ ÀÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄ
ÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄ 100 200 300 400 500 600Hz
The exact centre frequency is set on the multiturn preset by setting identical
losses at 290 & 310Hz, on the 1st op amp's test point and then the 2nd. The
overload detector pick of point is moved to an earlier stage and made it more
sensitive, so it operated before any clipping occured.
DISPLAY
Polar display uses 16 LEDs in a circle for ease of navigating and 1 in the
middle to indicate power. As well as that there is a 0-360ø bearing indication.
ÚÄÄÄÄÄÄÄÄÄÄÄ¿
³ 360 .úúú. ³
³ : . :³
³ ! .'ú...ú'³
ÀÄÄÄÄÄÄÄÄÄÄÄÙ
An overload LED indicates if there is too much AF drive.
IN PRACTICE
Anoyingly my scanner gives different 300Hz delays (phases?) between wideband &
narrow band FM modes, so the barring changes if the bandwidth is changed. But
I found on narrow band FM mode, the heavily clipped wideband FM AF, RDF 300Hz
signal is fine. So I added a dedicated unsquelched narrow band FM AF O/P for
RDF feed, and I can still listen to the AM & WBFM modes.
The main problem is RF multipath, if any of the 4 aerials sees an upset RF
field (nulling) the result is usless!
See my tech bul "2M DF ing with just a FT290r", & "Doppler DF Aerial Design"
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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