To : TECH@WW

By G8MNY (Updated Jan 15)
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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 & 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 & 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 camÿ unmounted outside the unit, so I put it in a diecast with magnetic underside & 4 BNCs, but I also modified it with 4 step up broad band RF transformers for more gain & terminated them on the PCB with 470R instead of the 47Rs & 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 & then the 2nd. The overload detector pick of point is moved to an earlier stage & made it more sensitive, so it operated before any clipping occured.

DISPLAY
Polar display uses 16 LEDs in a circle for ease of navigating & 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, & 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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