G8MNY > TECH 12.08.18 08:30l 131 Lines 5666 Bytes #6 (0) @ WW
BID : 33040_GB7CIP
Subj: 4 Aerial Doppler RDF
Sent: 180812/0629Z @:GB7CIP.#32.GBR.EURO #:33040 [Caterham Surrey GBR]
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 ->
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
Ţ1Ů They give the 4 phases of
Ů Ţ smooth gain changing bias
.-~-./ 0° \.-~-, for gate 2 of the 4 dual
. .-. gate mosfet preamps mimicing
Ţ Ţ2Ů a spinning aerial rotating
Ů Ů Ţ 300 times per second.
Ţ Ů Ţ3Ů
Ů Ţ Ů Ţ
Ţ Ů Ţ4Ů
Ů Ţ Ů Ţ
Ant 1 Ant 2
┌─┴─────┴─┐ 6 way ┌─────────┬────────┬──────────┬───────────┬─────────────┐
│Four Gain├==========┤Waveform │ Crystal│ Phase │Calibration│Heading Deg &│
│Modulated│ │Generator│Timebase│Comparitor│ Delay │Polar Display│
│ Preamps ├=======┐ └─────────┴────────┴────┬─────┴───────────┴──────┬──────┘
└─┬─────┬─┘ coax │ │300Hz │Kill
│ │ ┌┴──────┐ ┌──────┴─────┐ No 300Hz Squelch │Display
Ant 4 Ant 3 │Scanner├────────────┤ AF Signal ├──────────────────┘
│ Rx │ 300Hz + AF │Conditioning├─Overload LED
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!
------------- ------------- -------------
| ' ' | | . . | | |
|  | |  | |  :: |
| . . | | ' ' | | |
------------- ------------- ------------- ˙ = 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
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. Mine preamp came 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.
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...
┌───────100K─┐ dB │
10n │ │\ │ 0 ┤ .˙.
In──┤├─100K─┬┴────┤- \ │ │ Ţ Ů
│ │ >┬─┴─┬─ -10┤ Ů Ţ
u1 === ┌─┤+ / │ o │ | |
│ │ │/ │ Test -20┤ / \
├───)────┤├─┘ Point │ / \
220 │ u1 -30┤ .' '.
│ │ │ .' '.
100R │ -40┤ _ .˙' '˙. _
Preset │ └────┬────┬────┬────┬────┬────┬──
──────────┴───┴──── 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 clipping occured.
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 0-360° bearing indication.
│ 360 .˙˙˙. │
│ : . :│
│ ! .'˙...˙'│
An overload LED indicates if there is too much AF drive.
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
RFD 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.
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
Read previous mail | Read next mail