TDOA Antenna Switcher

This is a functional prototype of a upcoming Byonics product in our T-Hunting / RDF product line, alng with the PicCon and MicroFox 15. It is an TDOA (Time Difference Of Arrival) antenna switcher that, when combined with a radio receiver, will tell the direction to a transmitter via LEDs. The initial prototype does not yet have the LEDs, but is still useful, in that it generates an audible tone on an 2-meter FM receiver when the two antennas are not the same distance from the transmitter, and thus receiving the signal out of phase from each other. That tone will fade away when the two antennas are rotated as they become equidistant from the transimtter, and fall into phase.

The way to use this antenna switcher is to turn on the power, connect to a radio receiver on the proper frequency, and hold with the antenna elements going up and down, and rotate the boom of the antenna left and right until the 640 Hz tone goes away. When it does, then the antennas should be equidistant from the transmitter, and if you were to draw a line between the antennas, the transmitter should be on the perpendicular to that line right through the midpoint of the antennas.

Note that the transmitter being on that perpendicular line means that it could either be directly ahead, or directly behind. You would need to move some distance along the line of the antennas and take another bearing to determine which direction the transmitter actually was. In the next version, when the audio from the receiver is fed back into the chip and analysed, it will be able to tell if the signal is coming from the left or right when the tone is present, and light the appropriate LED.

This prototype used steel suspended ceiling rods as the 19" whip elements to make 2 2-meter dipoles. They can be folded down when not in use. Check back at this site in a few weeks, when we hope to have kits available.

ASM Code - for a Microchip PIC12F675

;                         T D O A 0 . A S M
; Byonics / Byon Garrabrant  N6BG  Feb 20 2014

	include	"P12F675.INC"


	cblock	H'20'			; system variables

	ORG	0			; start code

	call	H'3FF'
 	bsf 	STATUS, RP0		; Select Bank 1
	movwf	OSCCAL
	bcf 	STATUS, RP0		; Select Bank 1
	goto	main

delay					; delay 256*3+3 ins
	decfsz	delay1, F		; about 780 us
	 goto	$-1

#define		OUT_PIN			GPIO, 1

	movlw	H'07'
	movwf	CMCON
	bsf	STATUS, RP0		; Select Bank 1
	clrf 	ANSEL
	bcf	OUT_PIN			; make an output
	bcf	STATUS, RP0		; Select Bank 0

	bsf 	OUT_PIN			; output high
	call	delay			; delay about 780 us
	goto	$+1			; to match goto below
	bcf 	OUT_PIN			; output low
	call	delay 			; delay about 780 us
	goto	mainLoop


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