DTOA Switch
The first project is a direction finding assist.
The DTOA acronym is "Differential Time of Arrival".
The DTOA switch is connected between a pair of
antennas and a handheld receiver.
When the antennas are normal to the transmitter (i.e. electrically equidistant)
the receiver doesn't notice the DTOA switch. As the antennas turn
away from the transmitter, a squeal is introduced into the audio.
DTOA switch Schematic
DTOA switch Parts List (web page)
DTOA switch Master Build Record
DTOA switch DigiKey order spreadsheet
This file can be dropped directly into the
DigiKey ordering system
KC0JFQ Pages for this project
The last images in the group are the base of the prototype antenna.
The antenna is semi-rigid brass tubing. The main part of the vertical
element slips into the fixed tube mounted to the base.
The long antenna elements are stowed for transport in this image.
Clik the image to get to the high resolution image.
The two antennas are mounted on a yardstick about
1⁄
2
wavelength apart, although this spacing is
not particularly critical.
The fixed portion of the antenna has a brass spacer that is tapped 4-40 solderd
at the base to provide secure attachment, this being visible in the image.
The coax connector in this image is a BNC, but an SMA connector may be substituted.
The connection from antenna base to the coax, shown in the far right image,
is cut from the main circuit board (that is two circuit boards are nominally required).
You see on the trimmed coax boards that a 0.125" hole is provided to attach
the antenna base. Either a BNC or an SMA may then be attached.
The two non-plated holes provide for mechanical attachemnt.
In our prototype #4 pan head sheet metal screws along with some short nylon
spacers are used.
A short piece of 1" stock is glued to the inter-antenna spacer
(i.e. the two yardsticks glued together) to provide a bit of support for
the antenna base.
The circuit board is used as a drill guide to set the holes before
being assembled to the antenna and the spacers.
The two nylon spacers are present only to provide clearance for the solder
joints on the circuit board.
Operation
Connect the switch to the receiver antenna input.
With power off, the receiver will act normally.
If you have mounted the antenna elements 1⁄2
wavelength apart, the receiver will null with the elements
lined up with the signal.
Once switched on, the receiver will receive normally when the antenna elements
are normal to the signal. Your line-of-position is perpendicular
to the line between the two antenna elements.
As you move your line-of-position away from the source, a squeal will be introduced and
be heard in the received audio.
The system is sensitive to reflections, much more so than a single
antenna.
As you move further away from the transmitter you may see reflections
that have signal strength similar to the transmitter.
Swing the antenna array back and forth to find the point where the
squeal introduced by the switch is minimized.
As you get closer to the transmitter this will be more pronounced.
Keep in mind that the line-of-position has
front-to-back ambiguity, so you need several lines-of-position
to establish the direction of the transmitter.
When used with low power transmitters you may see the effect your
body has on reception, the front-to-back sensitivity may change.
You will still see the null if you turn around, but it will sound
slightly different.
