Technical Description
The DF measures signal strength with the help of the reciever, which must be slightly modified. An S - meter signal must be ( internally ) available, ( in one form or another ) and this signal must be connected through a 100K ohm resistor to the antenna terminal of the reciever. Depending on the reciever employed, it may also be necessary to install a 1000 pF capacitor, to provide ( required ) DC isolation for this signal.
The S - meter signal is coupled back to the DF unit through the center
conductor of the coaxial cable which ( also ) provides the RF signal to the
reciever input. This eliminates the need for a separate cable.
The S - meter signal is not directly measured, but instead is used to drive a closed CONTROL LOOP inside the DF. This control loop ( in turn ) drives a PIN diode attenuator, installed between the antenna output and the reciever RF input. This PIN attenuator has over 50 decibels of dynamic range. ( actually closer to 60+ decibels ) The control loop and PIN attenuator act together to maintain a constant RF level at the reciever input.
By measuring the drive applied to the PIN attenuator, the signal strength can be accurately measured.
This method might seem complex, but it has several advantages... Primarily, it
eliminates the need to accumulate detailed information about the S - meter
response curve for the reciever employed... calibrated measurements of the
signal strength ( with this method ) only requires detailed knowledge of the
response characteristics of the PIN attenuator, and this information will
NOT depend on the type or model of reciever employed.
Furthermore, many reciever S - meter circuits have very limited dynamic range... in some cases less than 30 decibels. Using the PIN attenuator allows the DF to operate over a much wider dynamic range. Additional range can be easily achieved with ordinary ( external ) attenuators, which can be inserted between the Yagi and the DF input.
The attenuator itself is derived from Agilent Application Note AN1048, and two sections are employed in this design, resulting in an insertion loss of approximately 6 decibels at 150 MHz. The attenuator described in the app note exhibits excellent stability of input / output impedance, across multiple octaves of frequency, and across all values of attenuation. The actual performance of the attenuator in the YagPlot DF has not been independantly tested, but its design is based directly on the Agilent AppNote.
Information HAS been independantly accumulated about the drive signal applied
to the attenuator, and the resulting attenuation value, at 150 MHz. This data
( in 0.5 decibel steps ) has been used to build a "lookup table" into the DF,
so that the values "reported" by the DF express the actual amount of attenuation
employed, in decibels. This value is displayed in the top right corner of the
Palm DISPLAY page, during the acquisition phase.
The "reference level" for these decibels is set with two trimpots ( course and fine adjust ) located on the front of the DF unit... These trimpots set the signal attenuation "threshold level"... the level at which the attenuator begins to "throttle back" the RF signal, in order to maintain a constant RF level at the reciever input. The proper settings for these trimpots will depend on the type of reciever employed.
Typically, these trimpots would be set to a level somewhat above the noise threshold of the reciever employed, so that the complete absence of any signal will cause the PIN attenuator to drive to minimum attenuation. ( as indicated by the very low pitch of the "beep" tone in the Palm display ) Adjusting the trimpots with this method does not require any specialised test equipment, but it results in a decibel scale with an unknown "reference level", so the decibel values displayed are ( therefore ) expressed in "relative" decibels. ( reference level = unknown ) This is not a liability for ordinary "T - hunting" purposes.
If a calibrated ( low level ) RF source is available, and it is used to adjust these trimpots, then the reference level can be determined with some accuracy, and the resulting ( decibel ) display values can be interpeted in "absolute" decibels. For example, if a -100 dbm RF source is used, and the trimpots are adjusted to begin attenuation at this value, then an indication of 35 decibels on the Palm display corresponds to an RF level of -100 + 35 = -65 dbm.
Technical Notes and a few Caveats
The DF bearing data only has a bearing resolution of ten degrees. Higher bearing resolution was easily possible when I designed the DF, but the time required to complete a "round robin" ( 360 degree rotation ) was painfully slow, with higher resolution. Intermediate bearings are "rounded off" ( up or down ) to the nearest ten degree bearing.
As it is now, the time required to complete a "round robin" is ( with a little practice ) about 20 seconds, which may seem "slow", for some users. Faster rotation speeds will result in "holes" in the DF display, due to "missed" readings on some bearings. The fastest possible speed is probably about 15 seconds, mostly due to speed limitations in the Palm display software.
If more than one reading is taken on a particular bearing, the display software uses only the strongest signal value... effectively, this is signal "peak" detection. This helps to deal with signal strength variations, either accidental or deliberate.
Once a complete dataset is accumulated, the final display "magnifies" the
data to fill the screen. This is done by scanning the data to identify the
strongest and weakest signal values, among the 36 measurements. This achieved,
the data is "re-scaled" to fill the screen, and then displayed. Using this
method, the largest signal approaches the outside azimuth scale, and the smallest
signal represents the center of the screen. This method enhances the "features"
of the directional pattern, and allows antennas with even a modest
directional pattern to yield useful results. It also serves to magnify the
( very small ) pattern of a weak signal.
One "side effect" of this method is the adverse effect of a "missed" DF bearing... if a complete dataset ( i.e. data for all 36 bearings ) is not available when the final display is invoked by tapping the screen, the "missed" bearings will ( by default ) represent the "weakest" signal, used in the "magnification" routine... this can lead to loss of magnification for the rest of the data.
The calibration data for the PIN attenuator was carefully accumulated at 150 MHz, using a professional service monitor, ( Singer / Gertsch FM-10C ) but the monitor has been ( "legally" ) out of calibration for some years. I intend ( eventually ) to double check and "refine" the calibration data, ( using a certified instrument ) when time permits. I suspect the "absolute level" errors are presently no more than 2 decibels, and "relative level" errors are less than 1 decibel. Again, the calibration was performed at 150 MHz.
The compass mechanism is not gimballed, and is vulnerable to magnetic dip, which is the amount of vertical "tilt" of the local magnetic Earth field. The tilt where I live is about 30 degrees from horizontal, and is slightly noticeable in the operation of the DF... Due to magnetic dip, it it important to try and keep the Yagi boom fairly "level" as it is rotated... tilting the boom up or down will have an effect on the compass readings.
The effect is more severe in some directions than others, ( mostly along two opposite bearings ) and a little practice will reveal these two directions. Once identified, it may be necessary to sweep through these areas somewhat more slowly than would otherwise be required, to avoid "missing" a bearing. The problem becomes more severe as the distance to the magnetic pole decreases... users in the northeast U.S. and eastern Canada will observe this problem with greater severity.
Closing Remarks
In general, this DF won't make you a better hunter, but it could help to "sort out" reflections and multipath in some intense situations. In any event, it turns a primitive "voodoo dance" DF ritual into something resembling a scientific activity.
Obviously, it can also be employed in non - DF situations, possibly as a calibrated ( bench ) micro-wattmeter, or for antenna development work. Antenna work ( in particular ) would be significantly accelerated by this device, since the effects of "tweaking" an antenna ( on the radiation pattern ) can be quickly evaluated.
I intend ( eventually ) to pursue some airborne applications, involving CAP SAR operations. The "portable" nature of this equipment eliminates many legal issues involved in the installation of equipment on aircraft. This DF would only require the installation of an ordinary COM antenna, probably at a location directly forward of the tail, so that the "shadow" of the tail would induce directional characteristics into the antenna.
A BNC connector can be provided for the new antenna, somewhere in the cockpit. The remaining equipment is portable, ( no FAA cert required ) and can be quickly installed or removed. I have heard rumors that yoke mounts for Palm PDAs are available, which ( obviously ) would serve very nicely in this situation.
For any further questions or inquiries, ( or comments ) please contact me via e-mail. Thanks...