(Frequently Asked Questions)
Except for special needs or applications we don't recommend this. Some who have tried it without our guidance have had poor results. If you need to build your own, read the following guidelines.
We don't recommend you build your own RF switcher, unless you ACCURATELY duplicate ours, including the board. At the higher frequencies lead dress becomes critical. The diodes and resistors must lay flat against the board, with the diodes centered over the ground strip of the board.
A shielded cross boom is important! It prevents seeing horizontally polarized RF, and thus reduces errors from reflections where the polarization has been shifted by the reflecting object.
Above 550 MHz don't use connectors for the four whips. We use 1/8 inch thick circuit boards, with brass whips soldered in, and a large clearance hole in the boom. Boards are screwed to the boom with sheet metal screws. If you wish to buy one, we now manufacture antennas which go up to 1300 MHz. We do not have kits for this antenna.
A metal vertical mast is fine.
Be an absolute fanatic about symmetry!
No. The antenna stops working. This is because the boom acts as a shield to prevent RF being picked up on the outside of the dipole feed coax. (See FAQ about metal cross-booms.) The remaining upper half of the dipole then has no counterpoise element, and no RF is fed into the coax. If you don't believe this, try tuning in a signal that is not full quieting, and then take off the lower whip. The signal will disappear!
You can use shorter whips, even rubber ducks, instead of the stock whips. Be sure all four are the same. The only effect will be loss of RF sensitivity. No effect will be noticed on DFing, unless the signal is weak to begin with.
1. It is important that the radio receiver you use to DF is accurately on frequency. When it is off frequency a few kHz, the received signal is not sufficiently centered in the receiver "S" curve. Quite often a person may not realize this by listening to the received signal because of the fact that these communications systems have a lot of audio distortion; the distortion due to being off-center in the "S" curve is not noticed. Take some effort to determine that your receiver crystal oscillator (used by the synthesizer) is accurately set. Remember that a crystal frequency error that at 150 MHz which does not cause significant mistuning can produce unacceptable mistuning at 800 MHz.
If you have a receiver with some form of "fine tuning" adjustment, try adjusting that to find the "center" of the transmitted signal.
2. I am suspicious of the quality of coax and coax connectors at 800 MHz. If those which are exposed to the RF wave front are leaking energy into them, this could cause a problem. Although I have not had to use it (yet!) RG-223 has much better shielding (double braid). Be sure the connectors are clean and screwed down tight.
3. On mast-mounted antennas, be sure the coax feed line running down your vertical mast is brought directly to the mast, and is taped to the mast (symmetrically to the DF antenna) and does not leave the mast until well below the antenna whips.
4. Bandwidth problems. If the modulation on the signal is wider than your receiver's bandwidth, the operation of SuperDF will be degraded, or unusable. Use a receiver with sufficient bandwidth. Some radios have a selection of NBFM or MBFM. A service monitor would be a good choice, as it both allows measuring the deviation, and adjusting to match it. REMEMBER: If you change the bandwidth of the receiver, the timing delay of the SuperDF must also be changed.
5. On foot in multipath situations, SuperDF will indicate what it sees at whatever fixed position it is in. SuperDF may behave seemingly differently at 800 MHz from what you experience at 150 MHz. This is likely to be several different bearings indicated at the same "location" while standing still and DFing on-foot (handheld). This is likely caused by bad multipath. As you swing your arm through a long arc (as you might do at 150 MHz) the antennas is swinging through an area where the cancellation and reinforcement of the multipath produces several standing waves in space. Therefore, a better technique is to hold the antenna up over your head, and rotate it around its own center-point, rather than swinging through a large arc at arms length.
Another method is to try walking at a brisk pace, with the Slow response selected, and the volume set low. Note which side seems to dominate, and steer in that direction. Or use tone display, and try to note which tone dominates. Use of tone display is safer; you can watch where you are going.
6. We have found that the same radio can require a change in the audio phase select switch (the A/B switch at the left of front panel) when the radio is tuned to a significantly different frequency. We believe this is caused by the radio changing from a high-side Local Oscillator to a low-side LO (or vice versa). Therefore, it is important to set up SuperDF at the frequency where it will be used.
Do NOT use a non metallic cross-boom. Shielding is important!!! What is being shielded is not the switching circuit, but the outside of the coax running through the boom to the dipole. This shielding is quite important. Without it, any horizontal RF component will be picked up on the outside of the coax and will be conducted to the dipole feed, where it will then enter the coax and be fed to the receiver. The antenna then becomes susceptible to horizontal components of the RF. If you are hunting a vertical signal, the only way there can be horizontal components is from the effects of the vertical signal being reradiated from of objects which are slanted from the vertical.
We recommend that you do not transmit through the SuperDF antenna. One Watt at 2 meters will not damage it. Two Watts may burn out the diodes. In both cases you will be radiating strong harmonics.
I DF using either a scanner receiver, or disconnect the microphone, or use a second antenna for transmitting that is selected by relay when I key the mike. If using a handheld, select low power to prevent damage if you forget and key the radio. The easiest way is to use a second radio and antenna for transmissions.
There is no computer compatible output. SuperDF is not an automatic RDF. It has to be rotated until it reaches the position where both whips see the same RF phase. It is then pointing at the signal. So in a very real sense, SuperDF is a "Phase Front Detector." It knows which way to rotate to reach bearing, and it knows when it has reached the bearing position. (I.e. it has reach the orientation of the phase front.)
SuperDF does have an error output voltage. This is a DC voltage which changes polarity and magnitude. Typically it is 35 to 100 mV at maximum bearing error, and drops to zero when on bearing. Maximum voltage is a function of the radio volume control setting. As the RDF antenna is further rotated, the voltage builds up again, but in the other polarity. This error voltage has been used in an automatic servo to keep the antenna pointed at the signal. If the pointing system which you might build has some form of angle encoder, that encoder output could be interfaced to a computer.
There is no completely universal RDF tool. I will outline here our recommendations for a set of tools for general RDF use.
(1) Start with a scanner which you have determined in your shop to have accurate frequency control. This is important when using SuperDF at 800 MHz.
A battery operated Service Monitor should make an excellent hunting receiver!!! It will provide more information than a scanner, as you can measure signal strength, and you can tune accurately even on a mistuned transmitter.
(2) Obtain at least one SuperDF system consisting of:
This will cover most hunting needs.
(3) Outfit at least one maintenance vehicle. Requires the operator of the RDF to be able to rotate the SuperDF antenna while in motion. Hunting in motion greatly increases the accuracy, and speeds up the hunting.
(4) Set up for "wave guide beyond cutoff" attenuator used in "Hot - Cold" hunting. See the section Hunting Inside of Buildings.
(5) A field strength meter with adjustable sensitivity. It should be broadband. Either this or the "wave guide beyond cutoff" attenuator is used for the last 100 feet or so.
Additional systems may be desirable, depending on whether you need to do triangulation, or cover a very large geographical area. (Los Angeles County radio maintenance has a SuperDF in each of its service vehicles - about 18).
An interesting tool which can speed up hunting inside buildings is simple to make. Obtain a large metal chassis box, a few inches deep. Make and mount a spiral-shaped dipole across the open face of the box. A handmade etched circuit board would do fine. Connected the feed points THROUGH a 1:1 BALUN to about 5 feet of light coax. Connect the coax to the field strength meter. This "shadow antenna" is broadband and is not sensitive to polarization. It can be used indoors for hot-cold hunting, and provides enough directivity to tell general direction (left-right, up-down).
If the signal is not strong enough for the "shadow antenna" to drive the field strength meter, then plug the antenna coax into a simple loop which is mounted part way down inside the "wave guide beyond cutoff" attenuator.
Do not try to hunt inside with SuperDF, unless it is a LARGE open space. Too many reflections.
No, not for DFing. The ONLY benefit from using PIN diodes is if you intend to transmit through that antenna. PIN diodes stay in conduction for a long time (compared to an RF cycle) after they are reversed biased, and therefore will conduct both halves of the RF cycle. While this is true, I still would be reluctant to transmit through them, as I suspect that they are not completely linear, and would therefore generate some harmonics.
Can the Adcock Antenna be used on VHF or UHF?
Theoretically yes, practically, no. There are problems that makes the Adcock impractical.
From my extensive experience of hunting in mountains, one MUST have the ability to take bearings while mobile in motion. The SuperDF does this very well, providing an integration of readings taken for about one second of travel-time. (This integration time can be increased by adding a capacitor.) This is usually enough integration to show a general trend of direction in a bad multipath area. The Adcock CANNOT do this.
NOTE: The Adcock is much better suited to use on HF, where there is not likely to be multipath.
Can SuperDF be used to hunt an ELT or other AM signal?
Even though the ELT at 121.5 and 243 MHz is AM modulated, the SuperDF connected to a NBFM receiver will hunt it very well. Some scanners will operate at 243 Mhz in FM mode. Look for one with a manual select for the detector type (AM/FM). If one cannot be found with a manual select, most scanners can be modified to force FM mode. This is usually easy to figure out once the schematic is obtained. (Schematics can be obtained for Radio Shack scanners.) See Hunting AM signals.
Would SuperDF be suitable for use in a Marine Environment?
SALT SPRAY The SuperDF was not designed for Salt Spray environment, however it can be adapted. We have worked out a way to waterproof the antenna by filling it with non-conductive and water-proof foam. There is additional cost for this treatment.
The control unit would have to be enclosed in a plastic bag, closing the bag opening around the cables with sturdy rubber bands, or kept inboard, away from spray.
Would such things as radar and GPS in close proximity affect its performance?
SuperDF needs to be mounted at your mast head, with nothing near it to cause unbalanced capacitances to the two SuperDF dipoles. As to radio interference, GPS is a receiving system, and probably won't be a problem. An operating Radar is likely to produce responses in the SuperDF by the Radar pulses modulating the diode switch in the SuperDF antenna. If this Rep Rate is not the same or a harmonic of the SuperDF antenna switching frequency (about 400 Hz), SuperDF's filters may be able to handle the noise. You may need to turn OFF the radar while DFing.
How easy is the equipment to use, is it portable, splash proof?
If mounted at Mast Head, SuperDF should be very easy to use. The pilot simply steers Port - Starboard, as indicated by the LED (or meter on the PRO control unit). When both LEDs are ON, or the meter is at zero center, the boat is moving directly towards the transmitter.
Since you would often be operating in full sun light, the PRO control unit (with its meter) would be easier to use, because the LEDs tend to wash out in bright sun.
Axis Cross-Coupling With this system, there can be axis cross-coupling. This means that as you both pitch and roll, a small error will will couple into the direction information. Watching the meter, and steering so that the meter swings about equal amounts left and right will reduce the effect of this error.
SHORE BASED Operation SuperDF has been used to triangulate on boats from high bluffs overlooking the sea. Power boats where then dispatched to the coordinates to effect the rescue. It showed to be about 3 times more accurate than the Polaris system (which I think is now out of production). If you have line of sight condition from a hill-top, and have a good clean DF site, you should be able to set up and take readings with an accuracy of about +/- 1 degree. One degree translates into a mis-distance of 910 feet at 10 miles.
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A very early and much simpler SuperDF type device which I supplied to the local FCC was used mounted on a small power boat for policing marine frequencies in LA and Long Beach harbors. The Engineering In Charge reported that it worked very well, even when moving past larger boats.
George R. Andrews
President
Please feel free to communicate with any other questions or suggestions!
George R. Andrews (Russ, K6BMG)
BMG Engineering, Inc.
9935 Garibaldi Avenue
Temple City, CA
91780, USA
Voice 1(626)285-6963
Fax 1(626)285-1684 (24 hour automatic)
America OnLine: Grandrews
Web: http://members.aol.com/bmgenginc
(31 Mar 1998)
Send E-mail to grandrews@aol.com.
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