Presentation is loading. Please wait.

Presentation is loading. Please wait.


Similar presentations

Presentation on theme: "A REMOTE STEERABLE RECEIVING ARRAY FOR 1.8 MHz"— Presentation transcript:


TO AID THE RECEPTION OF WEAK DX SIGNALS. NEXT On long paths to 160m DX stations, signals often arrive from non great circle directions. This is especially noticeable on near antipodal paths, for example, the evening path to the UK over the Pacific. Without a directional receiving antenna, it is impossible to determine the arrival angle of the signal, and part of the purpose of this project was to be able to shed some light on this matter. NEXT The other reason for wanting a remote directional array was to improve the signal to noise ratio on very weak signals. The prospect of having a directional receiving system out in the country, away from urban noise sources, was very attractive. The old saying that “you can’t work them if you can’t hear them” is very true. NEXT

In the signal combiner, signals from the individual antenna array elements, are combined in such a manner as to form a lobe in the required direction. The output from the combiner is passed to a simple single conversion CW only receiver for 1.8 MHz. Note that by far the majority of DX traffic on the 160m band is on CW, because of the very weak signals normally involved. The receiver output is re-modulated on to an FM carrier and transmitted back to the home QTH via a 70 cm link. Commands to control the direction of the array, and the tuning of the receiver, are sent from the home QTH on the other half of the duplex link. There is a telemetry module to monitor some of the important remote site parameters. The remote site is powered from a solar charged lead acid battery. NEXT

4 NEXT This slide shows the complete equipment rack for the remote site.
On the top tier are the controller on the left, and the signal combiner and 1.8 MHz receiver on the right. On the lower tier are the battery, solar charger, 70 cm Tx and Rx NEXT

BEAM WIDTH NOT EXCEEDING 60 DEGREES. MUST BE STEERABLE TO COVER ALL COMPASS POINTS MUST FIT INTO A REASONABLE SIZE, eg 1 HECTARE The first design choice to be made in the project, was how to implement a directional array for the 1.8 MHz band. NEXT The design aim was for an array with a beam width not exceeding 60 degrees, . NEXT steerable to cover all compass points, NEXT and the design must fit into a medium sized paddock. NEXT

6 THE BEVERAGE ANTENNA Long low wire about 2-3m off ground
Needs to be about 2 wavelengths long. The first option considered was the Beverage antenna. The Beverage antenna was named after its inventor, and was first used in the 1920’s. It consists of a long wire quite close to, or on, the ground. The far end, which is the end closest to the required direction of reception, is terminated in a resistor of several hundred ohms, and the signal is taken off the near end via a step down transformer. The Beverage is a very widely, and successfully, used low noise antenna, and there are many hundreds of them in operation on the amateur LF bands. The length of the wire needs to be of the order of 2 wavelengths if the required directivity is to be achieved. This length is 328 m at 1.8 MHz. NEXT

7 This slide shows the horizontal radiation pattern of a 2 wavelength Beverage. The beam width is 61 degrees between 3 dB points. NEXT

8 An array of these elements could be constructed, that covers all compass points.
The slide shows 8 elements, but the desired result could be achieved with only 4, if the terminations and feed points were switched using relays. In any case, a piece of land 330 m by 330 m would be required, and this is an area of 10 hectares, the size of a small farm. Another alternative needed to be found. NEXT

The broadside endfire array is described by W8JI on his web site. It consists of 4 vertical elements spaced at the corners of a rectangle of 100 m by 21 m. The direction of fire is along the short side of the rectangle. The front pair and rear pair of elements are combined in phase, and then the two pairs are combined with a phase delay to give a null of the back. The array can be made from full sized (or nearly full sized) elements and used for both transmission and reception, or from very short elements if it is used for reception only. NEXT

10 100X21m BSEF ARRAY The slide shows the horizontal radiation pattern for the BSEF array. The beam width of the array is about 48 degrees between 3 dB points, and therefore easily meets the 60 degree target for the project. NEXT

11 This slide shows how 16 elements could be arranged to form an array that could be switched to one of 8 directions. The red dots represent the elements that would be used for north and south, the green dots for north east and south west, etc This array would provide excellent performance, but requires 16 elements, and about 800 m of coax. I decided to try and thin it out a bit to see if I would be able to reduce the cost and effort required. NEXT

12 This slide shows a thinned out version of the 16-element array, in fact just 8 elements are used around the circumference of the circle. The colour scheme shows which elements are used for which direction. For example the top left element is used in both the east/west antenna, and the north east/south west antenna. NEXT

13 92X38m BSEF ARRAY This slide shows the horizontal radiation pattern produced by this arrangement. The main lobe is about 54 degrees between 3 dB points, which is more than the 48 degrees achieved by the conventional BSEF array, but still well inside the 60 degree design target. The slightly increased width of the main lobe is because the distance along the long side of the rectangle is reduced from 100 m to 92 m. The sidelobes are all more than 20 dB down, which is slightly better than in the standard BSEF array. This configuration is the one chosen for implementation. NEXT

SMALL LOOP The next design decision was concerned with what to use for the individual elements. NEXT W8JI uses 6 m top loaded verticals with 4 radials each in his BSEF array. Tom has also introduced some resistive loading in an attempt to stabilise the response in the face of varying ground conditions. The addition of resistive loss causes some reduction in the signal level obtainable from the antenna, but the received signal to noise ratio is defined by atmospheric noise, so this is not a problem. Since the construction of 8 elements, with top loading and radials, represented a very considerable design effort, I decided to look for an alternative. NEXT I tried an element 3 m tall, with an FET preamp to remove the effects of the capacitive reactance. Such a short element has a series reactance equivalent to about 20 pF. When I first tested the short element and preamp out at home, I found the combination to be very noisy, and it seemed to be picking up all sorts of unwanted interference from nearby power lines, appliances etc. This result was very disappointing. NEXT I then tried a 3rd alternative, a small inductive diamond loop, about 1.2 m per side, and tuned to resonance with a series capacitor. This antenna was found to be much quieter, but, of course, does not have an omnidirectional radiation pattern, as required by the array. An additional problem is that there is a phase reversal between signals arriving from each end, which would have complicated the switching arrangement in the signal combiner. Since I intended to use the array at a remote location, I took both the short vertical, and the loop out into the country with a portable receiver, to compare their signal to noise performance away from suburbia. I found that they were identical in this regard, as all the noise sources were in the far field of each antenna. I therefore chose the short vertical/preamp combination as it was the simplest solution.

15 Each element is made from a 3 m length of 11 mm diameter hollow fibreglass tube, into which a wire has been inserted. The photo shows one of these elements with the bucket covering the preamp box, installed at the receiving site at West Melton. NEXT

16 FET BUFFER This is the circuit of the FET source follower buffer that I used with each element. The active devices are 2 J310 FET’s in parallel. The input network is a series trap to attenuate the 612 kHz signal from the Ouruhia medium wave broadcast station. If not attenuated, this strong signal generates a 3rd harmonic, which falls on MHz, right in the middle of the CW DX band. C4 resonates with L1 at 1.8 MHz, and eleminates the shunting effect of the trap for wanted signals. L1 is wound on a Micrometals T50-2 core. The signal is extracted through transformer T1 with R3 providing some gain adjustment so that the preamps can be matched. The DC supply to the preamp is fed along the coax, and inserted via L2. NEXT

17 This slide shows the circuit used to select the 4 active elements, and to combine the signals from them. Relays RL 1-4 select the 4 elements according to the direction required. The control circuitry for the relays is not shown, as it depends on the type of relays chosen. In my case, latching relays are used, and coil activation is controlled using a PIC processor. Signals from the first pair of broadside elements are combined in phase in T1, which consists of 4+4 turns on a FairRite two-hole binocular core. The 150 Ω resistor provides isolation between the two inputs of the combiner. (Note that I decided on a 75 Ω system, since RG59 coax cable is the cheapest available at 80 NZ cents/m, and a total of 440 m is required for the array). T3 provides an impedance transformation from the 37.5 Ω at the output of T1, back to 75 Ω. It is made up of 4 turns on the same FairRite core, and the input is tapped at 3 turns. DC is fed to the two selected elements (for use by the preamps via a 220 μH choke and the earthy end of T1, which is decoupled by a 100 nF capacitor. Signals from the second pair of broadside elements are combined in the same way. Relay RL5 selects which broadside pair is used for the front elements, and which for the back. In Fig 5, it is shown as it would be for directions N, NE, E, and SE. For S, SW, W, and NW, the relay is activated. The result is that the coax phase delay is always inserted in series with the front pair of combined elements, looking in the selected direction. The phase delay is made up from 40.2 m of the same RG59 coax cable. This cable has a velocity factor of 0.8, resulting in the required 110° delay. The attenuation of this length of cable at 1.8 MHz was measured as 1.7 dB. This is compensated by the pi attenuator consisting of the 15 Ω and 820 Ω resistors. Signals from the front and rear element pairs are combined in T5, with T6 again matching the output back to 75 Ω. NEXT

18 This is the block diagram of the remote receiver
This is the block diagram of the remote receiver. It is a simple single conversion design with the IF at MHz. This IF was chosen because of the availability of crystals for the filter. The fact that the IF happens to fall in the 80 m band is of no consequence, as the receiver is only intended for a single band. The antenna combiner output is fed to the input of the receiver, where it is amplified using a lossless feedback design for good strong signal performance. The signal is then filtered, to remove image noise, and mixed up to the IF in a diode ring mixer. The local oscillator port of the mixer is driven from an Analog Devices AD9851 direct digital synthesiser, clocked at 14.4 MHz from a small temperature compensated crystal oscillator. The DDS runs from about 5.38 to 5.50 MHz. Any questions about DDS will be answered at the end of the talk. The 4-pole IF filter has a bandwidth of about 400 Hz, and is home made from available crystals. The IF amplifier uses 2 obsolete Motorola MC 1350 IC’s, and the product detector uses an NE612 chip. AGC and audio are processed using op-amps. NEXT

19 This is a picture of the receiver and antenna combiner unit
This is a picture of the receiver and antenna combiner unit. The receiver board is on top, and the combiner underneath. The relays can just be seen on the combiner. The input amplifier, filter, and mixer can be seen at bottom left, with the post mixer buffer in the screen just above them. Next on the right is the crystal filter. At centre bottom is the IF amplifier, with the AGC processing at far right. The product detector and audio output stage are just above. NEXT

PROGRAMS DDS IN Rx SELECTS CORRECT ELEMENTS FOR REQUIRED DIRECTION NEXT The controller uses a 16F876 PIC from Microchip. NEXT The control code is written in WIZ C. This development system is available from Forest Electronic Developments in the UK, for about $150. I have written several radio projects in WIZ C, so buying the development system has been very worthwhile for me. NEXT The controller decodes the serial data from the home station, NEXT programs the DDS chip in the receiver, NEXT and selects the correct array elements for the required direction. NEXT

21 The picture shows the open controller box, with the PIC, and the DDS chip and reference oscillator.

Tx - TM8100 PA CTCSS TONE TO SWITCH ON LACK OF CARRIER FOR 4 s, OR A DIFFERENT CTCSS TONE, SWITCHES SYSTEM OFF. DATA LINK 4800 Bd RAW RS232 The UHF receiver/exciter is modified Tait TB8100 development equipment. It has a “sleep” mode, in which the power consumption is only 500 mW, thus making it suitable for remote solar applications. The modifications include the addition of a low noise amplifier, and removing a DC blocking capacitor in the audio output path, making it suitable for a medium speed data link. The sensitivity of the receiver for a conventional analogue FM signal, is about -123 dBm, or about 0.15 V, but for reliable decoding of the 4800 baud data used in the uplink, about -115 dBm is required. The transmitter output stage uses a modified Tait TM8100 PA. I removed one of the parallel output devices, which reduced the output power to 10 W, and lowered the current consumption to about 2 A on transmit. When “asleep”, the receiver has an idle time of 5 seconds, and then wakes up for about 40 ms, which is enough to detect the presence of an incoming signal. I use the CTCSS decoder to look for the presence of a tone to switch on the 160 m system, and the UHF transmitter. The system switches off if there is no uplink carrier for 4 s, or if it detects a different CTCSS tone for more than 15 s. The reason for the 15 s delay, is that the raw 4800 baud data can be interpreted as a CTCSS tone, and during initial testing, I found that the system was switching off at random. NEXT

23 POWER SYSTEM This slide shows a block diagram of the remote power system. A 45 Ah lead acid battery is charged by a second hand 33 W solar panel. The switched mode controller has 2 feedback loops. The first of these senses the output voltage, and backs off the charger when the battery terminal voltage reaches 13.8 V. The second loop senses the input voltage from the solar panel, and backs off the charger when the panel voltage is below 13.3 V. For this type of solar panel, maximum power is obtained when sufficient current is drawn to drop the terminal voltage to between 13 and 14 V. If the load current is too great, the terminal voltage drops off very quickly, and the power supplied is less than optimum. The solar power system has been working well since it was first installed in April of this year, even during winter months. (It should be said that the solar budget is fairly conservative, as the system is rarely used for more than an hour per day) NEXT

24 This picture shows the solar panel and its wooden frame
This picture shows the solar panel and its wooden frame. It is of the older type of construction with circular cells. More modern panels use cells that are nearly square, thus making better use of the available area. NEXT

The setup at my home station is much simpler than at the remote station. It consists simply of a second duplex 70 cm transceiver, a headphone amplifier, array controller, and a telemetry receiver. NEXT

The array controller and tuner is again based the 16F876 PIC. It has an LCD display showing the frequency of the remote receiver, the array heading, and the returned telemetry data. The receiver tuning is driven from a small shaft encoder made by Western Digital, and the array direction is controlled using a 4x4 keypad. NEXT

27 The photo shows the tuner in its die cast box.

28 This picture is taken from the output of a piece of propagation software called RadioMobile. This is a free package generated by VE2DBE. The picture shows the path between my home QTH in Brooklands, and the remote station in West Melton. The path length is 35 km. NEXT THE UHF LINK

29 This is the next output from the program, and shows the vertical profile of the terrain between the two stations. The estimated path loss is 172 dB. This figure is quite high, and has necessitated quite high gain antennas to achieve the required received signal margin of dB NEXT

30 This is a picture of the antenna at the remote end (The two antennas are identical)
They are 15-element quagi’s, dimensioned according to the original QST article on the quagi in the 1970’s. The antenna has a gain of around 15 dBi. The boom is made from fibreglass tube, and the elements are 3 mm diameter copper wire. NEXT

31 This is a close up of the feed point
This is a close up of the feed point. The feed shown here is via a sleeve balun, but I have recently removed the balun, as I changed the coax to a type with a solid outer. NEXT

32 TELEMETRY PARAMETERS - 160m RSSI, UHF RSSI, Batt Volts, Solar Volts.
SUPERIMPOSED ON AUDIO 300 Bd FSK, Tones at 1850 and 2150 Hz Telemetry has HP filter at Tx Audio has LP filter at Rx The system did not originally include telemetry. I found however, that I was always wondering about the battery voltage, and the UHF uplink signal strength at the remote end, and I had no measure of the 1.8 MHz S meter reading. I therefore decided to add some low speed telemetry. Since the 1.8 MHz receiver is CW only, the required audio bandwidth only extends up to 1 kHz. The FM downlink audio bandwidth extends up to 3 kHz, so I decided to see if I could introduce a 300 baud FSK signal, with tones at 1850 and 2150 Hz. Steps obviously had to be taken to ensure that the tones are inaudible in the headphones at the home QTH, so some fairly severe audio filtering was necessary. The telemetry has a high pass filter at the remote end, and the received audio is low pass filtered before it is passed to the headphone amplifier. The deviation generated by the telemetry is restricted to about 20% of available peak deviation, so as not to cause distortion in. the audio signal. NEXT

33 NEXT The picture shows the telemetry transmitter at the remote site.
The DC signals to be processed, are scaled and level shifted by a group of op amps, and passed to the ADC inputs of another 16F876 PIC. This PIC can process up to 6 analogue channels. The PIC generates a data frame consisting of 5 bytes, each representing one of the parameters of interest. The data is output as a 300 baud serial stream. The tones are generated using programmable dividers, driven from the PIC clock. The serial bit stream simply selects which of the tones to transmit. The data signal is high pass filtered using two 5-pole sections. Originally only a single 5-pole stage was used, but the data signal was still just audible, and would have been a nuisance when trying to decipher weak CW signals. NEXT

The input FSK audio signal is fed to 2 switched capacitor filters tuned to 1850 and 2150 Hz. The outputs of these filters are rectified, and fed to a comparator, which makes the decision on which tone has the greater amplitude. In this way the FSK signal is demodulated back to a raw data stream. The data is fed to yet another PIC, whose job is simply to convert the baud rate to 4800, so that it can be processed by the PIC in the tuner unit, and displayed on the same LCD as the receiver tuning and array direction. The reason for the data rate change is as follows. The tuner operates at 4800 baud, to give a smooth feel to the receiver tuning. However, 300 baud is the maximum rate that can be passed over an audio link when the audio band is being shared. In order to enjoy the convenience of using the same display for both tuner data and telemetry data, the rate of the latter needs to be translated up to the same rate as used by the PIC in the tuner. NEXT

As is always the case with big projects, there were several problems during commissioning. The first of these concerned the receiving array itself. When first tried, there was no noticeable front to back ratio, or any real indication of directivity at all. I had assumed that the preamps would all provide the same gain, but, of course, the gain is dependent on the forward transconductance (gm) of the individual devices. I found that the gain spread was, in fact, up to 3 dB. This was resolved by artificially slugging the output of each amplifier with a variable load, in the form of a 500 ohm pot. (Previously shown in the circuit diagram for the preamp). Initially all the pots were adjusted to maximum resistance, and the preamp gains compared. The one with the lowest gain was then taken as the reference. The gains of all the others were then adjusted by reducing the value of the variable resistance, until they all matched the reference. NEXT

- PICKUP FROM COAX BRAID Even after the preamp gains had been equalised, the actual pickup from each element was not constant. In the preamp design, some care had been taken to prevent signal pickup from the braid of the coax feeds. These are isolated from the antenna ground using a 220 uH choke. The impedance of the choke at 1.8 MHz, is of the order of 2 kohm. This value seems quite high, but, of course, the input impedance of the preamp is around 10 kohm, so the series choke is not as effective as expected. I had noticed that the pickup seemed to vary depending on the orientation of the coax in the vicinity of the preamps. It so happens that I had a number of 60 mm diameter toroidal cores, that had been lurking in my junk box for a number of years. I wrapped is many turns of each RG59 feed coax as I could through one of these, at the preamp end. I managed 16 turns. This step seemed largely to cure the unequal pickup problem. The permeability of the cores is unknown, but could easily be high enough to give over 2 mH with the 16 turns I managed to squeeze on, and this would give an impedance of 20 kohm, which proved to be sufficient to solve the problem. NEXT

37 Here is a sample of the toroids I used.

- HOSE CLAMPS SERIOUS OFFENDERS - METAL MOUNTING PLATE - PVC TAPE UNDER U BOLTS - COAX BRAID FADING ON UHF LINK - AIRCRAFT FLUTTER CAUSES DEEP SIGNAL FADES ON BOTH UP AND DOWN LINKS. - SLOW FADING DUE TO GENERAL TROPOSPHERIC PROPAGATION VARIATIONS Other problems encountered, were associated with the UHF link. NEXT The most serious of these occurs because of the use of a common Rx and Tx antenna at both ends of the duplex link. At the start of this project, I was completely ignorant of this type of problem, but salt mine colleagues have told me that it is quite common. In retrospect, this is not too surprising, as the transmitted signal at the home QTH is 50W, or +47 dBm. The receiver noise floor is around -127dBm, or 174 dB lower. If joints in the system are not near to perfect, any trace of micro arcing when the antenna and its feed are powered, introducing even the tiniest amount of noise into the system, will seriously degrade the receiver sensitivity. I found that the worst offenders in this regard were the hose clamps I used to fix the two halves of the antenna booms together, the metal boom to mast clamp, contact between U bolts and the aluminium mast, and the RG213 braided coax. The hose clamps have been removed. The boom to mast clamp has been replaced with a thick plastic version, I put PVC tape under the U bolts, and the RG 213 coax has been replaced with FSJ1-50, a quarter inch foam heliax cable. The other problem on the UHF link is fading, which is caused by both aircraft flutter, and general propagation variations. Both mechanisms cause deep fades of up to 15 dB, and are severe enough to cause dropouts on both up and down links. I suspect that there is no practical solution to this problem, apart from finding a site that is closer to home.

39 TESTING In order to test the operation of the array, I used a crystal controlled source, and loop antenna, as shown in the slide. Because of the relatively narrow pattern involved, it was necessary to place the source at least 1.5 km from the array, if the angular accuracy of the measurement was to be maintained. At this distance, with the relative inefficiency of the loop radiator, the signal from the source was only 10-20 dB above the noise floor. For this reason, the actual null depth was difficult to measure. On a 1:50,000 topographic map, I drew radials every 45° around a circle centred on the array location. I identified points where each of these radials crossed a public road. Some of these locations were up to 3 km from the array. Since I was alone when doing the measurements, it was necessary to drive to each of the 8 points, back to the array to do the measurement, and then out again to move the source to the next point. In a space of half a day, I covered a distance of 70 km, without going further than 3 km from the array. At each point where the source was placed, the array was switched to each of its possible directions, and the relative signal amplitude noted, using the AGC voltage on the remote receiver. Since the distances between each point and the array were different, the measurements had to be normalised to the maximum for that particular source point. From this information, I determined that the relative response at 45° away from the intended direction, varied between 4 and 5 dB. Since the 3 dB half beamwidth from the simulation of this antenna, is expected to be 31°, this result seems reasonable. It was also possible to determine that the back lobe levels were not excessive, and the worst case measured was 12-15 dB. This figure is not as low as predicted from the simulation, but, considering the problems experienced with matching of the element preamps, I was reasonably satisfied. NEXT

40 This slide shows a close up of the crystal controlled test source and loop feed.
A capacitor is placed in series with the loop to tune out its inductance, and the resistive part of the impedance is matched up to 50 ohms using cascaded transformers wound on the same binocular cores used elsewhere in the project. NEXT

41 I then performed a test from the home station, using my transmitter as a beacon. I attenuated the transmitter power down to about 100 mW for the test, to simulate a weak DX signal. The heading of my home station from the remote location, is about 060 degrees, as shown on the slide. I started with the array pointing North East, and transmitted my callsign followed by the letters NE, for the direction. I then switched the direction of the array in steps, in a clockwise direction, and transmitted the initials for the direction at each step. The sound file demonstrates this test. This demonstration shows that the array is quite directive. Sidelobe attenuation is especially good with the direction set to north. As a final note, I carried out this test during the middle of the day rather than at night. Even at the relatively close distance of 35 km, the amplitude of the night time sky wave component is more than sufficient to distort the results. END


Similar presentations

Ads by Google