The 8 Element Controller

The K7TJR Array

The controller for this array is best described by breaking it down into sections. The job of this controller is to combine all the signals from the 8 antennas and their amplifiers producing the pattern as predicted by the antenna analysis software. In order to do that the controller must select 3 certain antennas for a 107-degree delay (same as –253 degrees), 3 other antennas for a –107-degree delay, and 2 more for a zero degree delay. It then combines them all together for output to a receiver. It is important to understand that –253 degrees is exactly equal to +107 degrees. Both these values are used interchangeably throughout this description. By utilizing the value of –253 degrees it is a little easier to visualize how this design can track phase over a varying frequency range.

Early on in this antenna’s development it became obvious that common mode signals on the antenna inputs could destroy the pattern and RDF of this array. Being that the array has over 30db of front to back and utilizes a very low average 3D response it should be obvious that all stray signals must be attenuated to a high degree (well over 30db). With RDF calculated from average 3D response and a fixed forward gain it follows that for maximum RDF one must maintain as low as possible average 3D response. No stray pickup. Every signal path and every power supply path is filtered or supplied with common mode isolation. The 75-Ohm inputs for each antenna are fed through an isolated winding input 1:1 impedance ratio transformer. The ground connection is carried through the inputs with an isolation inductor of 500uhy providing over 5000 Ohms at 160-meter frequencies to impede any unwanted current flow into the ground of the controller.

The 1:1 wide-band transformers are used for common mode isolation while maintaining the input impedance of the controller at 75 Ohms. This loads each antenna/amplifier and its connecting cable with their characteristic impedance. Having the proper load on the cable makes the phase delay of the signals be only affected by the cable length and its velocity factor. Making all 8 connecting cables the same physical length and materials takes their constant and equal phase delay out of the picture. The only exceptions to the 75-Ohm signal paths are in the phase1, phase2, and 3 input combiners where the internal impedances are matched then stepped back up to 75 Ohms.

The Direction control inputs use a transistor and a series of diodes to translate the TTL logic level to a somewhat more protected input. The addition of diodes to these inputs allows an over voltage of plus or minus 50 Volts to be applied without damage. Actual switching level is at +1.4 Volts sourcing 0.5 milliamps like TTL. This allows the input to simply be switched by grounding the input with a BCD switch or any TTL device. Refer to table 2 for the switching sequence.

These direction control signals are supplied to the BCD to decimal decoder chip. The BCD 1,2,4, and 8 are translated to decimal 0 through 8. Each output (1 through 8) of the decoder chip is used for a single direction of the array. There are 24 relays used to select one of three signal paths for each antenna. Each antenna has three relays activated one at a time to either send the signal to the –107 degree path, the 107 degree path or the zero degree path. The output of the decoder chip for each single direction is connected through 8 appropriately placed diodes (64 diodes total) actuating the necessary signal relays. Being that no two antennas next to one another are ever connected to the same phase, two relay signal outputs can be connected in parallel. This makes 4 each outputs for two phases, -107 and +107 degrees, plus two outputs for the zero phases. However looking at the decoding table shows that only three of the outputs are active at one time leaving one input open circuited and unterminated. Antennas 1&2, 3&4, 5&6, and 7&8 are connected together creating 4 outputs to be summed together for –107 plus 4 outputs for +107. Only two outputs are needed for summing together the zero phases as there are only 2 antennas connected at any given direction. The matrix switch supplies appropriate outputs to activate the precision terminators on the combiner unused inputs.

Three antennas are fed to the –107-degree combiner, three are fed to the +107-degree combiner and two are fed to the zero degree combiner. An array of two input Magic Tee combiners are used to implement a 4-channel combiner for both the –107 channel and the +107 channel. A single two input combiner is used to combine the Zero degree signals. The output impedance of the combiners is stepped back up to 75 ohms for application to the +10db amplifiers. The +10db amplifiers use heavy feedback to provide a 75-Ohm input and output impedance. They also use a heavy bias current to keep the third order inter-modulation distortion to a minimum. Feedback resistance is adjusted to trim voltage gain for each channel. The output transformer of the +107 degree (also same as –253 degrees) amplifier is used to add a 180-degree phase lag for its channel. This does two things, one it minimizes the amount of delay cable needed, and it makes both the –107 degree and the –253 degree delay cables similar in length (107 and 73 degrees). This makes the controller phase delays match more closely the physical wave delays for varying frequencies as the wave travels across the antenna array. In effect this makes the array work over a wider frequency band than if just fixed delay lines were used. Because the zero degree signal-channel has fewer transformers in its path a little phase delay compensation must be added. The 10db amplifier in the zero-degree channel also has a phase trim adjustment.

After processing through their required delay cables all three signals are fed to a unique 3-input combiner. This combiner is fed to a 6db attenuator circuit, which evens out any impedance mismatches that may occur as a result of unknown impedances being connected at the receiver output. This makes sure that the delay cables provide the actual delay they were designed for.

Actual circuit schematic

This array has a rather low output level and depending on personal preferences can require at least another 10db of preamplification. A good high-level preamp should be used to minimize any intermodulation. This array is not low frequency limited and can pick up high levels of broadcast signals from local stations. A BC band filter can also be used at the output of the array before any preamp as well. It should be a 75-ohm design or use matching transformers.

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