Conclusions

The K7TJR Array

 

†††† In the quest for maximum performance of low-band receiving antennas it has become painfully obvious that there may be a point of diminishing returns for any given real estate. In the course of discussing the operation of this array in the previous pages it was mentioned more than once that as you get better and better performance from an array of this type that it takes very little signal ingress from an undesired source to change the array output. The main reason for this is that this array cannot have any induced signals that are over 30 to 40 db down from any undesired direction. Any added unwanted responses add to the antennas overall integrated average gain and therefore reducing itís RDF. I mention this because there are some other limiting factors based on antenna location that can limit the maximum RDF obtainable in a given location. I think I have reached the limit at my QTH at about 13db. While evaluating the array response I have also included transmitting verticals, power lines, local irrigation pipes, railroad tracks, and local power drops. This array is quite sensitive to any resonant vertical transmitting antenna making it an absolute necessity for detuning during receive. Simulating the transmitting vertical as far away as 700 feet still affects the RX array pattern by reducing either the front to back ratio or the RDF depending on the selected direction of the array. Detuning the vertical is quite successful for restoring performance. There are very long local power lines that follow my local private road and perpendicular to that a county road with lines several miles long. These lines form a corner about 600 feet or so from the array with the local road lines getting within about 200 feet. By varying the length of these lines and applying the occasional pole ground wire it became painfully obvious that there is an effect from these 40-foot tall lines. It would be impossible to simulate or predict the actual effects but suffice to say they are there and quite real. They were able to affect the array pattern by reducing the front to back ratio from 44db down to as low as 30db and affect the RDF by several db.

†††† I am not aware of any techniques for detuning the power company lines so I have concluded there may be a limit to the maximum RDF one can realize with a given set of dimensions from the array to local power lines. There is also a local power drop coming in to my shack from the closest power pole. There is a ground rod at the terminus making it look like an inverted L coming from the pole transformer. Simulating this on EZNEC also shows some affect to the RX array pattern. I suspect these wires could be possibly detuned by clamp on cores on the ground wire as in the simulation this wire had the greatest effect. I also simulated some local irrigation lines that are about 1000 feet long but grounded every 40 feet by a large wheel touching the ground. Fortunately I could see no effect from these lines. I have been unable to determine if any of the tin roofs or metal farm buildings could possibly have an effect on this array. By the way I have also determined that local power lines also affect my 1000-foot long Beverage antennas. This is the point where simulation meets reality.

†††† There is virtually no way to simulate all the effects that might be happening to our receiving antennas by their proximity to local untouchable obstacles. In my case with the local power lines and other objects I have concluded that for any single RX antenna on this property that about 13db is the RDF limit. There is some good news however as it is possible to make additional gains by using diversity receiving or even combining two identical arrays.

†††† The last top-band season was spent listening to the 1000-foot Beverages with one ear and the 8 element circular array with the other ear by using an Orion transceiver. Using this technique has brought a whole new readability to most weak stations. Barry, N1EU has recordings using this technique on his website. It is well worth a listen. I have a nagging desire to build a second 8-element array spaced several hundred feet away, hopefully gaining another couple of db over the 1000 foot Beverages. Even if one cannot find the room for an array like this it sure is worth some effort looking at shortened verticals with High-Z amps. They are pretty easy to get to work with up to 4 active elements and bit more difficult in an 8-element array like this. No ground radials needed !

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copyright K7TJR

 

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