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I would like to construct a 2m Loop with a sense antenna. I have built just a loop antenna with great success that is resonant at 2m finding null's works quite well on local repeater outputs.
I would like to further my project and create one with a sense antenna, similar to what 80m DF units have installed.
Has anyone done this or know the location to plans for building one?
A small loop antenna is bidirectional in the direction of the plane of the antenna and has nulls that are perpendicular to the plane of the antenna. Due to its bidirectional nature, there is ambiguity as to the bearing of transmitted signal when only one receive location is in use.
Through the addition of a so called sense antenna, the bidirectional pattern of the loop antenna can be transformed to a more unidirectional, cardioid pattern. The omnidirectional sense antenna is typically a simple vertical antenna. For a 2 meter sense antenna, this can conveniently be made as a quarter wave antenna although it can be, and often is, shorter.
To generate the optimum cardioid pattern, the signal from the vertical antenna and the small loop antenna should of equal amplitude and combined with a 90° phase shift. This will result in the null of the cardioid pattern that is 90° from the null in the small loop pattern.
The 90° phase shift is easily accomplished with a short length of coax cable that is electrically 90° long (or an odd multiple thereof). If you use Belden 9258 (their RG8X), this would be a cable length of 1 foot, 4.5 inches at 146 MHz. Add to this, the length of the coax coming from the loop antenna. So if you have 1 foot of RG8X coming from the loop, for example, you can combine the sense antenna with a 2 foot, 4.5 inch length of coax with a tee connector and then continue with any length of coax to your receiver. You could also create the 90° phase shift through a lumped circuit. The lumped circuit has the advantage of field adjustability although once set, it should not require further adjustment.
Generally the sense antenna will be more sensitive than the small loop antenna. Its sensitivity can be varied with a simple non-inductive variable resistor in series with the whip. This allows the antenna to be easily field adjusted. Others have had success with pruning the length of the sense antenna until the two match in signal strength.
Since the cardiod null is not as sharp as the loop null, the common practice is to allow the sense whip to be switched in and out of the circuit. This allows the initial bearing to be taken with the loop alone by finding the null of the signal and then with the whip switched into the circuit to confirm the direction. Alternating back and forth can fine tune the direction. The same procedure can be used to find the peak of the signal when the signal is too weak to discern the optimum null position.
