Parasitic Element Tuning Procedure
- Cutting parasitic element lengths to formula not
- Measure and prune parasitic elements to resonance
with antenna analyzer (MFJ-259, etc.)
- Other elements must be decoupled during tuneup
- Director resonance @ 2.00 MHz
- Reflector resonance @ 1.90 MHz
The parasitic elements needed to be carefully tuned to the proper resonant frequencies. Simply cutting the lengths according to formula or to the dimensions in the computer model is not accurate enough in the real world, when considerations such as the velocity factor of insulated wire and environmental effects are taken into account. We strongly recommended direct measurement of the resonance frequency of the director and reflector. To do this, the corner of each element is temporarily opened and an antenna analyzer (such as the MFJ-259, which we used) is inserted at this point. By injecting RF into the element with the analyzer and measuring the SWR vs. frequency, the resonance can be determined at the point of minimum SWR. The lengths of the horizontal director and reflector segments are pruned to the resonant frequencies determined by the computer model: 2.00 MHz for the director and 1.90 MHz for the reflector. (The high resonant frequency of the reflector may seem odd, but appears to be a consequence of the sloping geometry of the element). While tuning one element, it is important that the the tower and the other parasitic element not couple and corrupt the measurement. (We are measuring self-resonance of the element, not mutual coupling resonance). To do this the tower is electrically opened from ground at its base, and the other parasitic elements are best lowered or removed completely during tuneup.