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Old 27-Jul-2013, 11:53 PM   #45
tripelo
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 127
4NEC2 Analysis: YA-1713 & Homebrew Long Yagi

Analysis with 4NEC2 software can often help provide insight into antenna modifications and resulting performance.

A computer simulation of an antenna is an approximation that is no better than the models represented in the software. The software model is based on numerical parameters that represent an antennas physical dimensions and electrical specifications. Often the dimensions are measured on a physical model. Electrical specifications may have to be determined by experiment. It is rare that measurements and specifications allow software to fairly completely describe an antennas performance, but is often close enough for practical purposes.

The performance of Winegard YA-1713 without balun, was simulated, both as a single antenna and stacked at 43 inches. The plot below shows gain (dBi) and Standing Wave Ratio* (SWR) over the upper VHF band. For a single antenna, the maximum gain is about 12 dBi, which would be close to Winegard specifications of ~10 dBd.



Interesting that stacking at 43 inches shows higher gain (~2.9 dB) at the low end of the band. Recall that a stack distance of 43 inches was empirically determined as best (within +/- 18 inches) for reception of channel 13. The real world includes ground effects (mainly reflections) that were not included in the simulations. Variations in height above ground can dramatically affect antenna patterns, thus affecting perceived gain. The change in SWR from single unit to 2 units stacked, could be a result of mutual impedance or coupling between the two antennas. A stack distance of 43 inches is relatively close, and some mutual coupling could be expected.

The performance of the long Yagi without balun was simulated, both as a single antenna and stacked at 89 inches. The plot below shows gain (dBi) and SWR (relative to 300 Ohms) over the upper VHF band. For a single antenna, the maximum gain is ~>14 dBi, about 2 dB greater than the YA-1713. Stacking at 89 inches shows additional gain improvement approaching 3 dB over most of the band.



The following plot shows the mismatched gain for both the stacked pair of YA-1713 and the stacked pair of long Yagis.



Mismatched gain is the normal gain (shown earlier) that has been reduced by the effects of SWR (normalized to 75 Ohms). The numerical value of SWR represents the mismatch to the characteristic impedance of the system. In this case, the characteristic impedance is 75 Ohms. Any impedance presented by the antenna other than 75 Ohms reduces the amount of power that can be transferred from the antenna to the transmission line or to a system (preamplifier, receiver etc). The amount of power rejected due to mismatched impedance is the mismatch loss. Mismatched gain is the full gain minus the mismatch loss. Mismatch loss is present to some degree in all systems with SWR greater than 1. It appears this final gain figure is the equivalent of what has been called ‘Net Gain’ by Ken Nist, at HDTVprimer.

The mismatched gain shown in the graph is probably realistic for a system with good SWR with respect to the transmission line and tuner, or preamp (if one is used). If the transmission line and tuner, or preamp, have poor SWR with respect to each other, then the overall mismatch is statistically likely to be worse with greater mismatch losses causing gain for the system to be less than that shown.

* Standing Wave Ratio (SWR): A measured or calculated number that mainly represents an impedance mismatch. SWR=1 represents a perfect impedance match, numbers greater than 1 represent progressively worse impedance match. SWR is an indirect way to describe ‘Return Loss’. To have a ‘Return Loss’, there does not have to be a transmission line physically long enough to support an actual standing wave length.

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Attached Images
File Type: gif ya-1713 single vs stack.gif (9.6 KB, 4480 views)
File Type: gif Long Yagi-single vs stack.gif (10.0 KB, 4241 views)
File Type: gif Mismatch Gain_Long Yagi vs 1713.gif (9.4 KB, 3980 views)

Last edited by tripelo; 28-Jul-2013 at 2:33 AM.
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