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Old 6-Jul-2016, 3:33 PM   #106
tripelo
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 126
Part 5 (continued from previous post)

Specifications, Measurement, Analysis and Discussion


In an effort to reconcile the various gain and SWR numbers available for these two antennas, the following chart was composed.

For each of the two antennas and each channel (8 and 9), the gain and SWR are presented. The sources for the gain and SWR are listed in the left column.

The manufacturers numbers are derived from the technical documents found in the links provided in a previous post. Antennas Direct lists gain figures as dBi, very likely the Televes numbers are also in dBi.

The 4NEC2 results are from simulations described in the immediately previous post. The gain numbers (dBi) listed here are calculated to include mismatch loss due to SWR.

The DFW Measure numbers are a summary of actual measurements at 25 ft from a location in Garland TX (described in a previous post). The units of values listed as gain are dB, either positive or negative relative to that averages of the two antennas (per channel).





Summary and Discussion

These two antennas are of different design, but for the channels tested yielded relatively close gain and SWR numbers.

The Antennas Direct antenna consists of driven element loops (inner and outer) with a screen reflector. The loop is made of relatively thick conductor that helps to broaden frequency response (flattens SWR). This design is in some ways similar to two stacked dipoles (top element fed at ends instead of center fed). This stacking effect tends to compress vertical beam width, thus for a given amount of total gain, allows for a slightly wider horizontal beam width. This C5 antenna being a relatively broadband design tends to have a flat gain and SWR response across the entire VHF high band.

The Televes antenna is a conventional Yagi design with folded dipole driven element. This design tends to have increasing gain with increasing frequency, up to the design limit. According to Televes literature the design limit is 230 MHz (the upper channel in the European band). Conventional Yagi designs tend to have a flattened oval shaped aperture (area over which signal is gathered). This flattened oval-shaped aperture shape means more of the antenna gain is a result of horizontal beam becoming narrow. So, one would expect the beam width of the Televes antenna to become more narrow at higher frequencies (where the gain is highest).

Since much of the data presented in the above preceding chart is in agreement.

The above chart for these two antennas tends to indicate:

1. Antennas Direct and Televes gain figures are consistent with each other.

2. Manufacturers gain numbers are consistent with 4NEC2 simulation results.

3. Antennas Direct SWR values are consistent with actual measurements (at 25 ft. AGL).

4. Computer simulations combined with field measurements can be used to characterize antenna performance, in some cases.

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