Iterations on Inter-Boom Angle
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Why? A log-periodic with fully parallel booms technically violates part of the concept. In theory, every parameter should be expanding from front to back. Some are impractical to achieve, and so are often ignored. The inter-boom angle ψ, however, is easy to manage. So let's play with that in search of maybe a little more gain.

In addition, there is something further I want to know. In more than one antenna book I have found mention of bi-directional log periodics obtained by directly opposing the booms (ψ = 180°). So while we are playing around with ψ, let's go all the way, finding out how well that works.

All charts are thumbnails. Click on any to view it larger.

• Antenna: Log Periodic Sawtooth Array
• Polarity: Vertical
• Frequencies: 18-30 MHz; Analysis @ 24.89 MHz
• Impedance: 300 Ohms
• Length: 15-30 meters (depending on psi)
• Elements: Count = 15; LHA^* = 9.78°; Diam = 1.628 mm (14 AWG)
• Boom: Spacing = 200 mm @ feed point; Diam = 2.588 mm (10 AWG)
• Segments: 120/Lambda, 2047 total (*.ant)
• Analysis: NEC2 via Nec2Go, version 2.1.24
• Accuracy: 0.98 ≤ AGT ≤ 0.99
• Download: ZIP Archive Just only Nec2Go ANT files this time.

^* Leading Half-Angle. Ref. Definition

Inter-Boom ψ Effect on Gain and SWR.

Dual SWRs? In the 5th chart below you'll see two separate traces for SWR: one for 200Ω_z, another for 300Ω_z. That is because one expected Z_in provides the lowest average SWR, while with the other you obtain the nearest approach to 1:1. I thought both worth knowing, so authored my Perl script for making pretty SVGs from Nec2Go sweeps so that charts would present in this way, and do so automatically. Ref. Nec2Go Utilities

ψ = 0° (parallel)  Z_in near 300 Ω


ψ = 30°  Z_in near 300 Ω


ψ = 60°  Z_in near 300 Ω


ψ = 90°  Z_in near 300 Ω


ψ = 120°  Z_in between 200 and 300 Ω


ψ = 180°  Z_in varying greatly Ω

Conclusions: At ψ = 30° dBi is a wee bit more than ither 30° or 60°. Charts below will search for the sweet spot more narrowly.

With fully opposing booms (ψ = 180°) SWR shows something of an issue. One which gets worse as frequency increases. But this may be due to the model, whose elements start at 46MHz. My thought now is this. A more proper arrangment for opposing booms, would be if saw teeth did not stop at 46MHz, but continued all the way up. Thus to obtain a geometry where each boom's apex for the angle α should coincide. Or, as a possible compromise, we could possibly meeth the concepet part-way. Simply spread the two booms further apart. Achieve this by extending the feed wire between them, enough so that for the apex for each α would still coincide. That would be a very different, antenna, however, not comparable to the one here. An modeling experiment for later, perhaps?

Effects on Pattern of Radiation

  Boom ψ = 0° (parallel)   8.34 dBi Azimuth Gain

  Boom ψ = 10°   8.81 dBi Azimuth Gain

  Boom ψ = 20°   9.21 dBi Azimuth Gain

  Boom ψ = 30°   9.48 dBi Azimuth Gain

  Boom ψ = 40°   9.49 dBi Azimuth Gain

  Boom ψ = 50°   9.4 dBi Azimuth Gain

  Boom ψ = 60°   9.29 dBi Azimuth Gain

  Boom ψ = 70°   9.14 dBi Azimuth Gain

  Boom ψ = 80°   8.91 dBi Azimuth Gain

  Boom ψ = 90° (right angle)   8.59 dBi Azimuth Gain

  Boom ψ = 100°   8.17 dBi Azimuth Gain

  Boom ψ = 120°   6.94 dBi Azimuth Gain

  Boom ψ = 140°   5.2 dBi Azimuth Gain

  Boom ψ = 160°   4.42 dBi Azimuth Gain

  Boom ψ = 180° (opposing)   4.37 dBi Azimuth Gain

Conclusions: At ψ = 49° this LPSA exhibits a 1.15 dBi increase of gain versus parallel booms.

With booms opposing, the pattern is indeed bi-directional, but not symmetrically so. As with the unwelcome surprise when it came to SWR, this unwelcome assymetry may be the result of the two booms failing to share a common apex with regard to the angle α.