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Exploration of a design for an 80m vertical antenna - Part I

Introduction

This article explores the design of an 80m vertical antenna.

The design objectives are:

Design

Baseline

The traditional vertical antenna use in a four-square steerable phased array configuration is a ground mounted quarter wave vertical with many quarter wave radials lying on the ground or shallow buried.

Fig 1:

The gain of ground mounted verticals with shallow buried radials is quite dependent on the soil type, but typically increases with the number of radials. Fig 1 shows the gain of a quarter wave monopole over quarter wave radials shallow buried in homogenous 'average soil' (εr=13, σ=0.005Sm). It can be seen that gain improves with the number of radials, but the law of diminishing returns applies; whilst four radials is about 1dB better than two radials, 120 radials are only 0.14dB better than 64 radials.

This study takes the efficiency with 120 radials as the practical maximum. A goal of 80% of that efficiency or 1dB less for a 'full size' quarter wave antenna system could be achieved with 16 buried radials.

Elevated radials

An alternative to buried radials is elevated radials.

Fig 2:

Fig 2 shows the effect of radial height on gain for a quarter wave monopole with 3, 16 and 64 radials. It can be seen that gain can be poor, especially for configurations with few radials, but improves markedly once the radials are raised more than about 0.1m above ground, and gain is relatively independent of height up to a few metres in height.

Fig 2 suggests that an antenna system using just 3 quarter wave radials at 2.4m height has gain about 0.9dB down on 120 buried radials, and increasing the number of radials to 64 makes negligible improvement to system gain.

So, if the inconvenience of elevated radials can be tolerated, just 3 radials are sufficient for good performance (0.9dB down on 120 buried radials, or 81% of the practical maximum efficiency). Indeed, under some circumstances, elevated radials may be easier and cheaper to install, may have better service life.

Shortened vertical with elevated radials and capacity hat

Fig 3:

Fig 3 shows a shortened vertical with three elevated radials and a capacity hat formed of wires at the top end of the three support guys. In this case, the radials are quarter wave and vertical is 12m, 0.6λ/4.

The shortened antenna has slightly lower gain at 1.3dB down on 120 buried radials and full size quarter wave antenna, or 74% of the practical maximum efficiency, but is 60% of the height, and can be constructed on one central support and three posts to tie off the radials and guy ropes.

Impedance matching

An impedance matching scheme that lends itself to this application is a hi-pass L match, and the antenna length can be adjusted to supply the necessary series capacitive reactance.

Fig 4:

Fig 4 shows the combinations of feedpoint Rs and Xs that can be transformed to 50+j0 with just a low loss shunt inductor.

The feed point impedance of the modelled shortened antenna adjusted by tuning the top hat so that Rs,Xs fall on the blue line is 21.5-j24.7. It can also be seen from Fig 4 that the required shunt inductance is about 1.8µH.

The process on a real antenna is to measure the feedpoint impedance with an antenna analyser or the like, and adjust its length so that the combination or Rs and Xs fall on the blue line. The required Lp for Rs is given on the same graph. The inductor should then be trimmed to obtain an exact match.

Transmission line

The distance from the shack to the antenna will be around 160m. A length of LDF5-50A is available for the main run, and TLLC gives a loss of 0.37dB for a worst case load end VSWR of 1.3.

Dimensions

Rigging

Ideally, the radials should be as horizontal as possible. The natural shape that the wire forms is a catenary, and the catenary needs a minimum sag to assure survival under high winds.

Using Antenna wire catenary calculator, the minimum sag for 2mm bare HDC to withstand 40m/s wind speed (safety factor = 3.5) is calculated as 340mm or 40N (≈4kgf) tension with no wind.

Performance

Peformance was modelled in NEC 4.1.

Table 1: System performance
Element
Detail
Single
Antenna (dBi)
0.6λ/4 vertical, 3 elevated radials, capacity hat
0.10
Main feed line 160m of LDF5-50A -0.36
Fly lead 2m RG213 -0.02
System (dBi)

-0.28

Table 1 is a summary of expected system performance.

Fig 5:

Fig 5 shows the modelled radiation pattern in the vertical plane. The half power points are at 8° and 60° elevation.

Links / References

Changes

Version Date Description
1.01 05/10/2010 Initial.
1.02    
1.03    

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