General site information and help

Information on topics raised in portable-antennas.com site.

About the designers

Each of the designers listed here share a common structure and form, while presenting combinations of controls specific to the type of antenna covered.

In each case, the user is presented with a group of controls in the upper left portion of the page: these controls enable the user to completely specify the characteristics of the antenna, namely frequency, wire lengths, angles, and so on. Please note that all length dimensions are given, or calculated, in meters.

Example designer layout

In the upper right portion of the page is a graphics area where the modelled antenna is presented in a 3D context. The graphic can be zoomed, panned and rotated. Antenna feed-points are shown as colored markers; the user may also choose to toggle the appearance of a semi-transparent plane at the height (1.85 meters) of an average person, in order to gain some perspective on how large the antenna would be when erected.

In the lower half of each of the designers, a group of controls enable the calculation of an antenna's performance, and the presentation of charts and diagrams showing the performance in various ways, in any combination of the following:

Example antenna performance controls section

Calculation of an antenna's performance is achieved using a NEC4.2 (Numerical Electromagnetics Code v4.2) code-base licensed to the site.  Results of the calculations, and the resulting charts and diagrams, are entirely comparable with those presented by programs such as EZNec and 4nec2, for similar antennas.  Nonetheless, NO claims are made concerning the accuracy of such information or results; the graphics and data presented in this site are provided "as-is" for the general interest and edification of the user.  Refer to this site's Disclaimer for more information.

 

Notes on ground types, and their effects in modelling antennas

The specification of a particular ground type beneath an antenna plays a large rôle in determining the radiation patterns, also gain and impedance, exhibited by the antenna.

Ground types come in three basic flavours:

Ground types

Several basic real ground types have been established over the years, based on measurements made of the conductivity and dielectric constant (permittivity) of the ground in many locations, and representing soil types ranging from rocky and poor, through rich pastoral types, to types encountered in city/industrial areas.  In addition to these real ground types, two water types are given, plus a perfectly conducting ground, and the option to place the antenna in free space.  Here is a listing of the ground types used in this site:

Relative
quality		 Conductivity (S/m) Permittivity
(Dielectric
constant) (F/m)		 Surface soil type
Very poor 0.001 5 Cities, industrial areas
Poor to very poor 0.002 10 Sandy, dry, flat, coastal soils
Poor 0.002 13 Rocky soil, steep hills, typically mountainous
Average 0.005 13 Pastoral, medium hills, and forestation, heavy clay soils
Good 0.01 14 Pastoral, low hills, good soil
Very good 0.0303 20 Pastoral, low hills, rich soil
Perfect Infinite N/A Perfectly conducting ground
Fresh water 0.001 80 Fresh water, lakes
Salt water 5 81 Sea water, low sea beach
Free space N/A N/A Free space - antenna is in free space

Values for soil and water types in this list are selected from the table in the ARRL Antenna Book, 20th Edition, p.3-13.

Modelling limitations

For modelling of antenna performance, the real ground types used in this site are taken to represent soil in a flat ground surface beneath the antenna, and extending indefinitely to the horizon in all directions.  The program does not take into account terrain variations that may be very important to a given antenna situation (see e.g. the section below this one.)  The soil is assumed to be uniform in type and quality, down to indefinitely large depth - in reality, of course, the ground beneath the antenna is normally stratified and changes quality with depth.

The antenna designers in this site perform calculations using a NEC v4.2 engine to produce radiation patterns, VSWR diagrams, etc. and, for real ground types, employ a fast Sommerfeld/Asymptotic integral calculation method, which gives accurate results in most situations.   The calculations are accurate enough for signals in the upper HF region and above, since these signals penetrate the earth to shallow depths only.  For signals in the lower HF regions or below, the signals can penetrate the earth to considerable depths and can thus be variably affected by the changing soil quality at different depths.  Users should be aware of this when designing antennas for the low HF bands.

Effects of ground on radiation patterns

In general, the effects of ground on an antenna depends largely on the orientation of the antenna, and its' height above the ground:

 

General notes on the effect of ground slope on elevation radiation patterns for portable users

It is a widely-recognized fact that the slope of terrain in the vicinity of an antenna, as well as in the far-field of the antenna, can have a profound effect on the elevation radiation pattern of the antenna.

In general, an antenna set up at the top of a downward-sloping piece of ground - as may occur when activating portable from a summit - will exhibit an elevation radiation pattern (ERP) which will tend to follow the direction of the slope, and will be lowered by an amount less than, and in some cases equal to, that of the slope. The amount by which the ERP is lowered will depend on the ground conditions, and hence on the degree of roughness of the slope, as well as the conductivity and dielectric constant, of the soil or materials constituting the slope.

This lowering is due to the reflected ground wave from the downward slope interfering with the direct wave, effectively lowering the angle at which the antenna radiates its signal. The effect is noticeable with both horizontally and vertically polarized waves from the antenna, although the effect tends to be less in the case of vertically polarized waves.

Steeper slopes can cause the antenna's ERP to split into multiple lobes, rather than a single, well-defined beam. This splitting of the pattern can affect the antenna's gain, sometimes leading to higher gain than expected in some directions, and sometimes lower gain in some directions.

These few notes are of course no substitute for a formal study of this subject, which is very complex and beyond the remit of this site.  Nonetheless, the notes presented here should give the portable operator a good general idea of how slopes near to their antenna may affect their signals.

The ARRL offers a program called HFTA (HF Terrain Assessment for Windows) which is included in the CD included with some editions of the ARRL Antenna Handbook. According to their documentation Operating Instructions for HFTA, Version 1.04 ,

"HFTA is a ray-tracing program designed to evaluate the effect of foreground terrain on the elevation pattern of up to four multi-element HF monoband Yagis in a stack."

The program uses custom-defined terrain model files, based on USGS map data in DEM (Digital Elevation Model) or NED (National Elevation Database) seamless formats in the USA. In addition, usage of SRTM (Shuttle Radar Topography Mission) 3-arc-second format is described, allowing the user to generate terrain profiles for areas outside the USA.

HFTA has been used by many radio amateurs to model the terrain in different directions around their home QTH. This has allowed them to estimate the effects of the terrain on their signals and to permit them to better plan the arrangement of their antennas to achieve better results in DX and in contests. The amount of work involved is considerable, but can pay dividends for a particular home QTH.  Whether this program can be useful to the occasional activator of SOTA summits or POTA parks remains to be seen.