Hi Ed,
Can I quote the following Wikipedia entry from a damn good writer Arthur C Clarke?
Clarkeâs Three Laws are three âlawsâ of prediction formulated by the British science fiction writer Arthur C. Clarke. They are:
1 When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
2 The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
3 Any sufficiently advanced technology is indistinguishable from magic.
Invoking Law No 1, if you lay down enough radials (a wire that runs along a radius from the antenna base) (eg 40 off 0.2 wave long) the ground type is not relevant. The wires simulate a sheet in which the current flows on the top side only hence the underlying ground is not involved. All the ground out to the rf horizon is however involved in determining the low angle reflections. Look for a huge salt water swamp with no trees on the SOTA summit. DXpeditions often pick low islands or put their aerials on the beach to get the best signals.
Otherwise three wires at 120 degrees or 4 at 90 degrees orientation 0.25 wave long, well above ground are required. As current flows on the whole circumference of the radials they will interact with any nearby conductor or material with a relative dielectric constant greater than 1 even if below the radial. 10 wavelengths is the rule of thumb for the extent of the near field of an antenna but the interaction for radials is much reduced if the spacing exceed one wavelength.
A counterpoise is a wire or several parallel wires to âbalanceâ the antenna. Used for standby or city building based AM band station antennas. The wires are insulated at the ends and usually placed symmetrically under the antenna. The antenna itself was usually a wire suspended between masts in a T arrangement. An ATU is required. In amateur circles it is used mainly in single wire form to stop the microphone biting the operator.
Invoking No 2.
Because the nature of the radial spacing and the ground condition are variable and generally unknown some experimentation is required for field ops to get the best result. Or you could use a current balun and ATU at the feed point and get on with operating.
Invoking No 3.
It might seem like magic but antenna science is sufficiently well known to be able to be modeled accurately on a PC. The ARRL used to distribute a CD with an antenna analysis program with every copy of its Radio Amateurs Handbook. Reading a handbook on antennas - one with some theory, not a collection of build it projects - is strongly recommended before you go pruning your radials or running computer simulations. If you donât under stand the basics it does seem like magic.
Any antenna that uses the word âmagicâ in its name or advertising is probably rubbish.
BTW the simulation programs are a great way of filling in a snow-bound weekend.
73
Ron
VK3AFW