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Antenna Resistance

This version was saved 12 years, 11 months ago View current version     Page history
Saved by PBworks
on November 11, 2007 at 4:39:24 pm
 

Antenna Resistance

 

The losses of a full sized vertical radiator.

 

 

a. = the radiation circuit in free space

b. = the antenna radiator

 

The current path is shown on the positive half cycle.

 

R1 and R3 above are the main loss factors for a 1/4 wavelength vertical. R1 is quite low and so most of the loss is via the ground return resistance R3. As you will find, in LPAM terms on medium wave; the idea of erecting a full sized vertical is not feasible for most stations on medium wave.

 

 

When R2 is greater than R1 + R3 the efficiency of the antenna is at more than 50%. As the antenna is shortened down from it's 1/4 wavelength length, it's radiation resistance lowers. And so there is a point at which half the power is lost across R3. And if shortened even more then most of the power can appear across R3. And so the efficiency of shortened antennas will always be less than the 1/4 wavelength ideal length.

 

The thing then for improving a shortened antenna is to improve the ground conductivity. 16 radials, of 2 mm diameter, 10 meters long; buried 100 mm has a resistance of 4 ohms in ideal garden type soil. Given the soil is well mineralized. Yet remember not all soils everywhere across the nation are ideal. So if the soil resembles pasture soil then chances are you have some good ground for an antenna location.

 

Below are the calculated number of radials needed for the desired ohmic value on the right. Each radial is 10 meters long, 2 mm diameter, buried 100 mm deep.

 

  1. Ohms

64 = 1 ohm

32 = 2 ohm

16 = 4 ohm

08 = 8 ohm

04 = 16 ohm

01 = 20 ohm

 

When you compare all of the various grounding methods, the use of wire radials in a star layout around the mast. Of at least 16 radials. Proves to offer the least ground resistance as compared to grounding rods or wire mesh layouts. So this is the ideal ground return system for an antenna.

 

The radials are all attached to a 6 to 8 foot (or 10 foot) grounding rod driven vertically into the ground.

 

In a year or so, after the ground radials are installed then the ground settles fully around the radials. And the soil mineralizes around the radials. So the ground return system gets better with time. The only way feasible to improve the conductance of the ground is to add in powdered charcoal, and that would take a ton or two and is mostly infeasible. Though not impossible for someone somewhere.

 

After a six months you will want to go out to the antenna and retune the matching network. And then again in another six months.

 

Some people attest that rock salt works well if evenly distributed in the soils and mixed with a tiller. However you do not want to pour salt into the ditch with the radials in a concentrated form. It is caustic on the radials. And it does better if spread though the soils of the whole radial system area. And yes, it would take a ton or two for the amount of area a LPAM antenna covers. The element in the salt that increases the conductance is the sodium metal. Sodium is highly reactive with oxygen and water and will combine with the other mineral molecules. Charcoal powder would be ideal with rock salt if one could afford them both. It would take about 1 bag of rock salt every two to three square yards.

 

If you use an antenna bridge such as the MFJ 204B and an antenna with known factors such as R1, and measure it's feed point resistance. You can subtract R1 from the feed point resistance and derive the ground resistance of your area. This can help you to calculate the amount of power loss in your ground return resistance. More will be said about this later and an antenna design for use up around 11 meters will be discussed so that you can build the antenna and make a measurement of your local ground conductivity. 11 meters is ideal since the antenna is not long and hence manageable. Also it will be inexpensive to build due to it's size. And you do not need a license to emit a rf signal on 11 meters. The ground conductivity is pretty much the same from medium wave up into the HF band.

 

The reason you want to build the antenna is that if given the right sizes for each component in the antenna. Then you will know the ohmic losses of the radiator. And can subtract that from the total feed point resistance.

 

Losses of a short vertical radiator.

 

As can be seen from the above illustration, the short vertical radiator has five loss factors. The lower radiator section or mast resistance R1, the coil resistance R2, and the upper rod resitance R3. All in series with the radiation reisistance R4 and the ground return resistance R5.

 

R feedpoint = R1+R2+R3+R4+R5

 

The short antenna consist of enough radiator length and coil wire length to equate to 1/4 wavelength of wire. And so, the extra length is wound up into a coil. The losses of the coil are reduced by using a large wire diameter such as 4 to 8 mm, and a large coil form diameter.

 

 

The length of 8 mm wire or tubing used for the coil in the LPAM vision of sizes, used on medium wave, is quite long. Up to 30 or more meters in length. Stranded 4 to 6 or 8 mm aluminum wire is best if it is cheaper than aluminum tubing. Copper wire or tubing will be quite heavy given the proper wire diameters. You might be able to get by with stranded #12 copper wire. If you use a wire under sized in diameter make sure it is stranded so that the skin effect of the wire is reduced. Skin effect is a source of resistance. As you can see this will be quite an expensive part of the antenna to construct.

 

In constructing the coil keep in mind that it has to be light weight and yet sturdy. Have low wind resistance as much as possible. Yet have the physical sizes required to have low loss.

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