Don’s Antenna talks by N4DJ

Two things are very important to understand when putting up antennas. First the high current points like the center third of a dipole radiates most of your signal and should be high and in the clear. Low current parts radiate minimum and can be low or bent. Also on longer wires adjacent halfwave sections are out of phase and radiation tends to cancel or at best change the radiation pattern drastically. Knowing where the high current and low current points are is important to understanding radiation. On a single halfwave wire the radiation is broadside. On a full wave wire with current in one half of the wire is out of phase with current in the other half and broadside radiation is cancelled or at least significantly reduced.  The half square antenna is a good example of this. With the half square, current in the two vertical wires is in phase and adds up broadside to the antennas. Current at the center of the top wire is zero but current to either side of center is out of phase so ...

  We know several things that help us relate antenna height to distance best covered by that antenna. We will limit this to one hop F layer skip. This is a primary path for most cases. The height of the antenna determines the peak radiation angle. In most cases the radiation falls off about 3 dB when you go 15 degrees above and below that peak angle.  So we have an optimal range of 30 degrees where our signal is at its best.  We also know that the F layer of the ionosphere exists somewhere between 150 to 250 miles high. So we have a range of heights as well as a range of angles.  Consequently we can be pretty confident in the following: Since an antenna 1/4 wave high has a peak radiation angle at about 90 degrees or straight up, it will be best for zero miles out to about 300 miles. Since an antenna 3/8 waves high has a peak angle of about 55 degrees, it will be best from 125 miles out to 560 miles. Since an antenna 1/2 wave high has a peak angle of about 30 degrees,...

  The above chart may need some explanation. I have marked the chart up showing coverage of a horizontal antenna at 3/8 wavelength high. On 40 meters this would correspond to a height of close to 50 feet. At that height the maximum radiation is centered on roughly 55 degrees. Zero degrees being horizontal and 90 degrees being straight up. At angles just above 70 and just below 40 the radiation decreases by 3 dB. Normally these 3 dB points are used to define the limits of the beam width of an antenna. There is not much difference when operating within theses “3 dB points”. The chart shows the upper and lower limits of a 3/8 wave high dipole. The fact that is says dipole is not significant as this applies to all horizontal antennas and not just dipoles. Since the ionosphere height varies from about 250 to 400 KM the corresponding distance for any radiation angle will vary depending on this height. For the 3/8 high horizontal antenna we can predict the minimum and maximum distances th...

This is a plot of two full size verticals, a 40 foot high dipole and a Half Square antenna on 80 meters For all but low angle DX I think the dipole is the best antenna. It appears that the verticals may have a slight edge on dipole at low angles but due to the Pseudo-Brewster angle that may not be true. That all depends on the quality on the ground several wavelengths away from the antenna not the radial system. However, the Half Square is the clear winner for long distance DX and higher dipoles perform much better  on 80 meters than one only at 40 feet.

 I want to make sure everyone understands that you absolutely can not have gain in all directions. Anytime an antenna has power gain is one direction it must have a corresponding power loss in one or more other directions. I am not only talking about compass directions but elevation angles also.  When someone asks the question about what is the best configuration for a particular antenna, there is no answer unless we know the purpose or purposes of the antenna. A high gain antenna is probably only going to be best in only one direction and that is at the expense of all other directions unless you can rotate that antenna or put up one for every direction you desire! Even directive antennas have nulls in their elevation patterns. So a highly directive antenna pointed in a desired direction may be too high or too low to concentrate the signal where you want it. I tend to prefer less directive antennas for everyday amateur radio operations. The antenna nulls are much more signific...

Measurement of loss on coax is probably misunderstood by many but is basically simple. Let’s take a 100 foot length of RG-8X as an example. In the first case let’s assume that it is simply open at the far end. No connection at all. This is an “open circuit” condition.  If we apply a very small radio frequency voltage at the near or input end this voltage will travel down the coax at somewhat less than the speed of light. It could actually be 80% or 90% of the speed of light. When this signal reaches the far or open end of the coax it will be reflected. All of it will be reflected back towards the input end. If we have a reasonably good instrument it can measure both the magnitude of input voltage we started with and the magnitude of the voltage that has returned after being reflected. The magnitudes of these two voltages gives us a lot of information.  First of all we can calculate the standing wave ratio that exists at the input to the coax. We can also calculate the loss in ...