Don’s Antenna talks by N4DJ

  Antenna information. This is some basic fundamental antenna information that may help understand a little about how antennas work. First of all antennas need to be different sizes for different frequencies. For example a short antenna ( in Terms of wavelength) will not radiate as well or as much energy as a longer antenna. The basic radiation is proportional to the product of amps on the antenna times the length. The term is meter-amperes or feet-amperes. Given the same current on a wire a longer wire radiates more than a short antenna. Along the same lines, for a given length of antenna increasing the current will radiate a stronger signal. As long as the product of meters times amperes is the same the radiation will be the same. It must be noted that longer antennas (over 1/2 wavelength) will have different radiation patterns. For such long antennas the directive pattern will also depend on the location of the feed point. For example a one wave long center fed antenna will h...

  NVIS and velocity factor Lots of Hams build “NVIS” dipoles and place them just a few feet above ground. I want to shed some light on some of the issues a lot of them run into. First is the length of the antenna. In free space a halfwave is 492/frequency in MHz.  So for a 40 meter frequency of 7.2 MHz that gives the halfwave as 68.3 feet.  When using wire to build a dipole, the wire has a velocity factor that needs to be considered. The wire also has two ends where it’s connected to some kind on insulator. To take into consideration the effects of both velocity factor and end effects on the wire a factor of 95% is applied to the free space constant of 492. That results in the familiar formula of length in feet being 468/frequency in MHz.  The velocity factor of a wire changes as it gets closer to the ground. You can measure the velocity factor change by cutting a dipole for 7.2 MHz when it is reasonably high. You can observe the resonant frequency move lower and low...

 This is a link to a presentation I gave on antennas to a few clubs. https://youtu.be/rfWG16gNBaQ?si=YV9xw94m72hOChuV

  Calculating Capacitance There is a very simple formula for determining the capacitance of an air capacitor.  C (in pf)= 0.2248 times area of plate divided by spacing between the plates. If the plate area is 4 square inches (2 inch by 2 inch plates) and the spacing is 1/8 inch we get: C= .2248 X 32 =    7.2 pf for the two plate capacitor. For each additional plate you gain 7 pf. ( 7.2 pf if you want to be precise but just using 7 is close enough for most things) So with 3 plates it’s 14 pf 5 plates makes it 28 pf 10 plates makes it about 63 pf 11 plates is 10x7 = 70 pf The multiplying factor is the number of plates total minus 1 since it takes two plates to get the first 7 or 7.2 pf. Even after using 11 plates, the difference between using 7pf and 7.2pf is only 72-70 or 2 pf.  It is very easy to make a capacitor for something like an antenna tuner for your antenna. Many antenna tuners need capacitors in the neighborhood of 50 to 150 pf.  In the example abo...

  Antenna Gain, dBi, and Radiation Angle tidbits I compare all antennas to a full size halfwave dipole if possible. It’s very hard to beat a dipole if it’s is reasonably high in terms of wavelength. I also like to use the term dBi for antenna gain. There is a very good reason for doing this. First of all, for example, if you start with 100 watts at a point source it is very easy to calculate what signal strength you would have in any direction and at any distance. Although this omnidirectional     point source cannot exist in the real world, we can calculate what the signal would be if it did. That becomes our well defined reference point. It is also relatively easy to calculate what the signal strength would be from a dipole antenna in free space where it is unaffected by any reflections from the earth or anything else for that matter. Consequently we know with almost complete certainty what the gain of a dipole in free space is relative to that theoretical point so...

  Antenna thoughts August 2025 If one knows the basics of antennas then it becomes easy to choose what antenna is best for you. There is no one “best” antenna. There are very good antennas of course but even a very good antenna is not going to be right for all uses. I will discuss some antenna basics here and give some “rule of thumb” guidelines on how to think about antennas.  The halfwave dipole can be considered the basic building block of antennas. I think understanding the dipole is the key to understanding antennas. Also for antennas we need to think in terms of wavelength, not feet or meters.  For example a dipole 1/2 wavelength long and 1/2 wavelength high above ground is an excellent antenna. It has a gain of about 8dBi off the broadsides. It has a slight reduction in signal strength off the two ends. It’s peak radiation ( off the sides) is centered at about 30 degrees. The actual radiation is rather broad from about 15 to 45 degrees and gradually drops off above...