Don’s Antenna talks by N4DJ

 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...