Don’s Antenna talks by N4DJ

  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...

  https://www.youtube.com/@dandypoint I also have quite a few amateur radio videos on my YouTube channel @dandypoint It seems to help to see a video as well as simply read a blog. I am trying to do both.

  There are lots of questions about radiation from antennas or wires specifically. It is not well understood by many amateurs. There are several ways to try and explain it without using high level math. There are also many variables for any particular installation.  However, regardless of the installation, radiation is caused by and is proportional to the current on the wire. I and other engineers use the term Meter—amperes. To try and simplify a complex topic,   it is the integral of rms current over the length of the wire.  Another way is to say it is that its the area under a plot of rms current against distance along the antenna. If you look at a plot of current on a halfwave wire, regardless of where it’s fed, you see a current minimum at the two ends ( high impedance points) and a maximum in the center ( low impedance point). Most of the radiation is from that center part where the standing wave ( not to be confused with the standing wave on a transmission...

  The image below shows the current on a wire. The curve is the standing wave of current on the wire while the arrow represents the direction of the current. When the arrows point in the same direction the current is in phase and where the arrows point in opposite directions the currents are out of phase . Note on the half wave wire ( figure 2.8 B) there is only one arrow and that indicates the current is in phase. Where there are two half waves (a full wave long wire) the currents in each half wavelength section will be out of phase if the wire is end fed ( Figure 2.8 A) If, however the antenna is center fed (Figure 2.8 C ) currents in each half wave section will be in phase. If a receiving station is broadside to the full wave end fed wire , it will receive equal strength signals from each half wave section but these signals will be out of phase and tend to cancel each other. There fore there will be a null broadside. In the case of a full wave center fed wire, the receiving stat...