Re: A (late) New Year Quiz
Thank you to everyone who took part - there seemed to be quite a bit of interest.
Here are the answers:
1. Needs to be exactly a half-wave long to be resonantIn practice "no", because of something called the "end effect".
If we could make a dipole infinitely thin and somehow suspend it without any end supports, the current flowing at the ends would be zero and it would occupy an exact half-wavelength, calculated using 150/f metres (or 492/f feet). In practice, the thickness of the dipole and the presence of end supports means there is some capacitance at the ends of the dipole which allows a small current to flow; so a practical dipole must be slightly short of an exact half-wavelength for resonance - typically by 5% - leading to the formulas you often see quoted of 142/f metres or 468/f feet.
2. If resonant at F MHz is also resonant at 3*F MHz; e.g. 7MHz and 21MHzNot exactly. The "end effect" explained in answer 1 is obviously only present once on a wire. So a wire resonant as three half-waves would be:
150/f + 150/f + 142/f long; which is obviously not 3 times 142/f. So a dipole resonant as a half-wave on 7MHz will be "three-half-wave" resonant slightly higher than 21MHz. Folk who build a 7MHz dipole and expect to use it on 15m usually find it is resonant just outside the top of the 15m band.
Older editions of the Antenna Book took this into account by quoting a resonant length formula of:
492*(N-0.05)/f feet
where N is the number of half-wavelengths
3. Needs to be shorter when constructed from thicker wireYes - slightly. The thicker the wire, the more the end capacitance and the shorter the dipole needs to be for resonance. On an 80m dipole the difference in length might be around 6" between one made with #12 gauge wire and one made with #18 gauge wire.
4. Needs to be shorter when constructed from insulated wireYes. The velocity of propagation of a signal along an insulated wire is slower because of the dielectric properties of the insulation; so a resonant dipole ends up being shorter. The thicker the insulation, the slower the velocity. Typically the shortening effect is in the range 0% - 5%.
5. Needs to be much shorter - typically 66% - if constructed with coax, because of the cable's Velocity FactorNo - this is a common "myth". In fact on another radio discussion site someone recently put it forward as a way of making a 6m vertical much shorter!
Velocity Factors like 66% only apply when the signal is totally contained within the dielectric material of the coax cable; that's what happens when the cable is operating as a transmission line. But when you use it as an antenna, the current all flows as Common-Mode current on the outside surface of the braid; then, only a small proportion of the signal is affected by the dielectric constant of the outer jacket, and again the shortening effect will be just a few percent.
6. Is very efficient and typically radiates over 90% of the power fed to itYes. The only loss mechanism in a half-wave dipole is the resistance of the wire, and even with something as thin as #18 gauge the resistance is very small compared to the Radiation Resistance.
7. Is more efficient if it is shorter; so a 20m dipole is more efficient than a 40m dipoleYes - this one seemed to surprise most folk. Assuming both the dipoles are constructed from the same size wire, even though the efficiency of both is high, the 20m dipole is slightly more efficient. The RF resistance per unit length on 20m will be 1.4 times that on 40m because of the "skin effect"; however the 20m dipole is only half as long. So the length "wins" over the skin effect, and the effective loss resistance of the 40m dipole is greater than that of the 20m dipole - making it less efficient.
8. Feedpoint impedance drops as it is lowered, and very near the ground it's almost zeroNo - that's what many text books show, but it only applies over a perfectly conducting ground. Over real soil, the impedance begins to climb again as the dipole gets very near to the ground. Over average ground, a dipole's impedance never falls below about 45 Ohms, no matter how low it is.
9. Has an omnidirectional azimuth pattern - rather than a classic "Figure 8" - when it is lowIt depends! A low dipole transmits its strongest signal vertically, and in that direction it is pretty much omnidirectional. However it still transmits plenty of signal at lower elevation angles, and in those directions the pattern still approximates a "figure 8". So if you were trying to work DX with a low(ish) dipole, it still matters which way it is orientated.
10. Needs to be a half-wavelength high for best DX performanceNo - it needs to be higher! The best height for DX performance is the height at which the dipole transmits most signal at a take-off angle that matches the DX path. Typically that will be much higher than a half-wavelength. For example, over a long-haul DX path, a 20m dipole at 100ft will be almost 8dB stronger than one at 30ft (half-wavelength).
But give yourself a point if you interpreted the question as "at least a half-wavelength high"
Cheers,
Steve G3TXQ