[Antennas] Re: Dipole Suppression of Even-Ordered harmonics

2002年1月16日 19:05:42 -0500

Doc, KD4E, wrote:
> Let's explore this a bit, please? Does a dipole, and 
> perhaps slightly more so a folded-dipole, actually 
> "reject" even-ordered TX harmonics or does it partially 
> suppress them by failing to radiate them very 
> efficiently?

Actually the antenna itself, radiating on even-ordered 
harmonics, is not less efficient. However the feed 
impedance of a center-fed antenna whose total length is an 
even multiple of one-half wavelength at the operating 
frequency is quite high. It is the transmitter output 
impedance matching network, nominally designed for low 
impedance output, that provides the harmonic suppression. 
If a low impedance transmission line is used to feed the 
antenna, on even harmonics the VSWR will be extremely high 
leading to poor power coupling into the antenna, and 
increased losses in the transmission line. The result is 
that the harmonics are not radiated as well as the 
fundamental, even though the antenna efficiency is high. 
In this case, you have to view the transmitter's output 
network, the transmission line, and the antenna as a 
system, and say the system does not radiate the harmonics 
as efficiently.
> I ask because it makes for an interesting relationship to 
> certain amplifier designs. I forget off-hand which does 
> which but does not the circuit selection in an amp (e.g. 
> push-pull) lend to the creation of even or odd harmonics?
> If one then chose an amplifier design that favored even-
> ordered harmonics one could only use that amp with a 
> dipole and thus compensate for some (how much?) of the 
> garbage created by the amp with careful antenna design 
> rather than lossy and costly filters.

Push-pull amplifiers tend to cancel even-order harmonics in 
their output stages. Odd-order harmonics are still 
present. The input impedance of center-fed antennas whose 
total length is an odd multiple of one half wavelength is 
generally low, providing little extra feedline attenuation 
due to high VSWR on the odd-order harmonics. Thus the 
push-pull amplifier provides no added harmonic suppression 
here.
While an interesting question, the point is somewhat moot 
as the FCC requires adequate harmonic suppression from the 
transmitter, independent of what antenna is used.
However, if you look at the problem of maintaining spectral 
purity of the transmitter, you will find that the harmonic 
suppression properties of a given impedance matching 
network in the transmitter can be affected by the actual 
load the transmitter sees. We generally think of the 
network providing matching from the output tube or 
transistor to a 50 ohm or similar load. When a real 
antenna and transmission line are the actual load the 
network sees, the network may see close to its design load 
at the fundamental frequency. But on harmonics, the load 
may vary wildly in impedance from extremely low to 
extremely high, and the harmonic suppression obtained with 
a purely resistive load may be quite different that 
obtained in actual usage.
 73, Barry WA4VZQ [email protected]