Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA6MHZ, Jan 4, 2012.
A friend of mine nicknamed it the still
I would point out that (Vfor-Vref)/(Ifor+Iref) references the reaction between the feedline (and transmitter source impedance, which one normally would assume would be equal to feedline impedance) and the fed antenna -- not the currents and voltages within the antenna.
Voltages and currents within the antenna will vary considerably, depending upon whether or not the antenna is tuned to resonance, upon where the coil is placed, upon relative diameters of elements (i.e., bottom is usually larger diameter than top), and upon where one chooses to measure.
One can de-tune the antenna to add inductive or capacitive reactance in series with the radiation resistance + Ground Loss + Coil Resistance to make the antenna "look like" 50 Ohms -- but you no longer have a resonant system. The only things one has accomplished by doing that are to lower the radiation efficiency -- and alter the radiation pattern. The better solution is to step up the impedance of the feedpoint so that it will match the feedline, while maintaining resonance (or near resonance) in the monopole.
Are you implying that you can lower SWR by increasing reactance?
Sorry, that's a false statement. The equation works wherever forward waves and reflected waves superpose in a distributed network to form a virtual impedance whether in a transmission line or on a standing wave antenna. Here's what Balanis says in Antenna Theory:
"The sinusoidal current distribution of long open-ended linear antennas is a standing wave constructed by two waves of equal amplitude and 180 degrees phase difference at the open end traveling in opposite directions along its length. ... The current and voltage distributions on open-ended wire antennas are similar to the standing wave patterns on open-ended transmission lines."
"Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backwards) and represented by traveling wave currents, If and Ib in Figure 10.1a." (page 550 in the 3rd edition)
And here's what Kraus said about "Standing Wave Antennas" in Antennas:
"A sinusoidal current distribution may be regarded as the standing wave produced by two uniform (unattenuated) traveling waves of equal amplitude moving in opposite directions along the antenna." (page 464 in the 3rd edition)
"It is generally assumed that the current (amplitude) distribution of a thin-wire (standing-wave) antenna is sinusoidal, and that the phase is constant over a 1/2WL interval, ..."
Standing wave antennas are one of the least understood subjects in amateur radio. For instance, a #14 horizontal wire 30 feet in the air has a characteristic impedance, Z0, of 600 ohms according to the single wire transmission line formula. Knowing the Z0 and assuming a feedpoint impedance of 50 ohms = (Vfor-Vref)/(Ifor+Iref) for a 1/2WL dipole, one can calculate that Iref at the feedpoint equals about 90% of Ifor at the feedpoint and the same is true for Vref = 0.9(Vfor). Pref on the antenna is about 80% of Pfor which calculates out to an SWR on the 1/2WL dipole of about 20:1 at the feedpoint. Of course, the SWR is infinite at the ends of a single wire dipole. After all it is a Standing Wave Antenna.
You might want to reconsider that.
If we have a resonant antenna of 20 ohms or any other resonant impedance, and we add series reactance to the antenna in an effort to increase impedance, the SWR simply increases. It has the lowest 50 ohm SWR when impedance is 20 j0, and would get worse with 20 +j30 or any other value.
If we did that, radiation efficiency of the antenna would not change unless the reactance also added loss resistance. We might have more problems feeding the antenna from the source from losses in the line or coupling problems, but the antenna's efficiency does not change.
It's true we can alter current distribution in the antenna and change efficiency, but that is because we are changing current through resistance somewhere.
Yes, one can mix theoretical formulae so that they represent what one wished that they represented, but reality depends upon application of relevant mathematics to the problem.
When one takes a "single wire in the air", one does not (by definition) have a dipole -- which requires TWO elements. Your argument is obviously flawed and specious. Actual relevant formulae are derived by rigorous application of observed results versus those predicted by the formulae. When reality does not agree with the formula, the formula is brought into line with reality -- which is how we know that a half wave dipole in free space presents roughly 70 Ohms purely resistive impedance at resonance. I say "roughly" because conductor diameter does impact the exact value. When placed over real ground, the impedance at resonance is dependent upon the ground conductivity and the distance of the antenna from ground -- but it still oscillates about 70 Ohms with excursions becoming smaller and smaller as the antenna is raised above ground -- until it essentially settles at the free space value of 70 Ohms.
When you take anything out of context, you can "prove" any point -- but context is crucial to understanding.
I would also point out that the distribution in a center-loaded antenna is FAR from sinusoidal. Most of the current is forced below the coil, making the degree-Ampere area of the bottom section a rectangle, and current tapers to zero at the tip, making the degree-Ampere area of the top section a triangle.
Totally incorrect. VSWR = 1:1 when Zo of the line is matched by the impedance of the antenna -- i.e., 50 Ohm line connects to an antenna (load) of 50 Ohms. If the 50 Ohm line is connected to a 20 Ohm antenna (pretty close to a 20 Meter 8 foot center loaded mobile input impedance), the VSWR is 2.5:1. If a reactance of 30 Ohms is added in series (which can be done by setting the upper antenna rod for lowest VSWR), the transmitter / feedline will see a match, but less power will be transferred to the antenna.
If, on the other hand, we resonate the antenna and employ a matching system (such as paralleled 72 Ohm quarter wave lines = ~ 36 Ohms) to transform the 20 Ohms to 50 Ohms, we transfer full power to the resonant antenna and present a 50 Ohm load to the transmitter.
Here's what happens when you add reactance to some resistive loads:
Seem you are saying that 20+j30 = 50 ohms resistive? If so, time to stop posting and open the technical books.
You obviously don't have The IEEE Dictionary handy. Those two poles in a dipole are NOT two physical elements - they are two electrical poles, one at each end of a 1/2WL wire whether the wire is broken in the middle or not. FYI, the solid parasitic elements on a Yagi antenna are dipoles because they have an electrical pole at each end of each parasitic element.
Is that 70 ohms a resistor or is it a virtual impedance equal to (Vfor-Vref)/(Ifor+Iref)? (A rhetorical question so no answer required)
That's only because you are plotting the physical length on the X axis. If you plot the electrical length on the X axis, the results become obvious (although it takes two sinusoidal terms to describe the current distribution in a loading coil). EZNEC will tell one, turn by turn, what the current amplitude and phase is.
Using the inductance calculator at: http://hamwaves.com/antennas/inductance.html
one can come up with Beta, the axial propagation factor for a loading coil and one can convert Beta to Velocity Factor using the following equation:
VF = (4*freq)/(191*Beta) where freq is in MHz.
One will find that the VF range of a 75m loading coil is ~0.02-0.04 so a 10" loading coil is actually ~30 degrees long electrically and has a current distribution that is two sinusoidal terms added together.
One needs to understand the sinusoidal current distribution on the following dual-Z0 shortened 1/4WL stub in order to understand the sinusoidal current distribution on a loaded antenna. When you come to understand how a stub that is 37.7 degrees long physically can be 1/4WL long electrically then (and only then) can we discuss this subject rationally.
You only need to take a quick look at a Smith chart to see that adding series reactance to any resistive load must make the SWR(50) worse. I don't know why anyone would say otherwise?