19
Clearly capacitance is the key
Capacitance is just one part of it. The gamma match in your question is three things:
A sort of folded dipole, performing an impedance step-up
A parallel shorted transmission line stub, adding shunt inductance
A series capacitance
An equivalent circuit is:
simulate this circuit – Schematic created using CircuitLab
So ...
14
A gamma-match serves a triple purpose:
As a small diameter wire parallel and in close vicinity with the main radiating element, it will carry only a fraction of the main element current while being exposed to the same electrical field strength. This turns it in an effective up-transformer of the antenna input impedance.
It also forms together with the main ...
13
How does moving a feedpoint off-center in a dipole affect the resonant frequency and resistive load?
I'm going to assume we are discussing ideal, resonant dipoles.
Consider what's happening inside the dipole. Say you are transmitting a carrier. Say at one point in this carrier's cycle, the voltage is shoving all the charge carriers to the left.
What makes the dipole resonant is this: those charge carriers get pushed down the wire. As the approach the end, ...
10
You have aptly discovered why a balun is necessary when feeding a dipole with coax. You are right to think the book is wrong, because it is. With a coax feed and no balun, the current distributions on the dipole are not equal because some share of the current that should be on the right half (connected to the shield) of the dipole is instead flowing on the ...
10
A transformer can help and could also make things worse. I have some bad experience with this.
The way that a radio is damaged by static is simple:
Something charges up to a high voltage, and then there's a sudden breakdown resulting in a high voltage on the first transistor of the receiver.
This can happen in a few ways:
A long-wire or monopole type ...
9
The increased bandwidth of a folded dipole is almost entirely due to the extra thickness. Two parallel elements behave as a thicker single element. There is a small contribution too from the combination of the reactances of the transmission-line mode and radiator-mode acting in opposite directions.
9
10⁻⁴
which I think is too low power.
Why? Gut feeling says that is actually pretty good a transmission for such a distance.
Notice how your free space path loss is
$$\left(\frac{\lambda}{R}\right)^2\cdot\frac1{16\pi^2}\approx \left(\frac{\lambda}{R}\right)^2 \cdot \frac1{160}$$
and $\frac\lambda R=3\cdot10^{-3}, \left(\frac\lambda R\right)^2=9\cdot10^...
8
Loaded $Q$-factor
Like any resonant circuit, the bandwidth of an antenna is determined by its loaded quality factor, defined by $Q_{\ell}\overset{.}{=}\frac{X}{R}$.
The lower the loaded $Q$-factor, the broader the antenna's bandwidth will be: $BW_{-3dB}=\frac{f_{res}}{Q_{\ell}}$, with $f_{res}$ the resonant frequency.
Analysis of the loaded $Q_{\ell}$ of a ...
8
The efficiency of any antenna has a direct impact on the gain of the antenna. The gain of the antenna equally effects transmit and receive - a characteristic known as reciprocity.
The highest gain dipole is a 10/8 wave dipole. When you begin to shorten that dipole, its gain drops. When the dipole becomes very short (<<1/10 $\lambda$) it is called an ...
7
You don't need a balun, in that it will work and it won't hurt you. But your antenna will not work as well as it ought to.
When you are transmitting, the effect of a balun used to connect a coax (unbalanced) feed line to a dipole (balanced) antenna is to prevent the RF from the transmitter from returning down the outside of the feed line and go entirely ...
7
Trees do absorb some RF energy. So do buildings, hills, and other structures. There are very few cases where higher isn't better for antennas.
However, I'd guess that most amateur radio dipoles are below the treetops. Will some energy be lost? Yes. Will the antenna still work? Absolutely. Without the specifics of yours trees, and your antenna, we can't say ...
7
The antenna you describe is "omnidirectional" only in the xy-plane; it has zero radiation along the z-axis. Thus, your dipole is mounted vertically; i.e., x=0 and y=0 for all segments.
According to Antennas by John Kraus, the far E-field for a center-fed $\lambda/2$ dipole in free space is:
$$E = \frac{\cos\left({\pi\over 2}\cos\theta\right)}{\sin\theta}$$
...
7
Before getting to the questions you asked, an observation: the design doesn't appear to include a balun. Adding one is probably a good idea.
Is there a way to flatten the SWR in this range [80m] or is this graph typical?
It's typical. Because 80 meters is such a low frequency, the fractional bandwidth of the entire band is quite large. For this reason ...
7
And why is an antenna resonant, because the voltage and current are in phase or because the reactances cancel out, or both ?
Reactance ($X$), resistance ($R$), and the phase difference between voltage and current ($\theta$) are related by:
$$ \tan \theta = { X \over R } $$
So if reactance is zero then $X/R$ is zero, so $\theta$ must be zero, meaning voltage ...
6
A little capacitive reactance is what gives you the greater bandwidth.
In a regular ½λ dipole, the current that flows along the conductors are in phase. When we add the second conductor in a folded dipole, what we are really doing is extending the dipole. As a result the current in the new section flows in the same direction as those in the original dipole....
6
In tuning a trap there are two concerns which are somewhat contradictory:
You want the trap to have a high impedance on the target band.
You want losses to be low.
The objective of a trap is to reduce the currents on the inactive portion of the antenna to a negligible amount. This doesn't require an impedance that's as high as possible: it requires only an ...
6
The gutter will be in the near field of the dipole. It may affect the tuning of the dipole or its radiation pattern, but it will not "short out" the dipole as long as the dipole wires are not in direct contact with the gutter or downspouts.
You may have other conducting materials in the near field such as siding, sofette trim and conduit. Any of these may ...
6
...if you have a full wave dipole antenna, there would be no transmission...
Here begins the misunderstanding of antenna directivity, antenna efficiency, and antenna impedance. Let's analyze each one independently and then bring it all together using the above example.
Directivity
Directivity of an antenna can be defined as the power density the antenna ...
6
Look closer at the diagram. At the two wires coming from the source, the voltage is NOT always zero. The only way for the voltage at the center point to be zero is for the two source wires to occupy the same point which is impossible. Think about the voltage as an electric field which is indeed zero at the point halfway between the two wires.
6
The ideal number of turns depends on core material, geometry, and frequency. This is why you find such variance in how many turns should be used.
More turns increases the choking impedance up to a point, but decreases the choke's self-resonant frequency (SRF). Once the SRF goes below the operating frequency, adding more turns increases the distributed ...
6
I applaud you for diving into this topic. The notion of trap efficiency has been driven largely by antenna marketing FUD (Fear, Uncertainty, and Doubt) with implicative phrases such as "no lossy traps". Since then, hams have been wringing their hands at the thought of all of those losses in their favorite trap antenna. Here is a current example of such an ...
6
Your reasoning is not too far off.
Say you attach a signal generator to an antenna, and then probe the magnetic and electric fields at many places around this antenna in a test chamber. The ratio of the electric field strength to the magnetic field strength is called the field impedance.
For any antenna, several wavelengths away (in the far field), this ...
5
An end fed wire may not necessarily work BETTER for reception, but it should be effective and it is much simpler to put up. You might pick up a bit more hash noise from local RFI sources with an ungrounded configuration though, due to common mode currents induced on the outer shield of the feedline.
Feeding the wire through an isolation transformer, and ...
5
Baluns are designed to be transformers (like 1:1 4:1, 6:1, etc.) or choke baluns, and both.
For an antenna, the purpose of a choke balun is to create a high-impedance to common mode currents that would flow on the outside of coaxial cable shielding. These common mode currents can cause all kinds of problems such as RF in the shack, matching problems, and ...
5
An antenna of a different length, relative to the wavelength of the signal, will have a different radiation pattern — it will transmit or receive most effectively in a different direction.
image by user Dantor from Wikimedia Commons
(In the diagram, “L” is the length of the antenna and “l” is the wavelength.)
A regular dipole has a pattern like the yellow ...
5
The UK band plan for the 2m (VHF) band can be found here on the RSGB website. In the UK, the 2m band is between 144.000MHz and 146.000MHZ.
Of note is the section 145.5935 - 145.7935MHz, which are the repeater outputs. Depending on where you are in the UK, there should be at least one repeater that you can here there. Whether or not there is any activity is ...
5
To begin the discussion, it is helpful to understand the effects of shortening any antenna to a length below resonance. In all cases, the directivity of the antenna is reduced but this tends to be a fairly uniform reduction regardless of how much shortening occurs. The reduction in length also reduces the radiation resistance of the antenna. This is ...
5
Here is an analysis of a vertical, 1/2 wavelength, 2 meter dipole suspended at various heights above ground:
As you can see, as you raise the bottom end of the antenna from near ground level up to about 7 wavelengths, the gain increases. At the same time, the elevation angle of the maximum gain decreases (becomes closer to the horizon). As you continue to ...
5
The classic dipole is a half-wave antenna. This means that the total length of the antenna is lambda/2. So writing it as 1/2-lambda is OK from an English language point of view, but not IMO as a rigorous mathematical formula.
For a half-wave dipole, each side of the feedpoint is one-half of that or a quarter-wave.
5
The preferred term is effective aperture. Effective aperture is defined as:
$$A_e=\frac{\lambda^2}{4\pi}G \tag 1$$
where $\lambda$ is the wavelength of operation and G is the linear gain of the antenna.
So you can see from equation 1 that you need to include the gain of the antenna - this was not reflected in your formula. The gain of the antenna is ...
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