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An excellent question! Without diving too deep into the theory, let's start with a few basic terms.
The "signal" that an antenna is receiving or transmitting is called an electromagnetic wave. This is exactly the same type of wave as light. It is just that our eyes are sensitive to a narrow range of frequencies that we call light. Electromagnetic waves ...
25
"Balun" is a portmanteau of "balanced" and "unbalanced". Anything made to interconnect a balanced and unbalanced load can be called a balun.
A common-mode choke (like a length of coax wound over a ferrite ring) works as a balun because it inserts a high impedance in the common-mode without affecting the differential-mode. By ...
23
There's always a ground. Whether it's what you intend it to be or not is another issue...
A mag-mount antenna is grounded through capacitive coupling between that magnet and the metal it's stuck to. At VHF/UHF frequencies, this effect is adequate for good results which explains the popularity of these mounts. Some folks advocate adding a wire instead of ...
23
No. Many types of antennas may be unsuitable for transmitting.
A so-called active antenna contains an integrated low-noise amplifier (LNA) for amplifying weak signals. (This is very common for GPS antennas.) This amplifier cannot pass a signal in reverse, and could be damaged by transmit power levels appearing on its output. It is possible to have an ...
19
If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well?
No.
Assuming we're talking about a characteristic impedance of 50 ohms, a 50 ohm resistor (otherwise known as a dummy load) will show a SWR of 1:1, although it will almost certainly perform very poorly as either a receive or transmit antenna.
The low SWR simply tells you that ...
18
The $492/f$ formula is for an ideal antenna in free space, the $468/f$ is an estimate for real antennas at a reasonable height over ground.
The $492/f$ formula is a conversion from metric units to English units for the fundamental frequency and wavelength ($\lambda$) formula. $c = 3\times 10^8_{m/s}$ (the velocity of light) and $f =$ frequency --
\begin{...
18
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 ...
17
Your body acts as the ground when using a handheld radio. By holding the radio, you are capacitively coupled to the radio and make the other leg of the antenna. This is one of the reasons to hold the radio in your hand away from your body while using it, rather than using it while attached to a belt clip. (The other reasons pertain to RF safety.)
Of course, ...
15
What do you mean by "circuit"? Do you mean there's a loop of conductor from one side of a battery to the other, possibly with some other conducting components along the way? How about this circuit?
simulate this circuit – Schematic created using CircuitLab
Is there a circuit here? There certainly isn't any way to follow a line from one side of the ...
14
With the information you've provided, there is no way to tell. Feedpoint impedance and VSWR have no relationship to antenna efficiency. See What is the relationship between SWR and receive performance?
The feedpoint impedance also has no relationship to antenna efficiency. A "50Ω antenna" means the impedance at the feedpoint is 50Ω. The impedance at other ...
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, ...
13
Simplified answer:
There is no relationship between SWR and receive performance.
There is one condition for this simplification to be true: the received RF noise floor must be above your receiver's noise floor. Beyond this, anything you might do to increase the output from the antenna does nothing to increase the signal to noise ratio, which is a better ...
13
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
As has already been stated, this will add amplitude modulation to your signal — you will "achieve its rated gain in all directions" but only at the peak of the modulation; the average will be much less, equivalent to an antenna with no azimuth gain but a similar elevation gain pattern.
You and other readers might be interested to learn that this modulation ...
12
I'm going to approach this a little differently starting from roughly the same place. Here I am going to use a resonant $\lambda$/2 20m dipole driven by 100 W as the model.
Let's compute the current at the feed point of a dipole at resonance, this is found with the input power (100 watts) and the feed point impedance; which for our dipole is assumed to be ...
12
Short answer: electrically small antennas have a relatively low radiation resistance. With less resistance, the resonance is less damped, meaning a higher Q factor and consequently less bandwidth.
To expand on that a bit, consider a children's playground variety swing. It's essentially a pendulum, and depending on the length of the swing and the mass of the ...
12
Most antennas are reciprocal — they have the same properties when receiving as when transmitting. This means that in many ways, a good antenna design makes both a good receiving antenna and a good transmitting antenna.
(The biggest exception to this is active antennas which have an internal amplifier that only works for receiving.)
However, the properties ...
12
In theory, if you had lossless conductors in the antenna and a lossless matching network, your shortened, 1 foot dipole would have a gain of only ~0.7 dB less on 160 meters than the gain of a full size 1/2 wavelength dipole. But the world is far from perfect.
The efficiency of an antenna is defined as:
$$\text {Efficiency}=\frac {R_r}{R_r+R_l} \tag 1$$
...
12
The answer from a theoretical purist is probably, "No," because your diagram specifies zero radiation outside of the disk. Practically speaking, there will always be some radiation above and below the disk and the shape of the disk will not be as perfect as you describe.
A well known solution that approaches your requirements comprises multiple ...
11
It's very difficult to predict the impedance of an end-fed wire, other than to say it's high. Usually it's determined empirically.
You are looking for a theoretical formulation. Consider, the feedpoint is a voltage source which makes a difference in electric potential between to things. The end of the dipole, and...what?
simulate this circuit – ...
10
Yes, a random wire can be a practical antenna. Anything conductive can be loaded up. Somethings work better than others. But if you want to play around, there's no reason the antenna has to be an "antenna".
There are, of course, some drawbacks:
You must have a tuner.
You might not get a match on every band you'd like to work.
For efficiency, you need a ...
10
So, by design, the elements of any Yagi have a zero current going through the center point. That's pretty obvious be symmetry: assuming you excite the "left and right" halves of the driving dipole with exactly opposing voltages, everything should be symmetrical across the plane through the middle of that dipole.
Hence, if you approach that plane from left ...
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
The difference is the "Velocity factor". A 36cm long physical coax wire of this type is electrical 54.4cm long.
Different types of wire have different velocity factors.
https://en.wikipedia.org/wiki/Velocity_factor
9
As one solution, you can combine the antennas with a power divider. See How to combine two 50 Ω antennas such that they appear as one 50 Ω load?
This makes your pair of antennas into a phased array. If there's no overlap between their coverage, you effectively lose half your antenna gain, or 3 dB. This is because on transmit, half your power goes into the ...
9
A parabolic reflector reverses the sense of circular polarization. so your feed must have the opposite sense or "handedness" to the incoming signal to avoid significant polarization loss.
9
Antennas are often resonant. Their physical dimensions are adjusted so standing waves develop at a particular frequency, like a bell rings at a particular tone.
Feedlines are not usually resonant. Usually an engineer ensures the end of the feedline is terminated (by the antenna or the radio) with an impedance that matches the characteristic impedance of the ...
9
The antenna itself won't be significantly affected by the water.
However, waterproofing the coax connection is essential. If this is not done, water will creep inside the coax by capillary action and ruin the coax.
There are several products that can be used, for some examples see 3M's application guide. However all have three elements:
An underlying ...
9
Ham operators often tell me that comparing gain to an isotropic radiator isn't much use because it's only a theoretical antenna, is this true?
No, that is not true. I generally find that hams that denigrate or choose to ignore the isotropic antenna or references to it, simply don't understand its central place in antenna engineering. Certainly the isotropic ...
9
The results depend on the two bands you choose. Frequency ratios of 2:1 are a good choice because the longer dipole, which is a full wavelength at the higher frequency band, will show high impedance on that band, while the shorter dipole, which is only a quarter wavelength at the lower frequency band, will show a high (capacitive) impedance on that band.
...
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