30
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How does the signal know where the cable ends and the antenna starts?
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 ...
- 18.5k
26
votes
Why do amateur radio operators call an RF choke a balun?
"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 ...
- 51.4k
23
votes
Accepted
Can every antenna be used both for transmit and receive?
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 ...
- 24.2k
14
votes
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Vertical Yagi spun at 1000 RPM: gain properties?
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 ...
- 24.2k
13
votes
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Are there any antenna that can create disk like lobes?
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 ...
- 7,808
12
votes
Accepted
Why does an "electrically small" antenna have narrow bandwidth?
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 ...
- 51.4k
12
votes
Accepted
What is the impedance of an end-fed half-wave antenna?
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 ...
- 51.4k
12
votes
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Does a receiving antenna need radials?
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 ...
- 24.2k
12
votes
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Antenna Efficiency
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 ...
- 18.5k
11
votes
Accepted
Conductive boom vs non conductive boom: what's the difference on a Yagi?
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 ...
- 15.2k
10
votes
Accepted
Dipole Antenna Current Distribution at any Time Instant
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 ...
- 51.4k
10
votes
antenna in the rain
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 ...
- 51.4k
10
votes
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Why is my calculation for added length of coax for a double cross antenna different to everyone else's?
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/...
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10
votes
How do VHF antennas compensate for change in frequency in Frequency Modulation?
All modulated signals, not just FM, have a bandwidth. The antenna has a bandwidth, which must be wider than the modulation of the signal you are using.
Note that receive bandwidth and transmit ...
- 5,536
9
votes
Wider wire = picks up more energy, yah?
In short, no.
In long, it's really complicated. The wider your wire, the wider the bandwidth, due to the increase in available paths. Wide multi-strand wire does better against skin effect (more ...
- 91
9
votes
Two antennas on different sides of a building
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. ...
- 51.4k
9
votes
Accepted
Why does not a bigger antenna receive more power?
Argument A: reciprocity
By reciprocity, we can flip this question around and ask it from the perspective of the transmitter. Assuming we've done the obvious like minimize feedline losses, and we're ...
- 51.4k
9
votes
Accepted
Does a parabolic reflector dish change circular polarity?
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.
- 7,808
9
votes
How does the signal know where the cable ends and the antenna starts?
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. ...
- 51.4k
9
votes
Does a 1/2 wavelength end-fed antenna need a good RF ground?
An end-fed half-wave operates on a principle that, for lack of a better name, I will call "voodoo".
If you look at the standing wave on a half-wave element in isolation, you see that the center is ...
- 11.5k
9
votes
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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?
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 ...
- 18.5k
9
votes
Accepted
Will electrically joined dipoles of different lengths, at right angles, behave as a multiband antenna?
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 ...
- 7,808
9
votes
Why are an antenna’s nulls sharper than its lobes?
To increase an antenna's gain, the elements of the antenna must be arranged such that they interfere constructively in the intended direction, and destructively in every other direction. This is ...
- 51.4k
8
votes
Aluminum vs copper antenna element lengths
Besides mechanical differences, the primary difference between aluminum and copper in antenna construction is RF resistance. Copper will have slightly less RF resistance for the same surface area. ...
- 18.5k
8
votes
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Can an antenna be too powerful for certain receivers?
Would the portable ~$40 Tecsun get damaged somehow by a 100-foot antenna because it's designed to work with shorter, portable antennas?
Unless you are within a kilometer or two of a very high power ...
- 18.5k
8
votes
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Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?
The SWR is related to the reflection coefficient $\Gamma$:
$$ \Gamma = {Z_L - Z_0 \over Z_L + Z_0 } $$
$$ \text{VSWR} = {1+|\Gamma| \over 1 - |\Gamma|} $$
where:
$Z_0$ is the feedline impedance, ...
- 51.4k
8
votes
Accepted
What is the advantage of making an antenna resonant?
Resonance or non-resonance does not have a direct effect on the efficiency or gain of the antenna. A resonant antenna is one that has only resistance without any reactance (capacitive or inductance) ...
- 18.5k
8
votes
Accepted
Will transmitting on this antenna cause problems?
It won't work very well.
antennas often work well if they're on a third harmonic of the transmit frequency
This is calculating the third harmonic the wrong way around. A better statement would be ...
- 10.3k
8
votes
What kind of loop antenna is this, and what is the relationship between its diameter and wavelength?
It is a 2-meter halo antenna as built by Mike Fedler N6TWW: the image appears to have been lifted from his page describing the build. It's 40" in circumference, and the boom length (slightly more than ...
- 11.5k
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