19
votes
What is a Gamma match in the context of the driven element of a Yagi antenna?
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 ...
- 49.8k
17
votes
Accepted
Hanging a dipole from a tree not code compliant?
225.1 Scope. This article covers requirements for outside branch circuits and feeders run on or between buildings, structures, or poles on the premises; and electrical equipment and wiring for the ...
- 8,833
14
votes
Accepted
What is a Gamma match in the context of the driven element of a Yagi antenna?
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 ...
- 1,267
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 ...
- 49.8k
10
votes
Can a 1:1 transformer be used as a protection from static electricity?
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 ...
- 7,817
9
votes
Why do folded dipoles have greater bandwidth than ordinary resonant dipoles?
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 ...
- 91
9
votes
Accepted
How to calculate power transmitted by monopoles?
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}\...
- 13.3k
8
votes
Efficiency of short dipole receiving antenna
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 ...
- 18.1k
7
votes
Do I need a Balun for a single band QRP antenna?
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 (...
- 22.9k
7
votes
Dipole antenna radiation field equation
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 ...
- 7,685
7
votes
Accepted
3 Band (80-40-20 Meter) Fan Dipole Tuning Question
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 ...
- 49.8k
7
votes
For a half wave dipole antenna, what's the value of the reactances that cancel out when the antenna is resonant?
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 ...
- 49.8k
6
votes
Accepted
What frequency to set trap on?
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 ...
- 49.8k
6
votes
proximity of rain gutter to dipole antenna
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 ...
- 18.1k
6
votes
Accepted
How the performance of the half wave dipole antenna changes as the length changes?
...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 ...
- 18.1k
6
votes
How can the voltage at the center of a resonant half wave dipole be zero if the input impedance is 75 ohms?
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 ...
- 1,646
6
votes
Accepted
How to determine number of turns for a 1:1 balun?
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 ...
- 49.8k
6
votes
Accepted
Estimating the efficiency of a shortened (loaded) antenna
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"...
- 18.1k
6
votes
Accepted
Does a loop antenna have less susceptibility to atmospheric noise than an open ended antenna?
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 ...
- 49.8k
5
votes
Accepted
How is a 1:1 current balun different from a choke?
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 ...
- 2,868
5
votes
Accepted
Dipole versus ungrounded end fed for HF RX
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 ...
- 66
5
votes
What happens if an HF dipole is made longer?
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 ...
- 22.9k
5
votes
How to find transmitting (voice and CW) Amateur stations in UK?
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 ...
- 4,291
5
votes
Using two short mobile HF antennas to make a vertical dipole
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 ...
- 18.1k
5
votes
Accepted
How low can a vertical dipole be mounted?
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 ...
- 18.1k
5
votes
The right antenna size
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 ...
- 1,023
5
votes
Effective area of a dipole
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 ...
- 18.1k
5
votes
How can the impedance of a half wave dipole at its resonant frequency be purely resistive when the voltage and current are seemingly not in phase?
Andrew, the typical graphics showing standing waves are showing voltage and current distribution along the wire, not phase shift from each other. Voltage and current are always in-phase at every point,...
- 51
5
votes
Accepted
Can anyone tell me how to make a 35 m long wire antenna work on 80, 40, 20, 15 and 10 metres?
I suggest you feed it in the center with open-wire (ladder) line and a balanced antenna tuner. (I have done that for many years). With the right tuner, you can obtain a near-perfect match anywhere on ...
- 7,495
5
votes
Estimating the efficiency of a shortened (loaded) antenna
"The ARRL Antenna Book" uses the lumped circuit model for HF loading coils which is an inaccurate thing to do in a distributed network circuit with reflections. The helix model in EZNEC presents a ...
- 1,646
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