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16 votes
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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 ...
hobbs - KC2G's user avatar
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10 votes
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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 ...
Phil Frost - W8II's user avatar
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 ...
tomnexus's user avatar
  • 10.9k
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 ...
Glenn W9IQ's user avatar
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8 votes
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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}\...
Marcus Müller's user avatar
7 votes

How is a 1:1 current balun different from a choke?

A choke is an inductor which is used to block high frequencies while allowing DC to pass. All chokes are inductors (though sometimes more than one inductor), but not all inductors are chokes: to be ...
Phil Frost - W8II's user avatar
7 votes
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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 ...
Phil Frost - W8II's user avatar
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 ...
Brian K1LI's user avatar
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7 votes
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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 ...
Phil Frost - W8II's user avatar
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 ...
Phil Frost - W8II's user avatar
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 ...
Glenn W9IQ's user avatar
  • 18.6k
6 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 ...
Scott Earle's user avatar
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6 votes
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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 ...
Glenn W9IQ's user avatar
  • 18.6k
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 ...
Cecil - W5DXP's user avatar
6 votes
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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 ...
Phil Frost - W8II's user avatar
6 votes
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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"...
Glenn W9IQ's user avatar
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6 votes
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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 ...
Phil Frost - W8II's user avatar
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 ...
Kevin Reid AG6YO's user avatar
  • 24.5k
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 ...
Glenn W9IQ's user avatar
  • 18.6k
5 votes
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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 ...
Glenn W9IQ's user avatar
  • 18.6k
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 ...
mike65535's user avatar
  • 1,035
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 ...
Glenn W9IQ's user avatar
  • 18.6k
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,...
user15838's user avatar
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 ...
Mike Waters's user avatar
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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 ...
Cecil - W5DXP's user avatar
5 votes

What is the equation for the arm lengths of a half wavelength dipole antenna?

There are several factors that come into play in determining the length of a center fed 1/2 wavelength dipole - all of which center on the notion of a near resonant feedpoint impedance. To begin with,...
Glenn W9IQ's user avatar
  • 18.6k
5 votes

What are the effects of a ground plane under a horizontal dipole?

If the container is small relative to wavelength, then it won't have much effect at all. If it's very large (say, infinite) then it will reduce ground losses by providing a more conductive ground ...
Phil Frost - W8II's user avatar
5 votes
Accepted

Will using a 1:1 balun at the feedpoint of a center feed dipole result in no common mode current on the outside of the coax?

Depends on how much current you mean by no current. But in general, unless the dipole is actually perfectly symmetrical, in a perfectly symmetric environment, above a symmetric ground, with the ...
hotpaw2's user avatar
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5 votes
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Can a 1:1 transformer be used as a protection from static electricity?

Assuming that power losses in the transformer are acceptable, and that it's placed after 1:1 current balun (which eliminates common-mode current), is there any reason why it might be a poor solution? ...
Phil Frost - W8II's user avatar
5 votes

What is the impedance of an off-center fed resonant dipole?

Both the real and the imaginary terms of the complex impedance across the feedpoint terminals will change as the feedpoint moves away from the electrical center of the antenna. As an example, a NEC4....
Richard Fry's user avatar
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