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Questions tagged [antenna-theory]

Designing antennas, selecting antennas for specific applications, properties of a given antenna design.

4
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2answers
76 views

How can I calculate current distribution on an end-fed dipole antenna in theory?

I understand that in a half-wave center-fed antenna current distribution has a sinusoidal wave approximately, so the current in the center has max value and it has to become zero at the ends of dipole ...
3
votes
1answer
73 views

Are there reasons why a receive-only wire antenna should not be λ/2 in length?

The answers to this question explain why it's difficult to transfer power from a transmitter to an antenna that is λ/2 in length (or a multiple for long wires, or as half a dipole's length). Are ...
3
votes
2answers
81 views

Yagi vs Parabolic Dish for 2.4GHz Campus Wifi from 1,800ft away?

My apt balcony is 1,812 feet exactly from my campus's Wifi access point behind a triple pane, brick, 4th floor building window. There is a clear line of sight. I was interested in seeing if I could ...
2
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3answers
125 views

Why long wire antenna length shouldn't be a multiple of λ/2?

Numerous sources say that LW length shouldn't be a multiple of λ/2 for any frequency in interest. This will cause the antenna to have a high impedance at this frequency. Since the coax cable impedance ...
2
votes
1answer
63 views

What happens to the transmitted power cancelled out by a yagi antenna's reflector?

For a yagi antenna, for transmit, does the waveform traveling away from the driven element towards the reflector get cancelled out by the reflector and then simply disappear ? Or is that cancelled ...
-1
votes
1answer
40 views

Skin effect for a modified antenna with a high frequency pulsed DC

A direct current will be interrupted by a high frequency switch. It seems , that in a wire, connected to the source (the antenna rod), instead of pushed back- and forwards electrons, the electrons get ...
2
votes
1answer
62 views

If I point a yagi antenna at the sky, what will its azimuth radiation pattern look like?

I have a 4 element 10 m yagi antenna. If I point it at the sky, will its azimuth radiation pattern then become the same as its elevation radiation pattern when horizontally polarized?
1
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1answer
80 views

Which polarization for a Yagi antenna will have the best side rejection for received signals with vertical polarization?

I'm trying to null out vertical signals from a particular suburb which is about 40 kms away from my location by pointing a 4 element 10 m yagi such that the direction i want to null out is off to the ...
4
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1answer
62 views

How to model passive elements when simulating an antenna?

So I started writing an antenna simulator program to design antennas for my needs. To calculate the electric field vector at every point, I split the active element of the antenna into a large number ...
1
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2answers
56 views

What would the power at output at a distance of 1cm when transmitted at a power of 1dBm from the transmitting antenna?

If the transmitter and receiver are placed one centimeter apart, what would be the power at the receiving end if the tranmission power is 1dBm? Consider an omnidirectional antenna. And if it is a ...
4
votes
3answers
100 views

antenna in the rain

What kind of changes can be expected in the behavior (resonance, SWR, etc.) of a mobile vertical HF antenna between dry conditions and wet rainy weather? (assume a coax feed-line and low SWR in dry ...
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3answers
116 views

Reflection behaviour of the radiation as a function of the used voltage

According all the answers to a question about What is the advantage of making an antenna resonant different lengths of the antenna rod could be tuned to resonance with the antenna generator. You ...
8
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2answers
255 views

What is the advantage of making an antenna resonant?

I am constructing a small loop antenna; apprx 4' diameter, will be used with an antenna tuner (mounted at the feedpoint) and 100W transmitter, and hope to use it on 20m, 40m, and maybe other bands if ...
2
votes
1answer
77 views

How do antennas behave as they are moved to near-field distances?

How do antennas behave as they are moved to near-field distances? Can they still receive a radio signal over a distance closer than a wavelength? Background: In this comment it is said, that the ...
7
votes
3answers
96 views

Can I use a helical ground plane radial in the same way that a loading coil works?

I'm working out the science behind mounting an antenna on a vehicle, and I came across the concept of ground planes. This answer says that longer is better but don't worry about it too much, but this ...
4
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4answers
194 views

Does a 1/2 wavelength end-fed antenna need a good RF ground?

I was searching for DIY end-fed antenna. Some of the articles I found say that for a 1/2 wavelength end-fed I need a "small to no counterpoise" (with a lambda*0.05 m counterpoise), but some other ...
1
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1answer
47 views

How to measure the power emitted from an HF antenna in near-field WPT?

I'd like to estimate the power density from a 13.56 MHz power source in a near-field WPT (wireless power transfer) scenario. Using a spectrum analyzer with a receiving antenna will give the E-field ...
3
votes
3answers
108 views

How thick should counterpoise wires be?

I would like to use a shortened whip antenna with a coil in it's base, e.g. OPEK HVT-400B. My transceiver maximum power is 100 W (and antenna's - 120 W). To make the antenna work I need counterpoise ...
2
votes
2answers
121 views

Fan dipole radiation pattern

I'm looking at dual band (2 m & 70 cm) antenna designs for my next project. A very interesting one to me is the Mighty Woof, which is essentially a fan dipole made of copper pipe. Would the ...
3
votes
2answers
106 views

Will wrapping an antenna around my house next to metal affect it?

My unit is 25' by 25' each side and the front is 22' across. The overhang is about one foot with aluminum and vinyl underneath. If I wrap an OCF dipole (fed with 4:1 balun) around this overhang in ...
17
votes
5answers
4k views

How does the signal know where the cable ends and the antenna starts?

I know very little about antennas. I know, that for a certain frequency the antenna needs a certain lengths for good sending/receiving conditions. Often the antenna is at the end of a cable. Why doesn'...
2
votes
2answers
79 views

Is there any difference in the voltage and current distribution for a half wave dipole fed in the middle, or fed with a gamma match?

Is there any difference in the voltage (potential difference wrt center) and current (rate of flow of electrons) distribution for a half wave dipole fed in the middle (dipole cut in the middle,) or ...
1
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2answers
197 views

Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?

Following is my understanding of some theory and the questions are at the end. Is this right ? ... A standing wave is an AC waveform oscillating at a particular frequency which has a varying ...
1
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3answers
87 views

If the voltage at the center of a half wave dipole isn't zero, then how do Plumber's Delight yagi antennas work?

The voltage at the center of a resonant half wave dipole can't be zero because the current is at a maximum in the middle. If the voltage was zero then there would be zero current right ? How then can ...
0
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2answers
84 views

Does a vertically or horizontally polarized electromagnetic wavefront have a thickness?

Does a vertically or horizontally polarized electromagnetic wavefront have a thickness ? Or is it really thin and just the height or width of it's wavelength ?
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2answers
60 views

Does a horizontally polarised yagi antenna have the same front to back ratio for horizontal and vertical received signals?

Can anyone answer this question : I have a 4 element yagi antenna which is horizontally polarized. Let's say it has a front to back ratio of 20 dB. Does that front to back ratio apply to received ...
3
votes
4answers
137 views

What is the voltage on a half wave dipole measured with respect to?

As I understand, voltage is a measure of the potential difference between two points, and voltage causes current to flow. I see lots of descriptions on how dipoles work with lots of references to the ...
0
votes
1answer
106 views

How does a gamma match on a dipole work as a balun?

I have a resonant half wave dipole which is one continuous length and not separated in the middle, fed using a gamma match, and with the braid of the coax joined to the exact electrical center of the ...
1
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2answers
123 views

What is Electrical length in RF and how to electrically cut and measure the RF cable in degree?

What is electrical length in detail? Why is it so that 1/4 wave , 1/2 wave dipole antenna works properly and not a full wave antenna? What is the role of electrical lengths in degree?
7
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3answers
216 views

How can the voltage at the center of a resonant half wave dipole be zero if the input impedance is 75 ohms?

If the input impedance of a resonant half wave dipole at the feed point is 75 + j0 ohms, then if an AC sine wave voltage V is applied to the feed point, how can the voltage at the center be zero ? ...
2
votes
4answers
179 views

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?

A half wave center-fed dipole has a resonant frequency where the input impedance appears to be purely resistive. How can this be, when the voltage and current distribution along the length of a half ...
4
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2answers
354 views

Does a parabolic reflector dish change circular polarity?

I've been receiving satellite signals in the L-band (~1.7GHz), recently. These signals are RHCP. So far, I've been using a DIY helical antenna in axial mode, and a patch antenna with cropped or ...
3
votes
5answers
153 views

Gain difference between half-wave and infinitesimal dipole?

In discussion around another Q&A here, Glenn W9IQ commented: The directive gain difference between a half wavelength dipole and an infinitesimal dipole is only 0.39 dB I'm curious where this ...
0
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2answers
144 views

If one were to add reflector elements above and below a dipole would it change the direction to horizontal?

Since adding a reflector and a director horizontally to a dipole (as in a three element Yagi) causes the beam to reflect horizontally, what should happen if reflectors above and below the dipole were ...
2
votes
1answer
93 views

Stacking DB8 “bedsprings”

I'm trying to receive a UHF TV signal 86 miles away. Considering the height of the transmitting antenna and the height of the receiving antenna, I should be able to just make line-of-sight between ...
0
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2answers
104 views

antenna theory for beginners [closed]

I am new to amateur radio and am having trouble getting my head around antenna theory. Can anyone suggest a good 'introduction to antennas' that doesn't get too technical? Thanks.
2
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1answer
88 views

What does co-phasing of cross-polarized antennas mean?

I was working with the 3GPP release 14.6, and it mentioned that modern communication systems have cross polarized antennas, and in order to perform beam-forming we need to co-phase our pre-coder. Pre-...
4
votes
2answers
68 views

What are the effects of different feed-line incidence angles?

Consider a dipole antenna fed by a coax (say, longer than the antenna) with a signal reasonably close to the dipole's resonant frequency. Various sources say that the feed-line should approach the ...
7
votes
5answers
194 views

Are there formulae to calculate capacitance and inductance of an antenna?

How to calculate internal impedance of a dipole antenna? Its resistance has two components ie wire resistance and radiation resistance, both these can be calculated using formula. Are there formula to ...
3
votes
1answer
131 views

Twisting copper wire for a less-flexible DIY antenna

I am making DIY antenna for a home project. The antenna is going to be "V-dipole" with leg length about 53cm. Unfortunately, I didn't find a thick copper wire, but I have a lot of thin core from tv ...
2
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3answers
102 views

Do PCB antennas suit for NOAA and ADS-B?

I saw many designs for creating DIY antennas for NOAA (~137MHz) and ADS-B (~1090MHz) but never seen it in a PCB implementation. So I have a rookie question: why nobody uses PCB antennas for such ...
2
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2answers
149 views

G5RV Jr. fed with ladder line and coax

I am a new ham and I was just given a G5RV Jr. I have about 20 feet of coax plus about 60 feet of 450 ohm ladder line I can feed it with. Can I extend the G5RV Jr. feed line with the ladder line I ...
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2answers
60 views

vertical 40m antenna takeoff angle

I built a 40m vertical using the W8AMZ design https://www.amateurradiosupplies.com/v/vspfiles/assets/images/40m_vertical_monopole_-_manual.pdf Seems to be working for the most part. But I'm not sure ...
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2answers
87 views

True near field distance for a short monopole antenna?

Antenna properties: Height x Diameter = 3.3 inches x 1.4 inches Test Frequency: 900 MHz (Wavelength = 33 cm or 13 inches) I see some conflicting formulas to calculate the near field distance for ...
5
votes
4answers
252 views

Why does not a bigger antenna receive more power?

According to http://www.antenna-theory.com/basics/aperture.php, the power received by an antenna depends on the power density of the incoming wave, the wave length (which I consider to be fixed) and ...
2
votes
1answer
158 views

CB Radio Ground Plane (or lack thereof)

Notice the vertical antenna on the truck. I believe this first antenna is referred to as a "dipole", mounted vertically. CB to coax, coax to antenna. The shell of the truck is fiberglass. Even the ...
2
votes
1answer
117 views

What type of this antenna is it?

Could you please tell me which kind of this antenna is? I supposed it is a modified dipole, but not quite sure. Is there any available equation for calculating the original dimensions of this type of ...
6
votes
1answer
102 views

Two antennas on different sides of a building

I live in the 4th floor of a building with one side facing south and the other north. I am able to use my 2M mobile base transceiver with a mobile magnet mount antenna placed just outside the window. ...
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1answer
139 views

How do I understand this figure about antenna pattern? [closed]

Why the back lobes have such a large area??? I think it may be like https://www.cisco.com/c/dam/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.doc/...
1
vote
1answer
74 views

Can the Q of a loop antenna be changed?

Can the Q of a loop antenna be changed? If so what are some common techniques? (Formulae appreciated).