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Questions tagged [antenna-theory]

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

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59 views

What are Smart Antennas?

What are Smart antennas. Are them used in amateur radios and their significance in amateur radio communication over the conventional antennas?
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2answers
69 views

Does a loop antenna have less susceptibility to atmospheric noise than an open ended antenna?

I always get the feeling that an antenna which has a loop for the driven element such as a folded dipole or quad will pick up less static or 'high impedance' noise than an antenna that has free ends ...
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3answers
92 views

Are standing waves on a transmission line RF or DC?

I am very confused about Standing Waves on transmission lines. Wikipedia says "a standing wave, also known as a stationary wave, is a wave which oscillates in time but whose peak amplitude profile ...
3
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4answers
95 views

Is there a way to explain antenna radiation patterns on receive without reciprocity?

I don't consider myself an expert, but I have a fairly decent understanding of radio. I belong to a club that includes a number of people who have less theoretical background. I understand that ...
5
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3answers
202 views

Determining Impedance With An Antenna Analyzer

I need some help interpreting some information from my antenna analyzer (RigExpert AA-35). I want to use the information to impedance match my AM receiving antenna system to AM radios. No ...
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4answers
106 views

Cell tower identifier antenna isolator cannon

There are several cell towers on the hills around me. Usually I just walk around their base in a circle with one of the many CellID apps to get their ID. Then I can impress my friends, "That's tower ...
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3answers
93 views

Can I measure position with antenna/receiver?

I hope this question is not out of bounds. I'm a EE with little RF and you guys seem to know a LOT about RF... I have a small vehicle traveling in a straight line at low speed. I need to know when it ...
2
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1answer
87 views

Bow Tie antennas impedance

I see on the internet that a radio amateur made bowtie antennas for transmitting and receiving on the 2 meter and 10 meter bands. They produced very good SWR bandwidth when fed with 50-$\Omega$ coax. ...
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1answer
64 views

What is the radiation pattern of an EH antenna?

I haven't been able to find NEC cards or pictures of radiations patterns. I'm used to seeing them as polar diagrams, however, the only description I've gotten is "[t]he modification of the loop ...
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1answer
90 views

Can I make ferrite paint? Will it shield my antenna from my roof?

I have a 3 element vertical Yagi mounted on a rotator, and in one direction the roof of the house gets pretty close and messes up the SWR a bit. If i mash up a few type 43 material ferrite cores and ...
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4answers
105 views

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?

When you look down on an isotropic radiator from above, ie : the azimuth view radiation pattern, it's a circle. The same as looking down on any omni directional antenna. So when comparing the gain of ...
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1answer
234 views

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

I was operating under the impression that the arm length of a half dipole antenna is λ/4 while making one that operated at the 145Mhz frequency using a piece of 9.525 mm (3/8 inch) copper tubing, at a ...
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3answers
163 views

Estimating the efficiency of a shortened (loaded) antenna

Recently I've build a trap dipole for 20/40/80m bands. I followed an article by John DeGood, NU3E and got following measurements: From balun to 20m trap: 485 cm From 20m trap to 40m trap: 362 cm From ...
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2answers
75 views

How can a vertical Yagi for 10 m be mounted on on a vertical metal mast and be made to work properly?

From a reply to a previous post I've been told that a horizontal tube half a wavelength long mounted right at the junction between the boom on a vertically mounted yagi and a vertical metal pole will ...
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2answers
53 views

What is the theoretical and actual cross polarization attenuation from horizontal to vertical polarization?

Can anyone tell me what the theoretical and actual cross polarization attenuation from horizontal to vertical polarization is ? The reason i ask is that i have the idea that for receive a vertical ...
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1answer
57 views

What EXACTLY does front to back ratio mean?

For a yagi antenna for example, what does front to back ratio mean exactly ? Does it mean the reverse gain at 90 deg behind the antenna compared to the forward gain at 90 deg in front of the antenna ?...
3
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2answers
121 views

What does dBi mean?

Can someone tell me what dB gain over an isotropic radiator means? Let's say I have a 3 element vertically polarized 10 m Yagi (which I do) which is meant to have 7.5 dBi gain. Which of the ...
3
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3answers
117 views

What happens to a vertical Yagi when you mount it on a vertical metal pole?

How much difference does a metal pole make when it's sitting in the middle of all the lower elements of my beautiful new highly optimized to the max YO.EXE 10 m yagi ? I get it that the closeness of ...
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1answer
37 views

Would an infintely thin horitzontal yagi with zero height respond to vertical signals?

I can hear vertical local signals (not skip) on my horizontal yagi, and when i point it at them their signal goes up, so my horizontal yagi has some gain for vertical signals. If the vertical ...
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1answer
52 views

Can radio waves be made to travel as or along a particle beam? [closed]

Can a laser be used as an antenna? Can a beam of any type extend transmission distance of an electromagnetic field? https://medium.com/starts-with-a-bang/this-is-why-the-2018-nobel-prize-in-...
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2answers
96 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 ...
5
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1answer
92 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 ...
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2answers
101 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 ...
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3answers
147 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 ...
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1answer
72 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 ...
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1answer
43 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 ...
3
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1answer
66 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?
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1answer
90 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
63 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 ...
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2answers
59 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 ...
6
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3answers
176 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 ...
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2answers
288 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
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1answer
79 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 ...
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3answers
112 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 ...
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4answers
427 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 ...
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1answer
51 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 ...
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3answers
176 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
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2answers
158 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 ...
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2answers
134 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 ...
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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
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2answers
83 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 ...
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2answers
240 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 ...
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3answers
91 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 ...
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2answers
91 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
66 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 ...
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4answers
162 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 ...
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1answer
181 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 ...
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2answers
220 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?
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3answers
238 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 ? ...