Questions tagged [antenna-theory]

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

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4answers
911 views

Are there any antenna that can create disk like lobes?

Are there any antenna that have disk like lobes : Or are there any antenna that have very thin directivity, say lobes be no more wide than few mm.
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3answers
44 views

Loop a long wire antenna or EFHW

For a typical EFHW antenna, let's say we are in a very space-restricted environment, says a hotel room. I am thinking of the idea loop around the wire around the room, maybe even many loops. This is ...
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1answer
33 views

The HPBW of a helical antenna

can someone tell me what are the formulas of HPBW for a helical antenna? I've searched a lot and I saw that there are two different formulas of HPBW for the helical antenna, this one, and this one, ...
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2answers
72 views

What does antenna frequency range mean in practice?

I'm trying to wrap my head around software defined radios and antennas and noticed that lots of antennas are described in terms of a frequency range. For example, this one is described as operating ...
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3answers
73 views

Yagi Antenna - Elements Length

I am currently modeling a yagi antenna using Matlab and after I optimized the antenna, it gave me a director length shorter than the exciter itself (even if the optimization gave me a good gain). From ...
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3answers
83 views

Radio Waves propagation (Intuitive Understanding of Wave propagation)

I have few similar questions regarding Radio waves communication particularly FM and AM type of Radio wave communication. Please bear with the naivety of the questions. Questions: Radio waves in FM ...
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1answer
84 views

How do dual band J poles work?

My understanding is a J pole is a $\frac{1}{2} \lambda$ dipole attached to a $\frac{1}{4} \lambda$ impedance matching element. How does this dual band work? What acts as the impedance matching element ...
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1answer
89 views

Heat-shrink as dielectric for bazooka balun?

I'm intending to make a sleeve balun (aka bazooka) for 2m, but rather than keep the lossy coax jacket as a dielectric (I have some RG58), was considering to remove it and replace it with heat shrink ...
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1answer
70 views

How does this W6NBC “derivative of the classical slot antenna” work?

Via the https://ham.stackexchange.com/a/9883/1362 answer regarding a slot antenna for a 2m mobile application, I was introduced to this (pdf) interesting antenna design: The designer John Portune (...
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1answer
85 views

Questions about effective isotropic sensitivity(EIS)

Recently, I read something about EIS (effective isotropic sensitivity) and TIS (total isotropic sensitivity). I'm trying to understand the concept, but I find it's quite hard for me. In the book ...
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3answers
157 views

Ground-Plane radial sizing, is there such thing as too long?

I'm just trying to make sure I understand the theory correctly. I have some triband VHF/UHF/800MHz antennas, I need to mount on a tower so I plan on using a "mobile to base" conversion kit. ...
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2answers
97 views

How does this quarter-wave stub balun work?

I was searching for 1:1 balun designs that would be efficient at VHF yet could easily be made from commonly available materials when I came across these two: While the design on the right seems ...
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2answers
71 views

How to match transmitter to Beverage antenna

Beverage antenna consist of pair of transmission lines - one being the "antenna proper", the other ground. The antenna is being fed via open feed line ( approximately 450 Ohms ) - one side ...
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4answers
107 views

Why is a half wave dipole resonant?

Is the following explanation correct ? RF energy with a wavelength equal to twice the electrical length of the dipole is applied to the feed point at the center of the half wave dipole. To understand ...
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3answers
97 views

Confusion about dipole standing waves and feed point impedance

The animation below from Wikipedia shows the standing waves of voltage and current on an ideal center fed resonant half wave dipole in free space. The red and blue curves show the change in amplitude ...
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2answers
112 views

Double Cross antenna: what's wrong with my model?

Important note: After consulting another source, I learned that the article calls for the first pair of opposing dipoles to be fed in phase, with the second pair fed in quadrature to the first pair (...
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1answer
75 views

For a center fed half wave dipole operated at resonance, are the voltage and current in phase only at the feed point?

For a center fed half wave dipole operated at resonance, the voltage and current at the feed point are in phase in time and there is zero reactance present in the impedance seen at the feed point. For ...
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2answers
112 views

Why do horizontally-polarized RF waves reflect better off ground than vertically-polarized waves?

It's a well-known fact among hams who use HF frequencies that horizontally-polarized RF waves reflect better off of ground than vertically-polarized waves. This explains why ground conductivity is so ...
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1answer
97 views

Outdoor Terrestial Digital TV Receiver Antenna: 500-700 MHz Band, Can I Make It Work Better?

See the unconventional antenna shown below, The whole antenna is made of two mirror-imaged Alum metal stampings on either side of a 2-cm hollow 31-cm long rectangular plastic boom. The antenna top and ...
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4answers
257 views

Does a resonant antenna work better than a non-resonant antenna?

Some books on antenna fundamentals i've read recently say that, for a half wave resonant dipole when transmitting for example, the traveling wave of movement of charges reflected back from the ends of ...
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4answers
219 views

For a half wave dipole antenna, what's the value of the reactances that cancel out when the antenna is resonant?

According to a few descriptions of antenna fundamentals i've read recently including that in Wikipedia and the ARRL handbook, when a half wave dipole is resonant, the inductive and capacitive ...
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3answers
336 views

Common-mode current and choke placement

Why are chokes usually placed at the feedpoint, can't common-mode currents occur below the choke as well? I understand it's placed at the feedpoint to keep everything balanced by eliminating common-...
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4answers
100 views

Variable polarization Yagi using two antennas on the same boom, joined together with variable feedpoint phase?

I have been trying for ages to find a way to mount a 10 m vertical Yagi properly, so that the mounting pole doesn't stuff up the impedance and radiation pattern, and have had no luck so far. Apart ...
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4answers
343 views

Any reason not to choose a discone antenna over a Wide/ MultiBand mobile antenna for TX/RX operations

We are setting up a all-band radio in an emergency com center that's intended most for amateur band use, but since its' for emergency use will also be used in the public safety bands; (150-170)MHz, (...
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1answer
38 views

On a horizontally setup 20m delta loop I am looking to determine the calculations for the proper length of a feed wire

Base of loop will be about 6 feet above ground. RG-8 or 213 will be used for the feed wire. Ugly balun as well.
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2answers
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Antenna Basics - Open/Short Circuit & Half-Duplex/Full-Duplex

I am beginner in the Antenna theory and I have a few basic questions. Please help me to clarify my understandings related to the basics which I have read. And this question has 3 basic questions. ...
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3answers
121 views

Need antenna for mobile mag-mount LPFM broadcast 88.3 MHz

I am doing some temporary FM broadcasting for car events such as COVID-safe car parades and drive-in movies. The terrain is horrible, urban areas with lots of buildings. The cars snake around a city, ...
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1answer
145 views

Can half of a directional antenna (Yagi-Uda, HB9CV) be replaced by a ground plane?

Consider a center-fed dipole antenna. It‘s well understood how the bottom element can be replaced by a ground plane (in practice, generally a few rods but in principle a conducting plate would work as ...
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1answer
121 views

How do I tune my EFHW 40-10 to have the lowest SWR in the 40, 20 and 15 meter bands?

This is a reading from my RigExpert AA-35 ZOOM of my EFHW 40-10 inverted L with radials, and feedpoint 2 feet above the ground. Any thoughts as to how I can tune or move this antenna to get the ...
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2answers
208 views

Designing a helical antenna for a dish

I'm interested in a dish antenna with an axial mode helical feed for receiving video. My target frequency is about 1280 MHz or the 1.2/1.3GHz range. I've been able to figure out helical basics, like ...
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4answers
108 views

Relation between antenna physical size and gain

I would like to know whether increasing the size of the antenna help increase its gain. Does this hold only for certain kinds of antennas like parabolic or horn antennas? And if it is true, can you ...
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5answers
115 views

Can I use a 1/4 wave decoupling stub to enable mounting of a vertical yagi on a metal pole?

My understanding is that the ground plane radials used on, for example, a free standing 5/8 wave 10 m vertical antenna mounted on a metal pole, are actually 1/4 wave length decoupling stubs, the ...
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1answer
66 views

Why do some 1/2 wave collinear designs have a 1/4 wave element?

I think I understand the basic principles of a collinear antenna. How the Coaxial Collinear Antenna Works? seems to describe it well. However, I've just repaired a 1090MHz (ADS-B receive only) ...
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0answers
54 views

Would a phase compensated sinusoidally spaced folded dipole present less noise to a receiver?

I am attempting to design an antenna similar to a TTFD, double zepp or array of end fed wires, with a sinusoidal, with the period of the wavelength, spacing that causes additive waves on the target ...
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2answers
43 views

Connecting Directional Antenna To Two Antenna Ports on NIC

I want my n/ac Wi-Fi card to operate in a spatially directional manner. To achieve that I want to use a directional antenna(s). There is a slight complication though, because my Alfa AWUS036ACH ...
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2answers
161 views

Magnetic loop antenna : loops vs selectivity

I've built 2-turn magnetic loop RX antenna using 5 meters of 6.5mm copper tube. I was able to tune it with a variable air-spaced capacitor in the range of 1.8-10Mhz, but I noticed that as tuning ...
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2answers
87 views

Loop on ground for transmit?

If a large horizontal loop antenna, insulated but laying at ground level, works as an MF and low HF receive antenna, why (even though it has very high ground losses) can't it be used for transmit (to ...
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2answers
93 views

How do I use a signal generator + oscilloscope to measure feeder length?

I could relatively easily take the feeder down and use a tape measure but it’s more of a challenge to learn how to measure the length using my oscilloscope -Red Pitaya - but I have to admit I have no ...
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1answer
90 views

Why does damping between two antennas increase with frequency?

While studying for a HAM license exam I ran into a question that I am having trouble with answering. There are two aspects that I cannot explain, given my current admittedly little understanding of ...
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1answer
77 views

144/430 MHz antenna on 155 MHz?

We have a local radio shop that started selling us Relm P150s with a high gain 9 inch antenna, and those are ok. He also has thrown in longer 15 to 14 inch dual band antennas saying use those in ...
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0answers
58 views

Circular polarized antenna feed changes as I rotate it?

I had a 100cm prime focus dish laying around and I thought of making a circularly polarized feed for it, since it seemed pretty straightforward from what I've been reading. Helical feeds are supposed ...
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4answers
187 views

Indoor HF/VHF/UHF antenna, have someone an Idea?

My wife doesn't think that having tall antennas is acceptable. Does anyone have a solution for this? Our house is 16 meter long and 10 meter wide. I have an antenna tuner (if it can help).
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2answers
83 views

Estimating near-field electric field strength at LF (137 kHz)

I'm experimenting with low frequencies, but due to the inherent nature of LF, I strive for more transmit power and bigger antennas. Now of course I want to comply with regulations, which allow (in my ...
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1answer
204 views

Is a ground plane antenna a poor choice to use in a forest?

Some time ago, when I just started with amateur radio, a few more experienced hams told me that an inverted-V works better in the forest than a ground plane, and that it has something to do with the ...
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1answer
85 views

Solid angle for different E/H plane beamwidths

The solid angle formula calculates the surface area on a unit sphere, from projecting a rectangular patch onto the surface of a sphere. Image source This is calculated using the azimuth angle $\phi$ ...
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2answers
193 views

For a resonant 1/2 wave dipole, does the reflections being in phase with the applied signal cause the zero reactance seen in the feedpoint impedance?

For a resonant 1/2 wave dipole when used for transmitting, does the fact that the reflected waves on the antenna are in phase with the applied signal result in zero reactance at the feed point, or ...
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1answer
91 views

Is shortening a half wave dipole to make it resonant by removing the inductive reactance a good idea?

A dipole which is exactly one half wave length allows reflections within the antenna to be in phase with the applied signal. To me it seems that this situation gives an antenna which will produce the ...
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2answers
115 views

For an antenna when used for transmittiing, what is the optimum radiation resistance?

For a 100 % efficient antenna, the real part of the impedance is equal to the radiation resistance. If the antenna were a dead short with zero impedance then the radiation resistance would be zero. ...
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2answers
186 views

Modelling a folded dipole

I am looking at using NEC to model a $0.5\lambda$ rectangular folded dipole, for closely spaced larger sides (choosing a separation of $0.01\lambda$). In Antenna Theory, Analysis and Design by ...
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2answers
125 views

What exactly makes an antenna resonant?

When an antenna is resonant, the impedance seen at the feed point is purely resistive and has no reactance. What are the physical properties of an antenna that actually make it resonant and how do ...

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