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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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23
votes
4answers
8k views

What is the relationship between SWR and receive performance?

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well? And what about the converse, will a well performing receive antenna show a 1:1 SWR? I've wondered this for a while, ...
9
votes
6answers
27k views

What is a Gamma match in the context of the driven element of a Yagi antenna?

How (and why) does a Gamma match work, when used on the driven element of a Yagi antenna? As shown here: (source: http://www.iw5edi.com/ham-radio/?2-element-yagi-for-10-meters-band,49) The article ...
18
votes
1answer
3k views

How does moving a feedpoint off-center in a dipole affect the resonant frequency and resistive load?

Specifically, I understand that OCF dipoles change the SWR, but does it do this via reactance or resistance, or both?
16
votes
3answers
8k views

What is the peak voltage at the tips of a dipole antenna?

What is the peak voltage present at the very end tips of half-wave dipole antenna in free space, and how might this peak voltage relate to transmitter type, transmitter power, RF frequency vs. antenna ...
14
votes
4answers
4k views

What does the addition of a shield to a small loop accomplish?

A small shielded loop is an antenna with a loop of wire, smaller than 1/10th of a wavelength in diameter, also surrounded by a shield. Often, they are constructed from coax. There is a gap in the ...
5
votes
2answers
10k views

What is the aperture efficiency of an antenna?

For reflector antennas, I see a number called aperture efficiency. What does it mean?
20
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2answers
3k views

How do Handheld antennas work without ground?

Dipoles have the 2 wires going to each leg of the antenna completing the circuit. Big AM towers use the second wire as a ground plane But how to handheld rigs transmit and recieve with just one wire ...
13
votes
4answers
3k views

How does a folded dipole work?

A folded dipole is like an ordinary dipole, but with the ends extended and folded back, until they meet. Although it looks like a loop, I'm told it behaves similarly to a dipole. How does this thing ...
6
votes
2answers
805 views

Dipole Antenna Current Distribution at any Time Instant

Above is a connection of half-wave dipole antenna. The book states that the current profile is same at any given time instant on both the radiators, each of quarter wavelength length. Now my ...
4
votes
2answers
6k views

How to calculate single-stub match (general case)?

I'm trying to build a 1/2 wave "Coaxial J-Pole" antenna, similar to what is described here: Such an antenna would be great for stringing up in a tree when operating remotely, no tuner needed. The ...
8
votes
6answers
6k views

What is the impedance of an end-fed half-wave antenna?

It is my understanding that the feed-point impedance ($Z_a$) of an end-fed half-wave antenna is dependent on at least the following factors: $L_a$: The length of the radiating element of the antenna. ...
15
votes
4answers
13k views

Why do folded dipoles have greater bandwidth than ordinary resonant dipoles?

This is something I've read in passing, but never encountered an explanation of why. For example, Wikipedia says: Another common place one can see dipoles is as antennas for the FM band; these are ...
9
votes
2answers
734 views

If two antennas of 50 Ω and 377 Ω have VSWR=1:1, then which one is more efficient?

If there are two antennas of 50 Ω and 377 Ω (same as the intrinsic impedance of air) impedance with 0 $\Omega$ ohmic resistance, and they are exactly matched with their transmission line, then which ...
6
votes
5answers
9k views

When and why does the size of a ground plane or radials matter?

Consider these three cases: My understanding is that when building or installing an antenna designed to operate over a ground plane (e.g. a quarter-wave vertical), the size of the ground plane does ...
5
votes
2answers
4k views

What is the impedance of a 1.25 λ dipole antenna?

I would like to make a dipole antenna. This particular antenna is used for reception only; no transmission is involved. I would like to receive as much power as possible by this antenna as it is used ...
6
votes
3answers
491 views

Can an antenna be too powerful for certain receivers?

I'm brand new to shortwave radio and have been researching what kind of a receiver to get. I'll use it on the antenna I'll build (longwire, 50-100 feet) for shortwave listening; no transmitting. Right ...
4
votes
1answer
54 views

Voltages in a beam antenna's passive elements?

This question has good answers for the voltages reached in portions of a dipole antenna during transmit. In a beam (Yagi or Yagi-Uda) antenna, do points along passive or parasitic elements (...
3
votes
1answer
240 views

Is it possible to make a shield that blocks time-varying electric fields, but not time-varying magnetic fields?

Take, for example, a shielded loop antenna. It is often claimed that the antenna is inside the shield, and the shield blocks electric fields from reaching the antenna, while allowing magnetic fields ...
2
votes
5answers
484 views

What happens if an HF dipole is made longer?

Does a longer dipole (than half wavelength), for example twice as long, tuned to 50 Ω using an antenna tuner perform better than a half wavelength dipole for the same frequency?
1
vote
4answers
103 views

Which 10 m antenna will give the most rejection in one direction for received signals which are vertically polarized?

I want to null out a particular direction so that i can't receive some vertically polarized interference on 10 m coming from that direction. Which 10 m antenna will give the most rejection in one ...
11
votes
4answers
2k views

Are antennas circuits?

For an antenna to work, doesn’t charge have to flow through it? A current must be present. To me it indicates it must be a circuit, but how is a dipole a circuit? If they are not circuits, then how ...
17
votes
1answer
19k views

How does a mag mount antenna achieve a ground plane?

I've been thinking for some time about magnet mount mobile antennas and wondering how they establish their ground plane. Consider that many mag mount antennas have some kind of protective coating on ...
8
votes
2answers
469 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 ...
8
votes
2answers
3k views

How far does an antenna need to be from metal to avoid problems?

Living in an apartment, I'm struggling to get good antennas set up, especially for HF. Worse yet, I'm not allowed external antennas (but am willing to try something low profile on the balcony), my ...
5
votes
8answers
7k views

How does the length of a metal sleeve around a (shieled) coax cable affect antenna performance?

A post on http://adsbexchange.com/forums/topic/beginners-2-cantenna-easy-diy-antenna-to-improve-rangeplane-count uses a $\frac{\lambda}{4}$ sleeve made of a pop can beneath a $\frac{\lambda}{4}$ wire ...
17
votes
3answers
2k views

What is a “random wire antenna”?

Some antenna tuners claim to match any “random wire” to any HF frequency. Can one really use a random wire as a practical antenna system? From everything I have heard, the resonance of the antenna (...
12
votes
6answers
21k views

Calculating Antenna Length on the FCC Exam vs. in Reality

I have seen the following formulas for calculating half and quarter wavelength antennas: Formula for $\frac{1}{2}$-wavelength antenna (in free space): \begin{equation} \mathrm{Length~(feet)} = \frac{...
9
votes
2answers
2k views

Can every antenna be used both for transmit and receive?

Assuming I have an antenna for receiving GPS signals, can I use it also for transmit — to create a repeater in a tunnel (or some place with no reception)?
8
votes
1answer
3k views

How does a slot antenna work?

How does a so-called slot antenna work? How can one construct one? What might be the radiation pattern and efficiency compared to a wire antenna? And, if a small slot antenna "works" at mobile ...
5
votes
1answer
1k views

How to calculate or estimate surface area of required ground plane for a mobile antenna?

I don't have conditions for installation of a fixed 2 m/70 cm antenna at home, so I'm thinking of using a mobile magnetically mounted antenna on a conductive plate instead. So my question is: What ...
4
votes
1answer
222 views

How far the near-field reactive region extends from the transmitting antenna?

I am doing research on near-field harvesting. In my situation the transmitting antenna is a dipole antenna operating at 2.45 GHz and its length is 14 cm long. I am trying to find how far near-field ...
3
votes
3answers
2k views

Using inner conductor of a coax feedline as an antenna

Can I use the inside conductor of coax to radiate RF energy? My in-laws have some sort of attic antenna installed many years ago for broadcast reception, which I would love to temporarily co-opt for ...
2
votes
3answers
302 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
2answers
1k views

How do you calculate the field strength at a given distance?

Given this question from the full license exam in the UK: What is the field strength at a distance of 7m from an antenna with an effective radiated power of 100W? in which the answer is: 10 V/m ...
7
votes
3answers
194 views

Passively reducing ground loss?

Would a disconnected wire or conductive mat below a low height antenna reduce ground losses? A tuned length of wire under a wire antenna might be suitable for NVIS gain. But would a bunch of space ...
6
votes
1answer
176 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. ...
5
votes
1answer
394 views

Using a loop antenna on a harmonic of the intended antenna frequency

I bought a MFJ 1786 Magnetic Loop Antenna, which is made for 10 thru 30 MHz. It is not meant for use on 40 meters. The antenna has its own built in tuner. I have a Yaesu FT-890AT with built in antenna ...
5
votes
5answers
2k 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 ...
5
votes
1answer
583 views

What happens to the “Yo-Yo Tenna” when you coil up its ends for use on higher frequencies?

I have gathered what is basically a Yo-Yo Tenna using some Camco laundry reels and a spool of cheap speaker wire split in half (pretty much exact duplicate of this build as it turns out). I've got a ...
4
votes
4answers
204 views

Why does an RF Amplifier fail when the SWR is high?

Exactly why does an RF output transistor fail if the SWR is bad ? Is it A because the impedance mismatch between the transmission line and the antenna results in maximum power not being transferred ...
3
votes
4answers
201 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 ...
3
votes
1answer
296 views

Understanding of NFC Reader Antenna matching

I have been designing a 20 x 20 cm square NFC reader antenna which looks like this: In order to make it work and detect NFC tags at certain distance, it needs to be matched. To do that I was trying ...
3
votes
2answers
703 views

How low can a vertical dipole be mounted?

An answer to this question suggests that raising a vertical dipole sufficiently can create the effect of a phased array. How about the opposite. How low can a vertical dipole be mounted? Can a ...
2
votes
1answer
67 views

What sort of antenna would use a bicone or lozenge-like arrangement of vertical wires?

In another QA here, there's a picture of an antenna I identified as a "discone": What are these antennas used for? One commenter noticed that antenna was actually a "Discone, but no disc, just the ...
2
votes
4answers
197 views

What is the maximum and minimum of antenna gap of dipole antenna?

Consider the picture below is design of half wave length (λ) dipole antenna. I read some explanations saying that it is the best design to get the optimum transmit power. Half wave length (λ) is from ...
2
votes
2answers
1k views

Field Strength calculations with known antennas

I can receive a calibrated decibel milliwatt signal level from the output of any antenna at a given frequency. With a given antenna gain (dbi) in the direction I want to receive the signal (...
1
vote
2answers
430 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
vote
3answers
2k views

Can I build a mini passive repeater to improve my WIFI reception at home?

The problem There's a concrete wall between my computer and my router (Linksys E900) which affects my WIFI reception pretty harshly, computer receives signal is at -65 dBm, at this rate i get around ...
1
vote
0answers
94 views

Antenna Impedance calculation for low frequency [duplicate]

If I have an end fed long wire in free space, say 100 ft, how do I calculate its $R$ & $X$ at different frequencies?
0
votes
1answer
448 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 ...