Questions tagged [antenna-theory]

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

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2
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1answer
563 views

Will adding a balun to a full-wave dipole help?

I have made a dipole for 40 m, no swr to speak of on 7040 kHz, but I've been told that it should also work on 20 m as it is 20 meters long (2× 9,92 m) but there I get SWR in the 7-9 range. Will this ...
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1answer
99 views

Directional antenna overlapping main lobes

In a 2-Rx by 2-Tx directional antenna system, say both transmitters are transmitting concurrently as in Fig. The main lobes from the two transmitters overlap. How does this affect the resultant signal ...
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2answers
255 views

Does antenna array structure effect overall radiation resistance?

If we consider a basic half wave dipole array does the radiation resistance of array depend on distance between two elements and phase we feed?
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1answer
82 views

What parameters are currently possible in a directional antenna less than 5mm long?

I'm curious what kind of directional antennas could fit inside a cell phone, which would require it to be 5mm or less in its longest dimension. I know that the length of a directional antenna is often ...
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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 ...
4
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3answers
412 views

Receive directionality of a sub-optimal Yagi

How do I build the best antenna for a given VHF frequency given that an optimal design is too large? I am trying to understand the properties of a directional antenna of sub-optimal design. All the ...
11
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3answers
5k views

Conductive boom vs non conductive boom: what's the difference on a Yagi?

I designed a Yagi antenna using 4nec2, assuming that I would use a non conductive boom. If I use an aluminum boom but I insulate the elements from the boom, can I keep the same design? How does it ...
9
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2answers
3k 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)?
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2answers
519 views

Antenna gain vs. radiation efficiency in 4nec2

I'm trying to evaluate HF antennas for low angle performance to pick from a list of designs that will fit in my limited space. I'm using 4nec2 to simulate them and compare the patterns. When your ...
0
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1answer
716 views

Which matters most in a fan dipole: Overall linear length or actual wire length?

I'm in the midst of constructing a fan dipole for 80, 40, 30, 20 and 10 meters. I have everything laid out and am about to start cutting wire. I have an ugly balun (RF choke) wound at the feed point ...
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2answers
643 views

Antenna length in relation to lambda length

I'm pretty new with antenna stuff. I'm not sure how to understand antenna length in relation to lambda with dipole antennas. If a simple dipole antenna is lambda/4, has it a total length of lambda/2 (...
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1answer
748 views

VHF / UHF - Terrestrial Circular Polarized Antenna

I've been doing mostly HF for the last 20 years and just recently installed a 2m/70cm mobile rig in my shack for base station work. I like to work FM simplex and repeaters when the HF bands are not ...
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3answers
3k 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 ...
5
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1answer
678 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 ...
3
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3answers
169 views

Antenna proximity limit to increasing signal strength via additional receiving antennas?

I have a 900Mhz transmitting antenna that is being received by one receiving antenna at the moment. I want to add a second (and possibly third, fourth, etc) antenna to increase the signal strength. ...
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3answers
362 views

Bandwidth, Q-factor, Radiation Eff calculations

Is it possible to give approximate answers about what an antenna is capable of? There are a lot of formulas out there, but every single one I found is based on actual measurement; input, output, SWR ...
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2answers
1k views

Is it possible to calculate the Q factor for an antenna type, given a frequency?

With only pen and paper, is it possible to calculate the ideal Q-factor for e.g. a dipole antenna, loop etc, given a frequency?
3
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3answers
4k views

How to produce a NEC antenna file (and what software to use it with)?

So I'm really trying to get to grips with antenna radiation patterns, and more importantly things that are to close to the antenna that might interfere. I've downloaded Qantenna and 4NEC2 to try and ...
5
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1answer
811 views

Minimum thickness of whip antenna

I'm looking at designing a communication system using very low power transmitter (0.1 W) and operating in the 400 Mhz range. For my uses I think a quarter wave whip antenna would work well, so I've ...
4
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1answer
243 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 ...
4
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1answer
728 views

Correct number of elements in 4nec2 antenna simulation

I am designing a 1.25-wavelength dipole antenna with the 4nec2 software. What I noticed is that the input impedance of antenna is changing greatly when I change the number of segments. How many ...
2
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1answer
170 views

Dual Band Antenna Matching

I am building a wideband dual band omni for school. My antenna is about 2m long. The antenna length basically has two sections (divided by ports) - from 0 - 1.8m (This length is for 20-180MHz) and ...
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1answer
133 views

Query about double stacked circular polarized yagis

I have heard that stacking two or more antennas help in increasing the reception gain. Will this be true for transmission? For example, in transmission using double stacked yagis, wouldn't each ...
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2answers
5k 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 ...
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2answers
12k views

What is the aperture efficiency of an antenna?

For reflector antennas, I see a number called aperture efficiency. What does it mean?
8
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1answer
4k 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 ...
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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 ...
11
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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 ...
8
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4answers
4k views

What's the difference between “minimum SWR” and “resonance”?

Is the point of minimum SWR equivalent to resonance? Exactly what does resonance mean, anyway?
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1answer
267 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 ...
3
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1answer
88 views

How much distortion would aluminium wire stays for the centre-pole on my inverted-vee cause?

I used nylon rope from the nearest hardware store to stay the centre-mast on my inverted-vee for the 20. The centre-pole is a long long looong bamboo. The trouble is rain here really isn't friendly to ...
5
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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
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1answer
1k views

Does a larger Biquad antenna give better results?

Consider a biquad antenna used for receiving, not for sending. They are typically built with each side having a length of $\lambda / 4$. If a biquad antenna was built with a side length of $\lambda / ...
8
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3answers
2k views

What sort of radiation efficiency can one expect from a folded dipole?

It is pretty well established that folded dipoles have much greater acceptable-SWR bandwidth than ordinary dipoles. Radiation efficiency is simply the quotient of radiated power to the antenna ...
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1answer
394 views

What is the best resonator in Yagi antennas?

As we saw many design for this Yagi feed point. Q1 . Which one would be the best resonator / driven? Gamma match, T-match, delta match, folded dipole, quad, loop? Q2. And what is the reason if ...
3
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1answer
1k views

How does adjusting the sleeve on antenna traps tune an antenna?

Take for example, the Hustler 4BTV. It has traps formed by an inductor, covered by a sleeve that's connected at the lower end, and the upper end is covered by a non-conductive cap: The tuning ...
6
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2answers
442 views

How to increase antenna aperture?

I know that in the short wavelength region, where parabolic antennas are used, the aperture can be increased simply by making the dish larger, as is the case with the radio telescopes. But how to do ...
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7answers
31k 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 ...
3
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2answers
11k views

Spacing between elements on a Yagi antenna

Does anyone know what the spacing between elements on a beam antenna should be? I built a 146 MHz 3 element beam following instructions, and the spacing was 16" between the driven element and the ...
6
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1answer
287 views

How to stack circular polarised antennas

I'm interested in stacking a pair of circularly polarized "crosshair" antennas. Do I construct such as both antennas are in phase with one another then match the two signals in phase via the coax line,...
10
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1answer
2k views

How does a “chip antenna” work?

How does a tiny surface-mount "chip antenna" for the 2.x thru 5.x GHz frequency bands work? What allows such a tiny item to radiate or receive RF with some amount of efficiency? What allows a chip ...
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1answer
122 views

Comparing antenna effeciency

Let's say I have three dipoles, one is resonant at some frequency and is matched at the antenna, the second is shortened with loading coals and is also matched at the antenna, the third is shortened ...
9
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1answer
2k views

Antenna RX vs. TX

If I only want to receive radio signals and not transmit, are there any differences in the way I design the antenna I need to use, or is it the same either way? I was going to pick up a Realtek ...
8
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3answers
653 views

How would I calculate antenna gain?

The concept of antenna gain is pretty important for calculation of link budget and is something I've had contact with. I know that it tells me how focused the waves emitted by the antenna are and I ...
16
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3answers
9k 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 ...
9
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2answers
752 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 ...
9
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3answers
4k views

Is a multiple-wavelengths antenna possible in UHF and up?

I know that in the High Frequency regions of the ham bands, the antennas have to be sub-wavelength because most people just can't put up a 160 meter full-size antenna. Let's say we're in the 23 ...
13
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5answers
4k 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 ...
14
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4answers
5k 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 ...
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5answers
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 ...

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