# Questions tagged [antenna-theory]

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

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### 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 ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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)?
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 ...
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 ...
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 (...
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 ...
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 ...
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 ...
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. ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
12k views

### What is the aperture efficiency of an antenna?

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

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 ...
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 ...