Questions tagged [antenna-theory]

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

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6 votes
7 answers
20k 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 ...
3 votes
3 answers
15k 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 ...
0 votes
2 answers
119 views

How to determine the best length for a short dipole antenna

If I were to design a dipole antenna that's significantly shorter than usual for the frequency, how can I determine the best length to use? In other words, given a frequency and a maximum length (...
6 votes
3 answers
1k 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 ...
5 votes
3 answers
2k views

Does a receiving antenna need radials?

I am very new to radio and am using two 433MHz 1/4 wavelength monopole antennas to send and receive data. On the transmitting antenna I'm going to add radials (as it is the standard in my application)...
15 votes
8 answers
14k 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. ...
7 votes
1 answer
271 views

How does a Large Loop Antenna work physically?

I am learning about loop antennas at the moment. I read that the response of a small loop Antenna is proportional to the rate of change of magnetic flux through the loop which causes a uniform current ...
2 votes
1 answer
352 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?
-3 votes
2 answers
145 views

What frequencies can a GNSS (Global Navigation Satellite System) antenna pick up?

Am sure that a GNSS antenna can pick up other frequencies than GNSS ones, so is there anyone who tested that theory?
20 votes
4 answers
15k 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 ...
3 votes
1 answer
335 views

What is the antenna design and gain for the WA1 U/V band antenna on the ISS?

My son KJ7NLL is working to contact the ISS using an antenna we built together, and we were wondering the approximate minimum transmission power to reach the ISS. If I understand correctly (and ...
2 votes
1 answer
173 views

Operation of an off-center fed vertical for 10 meters

I am looking for a smallish omni-directional 10 meter antenna for installation in my attic. I notice that MFJ sells one called MFJ-1790. They call it an off-center fed vertical, and it has two ...
2 votes
1 answer
64 views

What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?

From Understanding how antenna mismatch can damage a transmitter "The reflected wave appears to the transmitter's antenna port... as an AC voltage source... whose magnitude and phase depend on ...
2 votes
2 answers
322 views

Understanding how antenna mismatch can damage a transmitter

I know it's important to tune an antenna to the same impedance as the transmitter for maximum power transfer. But I've read that if you don't do so, you not only get less radiated power, but you can ...
2 votes
2 answers
191 views

Why does my 7 MHz antenna trap have an SWR of 8?

I made a 7 MHz antenna trap. On testing it the cut-off point was 6.687 MHz which is about right but the problem I've got is the SWR was an 8 and wouldn't go down to a resonant SWR. What have I done ...
9 votes
1 answer
444 views

Hardened UHF Antennas

Minuteman missile silos used an interesting hardened UHF antenna. What kind of antenna is this? Does anyone have a drawing with a cross section? Edit: Jay Moore added a picture to a different type of ...
2 votes
2 answers
752 views

Beamwidth of omni-directional antenna with gain

I am looking to buy a wifi antenna for a boat to connect to shore based wireless hotspots that may be "far" away. Due to the waves, I am assuming that the antenna mast sways +-15° from vertical. The ...
1 vote
1 answer
123 views

Combining antennas

Is it possible to combine 2 antennas, each being half the length of a normal antenna for a given band. To be connected to 1 mobile radio. Giving you 2 shorter antennas rather than 1 long antenna, ...
3 votes
2 answers
267 views

Avoiding radio damage with closely spaced VHF/UHF antennas

I've mostly been using a discone in the attic so far, but now I want to get proper antennas up for 2m/70cm and gmrs. I'll probably get common commercial vertical antennas. I'm not sure how much I ...
0 votes
1 answer
324 views

LTE (4G) - Is two (or multiple) antennas better than one?

I have two antennas. A "multiband" (1710~2170 Mhz, 22 dBi)... ...and a "fullband" (700~2600 MHz, 15dBi)... Taking into account that my modem (router) is "fullband" and ...
5 votes
6 answers
2k views

Does an antenna pick up signals of all frequencies or only its resonant frequency?

I heard that an antenna picks up signals of all frequencies, and then these signals are filtered by a circuit using an inductor and capacitor, and that this is how a radio is tuned. However, I also ...
0 votes
2 answers
135 views

Is the phase difference between the voltage and current of the standing wave on a dipole antenna constant along the dipole's length?

Everyone knows that a 1/2 λ dipole antenna has on it a standing wave of RF energy the voltage and current of which are about 90° out of phase with each other. The standing wave is circulating reactive ...
1 vote
2 answers
182 views

How is it possible for a halfwave dipole antenna to be resonant?

I've been learning about antennas recently and I have some questions about the halfwave dipole antenna. My first question is about its resonance; as I understand it everything that oscillates has a ...
19 votes
3 answers
34k 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 ...
2 votes
4 answers
3k 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 ...
3 votes
2 answers
73 views

Antenna Aperature for out-of-band frequencies

I am doing some calculations where I determine power at the input of Rx given some field strength at the antenna. I have a question about the antenna aperture: $A_e = D\frac{\lambda^2}{4\pi}$ $D$ is ...
3 votes
3 answers
494 views

Does Velocity Factor pose a significant cable loss?

We know that a lower Velocity Factor translates to using shorter runs of cable in resonant circuits. That apparently means that there is resistance to the flow of electrons, slowing them down. So ...
0 votes
2 answers
239 views

1/2 wave dipole instead of 1/4 wave + ground plane

I have a transceiver operating at 150MHz using a 1/4 wave vertical whip and horizontal ground plane at 90 degrees. My end user doesn't like the antenna sticking up in the air (trip hazard) so I'm ...
3 votes
1 answer
208 views

How do these ceiling mount dome antennas work? Are these discone antennas or horn antennas?

The small cone is connected to the larger one. The coax center cable is terminated into the small cone and the coax shield into the larger outer cone. The screenshots are from this video https://www....
8 votes
1 answer
157 views

S Parameters in HFSS

I'm new to HFSS Software and I'm trying to plot Scattering parameters for a dielectric resonator excited by a probe (shown in image). I know that the top cylinder has a higher dielectric than the ...
9 votes
3 answers
10k 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 ...
3 votes
3 answers
586 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 ...
1 vote
2 answers
304 views

Off-center fed long wire antenna

I want to feed a horizontal wire antenna (approximately 150' long at 50' or more above the ground) for multi-band HF operation with an off-center (25-33%) feedline. The element wire will have no ...
3 votes
3 answers
4k views

Are chokes necessary for VHF and higher frequencies?

I am trying to install a VHF/UHF antenna on my truck, and I have been studying everything I need to do that. I keep reading about chokes, in particular here: http://www.k0bg.com/choke.html However, ...
2 votes
2 answers
254 views

How can cross-polarization loss be determined given the angle between two antennas?

This question addresses the case where two antennas are exactly 90° cross-polarized, but not other angles. How can I determine the theoretical, and estimate the actual, attenuation from cross-...
0 votes
0 answers
66 views

LTE (4G) - Which external/outdoor antenna type is the best?

I've seen several antenna types out there, but I can't know for sure which model is the best for external/outdoor LTE (4G) use. I have to say that I don't understand anything about LTE (4G) antennas......
0 votes
3 answers
107 views

How does the waveform of electromagnetic wave affect the design of an antenna?

This is a question I thought about when trying to understand a design of an antenna. For different kind of waveform of electromagnetic wave propagating through space, how does one modify an antenna ...
1 vote
2 answers
63 views

Relation of Emitted Wave in Far Field Regions and Radiation Pattern with Antenna Design

As this post from electronics stack exchange suggested, EM wave emitted by the antenna tends to become spherical waves(plane wave with a local analysis) in the far field region regardless of the shape ...
1 vote
4 answers
521 views

Would it be possible to light up a bulb wired one side to a transmitting antenna?

Would a light bulb light up if wired to the end on an antenna ? If you would connect a light bulb on one of its poles to a quarter wave antenna like in the schematics below, would the bulb light up ...
5 votes
3 answers
723 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 ...
5 votes
4 answers
6k 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 ...
2 votes
1 answer
723 views

Calculating a real antenna gain from electric field strength?

Could someone please tell me if it is possible to measure the antenna gain simply by placing a field strength meter at a selected distance to my antenna, knowing the power applied to the antenna. ...
2 votes
5 answers
385 views

Minimal SWR at resonance

Why does a minimal SWR NOT indicate antenna resonance? Would SWR 1:1 be required to indicate antenna resonance?
2 votes
2 answers
380 views

Balance of gamma / delta match

A center fed dipole, with equal arm lengths, is a balanced device. If it is fed using a gamma match - does it change (the system) to be unbalanced?
1 vote
0 answers
222 views

What is the differece between a large loop antenna and a small loop antenna in terms of their response to an electromagnetic wave?

There is something that I don't get. I read that a small loop antenna (also called magnetic loop) responds more to the magnetic field than a large loop antenna does. I was wondering why this is the ...
5 votes
3 answers
948 views

does the shape of an antenna ground plane matter?

I would like to test a tranceiver setup I created before I mount it in my car. In my backyard, there's a length strip of metal; 10m in length, 10cm wide. Now I wonder if I can use that as a ground ...
6 votes
4 answers
2k 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
2 answers
145 views

What frequency ranges are most common for Amateur radio astronomy and why are most parabolic antennas tuned to such high frequencies?

What frequency ranges are most common for amateur radio astronomy? I am mostly interested in radios for the purpose of astronomy. In general I was expecting that the lower frequencies would be more ...
4 votes
1 answer
337 views

Parameters needed to calculate inductor value to achieve proper phase delay?

I'm thinking about trying a coil in my collinear 5/8λ antenna, instead of the 3/8λ phasing delay line between the lower and upper elements pictured below. I figure that a coil with a certain ...
1 vote
2 answers
529 views

Can fanned/parallel endfed antennas be used to get higher gain in some directions?

I have two pieces of antenna wire which are roughly 10 m long, with banana plugs at the ends, which I bring to hang in the trees when operating portable. I also bring a 49:1 unun and hopes that my ...

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