# Questions tagged [antenna-theory]

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

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71 views

### Coupling Capacitor of an Antenna

I have a simple question about general meaning & importance of the coupling capacitor in following part of a simple receiver circuit: Is it true that the proper reason why this coupling capacitor ...
125 views

### radials for 6 band Hustler vertical

I don't have room for quarter length radials for my Hustler 6 band vertical (10through 80 meters). Can I use 20 foot radials? If so, how many? The hustler is a Hustler 6-BTV 6 band HF vertical ...
102 views

### Antenna tuner at the transceiver output confusion

I am a bit confused. Let's take a HF transceiver for example. My understanding is that the output impedance of the transmitter is 50 ohms. The impedance of the coaxial cable feedline is 50 ohms, and ...
54 views

### Phasing stub length for colinear 5/8λ?

I've been designing a "Super J-Pole" for 10M in MMANA-GAL, but I'm designing it having two 5/8λ sections. However, I came across this picture: Various J-Poles types diagram indicating the ...
76 views

### Impedance miss-match short vs long section?

Let's say there is a 50 Ohm transmission line coming from a transmitter, suddenly the transmission line is spliced into a 300 Ohm line. From what I've learned a partial reflection occurs at this point....
36 views

### What is the signal of a receptor antenna irradiated with a plane wave?

Lets say there is a reception antenna connected to a detection system with a coax, and a transmitter sends a plane wave. What happens is that the antenna gets a certain power, following the [...
143 views

### Fractal antenna shapes that work for you? [closed]

I just answered this question, and I wanted to know if any particular shape has been found to be successful for anyone. There seems to be a lot of research on IEEE that indicates that there should be ...
117 views

### What is the minimum height required for a 60 meter NVIS antenna?

Various answers include 1/8th of the wavelength all the way down to 1/10th, but would 1/15th or even 1/20th of the wavelength be sufficient for a 60 meter (5 MHz) horizontal NVIS antenna?
133 views

### Will a HF beam antenna on rooftop building interfere with other antennas there

I am moving into an apartment on the top floor of a building and I intend to install a HF beam antenna for 20 to 10 meters, on the rooftop of the building. I notice that there are already some ...
70 views

### Mounting a hex beam on top of a high urban building

I am moving into a new apartment in the city soon which is on the 24th and last floor of an apartment building. I will have access to the roof of the building also. I'm still trying to understand ...
135 views

### Mounting a vertical antenna on a fiberglass mast

I have a multiband SE-X80 HF vertical antenna which works ok from 20 meters to 10 meters with a tuner. It is attached to the railings of my apartment balcony at the moment. This is the antenna : https:...
47 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 ...
86 views

### Why is there so little difference in the gains of Yagi Uda antennas with omnidirectional antennas? Shouldn't the power at the back be at the front?

Consider an omnidirectional antenna with X dBi gain. Intuitively I understand that the power that would have been dispersed in now-dead directions is directed around the antenna. What I don't get is ...
1k views

### How do VHF antennas compensate for change in frequency in Frequency Modulation?

I would like to understand how antennas specifically, compensate for variability in the frequency while receiving FM waves in VHF. The way FM works is that the receiver system - receiver and antenna ...
95 views

### Why don't the traveling waves on a dipole go back down the transmission line?

Why don't the traveling waves on a dipole go back down the transmission line ? Why does the standing wave on a dipole stop at the feed point ?
52 views

### Is this author correct about using a λ/2 of wire for a coil to get the proper delay on a λ/2 collinear antenna?

Is this author correct about using a λ/2 of wire for a coil to get the proper delay on a λ/2 collinear antenna? Improving the Super-J August 5, 2012 by John S. Huggins I agree with him that there has ...
420 views

### Grounding a vertical antenna from a balcony

I have a multiband vertical antenna mounted onto the railings of my balcony apartment high up on the 6th floor of an apartment building. The antenna is not grounded. This antenna comes without radials....
2k views

### Why are an antenna’s nulls sharper than its lobes?

Why, for realistic directional antennas, are its nulls sharper (deeper, narrower, etc.), than the radiation pattern’s lobes (local angular maxima)? Is there some physics or mathematical geometry to ...
315 views

### Can coax be run through the center of a radiating element without currents being induced on the coax shield?

I run a λ/2 vertical dipole for 20M that's fed to a 1.5:1 balun at its center with coax that is perpendicular to the dipole (parallel to the ground), for about 30 feet before it comes down to the ...
154 views

### Does anyone know of a compact ELF wave transmit antenna I can buy or make?

I need to transmit frequencies as low as 1Hz and I know the wavelength is the speed of light but wondered if there has been any breakthroughs in this field? I need a handheld antenna or at least one ...
234 views

### I'm looking for an effective beam antenna for both 20 meters and 10 meters

My favorite HF bands are 20 meters and 10 meters. I am currently using a multiband vertical antenna, but really it's not so great for DX'ing. I am looking for a good upgrade. I'm still learning alot ...
104 views

### Is there such thing as a center-fed longwire?

I want to make a longwire/Beverage receive antenna that will work on a wide range of bands (80-10m or wider). I've got a place where I can put a wire of at least 50m/150'. The only problem is — the ...
60 views

How do you measure the two to one bandwidth of an antenna using a Bird Watt meter? Tom Planer KJ9P
178 views

### Why are loopstick ferrite antennas commonly wound with Litz wire?

What is the advantage of using Litz wire to wind ferrite (loopstick) antennas? How much of an advantage in what characteristic does this type of wire provide? (over generic single strand enameled or ...
81 views

### How to compute the efficency of an omnidirectional antenna from its gain?

Some antennas have very limited specifications with no stated efficiency. For example, this antenna has the following specifications: Frequency Band 902-928 MHz VSWR <2.0:1 Impedance 50 Ω Gain ...
132 views

### What comes first, Voltage or Current?

Impedance can be defined as cause / effect or Z = E / I. This means that a difference in voltage potential causes a flow of current, the rate of current flow depending on the impedance. In a dipole ...
196 views

### For a resonant half wave dipole, is the phase of the applied RF voltage at the feedpoint in phase with the current reflected from the ends?

For a resonant half wave dipole, the voltage and current of the standing waves on the antenna are 90 degrees out of phase with each other. At resonance, is the phase of the current reflected back from ...
169 views

### Aren't all antennas technically dipoles?

Since all antennas require a ground, reference or otherwise, there are two poles for their operation. For a mag mount, the car body is the reference ground. A vertical cannot operate without a ground ...
41 views

### How might various conductive mast mounting configurations affect an electrically isolated rhombic antenna?

Assuming a rhombic antenna is: terminated for directionality electrically isolated at its mount point mounted to a metal mast partway up vs top of mast horizontally oriented (elements parallel to ...
52 views

### Impedance of V Antenna as Function of Angle

I'm trying to experimentally investigate the radiation resistance of a v-shaped antenna. When it's fully unfolded and the reactance is at resonance, the resistance is about 73Ω. When it's folded, it ...
246 views

### Is a resonant antenna inherently more efficient because it couples to free space more efficiently?

I saw an interesting assertion recently, that resonant antennas are more efficient because they provide a better impedance match to free space at the resonant frequency. I believe that to be untrue, ...
121 views

### Why do we need special geometry for telecommunication antennas?

In many youtube videos about building low power radio transmeters , I have seen a piece of regular wire or extension cord used as a antenna. But , it seems like telecommunication antennas have ...
320 views

### (When) does a transceiver re-reflect 100% of reflected power?

There has been a long discussion of one question here. Phil, W8II and I agreed that it's better to post a separate question. There are several sources (some editions of "The ARRL Handbook", &...
119 views

### What happens to the signal level if the antenna has high SWR?

I'm currently reading "Reflections: Transmission Lines and Antennas" by M. Walter Maxwell, W2DU. This book explains well why the energy is not lost because of high SWR during transmitting, ...
124 views

### what is main reason for formation of side lobes in directional antennas?

I would like to know why are side lobes formed along with main beam in an antenna. I asked one of my colleague and he explained me that its due to radiation of frequencies near to the required center ...
181 views

### Stacking Two UHF Yagi Antennas

Aloha! I have two HYS TC-YG05 Yagi antennas that were gifted to me and I would like to stack them side by side ( Collinear), in the vertical orientation for some local weather balloon telemetry data ...
66 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 ...
49 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 ...
135 views

### How to compute the required length and turns for an air-cored antenna

I'm experimenting with RF reception in the 433MHz band and have made the most simple "whip" monopole antenna with 17.6 centimeters of wire. Out of curiosity, I tried various designs for the ...
236 views

### Failing to detect Hydrogen line with 105cm dish

I've seen many examples of roughly 1 metre sized dishes being used w H+ sawbird and an SDR to detect and analyze Hydrogen line signal, and have been trying myself but can't seem to get anything beyond ...
200 views

### 1/8 wave 3-element 2m beam antenna

Here's what I am working on, and someone please tell me if I'm doing this right or wrong, but be kind and gentle. Center frequency 146.520MHz Driven element is 1/8 wave 10.25" long set at 6.125&...
228 views

### Using a parabolic dish for working the 70cm band - Am I missing something?

I have read a lot of times in this forum that C-Band TV dishes are not suitable for operating 70cm. However: Suppose that we have a 2,4m dish. By assuming a global efficiency of a 60%, our total gain ...
105 views

### Frequency selective surfaces working principle

I've seen that sometimes frequency selective surfaces are used to load a certain impedance to a certain antenna (for instance for input matching purposes). These surfaces act like a filter on the ...
271 views

### What do you call antenna that combines multiple Hustler resonators (using a VP-1) above a shared mast?

I know that a Hustler MO-3 54" mast with something like a RM-20S resonator on top is officially an example of a center-loaded quarter-wave vertical... ... but Hustler also allows you to mount ...
210 views

### Dual Polarization Yagi Antenna for 10 m

Imagine that someone has built a 4 element horizontal yagi antenna for 28 MHz with elements insulated from the boom, and with the driven element not split in the middle and using a gamma match to set ...
333 views

### Trying to identify and understand this antenna design - ground independent dual band vertical

This is the insides of a common dual band antenna 2m/70cm, made by diamond or nagoya. its generally labelled as: 1/2 wave radialless (144MHz), 2 x 5/8 wave radialless (430MHz) Its meant to be ground ...
79 views

### I have a 433MHz LoRa Dipole antenna mounted on a rocket. I am looking for a suitable Antenna to receive data from this. Any Suggestions?

The idea was to send GPS data from the rocket to the ground station at 433 MHz using a Lo Ra transmitter - mainly to help locate it once it lands and also to provide in-flight telemetry. I have a ...
58 views

### Excitation coefficient of a linear phased array

it's well known that in a linear and equispaced antenna array, the array factor is equal to: The array is then called uniform if $|I_n| = I$ is the same for all elements. The phase terms $\angle I_n$ ...