Questions tagged [antenna-theory]
Designing antennas, selecting antennas for specific applications, properties of a given antenna design.
307
questions
5
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1answer
41 views
How does this quarter-wave stub balun work?
I was searching for 1:1 balun designs that would be efficient at VHF yet could easily be made from commonly available materials when I came across these two:
While the design on the right seems ...
-1
votes
4answers
95 views
Why is a half wave dipole resonant?
Is the following explanation correct ?
RF energy with a wavelength equal to twice the electrical length of the dipole is applied to the feed point at the center of the half wave dipole.
To understand ...
2
votes
3answers
85 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 ...
2
votes
2answers
92 views
Double Cross antenna: what's wrong with my model?
Important note: After consulting another source, I learned that the article calls for the first pair of opposing dipoles to be fed in phase, with the second pair fed in quadrature to the first pair (...
0
votes
1answer
66 views
For a center fed half wave dipole operated at resonance, are the voltage and current in phase only at the feed point?
For a center fed half wave dipole operated at resonance, the voltage and current at the feed point are in phase in time and there is zero reactance present in the impedance seen at the feed point.
For ...
4
votes
2answers
92 views
Why do horizontally-polarized RF waves reflect better off ground than vertically-polarized waves?
It's a well-known fact among hams who use HF frequencies that horizontally-polarized RF waves reflect better off of ground than vertically-polarized waves. This explains why ground conductivity is so ...
0
votes
1answer
73 views
Outdoor Terrestial Digital TV Receiver Antenna: 500-700 MHz Band, Can I Make It Work Better?
See the unconventional antenna shown below, The whole antenna is made of two mirror-imaged Alum metal stampings on either side of a 2-cm hollow 31-cm long rectangular plastic boom. The antenna top and ...
2
votes
4answers
238 views
Does a resonant antenna work better than a non-resonant antenna?
Some books on antenna fundamentals i've read recently say that, for a half wave resonant dipole when transmitting for example, the traveling wave of movement of charges reflected back from the ends of ...
0
votes
4answers
202 views
For a half wave dipole antenna, what's the value of the reactances that cancel out when the antenna is resonant?
According to a few descriptions of antenna fundamentals i've read recently including that in Wikipedia and the ARRL handbook, when a half wave dipole is resonant, the inductive and capacitive ...
4
votes
3answers
188 views
Common-mode current and choke placement
Why are chokes usually placed at the feedpoint, can't common-mode currents occur below the choke as well?
I understand it's placed at the feedpoint to keep everything balanced by eliminating common-...
0
votes
4answers
95 views
Variable polarization Yagi using two antennas on the same boom, joined together with variable feedpoint phase?
I have been trying for ages to find a way to mount a 10 m vertical Yagi properly, so that the mounting pole doesn't stuff up the impedance and radiation pattern, and have had no luck so far. Apart ...
6
votes
4answers
316 views
Any reason not to choose a discone antenna over a Wide/ MultiBand mobile antenna for TX/RX operations
We are setting up a all-band radio in an emergency com center that's intended most for amateur band use, but since its' for emergency use will also be used in the public safety bands; (150-170)MHz, (...
0
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1answer
37 views
On a horizontally setup 20m delta loop I am looking to determine the calculations for the proper length of a feed wire
Base of loop will be about 6 feet above ground. RG-8 or 213 will be used for the feed wire. Ugly balun as well.
0
votes
2answers
72 views
Antenna Basics - Open/Short Circuit & Half-Duplex/Full-Duplex
I am beginner in the Antenna theory and I have a few basic questions.
Please help me to clarify my understandings related to the basics which I have read.
And this question has 3 basic questions. ...
1
vote
3answers
117 views
Need antenna for mobile mag-mount LPFM broadcast 88.3 MHz
I am doing some temporary FM broadcasting for car events such as COVID-safe car parades and drive-in movies. The terrain is horrible, urban areas with lots of buildings. The cars snake around a city, ...
5
votes
1answer
140 views
Can half of a directional antenna (Yagi-Uda, HB9CV) be replaced by a ground plane?
Consider a center-fed dipole antenna. It‘s well understood how the bottom element can be replaced by a ground plane (in practice, generally a few rods but in principle a conducting plate would work as ...
2
votes
1answer
102 views
How do I tune my EFHW 40-10 to have the lowest SWR in the 40, 20 and 15 meter bands?
This is a reading from my RigExpert AA-35 ZOOM of my EFHW 40-10 inverted L with radials, and feedpoint 2 feet above the ground. Any thoughts as to how I can tune or move this antenna to get the ...
5
votes
2answers
181 views
Designing a helical antenna for a dish
I'm interested in a dish antenna with an axial mode helical feed for receiving video. My target frequency is about 1280 MHz or the 1.2/1.3GHz range.
I've been able to figure out helical basics, like ...
3
votes
4answers
85 views
Relation between antenna physical size and gain
I would like to know whether increasing the size of the antenna help increase its gain. Does this hold only for certain kinds of antennas like parabolic or horn antennas? And if it is true, can you ...
3
votes
5answers
106 views
Can I use a 1/4 wave decoupling stub to enable mounting of a vertical yagi on a metal pole?
My understanding is that the ground plane radials used on, for example, a free standing 5/8 wave 10 m vertical antenna mounted on a metal pole, are actually 1/4 wave length decoupling stubs, the ...
2
votes
1answer
43 views
Why do some 1/2 wave collinear designs have a 1/4 wave element?
I think I understand the basic principles of a collinear antenna. How the Coaxial Collinear Antenna Works? seems to describe it well.
However, I've just repaired a 1090MHz (ADS-B receive only) ...
1
vote
0answers
54 views
Would a phase compensated sinusoidally spaced folded dipole present less noise to a receiver?
I am attempting to design an antenna similar to a TTFD, double zepp or array of end fed wires, with a sinusoidal, with the period of the wavelength, spacing that causes additive waves on the target ...
3
votes
2answers
41 views
Connecting Directional Antenna To Two Antenna Ports on NIC
I want my n/ac Wi-Fi card to operate in a spatially directional manner.
To achieve that I want to use a directional antenna(s). There is a slight complication though, because my Alfa AWUS036ACH ...
2
votes
2answers
138 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 ...
0
votes
2answers
64 views
Loop on ground for transmit?
If a large horizontal loop antenna, insulated but laying at ground level, works as an MF and low HF receive antenna, why (even though it has very high ground losses) can't it be used for transmit (to ...
3
votes
1answer
73 views
How do I use a signal generator + oscilloscope to measure feeder length?
I could relatively easily take the feeder down and use a tape measure but it’s more of a challenge to learn how to measure the length using my oscilloscope -Red Pitaya - but I have to admit I have no ...
4
votes
1answer
89 views
Why does damping between two antennas increase with frequency?
While studying for a HAM license exam I ran into a question that I am having trouble with answering. There are two aspects that I cannot explain, given my current admittedly little understanding of ...
2
votes
1answer
75 views
144/430 MHz antenna on 155 MHz?
We have a local radio shop that started selling us Relm P150s with a high gain 9 inch antenna, and those are ok.
He also has thrown in longer 15 to 14 inch dual band antennas saying use those in ...
2
votes
0answers
55 views
Circular polarized antenna feed changes as I rotate it?
I had a 100cm prime focus dish laying around and I thought of making a circularly polarized feed for it, since it seemed pretty straightforward from what I've been reading. Helical feeds are supposed ...
1
vote
4answers
153 views
Indoor HF/VHF/UHF antenna, have someone an Idea?
My wife doesn't think that having tall antennas is acceptable.
Does anyone have a solution for this?
Our house is 16 meter long and 10 meter wide.
I have an antenna tuner (if it can help).
2
votes
2answers
81 views
Estimating near-field electric field strength at LF (137 kHz)
I'm experimenting with low frequencies, but due to the inherent nature of LF, I strive for more transmit power and bigger antennas.
Now of course I want to comply with regulations, which allow (in my ...
4
votes
1answer
201 views
Is a ground plane antenna a poor choice to use in a forest?
Some time ago, when I just started with amateur radio, a few more experienced hams told me that an inverted-V works better in the forest than a ground plane, and that it has something to do with the ...
0
votes
1answer
80 views
Solid angle for different E/H plane beamwidths
The solid angle formula calculates the surface area on a unit sphere, from projecting a rectangular patch onto the surface of a sphere.
Image source
This is calculated using the azimuth angle $\phi$ ...
6
votes
2answers
187 views
For a resonant 1/2 wave dipole, does the reflections being in phase with the applied signal cause the zero reactance seen in the feedpoint impedance?
For a resonant 1/2 wave dipole when used for transmitting, does the fact that the reflected waves on the antenna are in phase with the applied signal result in zero reactance at the feed point, or ...
0
votes
1answer
82 views
Is shortening a half wave dipole to make it resonant by removing the inductive reactance a good idea?
A dipole which is exactly one half wave length allows reflections within the antenna to be in phase with the applied signal. To me it seems that this situation gives an antenna which will produce the ...
0
votes
2answers
112 views
For an antenna when used for transmittiing, what is the optimum radiation resistance?
For a 100 % efficient antenna, the real part of the impedance is equal to the radiation resistance.
If the antenna were a dead short with zero impedance then the radiation resistance would be zero.
...
5
votes
2answers
158 views
Modelling a folded dipole
I am looking at using NEC to model a $0.5\lambda$ rectangular folded dipole, for closely spaced larger sides (choosing a separation of $0.01\lambda$).
In Antenna Theory, Analysis and Design by ...
0
votes
2answers
121 views
What exactly makes an antenna resonant?
When an antenna is resonant, the impedance seen at the feed point is purely resistive and has no reactance.
What are the physical properties of an antenna that actually make it resonant and how do ...
-3
votes
2answers
206 views
How can a half wave dipole which has an impedance of 73 + j43 ohms ever be resonant?
Is it true that for a half wave dipole, cancelling out reactance using for example a gamma match does not make the antenna resonant at all, but rather just cancels out the reactance of what is ...
2
votes
1answer
47 views
Can I use a directional coupler to block reflected AC current from a mismatched antenna?
Is there any kind of directional coupler device such as you find in a SWR meter that only allows traveling AC waves of one direction to pass which i could use to block the reflected wave from my badly ...
1
vote
1answer
31 views
Can received and transmitted signals on the same antenna combine to produce standing waves?
Standing waves are seen on a transmission line when for example there is a change in impedance and a transmitted wave is reflected back from one end and then the incident wave and reflected wave ...
0
votes
1answer
86 views
If i transmit on the same frequency using two separate transmitters joined together will they both add up in a reciever to produce a stronger signal? [duplicate]
If i were to use a passive transmit coupler to connect two transmitters of equal power output to the same antenna, and connect one microphone to both transmitters, and then transmit on both at the ...
14
votes
1answer
2k views
Why do amateur radio operators call an RF choke a balun?
When using a balanced antenna such as a half wave dipole, with an unbalanced feed line such as coax, an RF choke can be used to block RF current emitted by the transmitter from splitting up at the ...
1
vote
1answer
75 views
Does an antenna work the same for pure sine waves as compared to other waveforms?
Antenna theory it seems is always described assuming that the transmitted or received signal being talked about is a pure sine wave usually at the resonant frequency. Standing waves for example are ...
1
vote
1answer
79 views
Can any one explain E,H plane definitions?
I read in some text books that
E-plane is formed by taking constant azimuth angle and scanning elevation angle from -90:90.
H-plane is formed by taking a plane perpendicular to E-plane.
What is ...
2
votes
1answer
96 views
Is the voltage at the centre of a dipole zero only at it's resonant frequency?
For transmit, the voltage at the center of a dipole not cut in the center and fed with coax and a gamma match is zero in the center at the resonant frequency. This is why apparently a 'plumbers ...
0
votes
1answer
43 views
What is the effect of coil Q on a shortened dipole?
I made a small yagi, with the driven element being a coil loaded dipole. What is the effect of coil Q in this situation?
The coils I built are 18 turns of 1.2mm enameled wire wound tightly with a ...
2
votes
2answers
210 views
Is an antenna resonant at the frequency of lowest impedance?
I understand that an antenna is resonant at the frequency where the capacitive and inductive reactance cancel out to 0. Only the resistive impedance remains at resonance.
Does this mean that the ...
3
votes
1answer
120 views
Ferrite Rod Antenna Array?
The ferrite rod is a reasonably effective receiving antenna for LW, MW, and lower HF spectrum. It is well known that a ferrite rod is much more effective than an air cored coil of the same dimensions. ...
3
votes
2answers
139 views
What kind of loop antenna is this, and what is the relationship between its diameter and wavelength?
At the bottom of webpages in the advertisement swamps I keep seeing this image of an antenna, accompanied by an offer that I won't be clicking on.
But the antenna itself looks quite intriguing. It's ...
