Questions tagged [antenna-theory]
Designing antennas, selecting antennas for specific applications, properties of a given antenna design.
335
questions
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4answers
154 views
Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole?
Lots of explanations i've read on the internet written by ham radio operators and in some text books state that a half wave dipole is resonant when the inductive and capacitive reactances cancel out.
...
0
votes
0answers
51 views
Long wire antenna end fed random length folded back on incline
Trying to lengthen my 10mtr. end fed to 39 metres in length. To do this I need the first leg (10mtr.) to run at a lower height approx (5mtr)turning back on a dog leg, and the final 29 meters then ...
3
votes
3answers
129 views
Use (TV?) Log-Periodic antenna for VHF
I found this old log periodic antenna that I assume was from a TV. Could I, or how would I, use this on the amateur radio bands? My main focus would be VHF, with UHF being secondary and HF would also ...
2
votes
2answers
83 views
Magnetic loop antenna: Which side of conductor is the current due to skin effect?
Contemplating a big magnetic loop antenna.
Would a wide copper strip be better than a tube with the same conducting surface?
Where will all the current be due to the skin effect?
Would a flat ring be ...
0
votes
1answer
45 views
Dual Rubber Ducky antennas
Can I use 2 identical rubber ducky antennas with a splitter on my CB HT radio to improve range?
0
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2answers
51 views
Is there an instance where you can directly feed a half wave dipole with coaxial cable without having to worry about balanced to unbalance?
Like the question says, can you feed a dipole with coax with no ill effects? Nothing else in the system, just the antenna and the cable.
2
votes
3answers
104 views
How do 'plumber's delight' type Yagi antennas have a bandwidth?
For Yagi antennas where the driven element is electrically connected to the boom like the parasitic elements, how can this function without diminished performance outside of the design frequency? In ...
1
vote
1answer
30 views
How to calculate of shielded loop antenna correction factor without extra antenna measurements
A lot of article about 60 cm shielded loop antennas and how to measure antenna factor using antennas with already known antenna factor.
But for example if I have one shielded loop antenna (...
1
vote
1answer
46 views
What length and number of radials should be used for a dual band vertical whip backpack antenna?
The Situation:
Very new Ham
I have a 2m/70cm 48" Abree whip antenna, originally designed to be connected directly to an HT
I would like to mount this antenna to the side of my hiking backpack, ...
3
votes
3answers
64 views
Recommendation for hobbyist flight computer radio antenna
I'm a CS college student working on the flight computer of my Rocketry Club's rocket. My flight computer has a 915MHz radio transceiver, and it needs to transmit data to my computer with another radio ...
2
votes
1answer
75 views
Substitute a car roof with radials?
I have a K40 Plus CB antenna, which is supposed to be attached to a car roof for the ground plane. However, I would like to set this up as a base antenna. Could I instead attach the ground to separate ...
1
vote
2answers
79 views
How many turns of wire inside 60 cm shielded loop antenna (9kHz - 30 MHz). How can I find this number without destroying the antenna?
Lots of information about shielded loop antennas is available on the internet. However, antenna manufacturers do not indicate the number of turns. How can I find the number of turns of a manufactured ...
0
votes
1answer
41 views
Stacked Yagi Gain calculation
I had a quick look at a practice exam for the license in my country and had a question on stacked Yagi antenna. I have been looking around but the only thing I can find is the mention of an additional ...
0
votes
1answer
40 views
Can I slant-polarize a bay antenna?
I am trying to work a 700mhz LTE signal with a DB2 uhf bowtie television antenna. I find the cellular signal to be horizontally polarized. Can the bowties simply be twisted 45 degrees in both ...
2
votes
2answers
91 views
How does a 40/15m dipole with capacitance hats work?
I followed an article Antenna Here is a Dipole [PDF] by James Hearly, NJ2L to build a dual band 40/15m dipole:
The antenna works more or less as expected. My 40m dipole was resonant at 7.030 MHz and ...
0
votes
1answer
47 views
60cm 9 kHz 30 MHz Shielded loop antenna behavior as a transmitting antenna
I know how shielded loop antenna work as a receiver antenna. (How behave electric and magnetic field).
But I can not understand how shielded loop antenna works as a transmitter.
Will it produce only ...
2
votes
4answers
949 views
Are there any antenna that can create disk like lobes?
Are there any antenna that have disk like lobes :
Or are there any antenna that have very thin directivity, say lobes be no more wide than few mm.
0
votes
3answers
66 views
Loop a long wire antenna or EFHW
For a typical EFHW antenna, let's say we are in a very space-restricted environment, says a hotel room.
I am thinking of the idea loop around the wire around the room, maybe even many loops.
This is ...
1
vote
1answer
72 views
The HPBW of a helical antenna
can someone tell me what are the formulas of HPBW for a helical antenna?
I've searched a lot and I saw that there are two different formulas of HPBW for the helical antenna,
this one,
$$ \mathbf{(HPBW)...
1
vote
2answers
83 views
What does antenna frequency range mean in practice?
I'm trying to wrap my head around software defined radios and antennas and noticed that lots of antennas are described in terms of a frequency range. For example, this one is described as operating ...
2
votes
3answers
81 views
Yagi Antenna - Elements Length
I am currently modeling a yagi antenna using Matlab and after I optimized the antenna, it gave me a director length shorter than the exciter itself (even if the optimization gave me a good gain).
From ...
1
vote
3answers
87 views
Radio Waves propagation (Intuitive Understanding of Wave propagation)
I have few similar questions regarding Radio waves communication particularly FM and AM type of Radio wave communication. Please bear with the naivety of the questions.
Questions:
Radio waves in FM ...
4
votes
1answer
91 views
How do dual band J poles work?
My understanding is a J pole is a $\frac{1}{2} \lambda$ dipole attached to a $\frac{1}{4} \lambda$ impedance matching element.
How does this dual band work? What acts as the impedance matching element ...
4
votes
2answers
131 views
Heat-shrink as dielectric for bazooka balun?
I'm intending to make a sleeve balun (aka bazooka) for 2m, but rather than keep the lossy coax jacket as a dielectric (I have some RG58), was considering to remove it and replace it with heat shrink ...
5
votes
1answer
94 views
How does this W6NBC “derivative of the classical slot antenna” work?
Via the https://ham.stackexchange.com/a/9883/1362 answer regarding a slot antenna for a 2m mobile application, I was introduced to this (pdf) interesting antenna design:
The designer John Portune (...
2
votes
1answer
104 views
Questions about effective isotropic sensitivity(EIS)
Recently, I read something about EIS (effective isotropic sensitivity) and TIS (total isotropic sensitivity). I'm trying to understand the concept, but I find it's quite hard for me.
In the book ...
3
votes
3answers
178 views
Ground-Plane radial sizing, is there such thing as too long?
I'm just trying to make sure I understand the theory correctly.
I have some triband VHF/UHF/800MHz antennas, I need to mount on a tower so I plan on using a "mobile to base" conversion kit. ...
5
votes
2answers
145 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
vote
2answers
90 views
How to match transmitter to Beverage antenna
Beverage antenna consist of pair of transmission lines - one being the "antenna proper", the other ground.
The antenna is being fed via open feed line ( approximately 450 Ohms ) - one side ...
0
votes
4answers
121 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
108 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
142 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
84 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
119 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
119 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
283 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 ...
-2
votes
4answers
328 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 ...
5
votes
3answers
538 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
106 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
376 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
votes
1answer
38 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
119 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
127 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
2answers
168 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
157 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
243 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
121 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
128 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
86 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
55 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 ...
