24
votes
Accepted
How does an SWR meter really work?
Dispelling the Myth
To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its ...
- 18.5k
12
votes
Accepted
What is the impedance of an end-fed half-wave antenna?
It's very difficult to predict the impedance of an end-fed wire, other than to say it's high. Usually it's determined empirically.
You are looking for a theoretical formulation. Consider, the ...
- 51.6k
12
votes
Accepted
Antenna Efficiency
In theory, if you had lossless conductors in the antenna and a lossless matching network, your shortened, 1 foot dipole would have a gain of only ~0.7 dB less on 160 meters than the gain of a full ...
- 18.5k
10
votes
Impedance ratio vs. SWR
As you can see from the following equation, it is definitely not that easy. What I would do is draw a 10:1 SWR circle on a Smith Chart and assume that your tuner can match all of the infinite number ...
- 1,646
9
votes
Accepted
50Ohm Coax presents 50 Ohms of impedance on which frequency?
For practical purposes, all frequencies.
If you cut a transmission line into infinitesimal segments, each segment can be modeled as:
simulate this circuit – Schematic created using CircuitLab
($...
- 51.6k
8
votes
Accepted
What exactly is impedance?
As resistance is to DC circuit analysis, impedance is to AC (or RF) circuit analysis. Now let's take a closer look at what that means, from several angles. I'm going to not include any of the math, ...
- 24.4k
8
votes
Accepted
Non-inductive resistors
For measurements the best resistor is a surface mount one - an 0805 resistor is non-inductive to a few GHz. Even better is to use two reststors, usually 100 ohms each, one each side of the coax.
A 1/8 ...
- 10.5k
7
votes
Accepted
Why does "high SWR" damage transmitters, instead of "impedance mismatch"?
The VSWR isn't the problem per se, it's just the impedance that appears at the transmitter's terminals. Take the load at the end of the transmission line, transform it according to the electrical ...
- 51.6k
7
votes
How is a 1:1 current balun different from a choke?
A choke is an inductor which is used to block high frequencies while allowing DC to pass. All chokes are inductors (though sometimes more than one inductor), but not all inductors are chokes: to be ...
- 51.6k
7
votes
Accepted
How can I calculate the 1/4-wave monopole impedance dependence on groundplane angle?
Once a ground plane antenna is 0.5 wavelengths or so above ground, there is no significant effect from the ground on the impedance of the antenna so it can be ignored.
A close approximation of the ...
- 18.5k
7
votes
Accepted
Coax or bifilar windings for a choke?
In most multi-band balun applications, there is rarely a need to maintain a perfect 50 ohm impedance within the balun. The feedpoint or input impedance is varying widely so another impedance bump in ...
- 18.5k
7
votes
Accepted
Can ferrite beads lower coax cable velocity factor?
No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside ...
- 7,818
7
votes
Accepted
Why do ham radio operators say that a full wave dipole isn't resonant?
Let's consider a simpler case by bringing the tips of a half-wave resonant dipole together so the two wires of the dipole are parallel, making a quarter-wave section of of balanced transmission line. ...
- 51.6k
6
votes
Accepted
How is a 1:1 current balun different from a choke?
Baluns are designed to be transformers (like 1:1 4:1, 6:1, etc.) or choke baluns, and both.
For an antenna, the purpose of a choke balun is to create a high-impedance to common mode currents that ...
- 2,988
6
votes
Can I use a CB antenna with a ham radio?
Congratulations on earning your General license!
You may get the antenna to work using the internal antenna tuner. Make sure the antenna is on a large groundplane. Try tuning it up with low power (&...
- 18.5k
6
votes
How can the voltage at the center of a resonant half wave dipole be zero if the input impedance is 75 ohms?
Look closer at the diagram. At the two wires coming from the source, the voltage is NOT always zero. The only way for the voltage at the center point to be zero is for the two source wires to occupy ...
- 1,646
6
votes
Can ferrite beads lower coax cable velocity factor?
For well-designed coax, the EM fields are confined to the space between the inside of the braid and the center conductor, i.e. the dielectric insulation region which affects the velocity factor. ...
- 1,646
6
votes
Ohms & coax cabling miscalculation?
First: if your multimeter reads 0.7 ohms with the probes shorted, it's not very trustworthy, and the rest of the calculation should be ignored. (Try putting a new battery in). The correct DCR for RG6 ...
- 11.7k
6
votes
Accepted
What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?
This would usually take a course in Transmission Line Theory, but we can give you the mechanical solution really quickly.
Behold: THE SMITH CHART (in impedance form):
Wow. A mandala! A fractal! What ...
- 15.5k
5
votes
What is the impedance of an end-fed half-wave antenna?
Wikipedia: Monopole Antenna
The monopole always has another set of conductors to which the
second wire from source is connected.
This second coundutor may be earth, or a metal object commonly known
...
- 389
5
votes
How is a 1:1 current balun different from a choke?
The name choke refers to the electrical component, whereas the name 1:1 current balun refers to the job it is doing.
There's more than one way to construct a balun.
There are purposes for a choke ...
- 24.4k
5
votes
Accepted
AN762 Amplifier Transistors
The tests they are referring to are for amplification characteristics like linearity and stability, not continuous thermal dissipation.
That said, unless you've constructed the thing in such a way ...
- 1,834
5
votes
Accepted
What is the actual loss in a feed line with high SWR?
Assuming no tuner losses, and assuming uniform loss throughout the line (which isn't true in practice, yet can be a practical simplifying assumption, more below), feedline losses in decibels are:
$$ -...
- 51.6k
5
votes
How can the impedance of a half wave dipole at its resonant frequency be purely resistive when the voltage and current are seemingly not in phase?
Andrew, the typical graphics showing standing waves are showing voltage and current distribution along the wire, not phase shift from each other. Voltage and current are always in-phase at every point,...
- 51
5
votes
Why are there so many coaxial cable impedances?
Glad to see this question posted! This has a really interesting answer which I'm not qualified to write a comprehensive answer to, so here's a rather unsupported answer to help you along until someone ...
- 24.4k
5
votes
Why are there so many coaxial cable impedances?
RF Cafe cites quotes Harmon Banning of W.L. Gore & Associates, Inc.:
In the early days of microwaves, around World War II, impedances were
chosen depending on the application. For maximum ...
- 7,818
5
votes
What is the impedance of an off-center fed resonant dipole?
Both the real and the imaginary terms of the complex impedance across the feedpoint terminals will change as the feedpoint moves away from the electrical center of the antenna.
As an example, a NEC4....
- 2,922
5
votes
50Ohm Coax presents 50 Ohms of impedance on which frequency?
The characteristic impedance of a line does change with frequency.
At high frequencies the loss components are effectively zero and $Z_0$ depends only on $L$ and $C$ (per unit length).
But at low ...
- 10.5k
5
votes
Is this result correct ? [ impedance cavity based on emtamaterials using CST Microwave]
Antenna efficiency over 130% would mean a perpetual motion machine, so probably not.
- 51.6k
5
votes
Antenna tuner at the transceiver output confusion
Having a transmatch (antenna tuner) at the feed point of the antenna can be an excellent way to solve the problem of a mismatched antenna; an antenna system with the transmatch at the feed point of ...
- 9,142
Only top scored, non community-wiki answers of a minimum length are eligible