23 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 ...
  • 18.2k
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 ...
  • 18.2k
11 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 ...
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 ...
9 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, ...
  • 23.1k
8 votes
Accepted

What happens in a low loss feed-line with a high SWR

For many modulations, the modulation is very slow compared to the propagation delay of the feedline. For example, SSB is typically limited to no more than 4 kHz. That corresponds to a wavelength of of ...
8 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 ($...
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 ...
  • 8,340
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 ...
  • 18.2k
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 ...
  • 18.2k
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 ...
  • 7,713
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 ...
  • 2,898
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 (&...
  • 18.2k
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 ...
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. ...
6 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. ...
5 votes
Accepted

Coaxial Folded Dipole Antenna Matching Impedance

The site above depicts a regular dipole, with a simple matching system. First, a bit about dipole impedance. The impedance of a dipole is 73 Ohms at resonance, but away from the resonant frequency ...
  • 8,340
5 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 ...
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 ...
  • 1,814
5 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 ...
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: $$ -...
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,...
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 ...
  • 23.1k
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 ...
  • 7,713
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....
  • 2,892
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.
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 ...
  • 8,932
5 votes

What is the difference between antenna input impedance and its radiation resistance?

No, they're separate. Radiation resistance contributes to the real part of input impedance, but so does ohmic loss (heating), and non-radiative coupling with the EM field produces the imaginary part ...
  • 9,552
5 votes

How can a low-impedance load can damage a FET final amplifier?

Step away from RF for the moment, and think about the basic operation of a single-supply transistor amplifier, or even a voltage regulator. simulate this circuit – Schematic created using ...
  • 23.1k
4 votes
Accepted

What's the difference between "minimum SWR" and "resonance"?

Good questions. But, I think another question to add is: What is more important, SWR or resonance? Given that this is the "Amateur Radio" forum, I think answers that are most meaningful to amateur ...
  • 2,898

Only top scored, non community-wiki answers of a minimum length are eligible