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24 votes
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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 ...
Glenn W9IQ's user avatar
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17 votes
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Can I use speaker wire as a transmission line?

I can think of a reason expect this not to work very well. The impedance of twin-lead transmission line is dependent on the ratio between the diameter of the conductors, and the distance between ...
Kevin Reid AG6YO's user avatar
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11 votes
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Attic Dipole - Impact of Ethernet over CAT5/6

Having Ethernet and your antenna co-located is not an ideal situation. But then most amateur antenna situations involve compromises. The general idea of the following recommendations is to take as ...
Glenn W9IQ's user avatar
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10 votes
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Does an antenna tuner remove standing waves from a transmission line?

An antenna matching network (aka "tuner") does not affect the conditions of the load (antenna) or the transmission line between the load and the matching network. The matching network transforms the ...
Brian K1LI's user avatar
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9 votes
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Why isn't twisted pair used for feedlines?

Balanced lines (of which twisted pair is a special type) really have an upper frequency limit; you can't use them to transport 1 GHz (well, you can, but the smallest variation in direction or distance ...
Marcus Müller's user avatar
9 votes
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How can a quarter-wave transmission line transformer be implemented with lumped elements?

It can be done! Remember that a transmission line consists of some self-inductance per unit length, and some capacitance per unit length, and the ratio of these determines the line's characteristic ...
Phil Frost - W8II's user avatar
8 votes
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Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?

The SWR is related to the reflection coefficient $\Gamma$: $$ \Gamma = {Z_L - Z_0 \over Z_L + Z_0 } $$ $$ \text{VSWR} = {1+|\Gamma| \over 1 - |\Gamma|} $$ where: $Z_0$ is the feedline impedance, ...
Phil Frost - W8II's user avatar
8 votes

Confusion about dipole standing waves and feed point impedance

So yeah, this image is confusing. There's actually a section on the talk page questioning its correctness. I've considered a few times just deleting it, because it's so damn confusing. The arrows are ...
Phil Frost - W8II's user avatar
8 votes

Can I use speaker wire as a transmission line?

It depends substantially on the particular speaker wire — available gauges range from 14-2 to 22-2, insulation materials and spacing vary, and the impedance is probably somewhere between 80 and 150 ...
hobbs - KC2G's user avatar
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7 votes
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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 ...
Brian K1LI's user avatar
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7 votes

Is it true that standing waves on a transmission line don't cause failure of transmitting devices?

This is a bit like saying, "it's not the fall that kills you, but the sudden stop at the end." Technically, it's not the standing waves per se that cause the damage. However, standing waves imply a ...
Phil Frost - W8II's user avatar
7 votes

What does an antenna analyzer tell me about these coaxial cables?

My name is Alex, I'm the head of technical support at RigExpert. This is actually an interesting question. Our engineers could not give an exact answer why the schedule in the second case behaves ...
Alexander Antonov's user avatar
7 votes

Waveguide size for 2.4 GHz

The cutoff frequency for the $TE_{11}$ mode in a circular waveguide of radius $a$ is $$F_c={c\over{2\pi}}{1.841\over{a}}$$ so for your pipe of 16 mm diameter, $F_c= 10.99 \text{ GHz}$ So as a ...
tomnexus's user avatar
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6 votes
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SWR Measured at the Transmitter versus SWR at the Antenna

Given the matched loss of the feedline and the SWR at the transmitter, we can calculate the SWR at the antenna in three simple steps. First convert the SWR at the transmitter to the corresponding ...
Glenn W9IQ's user avatar
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6 votes
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Does Coax Limit the Maximum SWR a Transmitter 'Sees'?

The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. ...
Glenn W9IQ's user avatar
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6 votes

Can a common-mode current exist on the inside of a coax shield?

It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed. Skin Effect When direct current (time ...
Glenn W9IQ's user avatar
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6 votes
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Why does ladder line generally have a lower loss than coaxial cable?

When the SWR is 1:1, the matched line loss of ordinary ladder line is lower than the matched line loss of ordinary coax because at HF, most of the loss is $I^2R$ loss, and the current magnitude is ...
Cecil - W5DXP's user avatar
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. ...
Cecil - W5DXP's user avatar
6 votes

At what watt level is it safe to use metal strips for modulated signals?

There is no such thing as a minimum power level at which a conductor will start radiating. If it radiates at all, then the radiation will be proportional to the power applied. Therefore, the maximum ...
Kevin Reid AG6YO's user avatar
  • 24.6k
6 votes

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

If you fed a dipole not with a sine wave but with an impulse, you'd see a wave travel down the feedline, to the feedpoint, and then "ring" several times in the dipole at the dipole's ...
Phil Frost - W8II's user avatar
6 votes
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Will there be common-mode currents if a coax followed by a ladder line is terminated in a dummy load?

Yes it radiates. How much depends on the lengths of the wires. The point at which you connect the coax to the two-wire line is a bit like a dipole feedpoint, with about half the voltage applied to it. ...
tomnexus's user avatar
  • 11.2k
5 votes

Why isn't twisted pair used for feedlines?

Twisted-pair is just a special case of a balanced (untwisted) feedline. The twisting reduces crosstalk between the multiple pairs in the cable assembly but doesn't otherwise change the fundamental ...
Phil Frost - W8II's user avatar
5 votes

Is there an optimum transmission line length for maximum power transfer?

Ideally, it doesn't matter where on the constant VSWR circle you are. The transmitter is an ideal voltage source with 0Ω impedance, essentially a short. There's no way to couple power into a short, so ...
Phil Frost - W8II's user avatar
5 votes

Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?

Can someone tell me EXACTLY how the presence of current on the outside of the coax changes the VSWR reading ? When RF current is flowing on the outside of a coaxial cable, the exterior shield has ...
Glenn W9IQ's user avatar
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5 votes
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Does the impedance along a transmission line change when there are standing waves?

VSWR is related to the magnitude of the reflection coefficient $\Gamma$: $$ \text{VSWR} = {1+|\Gamma| \over 1-|\Gamma|} $$ The reflection coefficient can be calculated from the load impedance $Z_L$ ...
Phil Frost - W8II's user avatar
5 votes
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Will using a 1:1 balun at the feedpoint of a center feed dipole result in no common mode current on the outside of the coax?

Depends on how much current you mean by no current. But in general, unless the dipole is actually perfectly symmetrical, in a perfectly symmetric environment, above a symmetric ground, with the ...
hotpaw2's user avatar
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5 votes
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Importance of Ferrite Size in Coax Choke Balun

When making a coax choke balun, does the size of the ferrite matter, and if so, why? Simplest answer: If the choke impedance is low enough to allow some common mode power through, then there is a ...
Chris K8NVH's user avatar
  • 1,042
5 votes

Confusion about dipole standing waves and feed point impedance

You're reading too much into the Wikipedia picture! I think it's been asked before. It's also wrong about the zero voltage at the middle. Voltage and current can never be exactly 90 degrees apart ...
tomnexus's user avatar
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5 votes
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Does SWR change along the length of a transmission line?

The $SWR$ is, in principle, defined as a ratio of maximum and minimum voltages that exist at different locations of the transmission line. These two different voltages $U_{max}$ and $U_{min}$ can be ...
Jens's user avatar
  • 181
5 votes
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When is it relevant to consider a trace as a RF transmission line

The general rule of thumb is anything more than 1/10th the wavelength should be considered a transmission line. At 446 MHz, that's 67 mm. Your circuit is much smaller than that, so there's not much ...
Phil Frost - W8II's user avatar

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