Questions tagged [transmission-line]

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Can I use speaker wire as a transmission line?

Speaker wire is really cheap and available everywhere, and it doesn't seem that different to 300 ohm twin lead. It seems to me it would make an excellent balanced transmission line. What would the ...
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Why do ham radio operators refer to current flow on the outside of coax as common mode current?

My understanding is that the desired mode of operation for a two wire transmission line is differential, where at any point along the line the addition of the voltage or current is zero, such that the ...
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1 answer
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How to make a 1:2 UnUn

Does anyone have a drawing or instructions on how to make a 1:2 unun like the one in the picture below ? I searched this question on this site and on the internet with no luck.
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How to Feed 2 x Co-Phased 4 Element Yagi Antennas?

I am wanting to build 2 x 4 element vertical yagi antennas for 28 MHz and co-phase them and feed with one 50 ohm coaxial transmission line. If i were to use a gamma match for each yagi and tune them ...
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3 votes
2 answers
144 views

How do you calculate peak voltage given VSWR caused by impedance mismatch in a 50-ohm system?

I would like to choose a feedline that is safe for the wattage at a given VSWR to avoid coax arcing. How do you calculate peak voltage given a peak power in watts and maximum expected VSWR from ...
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2 answers
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How should I think about "extra" impedances inserted within/across a transmission line?

I'm worried this might be two separate questions — or perhaps not any coherent question at all? — but some recent thinking about transmission lines made me realize I still don't know how to think ...
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9 votes
3 answers
210 views

Will there be common-mode currents if a coax followed by a ladder line is terminated in a dummy load?

Assuming an antenna system where a transmitter feeds an ideal coaxial cable which is then connected to an ideal ladder line without a balun: It is often said that common-mode currents appear in the ...
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2 votes
1 answer
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Why don't the traveling waves on a dipole go back down the transmission line?

Why don't the traveling waves on a dipole go back down the transmission line ? Why does the standing wave on a dipole stop at the feed point ?
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How to understand same SWR but different Tx/Reflected power

I'm tuning a wire dipole. Cross-needle SWR meter shows the same SWR on the lower end of the band as on the higher end. However, the needles move much higher on the lower end. They cross on the same ...
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3 votes
2 answers
67 views

When is it relevant to consider a trace as a RF transmission line

I'm building a simple Greinacher based RF Power meter for the PMR band i.e 446MHz. I intend it to work as a confirmation device and not something really acurate. The circuit is quite simple: The led ...
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2 votes
1 answer
184 views

Does SWR change along the length of a transmission line?

Wikipedia says "SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line". Standing wave voltage ...
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Apparent Paradox about wave reflection in a transmission line?

The transmission line is without loss of length λ / 4, the load $Z_L$ is a short circuit and $Z_G = Z_0$. At $t = 0$, an incident wave of voltage $V_1$ and current $I_1$ such that $V_1 = Z_0 * I_1$ ...
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5 votes
5 answers
395 views

(When) does a transceiver re-reflect 100% of reflected power?

There has been a long discussion of one question here. Phil, W8II and I agreed that it's better to post a separate question. There are several sources (some editions of "The ARRL Handbook", &...
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2 votes
0 answers
56 views

Printed Dipole Transmission Line Equivalent Model

let's consider a printed dipole on a substrate with a GND plane: I've seen many times an equivalent model which uses transmission lines like this (reference): This model is useful to understand the ...
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4 answers
426 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. ...
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4 answers
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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 ...
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3 votes
3 answers
470 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 ...
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9 votes
2 answers
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Importance of Ferrite Size in Coax Choke Balun

The question is quite simple: When making a coax choke balun, does the size of the ferrite matter, and if so, why? I have seen design notes that say to use a single FT240-43 toroid for powers up to ...
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7 votes
2 answers
462 views

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

Note: more measurements added after graphs below I recently acquired a RigExpert antenna analyzer and I am trying to learn how to use it to test cables. I took two cables to start with to try and ...
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-1 votes
2 answers
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At what watt level is it safe to use metal strips for modulated signals?

In my working design, I have a transceiver TX on low power; Then boost it in a preamp phase in the back. The way I'm sending the signals (RX and TX) through the radio is by a network of 1/8 to 1/4 ...
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1 vote
1 answer
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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 ...
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1 vote
3 answers
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Is it true that standing waves on a transmission line don't cause failure of transmitting devices?

Actual standing waves have nothing to do with causing the output device in a transmitter to fail, in fact it's the reverse or reflected current which helped to create the standing waves which is ...
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2 votes
2 answers
268 views

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?

Will using a 1:1 balun at the feedpoint of a center-fed dipole when using coax result in no common mode current on the outside of the coax ?
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1 vote
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398 views

Does the impedance along a transmission line change when there are standing waves?

I know the characteristic impedance can't change because it's determined by the physical properties of a transmission line. But the impedance seen when there are standing waves changes along the ...
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4 votes
1 answer
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Does an antenna tuner remove standing waves from a transmission line?

Does an antenna tuner remove standing waves from a transmission line ?
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4 votes
4 answers
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Why does an RF Amplifier fail when the SWR is high?

Exactly why does an RF output transistor fail if the SWR is bad ? Is it A because the impedance mismatch between the transmission line and the antenna results in maximum power not being transferred ...
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2 votes
2 answers
188 views

Voltage wave in transmission lines

The solution for the voltage in a transmission line can be written as: $$V(z) = V_o^+ e^{-jkz} + V_o^- e^{jkz}$$ The voltage $V(z)$ is the difference between the conductors of the line at a certain ...
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1 vote
1 answer
47 views

Passive receiver power needs & data rate

I'm working on a project to design a space-based low frequency array. It will only be receiving signals, and it will operate between 10MHz - 100MHz. It is made of short wave crossed dipole antennas. I ...
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2 votes
2 answers
105 views

Low mass, low frequency transmission line

I'm interested in a low frequency (10 MHz - 100 MHz) radio receiver array, made up of crossed dipole antennas. I'm working on a design project mainly focused on the mechanical engineering side of ...
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7 votes
2 answers
223 views

Can ferrite beads lower coax cable velocity factor?

For VHF and UHF, it seems reasonable to slip ferrite beads over a coaxial cable to act as a transmission line common mode choke. It also seems reasonable to use beads with the smallest diameter ...
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4 votes
4 answers
394 views

NMO bicycle mount antenna has unexpected SWR

I built a bicycle antenna mount featuring an NMO connector to allow quick changes. With a 2m 1/4 wave monopole it gives reasonable SWR results (1.4 or so in the band of interest), with a particular ...
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3 votes
2 answers
309 views

What is the main source of error for SWR meters?

I have updated this question to make it simpler because i haven't seen an acceptable answer yet. New Question. The impedance seen at any point on a transmission line is E/R. If the SWR isn't 1:1 ...
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2 votes
3 answers
377 views

Are standing waves on a transmission line RF or DC?

I am very confused about Standing Waves on transmission lines. Wikipedia says "a standing wave, also known as a stationary wave, is a wave which oscillates in time but whose peak amplitude profile ...
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0 votes
1 answer
160 views

Terminating double-shielded coax

When terminating coax (e.g. RG-400) that has an outer braid over an inner braid onto an SO-239 chassis mount, do we want to solder both the inner and outer braids together onto the ground of the SO-...
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9 votes
4 answers
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Why does ladder line generally have a lower loss than coaxial cable?

It is well understood in the amateur radio community that ladder line, window line, etc. have lower losses per foot than say RG-213 or LMR400, particularly on HF frequencies. It is also understood ...
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1 vote
1 answer
70 views

Effect of small amount of metal on twin-lead

I have a G5RV Jr. antenna on a fiberglass mast at 10m height. The only metal on the mast is a 1/8" thick steel guy ring with stainless steel snap links, plus a few small metal screws. Hanging freely,...
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1 vote
2 answers
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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?

Following is my understanding of some theory and the questions are at the end. Is this right ? ... A standing wave is an AC waveform oscillating at a particular frequency which has a varying ...
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4 votes
1 answer
257 views

Is this transmission line peak voltage formula published in CQ Magazine more accurate?

In his July 2018 article in CQ magazine entitled “Lightning (and EMP) Surge Protection for HF Radios”, Steven Karty, N5SK based much of his analysis for properly specifying a GDT (Gas Discharge Tube) ...
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5 votes
1 answer
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For air coax calculation, what is correct diameter metric vis-à-vis tube stock thickness?

I am interested in creating a square outer-conductor with round inner-conductor air coax post for a hex beam antenna. I found the extremely useful information at: How do I design a round or square ...
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1 vote
2 answers
297 views

Rotary Inductor Replacement

I thought it was time to build a balanced-balanced antenna tuner so I mechanically ganged two B&W 3852 rotary inductors to make a balanced L network. It appears that the 16 uH inductors I used ...
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18 votes
1 answer
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How does an SWR meter really work?

For ham radio operators, the SWR meter is a ubiquitous piece of equipment. There are dozens of standalone models on the market and most modern transceivers and antenna tuners have this functionality ...
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8 votes
1 answer
438 views

SWR Measured at the Transmitter versus SWR at the Antenna

It is fairly well understood that due to transmission line losses, the SWR as measured at the transmitter will be less than the SWR at the antenna. Some of the new antenna analyzers and some vector ...
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9 votes
3 answers
1k views

Attic Dipole - Impact of Ethernet over CAT5/6

I've been thinking about putting 20m and 10m dipoles up in my attic (HOA), but I also intend to soon put some Ethernet cable to a WAP (with PoE) through the attic. I was intending to feed this with ...
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10 votes
1 answer
282 views

Does Coax Limit the Maximum SWR a Transmitter 'Sees'?

A recent discussion about SWR led to the assertion that a length of coax cable will ultimately limit the SWR to which the transmitter will be exposed. By experimenting with an SWR calculation tool ...
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6 votes
4 answers
1k views

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

People often say common-mode current flows on the outside of the shield, while the current on the inside of the shield is always opposed by an equal but opposite current on the center conductor. This ...
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1 vote
1 answer
67 views

Can I use two voltage baluns to make a common mode choke?

I have two 1:4 voltage baluns. If I connect them together so that I have 1:4-4:1, is that the equivalent of putting a 1:1 common mode choke on my feed line?
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2 votes
2 answers
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Difference between Phase Velocity and Velocity of propagation in a transmission line

I'm confused by the difference between the phase velocity and transmission line propagation velocity. From https://en.wikipedia.org/wiki/Speed_of_electricity, in a good conductor, phase velocity (...
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4 votes
3 answers
553 views

Why is coax loss logarithmic?

With DC hookup wire, losses are directly proportional to length. If the resistance of 100m of wire is 10 ohms then the resistance of 200m of wire will be 20 ohms. But we measure coax loss in decibels ...
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0 votes
1 answer
69 views

Transmission Line Model of Co-ax Lines

I have got 2 questions on transmission line. In coax line, the centre wire is Copper, and the outer line is Aluminium, as far as I can see in the coax that I have. I have got couple of questions on ...
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3 votes
3 answers
455 views

How do transmission lines affect the impedance of some arbitrary load?

Let's say I have something like a non-resonant dipole, so that impedance changes depending on frequency and could take any number of values. Whatever that impedance is, how does adding transmission ...
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