Questions tagged [impedance]

Impedance is the measure of the opposition that a circuit presents to an alternating current when a voltage is applied. This is a force of much interest to the radio amateur especially surrounding the connection to antennas.

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-3 votes
2 answers
96 views

The idea that a non resonant antenna cannot accept current as well as a resonant antenna is a myth?

In a youtube video titled "Antenna Engineer Vs Ham Radio Myths! True or False?" the guy says this in the comments: The idea that a non resonant antenna cannot accept current as well as a ...
0 votes
1 answer
115 views

Input impedance of a shortened dipole antenna

I want to make myself a antenna for 40m and 20m wavelength band, but i don't have enough space for a full scale dipole, and even for more compact configurations like delta-loop or inverted v, so i ...
3 votes
2 answers
112 views

What should a NanoVNA show for twisted pair impedance in TDR mode?

Thought I'd try doing some actual experiments with twisted pair feedlines and also just some general "non-coax" measurements and testing, but not until after I understood a bit more about ...
15 votes
8 answers
14k views

What is the impedance of an end-fed half-wave antenna?

It is my understanding that the feed-point impedance ($Z_a$) of an end-fed half-wave antenna is dependent on at least the following factors: $L_a$: The length of the radiating element of the antenna. ...
5 votes
4 answers
2k views

What is the output impedance of a typical solid state ham transmitter?

I've often heard that the output impedance is 50Ω, and the load impedance should be matched to that and also 50Ω. While the reasons for the load impedance to be 50Ω are clear, is the output impedance ...
2 votes
1 answer
64 views

What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?

From Understanding how antenna mismatch can damage a transmitter "The reflected wave appears to the transmitter's antenna port... as an AC voltage source... whose magnitude and phase depend on ...
2 votes
1 answer
172 views

Impedance of Yagi boom model in NEC

When modelling a Yagi in NEC the radiation pattern is essentially identical whether a conductive boom is included or not, see pictures below. An explanation says this is a limitation of the method of ...
3 votes
1 answer
83 views

How do I couple two VHF Small Signal Amplifiers together?

I have managed to build single stage VHF amps ok using the transistor's S-Parameters to determine the load and source reflection coefficients for a specific gain and stability considerations. Simple ...
0 votes
3 answers
186 views

Ohms & coax cabling miscalculation?

I measured the resistance for 15 meters RG6 cable, which showed 3 ohms on my multimeter ! I've got 0.7 ohms resistance on the multimeter itself when touching the negative and positive. So 3 ohms - 0.7 ...
9 votes
6 answers
24k views

How is a 1:1 current balun different from a choke?

I have a 10m dipole and am going to feed it with 50 ohm unbalanced feed line (coax). My understanding is the feed point of this style antenna is about 50-75 ohms, and since it is a balanced antenna I'...
2 votes
1 answer
94 views

What this E(EL) symbol does mean?

I can't understand what this E(or EL) symbol does mean. I know that Zo means impedance and I can calculate trace width according it, but what E symbol does mean?
16 votes
12 answers
19k views

How can I safely transmit without an antenna tuner or SWR meter?

Just what the title says. An Antenna Tuner, henceforth AT, is almost a de facto piece of equipment in a shack; working the bands without one is tantamount to leaping off the diving board into a ...
2 votes
2 answers
171 views

Why measuring Voltage and Current becomes difficult at Microwave frequencies? Why measure Power?

I have been reading some Classic books viz. Techniques of Microwave Measurements - Carol Montgomery, Foundations of Microwave Engineering - Collins, and some other google searches and they all claim ...
17 votes
4 answers
8k views

Understanding coax impedance

I keep doing research, and trying to grasp it, but when it comes to RF impedances, and coax, I still have trouble trying to grasp it. So first, I think you have to have a 50 ohm coax for a 50 ohm ...
2 votes
3 answers
604 views

Why do ham radio operators say that a full wave dipole isn't resonant?

Every reputable antenna book that I've read says that dipole antennas are resonant at integer multiples of a half wave length. Wikipedia describes resonance as: the phenomenon of increased amplitude ...
0 votes
3 answers
434 views

SMA Connector Impedance

I am new to antennas, and am trying to create a monopole that connects to an SMA female to transmit a 2.4ghz wifi signal. I know that the antenna needs to be a multiple of 3.1cm in order to have the ...
4 votes
2 answers
926 views

Measuring actual feedpoint impedance for a given frequency

Background I'm attempting to tune an antenna with my NanoVNAv2. I calibrated it using the included open/short/(50-ohm)load standards by screwing them directly onto the NanoVNA one by one and running ...
2 votes
1 answer
75 views

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

I believe I understand how an excessively high impedance can damage a final output power MOSFET (voltage rises until avalanche breakdown). But how does an excessively low impedance do that? If the ...
4 votes
3 answers
461 views

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

I got to know that the input impedance of the halfwave dipole antenna is given as: Zin=1/Im* sin(β(H-|Z|)) where Im is the maximum current on the antenna( when excited with a 1 volt supply), H is the ...
0 votes
1 answer
79 views

How do you create a DC-blocking ground-shunt that looks inductive in an "L" match?

We are optimizing an impedance match with s2p models for an LNA and found that an "L" match with a serial cap and shunt inductor will optimize to provide +3 dB of output, versus using serial ...
6 votes
1 answer
334 views

Non-inductive resistors

When we use a "non-inductive resistor" to check the output impedance of say an unun, say 50ohm 1:1, what resistor type is actually non-inductive, or does it not really matter as long as the ...
1 vote
2 answers
406 views

Rudementary Impedance measurements for VLF antenna system

I am looking for ideas to improve my Impedance measurements I perform on my VLF antenna system. Due to circumstances my VLF experiments are usually "on location / in a field" so I do not have the ...
4 votes
1 answer
173 views

Looking for criticisms on my VNA-like SWR meter I am designing

I am in the process of building a SWR meter with VNA-like features. I'd be really interested to hear from fellow hams any input, suggestions, or tips they have regarding this before I go get it mass ...
2 votes
1 answer
1k views

Need advice for running coax for HF to my shack

I'm looking for advice for running coax cable to my shack on the second floor of my house. I'm planning to use this coax line for HF (recently upgraded to General!). I'm a newbie and I am very handy, ...
1 vote
2 answers
497 views

How do you calculate the impedance of a parallel-wire balanced feedline (ladder line)?

I've looked around and maybe I'm just searching for the right terms to find the answer. I know that the impedance of ladder line has to do with the distance between wires: What is the generalized form ...
5 votes
5 answers
390 views

Antenna tuner at the transceiver output confusion

I am a bit confused. Let's take a HF transceiver for example. My understanding is that the output impedance of the transmitter is 50 ohms. The impedance of the coaxial cable feedline is 50 ohms, and ...
12 votes
5 answers
1k views

How does an antenna transform impedances along its length?

If I insert some impedance (like a loading coil, or a trap) at the base of a monopole antenna, the effect on the feedpoint impedance is easy to predict: the impedance is in series with the antenna, so ...
4 votes
4 answers
1k views

Is an antenna always matched to free space impedance?

In this thesis the following model for an antenna is proposed: It is seen as a 2 port network, in which only port 1 is accessible (port 2 is an imaginary connection between the antenna and the free ...
4 votes
5 answers
918 views

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?

A half wave center-fed dipole has a resonant frequency where the input impedance appears to be purely resistive. How can this be, when the voltage and current distribution along the length of a half ...
7 votes
4 answers
1k views

How can the voltage at the center of a resonant half wave dipole be zero if the input impedance is 75 ohms?

If the input impedance of a resonant half wave dipole at the feed point is 75 + j0 ohms, then if an AC sine wave voltage V is applied to the feed point, how can the voltage at the center be zero ? ...
1 vote
2 answers
221 views

Does the self inductance and velocity factor of the elements of a dipole antenna change its resonant frequency?

Below are well known equations from Wikipedia for the resistance and reactance in the impedance of a dipole antenna. where $a$ is the radius of the conductors, $k$ is the wave number 2πf/c, $η_0$ ...
6 votes
1 answer
17k views

How to calculate single-stub match (general case)?

I'm trying to build a 1/2 wave "Coaxial J-Pole" antenna, similar to what is described here: Such an antenna would be great for stringing up in a tree when operating remotely, no tuner needed. The ...
1 vote
1 answer
132 views

Obtaining input/out impedance from S-parameters for a L network

This L network intend to transform Z1 to 50ohms. The design was made using a smith chart: The s-parameters plotted (S11 red - S22 green) in the smith chart is done sweeping the frequency from 200MHz ...
3 votes
1 answer
65 views

Impedance of V Antenna as Function of Angle

I'm trying to experimentally investigate the radiation resistance of a v-shaped antenna. When it's fully unfolded and the reactance is at resonance, the resistance is about 73Ω. When it's folded, it ...
1 vote
1 answer
668 views

Crystal Oscillator Output Impedance and Output Voltage

I am working on a project designing a 40 MHz radio transmitter, but I need some clarifications. I need to know how to calculate its output impedance. I know how to calculate common collector's output ...
0 votes
1 answer
242 views

Obtaining S parameters using S11 measurements?

Is it possible to obtain the S-parameters of a circuit with only S11 measurements (S11 measurement with a VNA)? My gut tells me that it should be possible by connecting open, short and characteristic ...
0 votes
1 answer
26 views

Is this result correct ? [ impedance cavity based on emtamaterials using CST Microwave]

In the contexte of my reaserch about antenna conceptions, I'm trying to simulate using CST Microwave a cavity antenna based on metamaterial structure and a slot antenna as a GND,However I got as a ...
0 votes
2 answers
2k views

How to use 75ohm (RG6) coax?

I know that there have been many similar threads on this, but I still need clarification. At the moment, the only thing I have is a 500' roll of RG-6 and a (very) tight budget, and I should be ...
2 votes
1 answer
345 views

Mutual impedance of two parallel dipole

Good Morning all, I want to calculate the mutual impedance of two parallel dipole. The first hypothesis is to assume that the dipole 2 is in open circuit, nothing difficult here I knew how to do it ...
6 votes
4 answers
2k views

50Ohm Coax presents 50 Ohms of impedance on which frequency?

Either there's a simple answer that I'm not finding in a spec sheet somewhere, or, I'm asking the wrong question!
5 votes
1 answer
437 views

Measuring Output Impedance of an RF (HF/VHF) Active Circuit with Hobby-level Tools

Is there a simple (by simple, I mean a hobby friendly method that trades some accuracy for expense) method of measuring the "actual" output impedance of an active or "hot" RF (HF to VHF) circuit? As ...
3 votes
2 answers
312 views

What is the impedance of an off-center fed resonant dipole?

Given a dipole approximately 0.48 wavelengths long, such that when fed in the center the feedpoint impedance has zero reactance, how does the complex impedance change as the feedpoint is moved away ...
3 votes
2 answers
924 views

Is an antenna resonant at the frequency of lowest impedance?

I understand that an antenna is resonant at the frequency where the capacitive and inductive reactance cancel out to 0. Only the resistive impedance remains at resonance. Does this mean that the ...
10 votes
4 answers
430 views

Why are there so many coaxial cable impedances?

Why do we have 50, 75, 62 and 92 Ω coaxial cable, to name a few? Why not just one standardised cable? Is there some technical reason or did Mr. Coax and Mr. Cable have a disagreement, like Mr. Tesla ...
9 votes
3 answers
2k views

Impedance ratio vs. SWR

The specs of the Elecraft T1 tuner say, that it can match a 10:1 SWR. What does that mean in terms of impedance ratio? What antenna impedance can it match to 50 Ohm then? Or is it not that easy?
4 votes
2 answers
398 views

Pi Matching circut for tube PA - what values?

I am working on my 80m and 40m CW transmitter and ran into a problem: I need to match the high impedance of PL504 to 50ohm coax, a Pi match seems to be the best, but I don't know how to calculate ...
1 vote
2 answers
156 views

Matching a 188 ohm antenna to an unbalanced input that expects a 50 ohm antenna

I am new to RF field and trying to use an Archimedean Spiral Antenna design from this paper to capture 915MHz RF for the P2110B power harvester chip. The estimated antenna impedance is 188 Ohm (see ...
1 vote
2 answers
3k views

How to best connect an FM transmitting GPIO pin to an antenna?

I'm trying to use a RaspberryPi3 as a FM transmitter, using pifm and/or PiFmRds to send audio in 88-108 MHz band. The RPi can then use the GPIO-4 pin to transmit between ...
7 votes
2 answers
494 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 ...
10 votes
2 answers
3k views

What is the actual loss in a feed line with high SWR?

It is well-known that it is unwise to have standing waves on the feed line between one's antenna and one's transmitter, unless it is a very low-loss type (e.g. ladder line), because the transmitted ...