# 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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19 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 ...
94 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 ...
63 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 ...
42 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 ...
145 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!
66 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 ...
117 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 ...
48 views

### Center-loaded vertical, high SWR at lower frequencies

I built a center-ish loaded coil-shortened vertical. The lower radiator is 3 meters, then the coil, then 5M more radiator, like this (Early prototype with shorter radiators and no covers for the coil ...
257 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 ...
177 views

### Why are there so many coaxial cable impedances?

Why do we have 50, 75, 62 and 92 Ī© 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 ...
981 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?
90 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 ...
147 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 ...
122 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 ...
874 views

### Coax or bifilar windings for a choke?

Making a common mode choke out of bifilar windings has proven rather difficult. For me the difficulty lies in getting the impedance to remain a constant 50 ohms from on the HF bands. I've tried 14 ...
669 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 ? ...
467 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 ...
61 views

### Reflection Coefficient to Impedance transformation on the Smith Chart

A certain RF transistor has a source - reflection coefficient with a magnitude of .105 and a phase angle of 160Ā°. When plotted on the Smith Chart, the normalized impedance is .825 +j .06 Ī©. What is ...
47 views

### 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 ...
32 views

### The 'plating material' materials and constants of the constants

The 'plating material' materials and constants of the Single-Layer Helical Round Wire Coil Inductor Calculator appear wired-in. There appears no 'un-plated' option. How to use this calculator to ...
542 views

### Antenna Efficiency

Intuitively, we all understand that an antenna at fractional wavelengths will not radiate well even with perfect impedance matching systems. From what I understand the efficiency of antenna has a ...
1k views

### How can I calculate the 1/4-wave monopole impedance dependence on groundplane angle?

I watched this video of a very nice explanation of a simple dipole and 1/4-wave monopole. In that video they show how you can impedance match the 1/4-wave monopole by bending the 4 (virtual ground ...
2k views

### Impedance effect of using 75 ohm coax between transmitter and an EFHW antenna

I have been given several hundred feet of very high quality 75 ohm coax. I need a run of about 36' from my QRP transmitter (FT-817 with elecraft T1 tuner) to an EFHW antenna (about 75' long). Running ...
3k views

### 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 ...
6k views

### Can I use a CB antenna with a ham radio?

I just passed General and purchased a FT-450D with an internal antenna tuner. While my ultimate plan is to use a G5RV, it will be a while before I can get it up. What modifications would I need to ...
174 views

### interdigital (finger) capacitor bank for fine tuning of differential loop antenna

I am attempting to implement a PCB trace antenna; namely the sample in this document. Thing is the 1.17 pF for Cp1 is quite hard to achieve in a cost efficient manner with 0603 caps (I loath those ...
75 views

### TX output power increase when fed through filter

I came across a weird case, I have a low power 440MHz repeater transmitter, which I thought to set at about 10 watts. I made this measurement using an accurate offset attenuator and service monitor. ...
1k views

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

I've often heard that the output impedance is 50Ī©, and the load impedance should be matched to that and also 50Ī©. While the reasons for the load impedance to be 50Ī© are clear, is the output impedance ...
2k views

### What exactly is impedance?

I see "impedance" used everywhere. Equipment has an input and output impedance. Antennas and feedlines all have this 'impedance' thing. For someone that understands DC electronics, Ohm's law and a ...
2k 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 ...
1k 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 ...
758 views

### Unexpected Resonance - Dual-Band Dipole

I tried to make a dual-band (2m/70cm) dipole this morning by following the instructions on this page: http://www.amateurradio.bz/2m-70cm_vertical_dipole_antenna.html, with the rather major significant ...
369 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 ...
8k 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. ...
8k 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 ...
100 views

### Antenna Impedance calculation for low frequency [duplicate]

If I have an end fed long wire in free space, say 100 ft, how do I calculate its $R$ & $X$ at different frequencies?
46 views

### What factors determine acceptable turning radius of RF cable operating into few GHz?

I want to understand what factors determine acceptable turning radius of RF cable. are the losses high for low frequency or at high frequency if cable is turned or twisted ? the cable i am referring ...
177 views

Is there any practical way to match the output of my PA which is 420 Ī© to my antenna which is 50 Ī© at 434MHz? I was thinking about adding a pi-network and using a VSWR Meter with the output of the ...
122 views

### What dimensions to choose when winding your own inductor

Suppose you want to make a wilkinson 3-way splitter with lumped components Given the following circuit, you need a 31.5nH inductor The inductor can be wound various ways: 3 wdg, 1 cm diameter, 2.36 ...
587 views

### How to feed an array of dipoles for an end-fire pattern

An array of dipoles can be fed to achieve an end-fire pattern. If the phase of the current on each element is equal, the array will have maximum radiation broadside to the array. If the phase of the ...
263 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 ...
3k views

### Why does “high SWR” damage transmitters, instead of “impedance mismatch”?

In the audio realm, where cable lengths are insignificant compared to the signal wavelength, I would understand an impedance mismatch damaging an amplifier as follows: Take an audio amplifier that ...
1k views

### Balun and impedance matching suggestions for dipole antenna

I'm a newbie who is attempting to build a half-wave dipole antenna that will be located in the rudder of a sailplane. 123.3 MHz is the most important frequency to me, although it's reasonable to ...
281 views

### RF6886 high power amplifier maximum reverse power

I'm designing a circuit which uses the RF6886 amplifier IC. What is the maximum reverse power that the amplifier can sustain? At what reverse power will the amplifier die?
244 views

### AN762 Amplifier Transistors

According to the MRF421 or MRF454 transistor datasheets, used in the AN762 HF amplifiers say that the transistors are "100% tested for load mismatch at all phase angles with 30:1 VSWR". Further, in ...
963 views

### Measuring Connector Impedance

Before discovering that connectors can have characteristic impedance, I ordered a bunch of adapters. I've noticed that I'm getting really sub-par performance out of the antennas I'm using and think I ...
516 views

### Necessary spacing for 1 kW Gamma Match on 144 MHz

I'm working on a 144 MHz antenna using a gamma match. The best result (resonance 50+j0 at 144.400 MHz) I reached is a 10 mm diameter gamma rod, 15 cm long and a capacity of 10 pF. I thought to obtain ...
195 views

### CC1125 reference design matching network using Smith chart

I am trying to confirm on a Smith chart the matching network for 169MHz frequency using an external PA and and 437Mhz with an external LNA. I am not able to end at the origin of the Smith chart using ...