# 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!
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 ? ...
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 ...
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 ...
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 ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
6k views

### Coaxial Folded Dipole Antenna Matching Impedance

In http://www.nsw.wicen.org.au/technical/projects/coaxial-folded-dipole-antenna, the guy is using a 50 ohm coax cable to build a folded dipole antenna and claims that this will enable use at 50 ohm (I ...
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 ...
682 views

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

I was recently reading an interesting article from the ARRL to get a better understanding of the SWR. The following case is still unclear: A low loss feed-line with an impedance that doesn't match ...
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 ...
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. ...
14k 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 ...
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 ...
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 ...
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 ...
10k views

### What is the relationship between SWR and receive performance?

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well? And what about the converse, will a well performing receive antenna show a 1:1 SWR? I've wondered this for a while, ...
6k 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 ...
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 ...
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 ...
1k views

### What impedance matching do I need between my 300Ī© feedline and Softrock receiver?

I will be assembling a Softrock Ensemble II HF receiver, and I know I'm going to want it on air as soon as I build it, so I'm challenging myself to build an antenna for it first. The plan is to do a ...
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 ...
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. ...
911 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 ...
4k views

### How does moving a feedpoint off-center in a dipole affect the resonant frequency and resistive load?

Specifically, I understand that OCF dipoles change the SWR, but does it do this via reactance or resistance, or both?
4k views

### What's the difference between “minimum SWR” and “resonance”?

Is the point of minimum SWR equivalent to resonance? Exactly what does resonance mean, anyway?
8k views

### Why do I need to tune an antenna?

I've heard that when an antenna is set up, it needs to be tuned to reduce reflective power back into the radio. Is this always necessary, and if so how would I go about tuning it?
212 views

### Do stub matching techniques still produce some loss?

I am using a double stacked Yagi-Uda antenna with about 57 Ī© impedance. The 50 Ī© feed line (coaxial) cables are combined using a tee connector. I observed, at the output of the tee connector, an ...
1k views

### Why is the risk of damage to a transmitter due to a mismatched antenna at lower frequencies greater?

A quick follow-up to this answer ... If the impedance mismatch is large, you risk damaging your radio, particularly on the lower frequencies ... I'm sorry to be so dense, but uh why is the risk ...
1k views

### What is the point of a SWR/power meter where the needles cross over each other?

There are SWR/power meters with two separate gauges, and others that have them mounted so the gauge needles cross over each other. Aside from looking cool, does the cross-over point of the two meters ...