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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8
votes
4answers
132 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 ...
6
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3answers
670 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?
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2answers
65 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 ...
4
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2answers
88 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 ...
4
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2answers
67 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 ...
5
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3answers
337 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 ...
7
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3answers
272 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 ? ...
0
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1answer
51 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 160°. When plotted on the Smith Chart, the normalized impedance is .825 +j .06 Ω. What is ...
5
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1answer
38 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 ...
2
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1answer
31 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 ...
4
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2answers
306 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 ...
4
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3answers
687 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 ...
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2answers
934 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 ...
16
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1answer
2k 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 ...
5
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2answers
3k 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 ...
3
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1answer
145 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 ...
2
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1answer
73 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. ...
3
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3answers
759 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 ...
5
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4answers
948 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 ...
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2answers
1k 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 ...
6
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3answers
919 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 ...
3
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3answers
468 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 ...
3
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3answers
288 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 ...
9
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6answers
5k 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. ...
4
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1answer
4k 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 ...
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0answers
89 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?
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1answer
42 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 ...
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2answers
165 views

Match 420 Ω to 50 Ω

Is there any practical way to match the output of my PA which is 420 Ω to my antenna which is 50 Ω at 434MHz? I was thinking about adding a pi-network and using a VSWR Meter with the output of the ...
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1answer
93 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 ...
3
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1answer
472 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 ...
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2answers
215 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 ...
9
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3answers
2k 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 ...
2
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1answer
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 ...
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2answers
244 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?
2
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1answer
208 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 ...
2
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2answers
597 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 ...
5
votes
1answer
456 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 ...
0
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1answer
172 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 ...
3
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2answers
6k 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'...
6
votes
1answer
145 views

What exactly does an unun do to the impedance of a transmission line?

If I have a load Z_L and a length of transmission line with impedance Z_0 I know how to compute the impedance at the input to the transmission line (using smith chart or using telegraph equation). ...
1
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0answers
52 views

Transformer secondary coil impedance [closed]

To transfer maximum power from the secondary coil to the load, I need to find secondary coil impedance. My load impedance 5 - J365. I would design a matching circuit between load and secondary coil, ...
0
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1answer
40 views

Impedances inside transmitter?

A 50 ohm match can be tricky outside a transmitter to match. One has to deal with ensuring the antenna , coax, and all connectors are all equal or close to the output impedance of the radio. Let ...
5
votes
2answers
310 views

Will measuring SWR through a feed line mislead me?

I know that except in the perfectly matched case, transmission lines transform impedances along their length; if I measure the impedance at the other end of my feed line, or even with a short jumper ...
2
votes
3answers
154 views

Do stub matching techniques still produce some loss?

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

Wideband impedance matching

I'm using an antenna that works in the range from 1 GHz to 3 GHz. I want to match impedances between a cable from the transmitter and the input amplifier and from the output amplifier and the cable to ...
3
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2answers
2k views

Stub impedance matching

What is the process of stub impedance matching and what are the elements included in the calculations relevant to it?
2
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1answer
214 views

Quarter Wave Transmission Line transformer

Can someone explain to me exactly how a quarter-wave transmission line transformer is put into action? I mean, how does it operate and what values are usually taken into consideration, calculation ...
3
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2answers
4k 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 ...
4
votes
2answers
4k views

What is the impedance of a 1.25 λ dipole antenna?

I would like to make a dipole antenna. This particular antenna is used for reception only; no transmission is involved. I would like to receive as much power as possible by this antenna as it is used ...
10
votes
4answers
888 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 ...