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.

Filter by
Sorted by
Tagged with
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 ...
Kevin Reid AG6YO's user avatar
  • 24.5k
27 votes
4 answers
20k 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, ...
K9KRB's user avatar
  • 642
19 votes
1 answer
5k 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?
Bill - K5WL's user avatar
  • 1,194
22 votes
3 answers
9k 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?
berry120's user avatar
  • 1,744
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. ...
Robert Quattlebaum's user avatar
18 votes
1 answer
5k 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 ...
Glenn W9IQ's user avatar
  • 18.6k
12 votes
3 answers
5k 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 ...
natevw - AF7TB's user avatar
8 votes
3 answers
1k 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 ...
ITChap's user avatar
  • 209
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 ...
Robert Quattlebaum's user avatar
4 votes
2 answers
927 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 ...
Bitbang3r's user avatar
  • 401
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 ...
VU2NHW's user avatar
  • 2,834
10 votes
2 answers
854 views

If two antennas of 50 Ω and 377 Ω have VSWR=1:1, then which one is more efficient?

If there are two antennas of 50 Ω and 377 Ω (same as the intrinsic impedance of air) impedance with 0 $\Omega$ ohmic resistance, and they are exactly matched with their transmission line, then which ...
kaka's user avatar
  • 203
5 votes
2 answers
7k 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 ...
Shiva Mudide's user avatar
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 ...
Phil Frost - W8II's user avatar
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 ...
Phil Frost - W8II's user avatar
10 votes
1 answer
2k views

How do I design a round or square coaxial transmission line to have a specific impedance?

Certain Hex Beam antennas use a coaxial center support post to send the RF to each antenna section, which is superior, in terms of maintenance, to a post with screw terminals connected by short ...
Adam Davis's user avatar
  • 12.1k
9 votes
2 answers
2k 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 ...
VU2NHW's user avatar
  • 2,834
9 votes
4 answers
7k 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?
Phil Frost - W8II's user avatar
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 ? ...
Andrew's user avatar
  • 3,685
6 votes
3 answers
3k views

Impedance Matching Different Feedlines

(I promise there will be a question in this wall of text. I'm just giving context.) I've been wanting to try my hand at building my own antenna system for the last few weeks and in doing research, a ...
Italic_'s user avatar
  • 183
5 votes
2 answers
560 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 ...
Kevin Reid AG6YO's user avatar
  • 24.5k
4 votes
2 answers
606 views

Resistive impedance with frequency

Full disclosure, I'm a little dyslexic. Concepts that are easy to most people usually seem ambiguous to me. I also have a had time expressing myself through writ so please forgive me if this seems ...
Turingrad's user avatar
4 votes
2 answers
1k views

What impedance matching do I need between my 300Ω 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 ...
Adam Davis's user avatar
  • 12.1k
3 votes
3 answers
1k 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 ...
William's user avatar
  • 803
3 votes
3 answers
529 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 ...
Phil Frost - W8II's user avatar
3 votes
1 answer
867 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 ...
tomnexus's user avatar
  • 11.1k
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 ...
Andrew's user avatar
  • 3,685
1 vote
0 answers
114 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?
Jerry Walters's user avatar
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 ...
not2qubit's user avatar
  • 378