# Tag Info

6

Reactance is the imaginary part of impedance. Together with the real part, resistance, impedance describes how a given load will respond to an AC voltage or current source. While for DC analysis resistance alone is sufficient for this purpose, for AC analysis we need to know not only the magnitude of the voltage or current, but the phase angle between them, ...

6

I would say in practice it is less important because hams use different types of antennas for HF and for VUHF. At HF, the same antenna is often used for many bands. The bands have large fractional bandwidth. Antennas are often simple wire, very thin compared to wavelength. And finally, because the wavelength is quite long, they are often too short for the ...

5

Very high. Harmonics work the other way around — antennas can be useful on higher bands than the design band, but almost never lower bands. A dipole cut for 40m might be good on 15m. An end-fed or a loop for 40m might be good on 20m and 10m. But pretty much any design of 40m antenna is going to be a half-size antenna on 80m, and very unlikely to be tunable ...

4

Receive power does vary with SWR; but for reception quality we're interested in signal-to-noise ratio. As long as you're getting enough receive power that the noise received from the antenna is greater than the receiver's own noise floor, getting more power from the antenna won't result in any noticeable improvement to your ability to copy. That's why we say ...

4

Let's say the antenna impedance on a given frequency is 100 Ohm, the feedline is lossless 50 Ohm, the transceiver input impedance is 50 Ohm. Between the antenna feed point and the feedline SWR = 2, 11% reflected power. This is a bit of dangerous thinking, because the distance between the antenna feedpoint and the feedline is zero. As such there can be no ...

4

The general rule of thumb is anything more than 1/10th the wavelength should be considered a transmission line. At 446 MHz, that's 67 mm. Your circuit is much smaller than that, so there's not much point in worrying about the trace impedance. Furthermore, everything right of the rectifier is DC. So moving the diode to be as close to possible to the RF ...

3

When tuning can be achieved then the only variables you have to optimize is the coupling and the compensation of the feed loop inductance (because k < 1). Just modify the area of the feed loop by bending the wires to a smaller or a larger area. You will end-up close to 50 Ohm. But resonance of the loop does not exactly correspond with lowest impedance. ...

3

I found a list of specifications for the TS-480SAT giving the matching range as 16.7 Ω – 150 Ω. Another list of specifications for the TS-870SAT says that model's matching range is 20 Ω – 150 Ω. Both ranges roughly correspond to a 3:1 SWR, which is typical for antenna tuners built into transceivers. External antenna tuners generally offer more tuning ...

3

I don't think the "matching to free space" should be taken so literally. I've never seen an actual equivalent circuit in a textbook or used it to derive any property of an antenna. Sure an antenna is the interface between the transmission line (which has an impedance) and free space (which also has an impedance, same units but different in nature). ...

3

The noise you hear in your headphones is coming from the antenna. When you tune the antenna and minimize the SWR, the losses between the antenna and the receiver are minimized, thus more of the noise received by the antenna makes it to the receiver, and you hear more noise in your headphones. (Assuming of course the receiver is set to a mode where power is ...

3

Well, according to the spec tables in the manual the thing expects a 50 Ω antenna impedance. Mismatch means a loss of signal energy. How much exactly depends on the length of the 75 Ω transmission line – this makes a system much harder to design predictably. I wrote all the following (up to the next horizontal line), then stopped to wonder "what might ...

3

I think the problem you will run into is that people will want this to be plug and play. They may not have a directional coupler, or know or feel like to calibrate it and all that other stuff. Even if they do it, they may wonder if they did it right and if the reading will be accurate or not. I think you will need to handle this part in your design and ...

3

Take three 1/4 wave sections of 75 ohm coaxial cable (let's say RG11) and solder together. You now have a 1/4 wave section of 25 ohm. Doing the math (25 * 25) / 12.5 = 50 ohm. Now use any 50 ohm coxial cable length to the radio. Ps: don't forget to take into account the 75 ohm cable speed factor to calculate the 1/4 wave section. Good luck. Luiz PY4ACP

3

An antenna that's Pretty short (0.08 wavelength), Not tuned, Mag-mounted to an inadequate counterpoise, Indoors with who-knows-how-much metal in its near field, could certainly have a 5:1 or worse SWR, which is enough to "peg the needle" on most meters I've seen.

3

The Question addresses a persistent confusion which is widespread especially in the ham radio community and can be tracked down to some published material (here no names!) and has survived since many years. However, a clarification can be straightforward and does not require complicated math. This answer starts from the “Total Feedline Loss” equation that ...

3

Interesting. Assume half-wave resonant antenna. The feed point in the middle (just a series source, or a transformer) results in 70 Ohm real impedance. When the antenna height is lower a match to 50 Ohm is sometimes possible. When moving the source insertion point in the direction of one of the ends the impedance grows up to beyond 1500 Ohm (depending on the ...

2

Smith chart shows that 50 Ohm antenna (assumed perfect 50 Ohm real!) connected to 75 Ohm coax cable can result in impedances between 50 and 112.5 Ohm. SWR 1: 1.5. So this so far not mentioned possible solution in this thread may be usefull for your problem: for a single frequency, or for a small frequency band, the use of cable with a multiple of half ...

2

I believe the answer currently is: yes it matters less, but it still matters. Variables include 1) the mode of operation and 2) the speed of the foldback circuit in the radio that provides the trigger for power reduction on a bad match. While modern transceivers all seem to have a foldback circuit to limit potential damage (for ex, see ic7300 operator manual ...

2

Missing in this discussion is the fact that optimum power matching does not necessarily correspond with optimum noise matching. Antenna tuner alignment on band noise is possible when the input impedance of the receiver is 50 Ohms real. Then maximum received noise corresponds to correct antenna tuner alignment.

2

The NanoVNA, and any VNA, when calibrated with open, short and load standards, can correctly measure through an arbitrary two port network, like a lossy, imperfect cable. Any combination of reflections and losses, like a series of cables, adapters, etc, can be reduced to a single reflection and transmission device, and fully calibrated. Calibration is done ...

2

Welcome to hamSE, Jason. Assuming you can match your rig to the load at the shack end of the transmission line, the importance of SWR depends primarily on how much transmission line loss affects your operations and how much money you're willing to spend on transmit power amplifiers, receive preamplifiers and lower-loss transmission lines to compensate for it....

2

You can get a good match by maximizing received noise, because that is the point where more RF noise might be transferred from the antenna to your receiver, and thus indicate where the SWR is lower (e.g. less noise is reflected back to the antenna at the frequency of interest). Tuning to minimize SWR might improve receiver performance by changing the ...

2

I think you've answered your own question. In case a, with an antenna with 2:1 SWR and an otherwise lossless system, 11% of the power will be reflected and re-radiated by the antenna, so 89% makes it to the receiver. In case b, all of the power will be delivered to the receiver. Antennas are reciprocal, so all losses in transmit are the same as losses in ...

2

This model makes two simplifying assumptions: losses are uniform throughout the transmission line, and there is a lossless tuner between the transmitter and the feedline adjusted such that the transmitter sees a matched load The first assumption isn't directly relevant to your question and is discussed in more details in the answer you linked. Now about ...

2

Let us assume a 50Ω Source feeds a 2-port network #1, which is followed by a 2-port network #2, which is terminated into 50Ω. The (modified) input reflection coefficient $s’_{11}$ of the (output-mismatched) network #1 is $$s’_{11} = s_{11} + \frac{s_{21}s_{12}\Gamma_{L}}{1-s_{22}\Gamma_{L}}$$ with (complex) s-Parameters of network #1, and $\Gamma_{L}$ for ...

1

If the antenna isn't lossless (e.g. not superconducting), then some of the received RF energy reflected back into the antenna due a feedpoint mismatch will eventually be dissipated as heat in the antenna, or re-radiated as RF. But for received signals this is possibly only a loss of nanoWatts or picoWatts. However, not only is reflected signal energy ...

1

A SWR meter will vary in swr with coax length if unbalanced as your coax length is around 1/2 wave length multiple. I have never seen an SWR measuring device that was designed to be used with a balanced load. This could certainly be done, but such devices are usually intended to measure SWR on a coaxial, i.e., unbalanced, line. For the purposes of SWR ...

1

Here's an example of what you could do with EZNEC+, a graphical user interface to the NEC2 computing engine. Note that the values below are not meant to represent a working design, they only serve to illustrate one use of the program. It may be possible to accomplish the same results with other (and free) software. The "Wires" pane shows that the ...

1

A possible answer came to me in a flash of insight. It occurred to me that the AGC in my Elecraft K2 does not try to make the volume in the headphones constant wherever I tune the receiver; if the AGC worked that way then the static of an unused frequency would be exactly as loud in the headphones as a contest station transmitting 1,500 W from 10 km away. ...

Only top voted, non community-wiki answers of a minimum length are eligible