18

Dispelling the Myth To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its circuitry that suggests this capability is flawed. We can show this empirically with two different experiments. Experiment 1 Connect a 100 ohm resistor directly ...


11

Having Ethernet and your antenna co-located is not an ideal situation. But then most amateur antenna situations involve compromises. The general idea of the following recommendations is to take as many precautions as practical to minimize the interference possibilities. I recommend that your Ethernet cable to your WAP be a CAT6 shielded cable (STP). While ...


10

An antenna matching network (aka "tuner") does not affect the conditions of the load (antenna) or the transmission line between the load and the matching network. The matching network transforms the impedance "looking into" the transmission line to a more desirable value, typically 50$\Omega$ for ham applications. If the network manages to achieve such a "...


8

Balanced lines (of which twisted pair is a special type) really have an upper frequency limit; you can't use them to transport 1 GHz (well, you can, but the smallest variation in direction or distance would have catastrophic effects, and the conductor distance would get pretty small). This can be seen in technical practice: 100 Mbit/s Ethernet (Fast ...


8

Executive Summary Assuming: air dielectric (insulator) 50Ω characteristic impedance Then for round coax, make the inside diameter of the outside conductor 2.302 times larger than the diameter of the inside conductor. If the shield is square, and the inner conductor is still round, make the inside length of one side of the shield 2.134 times larger than ...


7

For many modulations, the modulation is very slow compared to the propagation delay of the feedline. For example, SSB is typically limited to no more than 4 kHz. That corresponds to a wavelength of of 75 km. As long as the feedline is significantly shorter than this, then the delay due to the feedline is negligible. It may be easier to understand ...


6

It can be done! Remember that a transmission line consists of some self-inductance per unit length, and some capacitance per unit length, and the ratio of these determines the line's characteristic impedance: $$ Z_0=\sqrt{\frac{R+j\omega L}{G+j\omega C}} $$ Intuitively then, a lumped element implementation might look like some series inductance and some ...


6

The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. Since the SWR limit is the result of losses in the feedline, the efficacy of the 'high SWR fix' must be considered. The mechanism has to do with the trips that ...


6

The SWR is related to the reflection coefficient $\Gamma$: $$ \Gamma = {Z_L - Z_0 \over Z_L + Z_0 } $$ $$ \text{VSWR} = {1+|\Gamma| \over 1 - |\Gamma|} $$ where: $Z_0$ is the feedline impedance, usually 50 ohms, and $Z_L$ is the load (nominally, the antenna) impedance. The reflection coefficient is a complex number and thus takes into account the phase ...


6

When the SWR is 1:1, the matched line loss of ordinary ladder line is lower than the matched line loss of ordinary coax because at HF, most of the loss is $I^2R$ loss, and the current magnitude is inversely proportional to the characteristic impedance of the feedline. Of course, a high SWR results in higher standing wave currents and thus increases the $I^2R$...


6

No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside construction of the cable: the outside diameter of the inner conductor, d, the inside diameter of the outer conductor (shield), D, and the magnetic permeability,...


6

For well-designed coax, the EM fields are confined to the space between the inside of the braid and the center conductor, i.e. the dielectric insulation region which affects the velocity factor. Therefore, those beads have negligible effect on the differential signals. They do have an effect on the common-mode signals on the outside of the braid which has ...


5

Given the matched loss of the feedline and the SWR at the transmitter, we can calculate the SWR at the antenna in three simple steps. First convert the SWR at the transmitter to the corresponding magnitude of the reflection coefficient (Gamma), or MRC for short within the context of this answer. The MRC is the magnitude of the complex ratio of the reflected ...


5

This is a bit like saying, "it's not the fall that kills you, but the sudden stop at the end." Technically, it's not the standing waves per se that cause the damage. However, standing waves imply a mismatched load possibly outside the transmitter's specifications, which implies a potential for excessive heating or voltage in the transmitters, which can lead ...


4

A "Guanella balun" is really just a common-mode choke. simulate this circuit – Schematic created using CircuitLab For differential-mode currents, the current on one half of the transmission line is equal but opposite the current on the other half. Likewise, the magnetic fields due to these currents is equal but opposite, so they cancel, and so there'...


4

Perhaps some of my confusion comes from an audio background, where one has "balanced" and "unbalanced" cabling in a way that is easier for me to understand: an unbalanced cable is simply a signal and ground wire, while a balanced cable actually has a ground wire and two conductors whose voltages are opposite. That's not entirely accurate. What makes a ...


4

At 8W, the power from the radio is 9.03dB. With the cable losses of the receiving station being 1.5dB (1.41W), and the antenna receiving 1.733e-8W, it appears the received signal will never make it to the receiving station's radio. This seems to be the crux of your misunderstanding. You can't convert watts into decibels, or decibels into watts. A ...


4

Firstly I would note that your initial statement isn't true. Indeed, the resistance of a wire increases with length, but resistance isn't loss in the same sense of transmission lines where loss is measured in power or a ratio of powers. Consider: simulate this circuit – Schematic created using CircuitLab $$ I_\text{source} = I_{R1} = I_{R2} = \sqrt{1 ...


4

It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed. Skin Effect When direct current (time invariant current) passes through a conductor it tends to uniformly use the entire cross sectional area of the conductor. When alternating current (time ...


4

Ethernet cable has a minimum bending radius (check the specs of your cable). When feeding it through toroids, don't wind it tighter than that. Exceeding the bending radius causes the twists in the twisted pair to come undone somewhat, and will make the cable leak.


4

Coaxial cable impedance is determined by the dialectric between the two conductors and the distance between the faces of the conductors. So normally it is the outer diameter of the inner conductor and the inner diameter of the outer conductor. The facing surfaces being a controlling factor is due to the skin and proximity effects that tend to keep the ...


4

It is well understood in the amateur radio community that ladder line, window line, etc. have lower losses per foot than say RG-213 or LMR400, particularly on HF frequencies. Is it though? I'd say only sometimes. From the LMR-400 datasheet: From DX Engineering's 300 ohm ladder line datasheet: Compare the loss at say, 30 MHz: 0.7 versus 0.668 dB/100 ft. ...


4

A transmission line is linear, which means we can consider the voltage at any point to be the superposition of two other voltages. Asking what they are "physically" may or may not be a useful question. I could for example define: $$ V(z) = Ve^{jkz} + V_{DS}(t) $$ where $V_{DS}(t)$ is a function corresponding to the normalized amplitude of Pink Floyd's ...


4

VSWR is related to the magnitude of the reflection coefficient $\Gamma$: $$ \text{VSWR} = {1+|\Gamma| \over 1-|\Gamma|} $$ The reflection coefficient can be calculated from the load impedance $Z_L$ and the characteristic impedance $Z_0$: $$ \Gamma = {Z_L - Z_0 \over Z_L + Z_0} $$ The reflection coefficient, like the load impedance, a complex number with ...


4

Depends on how much current you mean by no current. But in general, unless the dipole is actually perfectly symmetrical, in a perfectly symmetric environment, above a symmetric ground, with the feed line at exactly 90 degrees all the way into the far field, and/or with the radio equipment and their ground connections also exactly 90 degrees perpendicular ...


3

From the symptoms you state of not being able to hear transmissions from identical setups only 4 miles from each other, and then being able to have one person receive with the rubber duck antenna, it tells me your J-Pole antennas are not behaving ideally on the frequency you choose. Consider that an antenna like a J-Pole has about a 10MHz bandwidth in the ...


3

I'm afraid that it is not correct that there is no energy transfer back to the source after an initial period. Wherever you have a mismatch on a transmission line, there is a reflection (at least partial) back to the source. A percentage of power is "actually" reflected back. It can be separated from the transmitted power using a suitable device like a ...


3

Ideally, it doesn't matter where on the constant VSWR circle you are. The transmitter is an ideal voltage source with 0Ω impedance, essentially a short. There's no way to couple power into a short, so whatever power is reflected from the antenna gets reflected back when it reaches the transmitter, so all you need to worry about is the impedance presented to ...


3

Wherever there is an impedance discontinuity, there are reflections. So indeed, even with your stub there are still reflections. However, the goal of a matching network, be it stubs, discrete components, or anything else, is to confine these reflections and the energy they contain to the smallest area possible. By reducing the amount of transmission line ...


3

"Stubs" are sections of transmission line which are usually less than a half-wavelength long and either shorted or open on one end. The two connections on the other end look like two terminals on a lumped impedance which can be either an inductor or a capacitor, depending on the length of the stub. For a short-circuited stub, the impedance is: $$ j Z_0 \...


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