Hot answers tagged

7

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

To aid in identifying a mystery ferrite core mix the most common method is to take some measurements to determine the initial permeability (µi). Then compare your results to a table of µi for common core materials. This will put you in the ballpark and likely more than close enough to make an educated guess as to what application(s) the ferrite core would be ...


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 ...


6

Following up on @dfannin's excellent answer: The most intuitive way of dealing with this would be: wrap your ferrite rod in halfway stable paper or so, something that certainly doesn't have high $\mu_r$ (gut feeling: baking paper is nice as it is very "slippery" on flat surfaces) make as many turns as you want around that; you're building a coil now, with ...


6

The K9YC paper you mention gives data up to 1 GHz on page 49: These data show for these core types the maximum number of turns is just one or two. Furthermore, it's quite difficult to achieve a choking impedance above 1 kΩ. That's not very much, and for many applications you'd want more. To get a higher choking impedance will require a larger core, and so ...


5

The primary issues with coax wound common mode chokes are; The inductive reactance of the choke can cancel out the capacitive reactance associated with the feedline shield thereby worsening the CM situation The resistive component of the choke is not sufficient such that the core undergoes an undesirable temperature rise The interwinding capacitance forms a ...


5

I don't think it will work to add suppression to the fence. The fence charger is designed to support heavily reactive loads by generating pulses every second or two. The pulses are formed by dumping the charge of a capacitor bank through a step up transformer. Any additional reactance will be a very minor load delta. My experience (several miles of e fence ...


5

you're on the right track to measure/calculate effective permeability. However, you need to use an inductance meter that can measure up to 1 nH accurately or so. If you're trying to use one of those $25 LCR meters , it won't have the accuracy or precision you require, plus you won't be able to zero it. Another issue is that you'll have a wide variance ...


5

I've had a couple turn up cracked when left outside in an enclosure that wasn't waterproof, but I can't say if that was freezing that did it, or just thermal cycles on a compromised toroid causing stress. These days I hit pretty much all of them with a light coat of decent spray paint, or even a light coat of flex-seal type sealant before putting them ...


5

It appears that all three 'scope probes are set to their "X1" position. As such, each probe includes the oscilloscope's internal 1M input resistance in parallel with about 20pF input capacitance. But more importantly, the capacitance of the probe's cable is in parallel too. The total 'scope parallel capacitance is roughly 100pf. Since there are two ...


5

When making a coax choke balun, does the size of the ferrite matter, and if so, why? Simplest answer: If the choke impedance is low enough to allow some common mode power through, then there is a possibility of overheating. The bigger cores either dissipate heat better or provide higher impedance. Another way to say the same thing: As long as the choke ...


4

SPICE modeling shows that replacing T2 and R with a 22.2-$\Omega$ resistor, the impedance looking into T1 is indeed 50-$\Omega$, as expected: Similarly, replacing the 50-$\Omega$ generator and T1 with a 25-$\Omega$ generator shows that the impedance looking into T2 is 25-$\Omega$: This is a result of the electrical symmetry about a "virtual ground" ...


4

"Ferrite" is not a fungible material. There are many kinds of ferrite materials, each with very different properties. To make a comparison, you need the material datasheets, which answer questions like: What is the relative permittivity? What is the loss at the frequencies you intend to use them? What is the Curie temperature of the material? You can see ...


4

For the purposes of amateur radio, ferrite materials are usually useful for making inductors where an air-core coil could not feasibly achieve the necessary inductance. Three relevant questions which the datasheet can answer for us are: what inductance will a coil on this core have? what losses will this core material introduce? at what current will this ...


4

That does look like a fine core to use, and the photo of your construction looks good. Your coax isn't causing any problems with noise. Try putting the choke at the feedpoint, not at the radio, in lieu of the ugly balun you have now. A choke at the radio will do little since the coax shield, connector, and enclosure of the radio already form a continuous ...


4

No. At best it will do nothing; if it did something, it would probably turn your roof from a relatively good reflector (which may mess with your SWR, and may distort your pattern, but the energy that doesn't go where you expected it at least goes somewhere) to a relatively bad reflector (which absorbs energy from your antenna and turns it into heat — ...


3

The answers here are excellent and I have little to add other than to emphasise / RFI problems are hard / Consider the case of the RFI problem discussed in Electronics Stack Exchange ( https://electronics.stackexchange.com/questions/402021/how-do-i-specify-a-low-rfi-led-driver/402029#402029 ). Everything has been ferrited to death (5000 ohm common mode ...


3

As it's name implies, a loopstick antenna is a magnetic loop antenna and is one of the best choices for an electrically short antenna; necessary because broadcast AM wavelengths are hundreds of meters long. Rather than a regular loop, a loopstick utilizes a large amount of (usually Litz) wire wrapped around a ferrite rod. This forms an inductor which is ...


2

Transformers are inductors. They just happen to have mutual inductance with another inductor. Like inductors, they also have self-capacitance, and so a resonant frequency. They also have loss, and consequently some maximum power handling ability which if exceeded causes the transformer to fail by overheating. As such, any parameter which is relevant to an ...


2

Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself or more simply, the inductance per unit length. Magnetic permeability is typically represented by the Greek letter μ (mu) and is measured in henries per meter. It's helpful to remember that inductance here is a complex number. In the context ...


2

Looking at an Amazon listing, it appears that the Degen 31MS active antenna has three ways to connect to a radio. The first is via a direct ANT jack, the second is via clips to existing wire (likely telescoping) antennas, and the third is some sort of ferrite bar coupler that you're mentioning. A brief search didn't bring up a user manual or anything like ...


2

So the problem here is that your setup is not sufficient to tell you what you want to know about the ferrite core. In my set up I have a ferrite core where I made a number of turns on. I connected the signal from my signal generator and one channel of the oscilloscope to one end of the wire to monitor the input. So in this setup your first probe is ...


2

I believe the choice of the core size is twofold. As from K8NVH good answer a minimum impedance is needed for the balun to do its job. A common mode impedance much higher than differtial ones involved makes sure the balanced to/from unbalanced convertion takes place. This somehow drives the size of the core for its mechanical dimensions shall allow the ...


1

https://www.alibaba.com/countrysearch/CN/ferrite-coil.html?spm=a2700.7724857.22.2.39a54807W9PFes Many Chinese suppliers offer these on Alibaba


1

My biggest mistake was keeping my search on everything about ferrite and balun online, and receiving various piece of confusing information of mixed quality. Meawhile, I didn't even take a look at the most basic and authoritative reference material: The ARRL Handbook for Radio Communication. Today, I just obtained a copy of the 2014 edition, and immediately ...


1

In addition to everything Phil wrote, a possible "quick fix" might result if you wrap several turns of every cable between the radio and the PC through a ferrite toroid. I have encountered several cases of RFI resulting from ground loops and other mismatches between pieces of connected equipment. You may need to "divide and conquer" the total, complex ...


1

In addition to what Glen W9IQ said, also consider using a Fair Rite #46 material mix. This is electrically similar to #43 and in some regions is much less expensive.


1

As Mike Waters points out, your application requires a balun since a 1/2 wave dipole is balanced and the coax is unbalanced. The purpose of the balun is to enforce equal current in both dipole legs. It performs this function by preventing current from flowing on the outside of the coax shield by presenting a high impedance path to exterior shield currents. ...


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