# Tag Info

26

"Balun" is a portmanteau of "balanced" and "unbalanced". Anything made to interconnect a balanced and unbalanced load can be called a balun. A common-mode choke (like a length of coax wound over a ferrite ring) works as a balun because it inserts a high impedance in the common-mode without affecting the differential-mode. By ...

12

You'll find voltage baluns in plenty of circuits. For example, the QRP Labs receiver module: HPSDR Pennywhistle (twice, T1 and T3): The Elecraft KX3: A voltage balun can do some things a current balun can not. Firstly, the turns ratio can be varied to provide impedance transformation. Some of these circuits use something other than a 1:1 turns ratio to ...

7

In most multi-band balun applications, there is rarely a need to maintain a perfect 50 ohm impedance within the balun. The feedpoint or input impedance is varying widely so another impedance bump in the mix typically has no detrimental effect. I highly recommend the use of coaxial wound over bifilar style for a 1:1 balun. Comparatively, the coaxial and ...

6

The ideal number of turns depends on core material, geometry, and frequency. This is why you find such variance in how many turns should be used. More turns increases the choking impedance up to a point, but decreases the choke's self-resonant frequency (SRF). Once the SRF goes below the operating frequency, adding more turns increases the distributed ...

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 ...

6

By the principle of reciprocity, if it works for transmit, it works for receive also. Possibly if your receiver and transmitter are separate devices you may end up with a situation where no common-mode current was detectable on transmit, and yet common-mode signals did appear in the receiver. However with modern equipment having both receiver and transmitter ...

6

If you do it right, the bead(s) shouldn't get hot at all, and shouldn't add much loss. The impedance of the bead(s) should be about 10 x the impedance of the dipole. The current on the feedline with no balun might be about half the antenna current. With the beads on it it'll be less than 1/10 of the curent, or 1/100 of the power. Small beads (3.5 mm inside, ...

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

There are balanced feedlines (twin-lead), and balanced antennas (dipoles), and similar things all described as "balanced". In this sense, balance means the two halves of the thing have equal impedance relative to ground. Here, ground does not mean whatever is called ground on the schematic. Rather, it means "the environment". Consider twin-lead for example. ...

5

The choke should go at the transition between unbalanced and balanced, which is where the RG-6 meets the ends of the loop wire. You talk of coiling coax: I suggest you coil it around a ferrite core and not just air. The trouble with air-core "chokes" is they have no resistance, only reactance, in common-mode. Depending on what the common-mode impedance is ...

5

From my current experience of using Cha F-Loop 2.0: Yes, you most definitely need an RFI choke to use a magnetic loop antenna, and a good one. Magnetic loops are not as good as you might think. Particularly they are very narrowband - you have to seat near it and tune the capacitor all the time. Or you need a remote tuning device. Despite the fact that some ...

5

You could rewind it, but the result may not necessarily be better. This is because the core must be optimized for different needs in each balun design. In what you're calling a "current balun", the idea is to maximize the impedance in the common mode. Ideally, that impedance is mostly resistive, meaning high loss. Saturation current isn't so important, ...

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 ...

5

A choked end-fed antenna is (or acts just like) a highly off-center fed dipole, where the length of any stub between the common mode choke and the unun/transformer/feedpoint sets the off center ratio and thus the impedance at the antenna feed point. Your current 1 foot stub is less than 1% of the antenna, whereas a few percent is usually recommended, that ...

4

I'm answering my own question after seemingly solving the problem, thanks to the helpful comments I received. Evidently, the ferrite rod core that I salvaged from a Hy-Gain balun was the culprit. I couldn't find any information on what kind of ferrite it is but it was manufactured about 30 years ago. I don't know much about baluns yet but earlier questions I ...

4

Looking at the pictures of the LDG RBA-1:1, it looks like the answer is yes. It is just a choke that can be used as either a balun or unun. Note that if the vertical has a resonant feedpoint resistance of 35 ohms, a 50 ohm unun will transform the 35 ohms to a slightly inductive impedance which will decrease the resonant frequency at the unun input terminal. ...

4

The advantage of using coax for a 1:1 choke-balun is the constant Z0 of 50 ohms. As can be seen in the following graphs, the only time a 1:1 choke-balun accomplishes a 1:1 transformation is when it sees 50 ohms at its output. Any other impedance at the output causes an impedance transformation because the SWR is not 1:1.

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

Forget about air-core chokes. G3TXQ's page that Phil links to in his excellent answer here shows just how much superior ferrite chokes are than air core choke baluns. Please visit k9yc.com and download rfi-ham.pdf. Jim Brown there on page 36 recommends a choke balun made by wrapping 8 turns of the coax at the antenna feedpoint through four stacked 2.4" ...

4

End fed antennas have become quite popular in recent years, primarily due to their convenient installation conditions. Many people subjectively report good results from the antenna. But how the antenna works is not often well understood. The end fed antenna often utilizes an unun at the feedpoint of the antenna. This is done because the feedpoint impedance ...

3

I cannot speak to the off-center type dipole feeds that use the 1:9 (impedance) transformers, but for the truly end-fed dipoles fed with something like a 1:49 impedance ratio, the answer is, it depends. I performed some simulations of an end-fed dipole system with an autotransformer and an additional wire representing the currents along the feed line. I did ...

3

That length of 450 ohm ladder line is important. In order to have a reasonable VSWR on the bands that the G5RV was designed for, you should not change it unless you want to cover different bands. The G5RV and the "Jr.", while good antennas on some bands, was invented before antenna modeling programs (such as EZNEC) were available. After they were, ZS6BKW ...

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 ...

2

Magnetic loops do not need a choke more than other kinds of antennas but I recommend you have one anyway especially if you will have coax running through your house. The reason to have a choke with a magloop is to reduce rf receive noise which can be picked up by the coax from local rf noise sources in your house. This is also valid for other types of ...

2

I strongly recommend "Building and Using Baluns and Ununs" by Jerry Sevick, W2FMI. I have built dozens of coaxial- and bifilar-wound transformers and chokes for use in high-power amplifiers and antenna systems based on the easy-to-understand information in this book. Specific mention is made of how to make reproducible bifilar-wound units over a wide range ...

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 ...

2

I attended a seminar on baluns --as applied to antennas-- at a local hamfest, given by no less a guru than Glenn Shultz, W9IQ. He nicely explained that the only place we need to use a voltage balun in an antenna system is for end-fed antennas. Every place else, use a current balun. And the simpler, the better.

2

Is it sufficient to construct two 1:1 common mode chokes ... then wire them together as described in the previous paragraph? It is nearly as simple as that. Here's a wiring diagram: In a perfect world, the transmission line wound around each core would have a characteristic impedance of $\sqrt{200*50}$=100-$\Omega$. Books on this subject by Sevick and the ...

2

Different beats have different attenuation at different frequencies. My gut tells me you want at least 6 dB, preferably more. Guys seem to wind their own inductors as RF chokes but not sure how much attenuation at the desired frequencies.

1

A balun effectively allows you to connect a balanced antenna (such as a dipole) to an unbalanced feeder (such as coax). You have a balun at the feed point of the antenna, with a coax socket on it, which means that the feeder going from the radio to the antenna is unbalanced. The HFV-5 is an antenna that presents a 50Ω impedance on its coax socket and it ...

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