How does this work and why is it needed. Watch this episode for more information on common mode currents, baluns, and RF chokes. Want your ham radio question answered? Leave a comment below or email kb9vbr jpole-antenna. A few weeks ago I purchased a J-Pole antenna and it seems to work very well with RG-8x that runs to my transceiver.
This question opens up the discussion on common mode currents, what a balun is, and why certain antennas benefit from using them.
Baluns can be very complex and there are entire books written on the subject. Baluns provide an important part of your antenna system by matching, or transforming the impedance of the transmitter to the antenna. Balun is a contraction of two words: Balanced and Unbalanced. Often they are used in connecting balanced antennas, like dipoles to unbalanced feed lines, like coaxial cable.
Coaxial cable is an unbalanced feed line. The RF energy current runs up the center conductor and returns back down the outer braid. But if there is a mismatch in some way, that is has a high SWR or some other issue, that energy can flip and instead travel down the outside of the braid.
Baluns and RF Chokes: Ham Radio Q & A Episode 3
This current running down the outside of the coax braid is what we refer to as common mode current. RF Energy travels up the center conductor and returned back down the inside of the braid. The outside of the braid is designed to keep the cable from radiating, like a Faraday cage keeps RF signals confined within it. If the signal is on the outside of the cable braid, then the coax turns into a radiator, or an antenna itself.
So, how do we keep the common mode RF energy from traveling down the outside of the cable? With a special type of balun, of course.
Remember way back to antenna theory For an antenna to radiate its RF energy, it needs to have an impedance that matches the transmitter. In the case of amateur radio, this would be 50 ohms. So we build our antennas to have a 50 ohm impedance. In an ideal world, the RF energy travels out the transmitter, up the feed line, into the antenna, and gets released into the ether.
The RF current that transported that energy then returns back down the inside of the braid of the feed line and completes the electrical circuit.
But, in some cases, you may have a 50 ohm antenna, but due to some type of imbalance or mismatch in your antenna system, not all of the RF energy is being released but instead returning back down the outside braid of the feed line. This causes the cable to radiate or exhibit high SWR to your transmitter. Secure the loops with electrical tape or zip ties. Placing this choke or balun near the feed point of the antenna will increase the impedance of the outer braid of the coax to ohms or more.
And since RF energy will seek the path of least resistance, it will be stopped by the choke. A second option is to use these snap on ferrite beads. These ferries clamp onto the outside of your cable and perform the same function as the loops of coax. So do you need a balun or RF choke with every antenna? The answer is no. It really depends on the type or style of antenna and the feed line you are using.
For example, a center fed antenna like a dipole is considered a balanced antenna. If you connected it to an unbalanced feed line like coaxial cable, a balun will be necessary to keep the common mode RF energy from traveling back down the antenna. You could connect an unbalanced ground plane to your unbalanced coax without the need of a balun or choke of any sort. But what about the J-Pole antenna referenced in our original question?
Ferrite common mode choke for 144MHz
This is where it gets very interesting.QRZ Forums. Hi all. I have tried an air wound choke, but found it not to do the job. I tested it with a crude method of touching each tip of the dipole and noting that touching the element connected to the centre of the coax had a bigger effect on impedance than touching the other one. Also, touching the bnc socket had a small effect on impedance, suggesting it was acting as part of the antenna.RF Choke 10 - 80 meter.
I tried 4 turns of RG around an unknown mix small ferrite and it seems to be very effective. I don't see this method used at VHF and was wondering why. I have only tested it with an analyser so far, but the antenna seems balanced and touching the shield of the BNC has no effect on impedance now and touching either end of the antenna results in the same impedance change, unlike previously. This is a driven element for a portable Yagi, so I wont be shoving hundreds of watts up it.
Will my little ferrite be OK with a max of w do you think? I have attached a photo below. M0UKDNov 8, I think that will probably work. I use no ferrites on my VHF yagis because they have baluns as part of the design, and the feedlines are very well decoupled.
But for a simple feed like a split driven element, what you've done ought to work fine. Without the 43 and 61 mix beads I was picking up intermod on all 4 bands and with them nothing; the 6M noise level went from S3 to zero, and no more paging beeps on 2M and no more TV spurs on and I can hear my EME signal when aimed at the negative horizon on all 4 bands and that is used as my regular system performance checks.
Ive never made an EME contact, no interest in it. KM1HNov 8, Carl, what is a or the negative horizon? AA9GNov 8, KM5FLNov 8, My current 6m and 2m yagis both use custom built ohm ferrite chokes. Google "k9yc". He's written a lot about ferrite use for ham radio. KF6ANov 8, Are not resistive chokes ferrites more effective and broadbanded then reactive air core chokes?
Amateur Radio Stack Exchange is a question and answer site for amateur radio enthusiasts. It only takes a minute to sign up. In another questioncommenters recommended that I look into adding a balun to a dual-band dipole to eliminate the feedline as a radiator.
As I understand it, a balun also functions as a choke, but is more efficient because When I try to find plans to build a balun, they all seem to specify a frequency range of 1. Let's clarify some terminology: a balun is any device used to convert between a balanced system ladder line, dipoles and an unbalanced one coax, vertical monopoles.
There are a lot of ways to make a balun, and a common-mode choke is one of them. A choke can also be used in balanced to balanced, or unbalanced to unbalanced connections where suppression of common-mode currents is merited. The choke approach or "current balun" as you'll sometimes hear it described is nice because it's small, easy to fabricate, broadband, and low-loss.
Ferite-wound chokes can be done on VHF as well. Jim Brown, K9YC has some measured data on ferrites. Look for page His data show that Fair-Rite mixes 31, 43, and 61 can all be made to work.
However, the effective permeability of ferrites drops with increasing frequency, so it becomes difficult to get a sufficiently high choking impedance for the choke to function effectively as a balun. You can always measure and see.
One approach is to keep the system unbalanced all the way. If you have an unbalanced coax feedline, then put an unbalanced monopole antenna on it. The three drooping wires are the "ground plane". Since the ground plane presents a very low impedance to the ground currents, that's where most of the current will flow. The coax common-mode is a relatively high impedance. Some antenna designs which aren't simple monopoles use a "skirt" which looks like radials on a monopole.
As in the monopole, the skirt looks like a low impedance ground plane, and the feedline is "hidden" under it. Other designs are the folded balunand the folded dipole. These designs work by getting the coax to attach at a ground potential point, while creating such a point through some clever arrangement.
A sleeve balun is another possibility. It uses a quarter-wave stub to insert a high impedance in the common-mode. There some antenna designs out there that claim no balun is necessary. Sometimes it's true the balun is intrinsic in the design and sometimes it's not. Look for an antenna model that contains a feedline such as W8JI did with the J-pole.
Or build the antenna and measure the common-mode current yourself. What you really want is a choke often called current balunas a voltage balun i.Several types of coiled coaxial cable chokes are used near the antenna feedpoint inline with the transmission line leading to the antenna. The objective is to minimize any common-mode current that might be present on the outer braid of a typical coaxial cable. In his article, I will try to bust some of the myths associated with Read More….
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The next figure shows the ferrite with its coax along with a similar length of RG for comparison. The instrument was configured to sweep from 10 to MHz. Calibration complete, the coax with ferrite was swapped in place. So really we are simply treating the RG as a large diameter wire. Indeed if we substitute a 10 copper wire for the coax the results will be the same. Figure 5 contains the results of the ferrite test with arrows highlighting values at VHF and UHF amateur radio spectrum segments.
Since the plain coax was also our calibrator, its transmission in green shows a nice flat line from 10 to MHz. The power attenuation observed through the ferrite in red is what one should expect. With a logarithmic frequency axis we see a more balanced slope on either side with a more textbook shape.
Could it better? Is that enough to manage feedline currents? Note this is a relative power measurement. The current attenuation for a given common mode circumstance drops by the square-root of power. Per the suggestion from a commenter below, the following graphs added Sept. Here we go….
As you can clearly see, the data uncertainty is much less than the larger trends shown by the ferrite under test. Remember, this data is NOT measured by a current sensor. It is derived from S21 power measurements. For testing like this we can likely assume for calculations a 50 ohm system impedance.
In the real world the current to power relationship will be a bit different. We must keep in mind measuring power through a choke requires knowledge of impedance to calculate current.
What is the ferrite material used in the N9TAX products? The sent email with that question remains unanswered. It is vitally important for the antenna user to understand there are many flavors of ferrite material.
Paired with how you apply the available ferrite products yields designs benefiting specific frequency ranges and uses.
Where a choke chokes
No one ferrite-construction topology is good for all frequencies and purposes. K9YC does as good a job as any on the various recipes and techniques available to help the antenna user make educated choices for particular circumstances. Recommended reading…. The ferrite used in this antenna appears to good for VHF only.
Round Cable Snap-Its K9YC would say you should aim for much more series resistance and he is certainly correct. Ok, this shows attenuation with the ferrite, as expected. What I was expecting was a measurement of common mode current, with and without the choke. How do you know the ferrite is being effective, if you don't know what the common mode current is? Inquiring minds want to know….Besides burning your fingers on the key or lips on the mic, it can also cause some devices to fault.
It may also be desirable to place a second one just before the coax enters the house. There are several ways to accomplish this.
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