This is a summary of the various ferrites we will examine over time, but only one can be the first for close up inspection. Figure 2 happens to show two type 61 chokes in series on the same test jig seen in the introductory article. Three tests were done with one ferrite in three positions: left, center and right. Figure 3 highlights the positioning of the single ferrite.
All the single ferrite tests, positioned right, center and left, overlap almost precisely with a little bit of divergence at the highest frequencies. The double ferrite tests reveal marked improvement overall with a little over 4 dB less power making it past the coax ferrite chokes. The two double choke plots overlap quite well, but begin to show an advantage at the higher frequencies where the two ferrites are positioned shoulder to shoulder. The MHz and above choking advantage over a single ferrite widens to just over 5 dB with two ferrites in series.
Assuming a 50 ohm system, figure 8 portrays the relative percentage of current allowed through by the five scenarios. Remembering that current is the square-root of power, the relative choking benefit in amperage is a less power.
Adding multiple ferrites in series significantly enhances the ability to thwart common mode current. The shoulder to shoulder chokes suggest this is true for the higher frequencies with the data above.
The best approach to achieve this is to wind multiple turns of the coax through a toroid ferrite with the goal of presenting a high impedance with as little reactance as possible. These tests merely reveal what most of us know. Chokes choke and more chokes choke more. The type 61 material does seem to provide good capabilities for the entire VHF band.
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Sharing is caring! Enjoy reading these similar articles: Effect of shields on ferrites Where a choke chokes Finding facts of Fair-Rite Ferrites. Leave a Comment Cancel reply You must be logged in to post a comment.The following chart presents the results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz. Amateur frequency allocations are indicated approximately by the vertical grey bands.
The colours of the bars indicate the magnitude of the CM common-mode impedance; however, depending on the style of choke and the type of ferrite material used for the core, that impedance might be mostly Resistive, mostly Reactive, or somewhere in between.
No black bars are shown for the air-cored chokes because their impedance is almost entirely Reactive apart from a very small band of frequencies around resonance. Reactive chokes have the disadvantage that they can "resonate" with a CM impedance path that is also reactive but of opposite sign - in some cases actually increasing the CM current flow rather than choking it; see the section at the bottom of this page for a detailed explanation. Resistive chokes have the disadvantage that if they have insufficient impedance to reduce the CM current to a very low value, there may be significant core heating.
Aim to choose a choke which has a high impedance and is Resistive over the frequency range of interest. For high power applications RG coax can be used in place of RG58 with little change to the choke impedances.
The method I use to measure choke impedance is shown further down this page. The results can be seen here:. Remember, a good choke will be high impedance and Resistive over the frequency range of interest.
Let's take the example of a 20m half-wave dipole erected 30ft above average ground. The centre conductor of the coax is connected to the left-side dipole leg, and the braid to the right-side.
Fair-Rite 2661480002 Type 61 Ferrite Slide-on Chokes
At the feedpoint, current flowing along the inside surface of the braid will split - some will flow into the right-side dipole leg and some will flow down the outside surface of the braid, depending on the relative impedances of the two paths.
EZNEC predicts that about 0. As we vary the length of the coax, the braid path impedance changes. When the coax is close to a quarter-wave long the CM path is high-impedance and relatively little current flows along the braid whether we include a choke or not; when it is close to a half-wavelength long substantial current flows if we don't include a choke.
The situation gets more complex with a multiband antenna - in fact the potential for a Reactive choke exacerbating the situation on at least one of the bands increases. The following table shows for a range of coax lengths from 20ft to 70ft on this model the braid current without a choke and with a worst-case inductive choke; it also shows the impedance required in a Resistive choke to keep the braid current 30dB below the level of the dipole current.
We conclude that a high value Resistive choke is the safe option for all scenarios. We also conclude that Rules-of-Thumb which equate the required choke impedance to some multiple of the differential-mode load impedance are unsound. My early attempts at choke impedance measurement used a Vector Impedance Analyser AIM with the choke directly connected from the measurement port to ground. However it's not ideal: very high choke impedances are outside the range where the analyser can be expected to be accurate, and despite careful calibration to the measurement plane the analyser always added the equivalent of a few pF of parallel capacitance; this significantly shifts the self-resonant-frequency of higher-Q chokes such as those wound on Type 61 material or air-cored.
It turns out that more accurate results can be obtained by measuring the attenuation the choke introduces when placed between a signal source and a load. So, for instance you could place the choke in series between a signal generator and an RF voltmeter; then, knowing the generator output and the RF voltmeter reading, you could deduce something about the choke's impedance.
However this simple scalar measurement will not tell you anything about the choke's complex impedance its resistance and reactancewhich as we have seen is vital for a complete understanding of how well it will perform.
Fortunately, a 2-port Vector Network Analyser can measure both the magnitude and the phase of the attenuation introduced by the choke, and that allows us to fully determine the choke's complex impedance. I use a VNA in the arrangement shown on the right. The jig typically adds the equivalent of 0. The VNA is first calibrated with the two clips shorted. Then the choke is connected between the clips, and a VNA measurement scan made between the required frequencies. The resulting S21 Amplitude and Phase data is then transferred to a spreadsheet to calculate the choke's complex impedance.
The derivation of the choke's complex impedance from S21 Amplitude and Phase is explained in this note.It was time to quantify this behavior in the lab. 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.Best western front desk checklist
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.Install package files
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.
Ferrite Beads, Common Mode Chokes, RFI by W6LG
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. No one ferrite-construction topology is good for all frequencies and purposes.
Where a choke chokes
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…. Hmmm, well I can convert the S21 measurements to amperage as a percentage. Let me see what I can add. You must be logged in to post a comment. This site uses Akismet to reduce spam.One website that has lots of information and even tutorials is Palomar-Engineers.Chinchilla cat breeders
Not all ferrite beads are the same quality or have the same effectiveness at a given frequency. When you buy ferrite beads buy only from a reputable dealer with a known history of selling to amateur radio operators; like Palomar-Engineers.
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How to shorten your wire antenna without cutting itHere are the details. Return to Web Store Index. These provide a better fit on all RG size cables than do the. I have decided that it isn't worth the one dollar difference in price to take a chance on hurting the cable. These are also very useful for smaller cables where you can wrap two or more turns through the core.
Each turn has the effect of adding an additional core. These meters have probes which open and close around a wire or cable, so they are fast and easy to use. When you find a wire or cable that produces a reading on the RF current meter, then it would be appropriate to use "snap-on" Ferrite cores on that wire or cable. You may need to use more than one. After applying the snap-on Ferrite core smeasure the RF current and note the improvement. These are the "big guns" in the attack on RFI.
They can help on both transmit and receive. Four snap-on cores are shown on each cable. In some cases, more may be needed. Each core provides around ohms of inductive reactance at most HF frequencies. As you can see from the chart above, it was useful from 1 through about 25 MHz. To cover the frequencies up to MHz you would have to switch to 43 material. Just as important, the impedance of the new bead is higher than previous types. We stock Ferrite Cores in the two most popular sizes with inside diameters with.Electric keg still
Both cores use the new 31 ferrite material. If you use some other ferrite mix, you won't get the best results. Also, you have to use a lot of Ferrite beads to equal these snap-on cores which are about 1. Use for any wire or cable up to.
Caution Caution: In mobile installationsit may not be safe to use your radio equipment when your vehicle is in motion.Building the noise reducing common mode choke with parts readily available
RFI can affect your vehicle's onboard computer. Check with the vehicle's manufacturer. Without proper grounding, solutions are difficult. It's important to know the difference between RF and electrical or d.
For important details be sure to read the information in Jim's Notebook. You'll need a jumper cable to connect Line Isolators to your equipment. We stock 18" RG-8X jumpers especially for this purpose.If you own an antenna fed with coax, you need RF chokes on the feedline! Using a balun with adequate RF choking impedance will improve the performance of your antenna, prevent pattern distortion, and prevent RF coming down the feedline, which can cause RFI.
Ferrite is a ceramic consisting of iron and metal oxides having high permeability. When a conductor is passed through the center of a ferrite bead, the impedance goes up with the SQUARE of the number of passes through the center.
Ferrite is made in different formulas, called mixes. The mix used will determine the best choking impedance for the desired frequency range. Mixes 31 and 43 are best for HF use 31 is better for the low bands, with mix 43 having a slight advantage from 14 to 30 MHz. I currently stock mix 31, 43, 61, 77, and 73 ferrite. Let me know if you are looking for something in particular. Chris at KF7P dot com. Check out our TECH page for lots of information on ferrite material and how to properly use it.
Useful baluns and chokes for coax. Large opening allows several passes through center without taking connectors off. Often a stack of several of these are needed to achieve target impedance. Specify quantity after adding to cart. Useful for small cables or one pass with RG8 size coax 43, 31, and 61 beads.
Small beads 61 and 73 only fit small coax. I put one of these excellent choke kits at my transceiver and another "pretty good" choke at the feedpoint of my G5RV Jr. It knocked a good 2 or 3 s-units off of the "band noise" I was experiencing. The product is well made and easy to put together, and the ferrite components are top-notch. I will be ordering more. Love the big mix 31 Ferrite Beads They really do the trick. Love Big Mix 31 works great!! Made a big difference on my transceiver. Purchased the "Biggest Clamp-On" for common mode rejection.
Both the price and service were excellent; Very competitive.
Amateur Radio Stack Exchange is a question and answer site for amateur radio enthusiasts. It only takes a minute to sign up. I have a Flex connected via RG coax to a inverted V fan dipole tuned for 40m and 20m on my roof.
I have an "ugly" balun up on the mast by the antenna. I further have the feed-line going into a lightning suppressor where the line enters into the house and is connected to my 8 foot ground rod. I realize that my coax is not 50ohm it's 75ohm but I'm operating at 80, 40, and 20m. My SWR is 1.
My understanding is that this should not be much of a factor on HF. In fact this coax is plenum rated so it has the extra shielding as well so it's very well guarded from external interference. Unless of course that extra shielding is having the reverse effect allowing even more RFI to flow down the shielding as it's acting as a fantastic antenna.
Hence why I'm trying to do what I'll explain next I'm trying to reduce various RFI on 80m and 40m some 20m coming into my shack and also eliminate RFI going out to various devices by my transmissions but have not had much success. I'm trying to use ferrite cores to help reduce some of this Mouser Link. Fair-Rite Link. I've played with these toroid cores on an off for the past 3 years.
I keep coming back to them as I'm hoping to reduce the RFI in my shack. However each time I've tried to use these cores, they've made exactly zero difference. For example I have a known source of RFI which is a birdie caused by my main 27" monitor an Apple model.
I see this birdie on 80m. I've tried wrapping the power cord multiple times and no luck. Zero effect. However this is just a birdie so I can deal with it. This is a Logitech system. It all works from a main sub woofer that all the speakers then connect too. I've disconnected all the speakers and still get the RFI through the main woofer.
I've wrapped the power cord of the woofer about a dozen times through two cores each. Zero difference on the RFI leaking into the speaker. As mentioned I use a Flex so having a PC and a speaker is essential.
Again I can get around this and use wireless headphones but it's not ideal. This was done with 14 THHN wire as detailed in the presentation above on page This was connected to my feed-line on one end and the other end went straight into my flex I have various other sources within my house that I'm attempting to deal with but I'm trying to mitigate a few of them with cores. But the cores just seem to make absolutely no difference.
Either I've got the wrong cores or I'm missing something and am not doing it correctly?
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