Common mode 1:1 choke baluns

prcguy

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If this post comes across as a sales pitch for MyAntennas brand common mode chokes, it kind of is my personal stamp of approval. I just came across an ad from Palomar Engineers for their new "MAXI-CHOKER" coax common mode line isolator in tubular form with "propitiatory multi-mix, multi-core, multi-turn technology". It advertises between 500 ohms and 5,000 ohms of choking impedance across the 160-6m amateur bands and up to 38dB of isolation. Bravo, good work Bob.

A few years ago I was chatting with Bob from Palomar Engineers at a friends hamster party and brought up the MyAntennas tubular common mode chokes and how much better specs they had than Palomar Engineers and that Bob otta take a look at the competition to see what they are doing. I knew MyAntennas was using some sort of multiple core, multiple mix, multiple winding approach to cover the entire HF band with terrific specs and Bob only had two stacked core, same material chokes that were kind of ho-hum in performance. Oh, and he had some snap on ferrites you could put around your existing coax.

Fast forward a couple of years and gee, the new Palomar common mode choke looks a lot like the MyAntennas line and with similar construction. However, the Palomar specs of 500 ohms to 5,000 ohms choking impedance is quite a bit less than the MyAntennas CMC-130S-3K with its 5,000 ohm to 10,000 ohm choking impedance that costs less. If you look at the top of the line MyAntennas CMC-130-3K it has a minimum of 10,000 ohms choking impedance from 3.8 to 30MHz and over 5,000 ohms at 160m and upwards of 15,000 ohms at 10m and up to 41dB of isolation. And it has about the same retail price of the Palomar MAXI-CHOKER.

I'm a huge fan of Danny Horvat and his company MyAntennas and still recommend his common mode chokes for not only the best bang for the buck, but the best bang period in snuffing out common mode RF problems on your coax. Always let the specs be your guide and if the common mode choke mfr doesn't specify choking resistance or isolation in dB, then they probably don't want you to know or they don't know how to measure it. I attached some web pages below with the brands and models I mentioned.




 

Ubbe

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But do you really need 40dB isolation? Isn't 20dB or even in some cases 10dB enough (100 times or 10 times supression)?

/Ubbe
 

prcguy

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You need as much as you can get and 10dB of isolation will probably not fix much if anything. Most of the tubular chokes simply have a string of ferrite beads over some coax under the tube and at best they peak out at about 20dB isolation mid band with much less at 1.8Mhz, maybe 5db and 28MHz might reach 10dB. I've had RFI problems where one of those did nothing and a better design like coax wound around a ferrite ring made a noticeable improvement.

The 8 to 10 turns around an FT-240-31 or 43 core types can give up to 30dB of isolation at some frequencies while keeping maybe 20dB at 10m and 40m and less at 1.8MHz and going from a bead over coax type to one or two rings wrapped with coax can make or break an RFI problem. I used these types for years but the clincher for me was replacing the ferrite ring type on a ZS6BKW antenna at the ladder line/coax junction with a MyAntennas CMC-130-3K and watching the noise floor and some birdies on my spectral display go down by a couple dB over most of the HF band. It was so disturbing I had to put the old choke back in line and swap it out again to believe what had happened. I already had what I thought was a good performing choke, and it was for many things, but I never knew I had an RFI up the coax problem until it was snuffed out by the MyAntennas common mode choke.

That is an example of what an extra 10dB or more isolation can do on top of an already good choke. I was so impressed I put another one at the radio end where the coax exits the room after passing by a bunch of computer and router stuff that was probably what induced the RFI onto my coax shield. The second choke at the radio end made a further but smaller improvement on my noise floor and I will take every fraction of a dB I can get.



But do you really need 40dB isolation? Isn't 20dB or even in some cases 10dB enough (100 times or 10 times supression)?

/Ubbe
 

Ubbe

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I don't understand. If a choke supresses RFI by 20dB, why do you only see a couple of dB improvement of RFI supression in the radio receiver? It surely must be something else going on. Are your total signal also reduced a couple of dB, less than a halv S unit? The signal to noise ratio might still be the same if the whole signal level are attenuated a couple of dB?

/Ubbe
 

Ubbe

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Yes, but I do not trust Bonito when he says to put his choke filter closest to the receiver and mentioned nothing about the antenna end of the coax.

A receiver are often a metallic box that keeps RFI out of the box. An antenna are completly open to all sorts of RFI that climbs up the coax to it. A choke filter probably makes the biggest difference at the antenna to keep the RFI from the outher coax shield to reach the antenna elements where it is most vulnerable, the connection to the transmission line. Having a choke at the receiver end are probably not neccesary in most cases, or makes too little difference considering the cost of the choke. Using ladder line closest to the antenna will create a huge distance between RFI and the antenna and makes a choke unneccesary at that end of the coax.

/Ubbe
 

prcguy

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I don't have an answer for this. Common mode chokes are tested in various ways on a test bench to get baseline isolation and resistive choking information. The real world seems to be different but the chokes with higher measured isolation can work noticeably better than those with less isolation. Going from a 20dB isolation to a 40dB isolation rating will not give you a 20dB lower noise floor and doesn't appear to reduce what looks like RFI on the coax by that additional 20dB. However, in my testing a 20dB isolation bead over coax might not do anything to reduce noise induced onto your coax but a MyAntennas version can make a noticeable improvement.


I don't understand. If a choke supresses RFI by 20dB, why do you only see a couple of dB improvement of RFI supression in the radio receiver? It surely must be something else going on. Are your total signal also reduced a couple of dB, less than a halv S unit? The signal to noise ratio might still be the same if the whole signal level are attenuated a couple of dB?

/Ubbe
 

Ubbe

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It's not snake oil. It has its purpose but question are what the cost/effect ratio are. When you transmit you want the choke filter at the antenna to stop any RF power from the antenna to ride on the coax braid and lead RF down the coax path. When you receive you want it at the antenna to stop RFI that the coax has picked up from the enviroment from reaching the antenna.

Having a choke filter at the receiver must have very little effect or probably none, as the receiver is shielded and the antenna input on the circuit board are, if not always it is at least extremly common, in a screen can to stop any interfering signals reaching it. After that the signal are amplified and are less prone to be affected from outside sources.

I would say that in most cases it's just one more thing to worry about, a pair of extra connectors that need proper sealment, a plastic tube that must not crack at the connectors or elsewhere that otherwise could lead to water instrusion. It must be properly secured so it doesn't move in the wind and with time could break the coax open.

They only make choke filters for shortwave? It is at least as much RFI problem at VHF and UHF but you never see any choke filters for those frequencies. People complain about a $600 price tag for a scanner so it could probably be hard to convince them to buy a $100 filter. People who listen to shortwave doesn't hesitate to buy $5000 receivers so probably much easier for them to part with an additional $100 for a choke filter.

/Ubbe
 

prcguy

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Here is my take on choke placement. For retaining the best radiation pattern on a dipole or Yagi, etc, the choke is best right at the antenna feedpoint. That will minimize effects of the feedline exiting the antenna and if there are RF currents flowing on the shield of the feedline the choke will stop it and the resulting and unwanted feedline radiation.

For keeping RFI induced onto the coax shield from reaching the antenna I think its best to place the choke near the antenna but not right at the feedpoint. Choking at the feedpoint with a good effective choke should stop the common mode currents at that point but its right at the antenna and the coax leading up to that point is in effect an antenna in very close proximity to the antenna that might radiate that noise into the antenna. I might place the choke outdoors and a ways down from the antenna feedpoint like maybe 1/4 wavelength down. This is just my opinion and is not tested.

Placing the choke at the receiver end can have various results depending on where RFI enters the coax. If the choke is right at the receiver and the coax later passes right next to and parallel to noisy computer cables, etc, then the RFI as common mode current can still flow up the coax to the antenna and get received. I'm not sure if and how the RFI might get into the receiver via the coax running to the receiver. I think it depends on the design of the receiver and its housing and what other cables might connect to the receiver, etc.

In my case I have a good, make that a great common mode choke right at my antenna feedpoint, then another near the receiver but after the feedline passes noisy computer cables. This will have the most attenuation of common mode RFI traveling up the coax to the antenna but not for RFI from my computer cables going the other way towards the receiver. I can do a future test with a third choke right at the receiver since I now have a stockpile of MyAntennas common mode chokes. I will report the results if I do this test.

It's not snake oil. It has its purpose but question are what the cost/effect ratio are. When you transmit you want the choke filter at the antenna to stop any RF power from the antenna to ride on the coax braid and lead RF down the coax path. When you receive you want it at the antenna to stop RFI that the coax has picked up from the enviroment from reaching the antenna.

Having a choke filter at the receiver must have very little effect or probably none, as the receiver is shielded and the antenna input on the circuit board are, if not always it is at least extremly common, in a screen can to stop any interfering signals reaching it. After that the signal are amplified and are less prone to be affected from outside sources.

I would say that in most cases it's just one more thing to worry about, a pair of extra connectors that need proper sealment, a plastic tube that must not crack at the connectors or elsewhere that otherwise could lead to water instrusion. It must be properly secured so it doesn't move in the wind and with time could break the coax open.

They only make choke filters for shortwave? It is at least as much RFI problem at VHF and UHF but you never see any choke filters for those frequencies. People complain about a $600 price tag for a scanner so it could probably be hard to convince them to buy a $100 filter. People who listen to shortwave doesn't hesitate to buy $5000 receivers so probably much easier for them to part with an additional $100 for a choke filter.

/Ubbe
 

Ubbe

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If you don't have a proper balun at a balanced antenna to connect to an unbalanced coax, you must use a choke balun at 1/4 wave from the antenna, or right at the antennas connection point. The balun must be long enough to cover the wavelenghts from lowest to highest frequencies used.

Not isolating the coax properly from the antenna will make the coax part of the antenna and both the directional loob, impedance and tuning will be off. If a choke balun are used it will also do double duty as a choke filter to stop RFI.

/Ubbe
 

prcguy

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There is a little more to this. There are thousands if not tens of thousands of dipoles in use all over the world without any choke balun and they work fine and they have no RF on the coax. How well they work depends some on the length of the feedline and how it exits the dipole.

If you take a typical center fed dipole and use coax cut exactly 1/4 or 3/4 wave or a multiple of that then RF on the grounded side of the dipole element can divide and flow on the coax because its resonant and low impedance just like the grounded element. It also depends on how the coax exits the dipole and if it exits opposite the grounded element and the coax is 1/4 wave resonant or multiple of that then radiation from the grounded antenna element and the coax can be reduced or possibly canceled out. In other words you would have made a ground plane by accident if everything meets a certain criteria.

If you have the same half wave dipole with no choke balun in the system and the coax is cut to a half wavelength or multiple, it will be a very high impedance at the feedpoint and most if not all RF current in the antenna will flow on the two halves of the dipole and not on the feedline. So its a crap shoot if you get RF on the feedline and it depends on several things. Using a good choke balun at the feedpoint will insure there is little or no RF on the feedline and that the antenna radiation pattern is not disturbed (as much) by the feedline.

If you don't have a proper balun at a balanced antenna to connect to an unbalanced coax, you must use a choke balun at 1/4 wave from the antenna, or right at the antennas connection point. The balun must be long enough to cover the wavelenghts from lowest to highest frequencies used.

Not isolating the coax properly from the antenna will make the coax part of the antenna and both the directional loob, impedance and tuning will be off. If a choke balun are used it will also do double duty as a choke filter to stop RFI.

/Ubbe
 

Ubbe

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The theory are that the RF, which is alternating current, flows in one direction using the inner conductor of a coax and in the other direction on the inside of the braid. The outside of the braid should be "cold". If there's something on the outside of the coax it is interference and unwanted RF that has to be stopped, by a choke filter.

What happens to that wanted RF at the inside of the coax when it attaches to a receivers metal box I don't know. The whole box radiate, but then you usally ground your receiver? Using a dipole arrangement directly coupled to a coax only leaves the element connected to the braid as a dumb counter poise and you can ground that coax directly at the antenna. But if you use a proper balun, with a transformer circuit that have both elements connected to it and they work together when their phase have been matched in the balun, then you start to get an efficient dipole antenna.

Anything you put on the outside of a coax can't do anything to the inside of it, right? It's probably a reason to that most good coax use double shielding, an inner foil for the good RF and an outher braid to insure that the RFI stays on the outside.

/Ubbe
 

W5lz

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A "choke-balun" is a combination of the two. A balun alone will never be a choke, same for that choke being a balun. I don't use a balun, I don't find them necessary. An 'unbalance' at HF generally means a slight skewing of the radiation pattern. In most cases that skewing isn't noticeable so no loss. It's also another point of failure. Chokes only affect the outside of coax, not the inside where the signal is, so why not if you've got the extra coax.
 
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