Coaxial cable choke and RF common-mode noise
Let us impassionately and realistically examine the effects of such coiled coaxial cable chokes that are often used near the antenna feed-point and figure out with rational scientific logic if they are truly as effective as one might be made to believe… At this point, some readers may feel impatient and say, stop beating about the bush, give us the bottom line… OK, the bottom line is NO! they don’t work well enough to feel jubilant or elated. Especially, on the lower HF bands like the 80m, or 40m. Even on the other higher frequency HF bands, they don’t perform as well as one might want. If the antenna is a multiband setup working through 80-10m, then such a coiled coaxial coil choke contraption is almost an eye-candy on most bands. Let us see why?
What is the basic objective of a coaxial cable choke at antenna head?
Many of the antenna setups are done while ignoring the fact that a typical coaxial cable transmission line when connected to an antenna, due to its unbalanced nature might produce an unwanted common-mode current that flows on the outer layer of the braid of the coaxial cable. This is true more often than we think. The magnitude of this common-mode current may vary depending on the type of antenna and its overall configuration.
In some instances, this magnitude may be rather unacceptably high enough to allow unwanted radiations during transmit to cause EMI/RFI in the neighborhood. Whereas, during receiving, it may result in a situation that could practically increase the undesired noise pickup to such an extent that it might result in a hopelessly noisy environment.
Every day, we come across radio amateurs who complain about horrendous noise levels on HF bands. Quite often they blame the antenna, at other times they blame their noisy urban environment. Though it is absolutely true that the prevalent noise level on the HF frequency bands in an urban area is indeed rather high, the fact is that it can be managed well enough to mitigate the detrimental effects by using proper techniques. Is the coaxial cable choke one of the effective techniques? Unfortunately, the answer is no. There are several other far more effective and robust methods to tackle the issue of radio noise due to common-mode currents.
What nonsense? The coaxial cable choke brought my SWR under control
Agreed! At this stage, some might say, I found the coaxial cable choke very useful. I had unmanageable SWR at my transceiver, however, after I installed a few turns of coax cable coil near my antenna feed-point everything became hunky-dory. My SWR became good and stable… Sure! it did. That’s because your SWR was never a problem in the first place. It happened to be good even before you made that coil.
One must understand that the SWR on a coaxial cable is determined not by the common-mode RF current that flows on the outer surface of the coaxial cable braid but by the differential-mode current that flows within the confines of the inner surface of the coaxial cable braid and the inner center conductor. The inner confined differential-mode current is what we want, while the outer layer common-mode current is the destructive villain. The magic lies in the fact that even though the braid is physically a single entity, the two RF currents that flow along the braid are totally isolated and independent of one another.
Really? If the SWR was indeed good, then why was the SWR meter going wonky? Why couldn’t I push out TX power? Why was my TXR power output folding back? The answer is simple… The excessive common-mode RF current that was flowing back over your coaxial cable was messing up your SWR metering function and consequently the transmitter’s ALC protection and foldback circuit. The incorrect SWR signaling on account of the common-mode RF flowback was misguiding your rig to believe that the SWR was bad. With a little help from the marginal choking effect of the coaxial coil, the common-mode cable current dropped below the threshold of malfunction. Therefore the SWR bridge and the associated electronic control circuits stabilized.
However, as far as the noise reduction is concerned, the coiled coaxial cable choke perhaps did very little… Yes, it did reduce the noise a bit but certainly not enough to be of enough real consequence. It certainly did not reduce the noise to the level that was potentially possible.
Fair enough! So, why doesn’t the coaxial cable choke do enough?
To answer the above question, let us quickly examine as to why did we put the coil in the first place? What was the objective?
The objective was to reduce (attenuate) the undesired common-mode current on the cable. Although there are various means of preventing the common-mode RF current from being generated in a typical antenna setup, we will skip that and cover it in another article or post. For the moment, let us try to focus on the problem at hand.
In any electrical circuit, one simple way to attenuate current is to place some impediment like a resistance (impedance) in series with the path of the current. In an RF scenario (AC circuit) like our coaxial cable, we have to do just the same. The coiling of a portion of the coaxial cable into several turns is expected to do just that. Since the common-mode RF current flows on the outer surface of the braid, the coaxial cable coil in the path will act as an inductor producing inductive reactance resulting in partially choking the current flow and reducing its magnitude. Remember, since the desired RF current (differential-mode) flows within the confines of the cable, it is not affected by the coiling of the cable. The inductance produced by coiling only acts upon the common-mode current. This is therefore called Common-mode Current Choke
So, the coil actually reduces the common-mode current to an extent. That’s good, Isn’t it? Yes, It is… But it isn’t good enough. We will find out why in a moment.
Before we proceed further, here is an illustration with a set of animated slides depicting typical coiled coaxial cable choke sections that are often used along with all types of antennas. They are typically 5-10 turns (at times even 20) of coaxial cable laid out in a single layer with an air core and with diameters usually ranging from 4-12 inches or so.
Let us now define and set our objectives so that a common-mode current choke becomes effective enough for practical use. If we wish to attenuate the unwanted common-mode noise pickup by the cable to make a substantial improvement, then we have to attenuate this current by at the very least 25-30 dB or more. Anything less than 25 dB is no good, though I would personally always like to ensure that it is more than 30 dB, preferably around 40 dB. The problem is that it is a very tall order for our typical coaxial cable choke. The choking capability of these coils falls far too short.
The common-mode RF noise picked up by the coaxial cable in an urban environment can typically be anything from an additional 4-5 S-units, especially on the lower frequency bands. This noise becomes more obvious when using a horizontally polarized antenna. Most HF antennas like the dipole, OCFD, Long-wire end-fed, Yagi, etc are usually horizontally polarized. On the other hand, most of the QRM is largely vertically polarized. Hence, the horizontally polarized antennas have a native property of rejecting a large portion of the QRM by virtue of polarization mismatch.
However, the coaxial cable with enough current imbalance to produce common-mode surface current is typically vertically oriented over a large section of its length. Hence, such a cable acts as an undesired antenna section having a perfect polarization match with the vertically polarized QRM. Thus, the cable on account of common-mode imbalance picks up most of the noise and adds it to the signal coming from the antenna. The coaxial cable becomes the real spoilsport.
Therefore, the concept of placing a coaxial cable choke inline with the transmission line to choke (attenuate) the common-mode current is a perfect idea. However, it is unfortunate that these coiled cables are invariably unable to provide the desired magnitude of attenuation that could have made them effective. The reactive impedance on account of their inductance falls short of what we need. That’s why, these coaxial cable chokes are not good enough.
Why can’t cable choke achieve desired 30 dB common-mode attenuation?
One of the primary reasons is that to achieve high common-mode attenuation, the choking impedance of the coaxial coil will have to be much larger than what these coils can achieve. For instance, to achieve a 30 dB current attenuation, it needs to be attenuated by a factor of 31.6. Similarly for 40 dB choking, the current must be attenuated by a factor of 100. In other words, if the load impedance (as in the case of a typical RX input) is 50Ω, then for 30 dB choking (31.6 times reduction), the choking impedance (reactance) of the choke must be 50×31.6 = 1581Ω. Similarly, for 40 dB choking (100 times reduction), the choking impedance has to be 50×100 = 5000Ω.
To summarize what we discussed in the above paragraph…
CMatt = 20Log(Zc/Zo)
CMatt is the common-mode Attenuation in dB.
Zc is Choking impedance of the the coiled coaxial cable.
Zo is Receiver Input impedance (usually cable impedance).
The following results are obtained by applying the above equation. A ballpark figure of the expected choking attenuation (in dB) due to an inline insertion of impedance in the coaxial cable setup may easily be found. Here are two examples…
- 30 dB current choking for 50Ω RX input would need 1581Ω impedance.
- 40 dB current choking for 50Ω RX input would need 5000Ω impedance.
Can a typical air-wound coiled coaxial cable achieve the above? Unlikely!
The impedance produced by the open-air coaxial cable choke by virtue of its inductive reactance at any given frequency increases with the increase in coil diameter and the number of turns. It also increases in proportion to the frequency of the signal. In other words, a large diameter coax coil with more turns will produce higher choking impedance. Moreover, this choking impedance will be higher at higher frequencies.
Having said that, a practical coaxial cable choke for use with HF antennas has around 5-8 turns for large diameter coils with a diameter of 8-12 inches. Coils with smaller diameters like 4-5 inches may be built on PVC formers with around 10-20 turns at the most. These are the optimum sizes, yet they fall short of our needs.
One might ask, why can’t I have more turns with a larger diameter to increase the inductance and therefore produce larger choking impedance? Unfortunately, it is not workable in practice. The primary reason is that these coax coils apart from creating inductance also produce capacitance. Due to the proximity of consecutive turns to one another, it produces what is known as distributed capacitance across the coil. The distributed capacitance appears in parallel to the inductance of the coil and produces electrical resonance.
The frequency of self-resonance (SRF) of a typical coaxial cable choke for use with an HF antenna would lie in the range of 8-12 MHz. Above this resonance frequency, the choking coil no more behaves like an inductor. The reactance becomes capacitive and the effective overall impedance of the coil starts falling beyond the resonance frequency.
The dilemma on account of the above-cited effect is that if we have more turns or larger diameter, then although the coil impedance will be higher at low frequencies, the resonant frequency will also reduce further making the choke less effective at higher frequencies. On the other hand, if we reduce the number of turns or reduce the diameter, then the resonant frequency will increase thus increasing the high-frequency threshold quite undoubtedly but the choking impedance will become less.
Let us see how much choking impedance can we obtain from typical coiled coaxial cable chokes on the HF bands. The HF bands below 20m are the most adversely affected bands due to elevated both natural QRN as well as the QRM component. The 80m and 40m bands are the worst affected. Here are a few simplified examples that do not account for the distributed capacitance of the coils that reduce their self-resonant frequency (SRF) and make the coils less effective beyond the SRF…
8 inch diameter, 5 turns of RG213 will have 9.4μH inductance.
80m band choking impedance is 207Ω, resulting in 12 dB choking.
40m band choking impedance is 413Ω, resulting in 18 dB choking.
20m band choking impedance is 826Ω, resulting in 24 dB choking.
6 inch diameter, 6 turns of RG213 will have 9.2μH inductance.
80m band choking impedance is 202Ω, resulting in 12 dB choking.
40m band choking impedance is 405Ω, resulting in 18 dB choking.
20m band choking impedance is 809Ω, resulting in 24 dB choking.
5 inch diameter, 10 turns of RG213 will have 19.8μH inductance.
80m band choking impedance is 435Ω, resulting in 19 dB choking.
40m band choking impedance is 871Ω, resulting in 25 dB choking.
20m band choking impedance is 1741Ω, resulting in 31 dB choking.
4 inch diameter, 20 turns of RG213 will have 57.8μH inductance.
80m band choking impedance is 1271Ω, resulting in 28 dB choking.
40m band choking impedance is 2542Ω, resulting in 34 dB choking.
20m band choking impedance is 5084Ω, resulting in 40 dB choking.
Those calculations above that are shown in RED represent scenarios that are not physically realizable due to the self-resonant frequency (SRF) falling below the operating frequency. The coil would no more behave like an inductor but more like a capacitor above the SRF.
Typical coiled coaxial cable chokes of the types we have discussed so far and the types that can be made using coaxial cables have the above limitations. As a consequence, usually, the choking impedance produced by such coils falls within the range of 200-600Ω, or perhaps up to 800Ω at best. This would result in unwanted common-mode current and noise attenuation in the order of 12-20 dB typical, or 25 dB at best. We would have been more comfortable if the attenuation had been 30-40 dB.
The maximum choke attenuation value cited above is only applicable on no more than one HF band for which the coil might be optimized. On the other bands, the attenuation will be far less.
Hence, the performance of a coiled coaxial cable choke is generally far below the requirements. It is quite an under-performer. Even though one may find a lot written about this choke arrangement in amateur radio literature, one should try to use better alternative solutions that yield promising results. consider using proper Balun or Unun as required for impedance transformation plus a common-mode choke using proper quality ferrite toroids or beads.