Choke-Baluns in amateur radio – How to get rid of interference (2025)

When your antenna cable becomes an invisible hazard

Do you get tingling fingers when operating the station? Do you burn your lips on the microphone? Does the standing wave ratio change when you touch the station ground and with the cable length? Do you cause interference to yourself and your neighbors? Does your QSO partner tell you: “There is RF on the modulation”? Then sheath waves on the antenna cable are the cause. Not only your antenna, but also the coaxial cable radiates!

If the antenna gets out of sync

If a symmetrical antenna is directly fed at the feed point without a balun using an (asymmetrical) coaxial cable, there may be problems with sheath waves depending on the length of the feeder and the routing and arrangement of the feeder to the radiator. This is the reason why a feed line that is as perpendicular as possible to the radiator and pointing straight down is always recommended! Otherwise, a path that deviates greatly from this can cause high frequency coupling to the shielding of the coaxial cable. The dimensions of the radiator and feed line together can also, purely by coincidence, lead to the entire system resonating at the operating frequency. In all these cases, the coaxial cable with its shielding sheath becomes part of the antenna, and as a result, the ground of the station is also exposed to RF.

Choke-Baluns in amateur radio – How to get rid of interference (1)

Choke-Baluns in amateur radio – How to get rid of interference (2)

Systematic interference avoidance – tracking down the real causes

When these problems occur, the usual solution today is to install a choke-balun (hereinafter referred to as CB). With its help, the disturbing symptoms can usually be eliminated reliably and effectively. Nevertheless, one should ask oneself what the actual causes of these disturbances are. As a first aid, the length of the feeding coaxial cable can be changed to try and find the cause.

The perfect cable length as a solution

The electrical λ/4 lengths (actual length times velocity factor) and their resonances and uneven multiples of these should be avoided. They intensify the problem of sheath waves. An ideal electrical cable length of λ/2 (or a multiple of this) x velocity factor (v = 0.66) often provides a solution. This electrical cable length at least transmits the conditions from the feed point of the antenna to the lower end of the cable at a ratio of 1:1. Unlike a length of λ/4 and odd multiples of it, the cable does not change the conditions at the lower end of the feed line.

Choke-Baluns in amateur radio – How to get rid of interference (3)

High-quality coaxial cable sold by the meter

The inadequate effect of earlier approaches

Before the wide range of versatile ferrite materials became available, a “cable coil” made of several turns of the feeding coaxial cable, directly in front of the feed point, was recommended. Due to their low blocking inductance, their effect was only marginal and at best noticeable on the upper shortwave bands. Even the feed choke according to W1JR, with a relatively low number of turns of the coaxial cable on a ferrite ring core, does not fulfill expectations on the lower shortwave bands for the same reason.

Current BALUN instead of voltage BALUN

A conventional voltage BALUN only balances the voltages and is not useful for sheath waves. Only the use of a 1:1 current BALUN is suitable to minimize the risk of sheath waves from the outset. True (current) balancing at the feed point is the first prerequisite for preventing sheath waves from occurring in the first place. Only then should you consider inserting a choke-balun afterwards.

The evolution of the choke-baluns

Initially, Choke-Baluns consisted of ferrite ring cores of high permeability threaded onto one end of the coaxial cable, close together. Currently, manufacturers have switched to ferrite tubes and, because of the expected heat build-up, to Teflon cables. The ends are equipped with SO-239 plug-in connectors for insertion into the feeder cable. The whole thing is fixed and held together by a heat-shrinkable tube. The ferrite material of this CB has such a high permeability that sufficient blocking attenuation is ensured even in the 160-m band. An CB also prevents unwanted resonances of the feed line or the entire antenna system consisting of the radiator and the feed line.

Choke-Baluns in amateur radio – How to get rid of interference (4)

A CB works as an absorber in which the power component that is attributable to the sheath wave is “burned” in the ferrite material. This part of the transmission power is thus lost for the radiation. Ideally, the CB remains cold or warms up only slightly and only after a long period of operation. If the sheath wave component is very high, it can cause an extreme, rapid heating. This is an indication that something is seriously wrong. In this case, it is advisable to look for the exact cause instead of “tinkering” with the symptoms.

Top or bottom – where does the block work best?

At the top of the feed point or at the bottom of the output of the transceiver or antenna tuner? Clearly, at the top of the feed point. After all, we want to get the feeder free of sheath waves. If we insert the CB at the bottom of the station, it also works at first glance. However, apart from an RF-free station ground, we have gained nothing. The sheath waves on the feed line are still there. Nevertheless, some operators insert a second CB at the lower, stadium-side end and leave it at that. By the way: If a coaxial cable is laid underground on the way to the antenna feed point, the outer damping of the shielding is intentional and quite helpful. It prevents both the generation of sheath waves and the penetration of interference!

The CB and its versatile application possibilities

Yes, in addition to sheath wave suppression, there are other effective application examples in which an CB can do a “good job”.

Endfed antennas as a special feature

Not only on strictly symmetrically fed wire antennas such as dipoles and the like can be done with a CB & co. Also, the problem with the sheath waves appears increasingly in the popular Endfed antennas. These half-wave radiators (and their harmonic multiples thereof) are indeed “end-fed dipoles”, but are similar to the classic Fuchs antenna and are also end-fed long-wire antennas of defined length. For one of them, everything works without an CB because, by chance, the length of the feeder line and the ground conditions are just right. For the other, a so-called “pigtail” (short radial) is needed at least at the top of the ring core transformer's ground connection. Finally, the third one needs at least one CB at the top, inserted into the coaxial cable at a certain distance from the transformer unit, in order to get the antenna to work in terms of the absence of sheath waves. Others even report a second CB at the lower cable end. In this respect, Endfed antennas in particular offer a wide range of options for experimenting with CB.

Choke-Baluns in amateur radio – How to get rid of interference (5)

Choke-Baluns in amateur radio – How to get rid of interference (6)

When RF is supplied to unbalanced antenna tuners

The next case is somewhat different, beyond the sheath wave problem. An unsymmetrical antenna tuner can be “upgraded” in terms of HF by connecting the transceiver and antenna tuner directly to a CB. The two-wire line to the antenna is then to be connected with “one leg” to the single-pole output of the tuner and with the other conductor to the former ground. However, with the CB balanced, the tuner's ground now carries the “full RF”. Except perhaps in QRP operation, this is a problem when operating the antenna coupler! Because of the shock hazard from the high voltage, the antenna coupler would have to be designed in an isolated manner or modified afterwards. In principle, this is also possible with an asymmetrical automatic tuner. However, the supply line for the operating voltage and a possibly existing control line would then have to be integrated into the CB.

Discover the best antenna tuners on the market

CB in the fight against QRM

Where interference, or local QRM, reaches the receiver via the cable sheath, many radio amateurs report a significantly reduced interference level when an CB is inserted before the transceiver or receiver input.

Choke-Baluns in amateur radio – How to get rid of interference (7)

The perfect CB – designs and applications

When browsing our category, please note that CB are referred to as “choke baluns” in the English-speaking world. All choke baluns have an input and output impedance of 50 Ω, i.e. a transmission ratio of 1:1. In this design, they are very broadband and suitable for the entire shortwave range.

We offer two different designs, all of which we manufacture ourselves: in a pot shape, weatherproof encapsulated or in an extended shape, with ferrite tubes on a coaxial cable, each with connectors on both sides in PL259/SO-239 standard. The former design is intended primarily for looping directly at the base of antennas. The extended “cable version” is primarily designed for use within the station. When mounted in a weather-protected location, it can also be used outdoors. In this location, it is best placed for effective sheath wave suppression in any case.

Huge selection of Choke-Baluns at WiMo

Without experiments it does not work

The CB eliminates the symptoms for the time being, but not always the causes. Sometimes the sheath wave problem, for example in portable operation, is also only of a temporary nature. With the help of an CB, you can initially transmit without problems in portable operation. Nevertheless, if you set up your equipment for a longer period of time, you should continue to search for the causes and eliminate them – after all, amateur radio wouldn't be amateur radio without your own experiments!

November 2024, Alfred Klüß, DF2BC

Choke-Baluns in amateur radio – How to get rid of interference (8)Choke-Baluns in amateur radio – How to get rid of interference (9)Choke-Baluns in amateur radio – How to get rid of interference (10)

Share this post with friends!

Choke-Baluns in amateur radio – How to get rid of interference (2025)
Top Articles
Latest Posts
Recommended Articles
Article information

Author: Greg Kuvalis

Last Updated:

Views: 6334

Rating: 4.4 / 5 (75 voted)

Reviews: 82% of readers found this page helpful

Author information

Name: Greg Kuvalis

Birthday: 1996-12-20

Address: 53157 Trantow Inlet, Townemouth, FL 92564-0267

Phone: +68218650356656

Job: IT Representative

Hobby: Knitting, Amateur radio, Skiing, Running, Mountain biking, Slacklining, Electronics

Introduction: My name is Greg Kuvalis, I am a witty, spotless, beautiful, charming, delightful, thankful, beautiful person who loves writing and wants to share my knowledge and understanding with you.