Solving Problems of RF FEEDBACK
Are you getting stung (RF burns) off of the transmitter and accessories? This problem usually occurs when you touch anything connected to the transmitter while transmitting. Such is an indication of RF power that is not radiated out of the antenna and is considered as a loss of transmitter power. This can cause interference to other equipment such as TV sets and stereo equipment. In this Part 1 article of the 2-part series, we will attempt to define the reasons for such interference and suggest ways to minimize or eliminate the problem during radio amateur operations.
Interferences of the First Kind
Let’s begin our analysis of the reasons of the “first kind” for RF burns and interference defined as being that which is caused by direct radiation from the antenna and absorbed by surrounding objects. All conducting material around an antenna will absorb RF energy from the antenna (Figure 1). The amount of absorption of any object will depend on the ratio of the size of the object to the wavelength of the frequency transmitted. The distance from the antenna and the amount of power that the transmitter is running will also determine the amount of transmitted energy absorbed by these objects, including by the operator. This is easy to demonstrate by connecting an RF voltmeter to the transmitter and any other metallic object in the immediate area of the antenna and ground system (Figure 2).
You could also connect one probe of the RF voltmeter to the operator’s body and get a reading. The voltage and phase of the measured voltages will differ in both phase and magnitude, depending on the variables mentioned above. This is why when the radio amateur touches the transmitter or other object within the RF field; the operator will be “stung” due to the “potential difference” of RF energy present. It does not matter whether the operator or the transmitter has the greater RF potential; it is the difference that causes the problem.
As a simple test, hold an NE-2 neon bulb barehanded and key the transmitter. The bulb will light up when there is enough RF being radiated from the equipment and surrounding area objects. This is particularly true if one of the NE-2 electrodes is touched to the transmitter case, or to any other metallic object in the vicinity, having a dimension that would resonate at the wavelength being transmitted. To determine how much “potential” each metallic object has, the use of an RF voltmeter will display the exact voltage.
In radio amateur operations this type of “interference” occurs mainly in the ranges of 17-6 meters, but is not limited to these ranges. So, why does the non-metallic human body experience these “stings”? Especially in these ranges, a peculiarity of the human body is that it makes a good antenna and can even resonate at some frequencies in these wavelengths. With rather low transmitter power (up to 50 watts), the condition can exist where significant RF voltage on the human body can be detected. This is primarily caused by the presence of a significant RF field within the immediate location of the transmitter and the operator. It is noted, however, that almost any frequency will generate an induced voltage of some level.
Whenever a wire antenna is directly connected to a transmitter or amplifier absent of a feed line, an electromagnetic field is created during transmission. A high electromagnetic field is created whenever the antenna is located close to the radio set. The more power used, the greater the electromagnetic field.
Also, when the major directional signal lobes of the antenna pass through the shack, this will cause interferences to the instrumentation, other equipment and to the body of the operator. This is due to the induced field that will generate voltage within the conductors present in the shack, and the human body becomes one of these “conductors”. RF burns are the result when the body comes in contact with the other “conductors”.
To eliminate the problem of interference, it is important not use a random length antenna or antennas without feed lines that are suitable for their primary range of operation. If it is absolutely impossible to use any other kind of antenna and some type of substitute antenna is used, then the power into the antenna should be kept low. The power most likely should not exceed 10 watts or at least be maintained at a level at which the safety of the operator and equipment would not be at risk. CAUTION: Pay close attention to this caveat regarding the use of low power! Start low and ease upward is the best approach.
Coax Feed Lines
Should RF interferences occur when using coaxial cable feed lines, it is necessary to reduce the power to a safe level to prevent RF burns occurring whenever the transmitter is touched. Whenever the problem is severe with the use of coax, the antenna either should be moved or the directional characteristics of the antenna changed if possible. If all else fails, try changing the transmitter location as well. Also, try to improve the ground system.
To combat interferences at professional communications sites that do both receiving and transmitting, the equipment is commonly placed in an electrically shielded and well-grounded building. The windows usually have metal shielding grids installed to increase the shielding. Even with this extensive amount of shielding, sometimes the RF leakage still gets inside and causes interferences of the first kind.
Extensive shielding of buildings is the most effective way to eliminate the problem of interferences of the first kind. We cannot equalize the RF potentials present on the transmitter and operator by using a common “ground” (Figure 3) like those used for electrical power (50-60 Hz) in the low frequency ranges commonly found in the household. Due to the distance “D” (between transceiver and the operator and the defined wire lengths in the ground system, L1 and L2 from the transceiver and the operator to the RF ground) and resistance “R” in ground between transceiver and the operator, there will always be an HF voltage difference between the body of the operator and the transceiver. This potential difference will cause the operator to get an RF burn if he touches the transceiver.
If the jumper L3 (Figure 4) is installed between the body of the radio operator and the transceiver, the length of the jumper “D” will show both inductance and resistance. This will cause a voltage and phase difference between the transceiver and the radio operator. There will still be an RF voltage difference between the operator and the transceiver.
Usually, it is not practical to use full electrical shielding of a radio set in radio amateur situations. But if the radio equipment is installed in a house made of reinforced concrete, which exhibits strong absorption of RF power, substantial shielding is already in place. As with commercial facilities, adding copper window screens provides more shielding and contributes to the reduction of the interferences of the first kind. To increase the shielding effect, connect these screens to the grounding system of the building such as the water pipes or steel reinforcement in the walls (if grounded), etc. If these “grounds” are not well grounded, then some alternate RF grounding system will be needed.
Compensation Method
The radio amateur may use the “compensation method” of eliminating RF from the case of the transceiver to help stop RF burns from occurring.This method consists of phasing out the RF potential between the transmitter and the operator of the set. It is done by properly feeding back into the transceiver an RF potential of the correct phase and voltage. By “tuning” the phase and voltage, at some point the voltage will cancel out the potential on the transceiver and the operator will not be “stung” when touching the transceiver or accessories (see Figure 5). As shown by the diagram, the length of wire L should equal 1.5 –3 meters, which can be determined by experimentation. It is desirable to have this wire suspended in free space such as hanging one end from the ceiling. It is NOT desirable to place this wire on the floor.
This conductor may be connected to an “artificial ground” such as the MFJ-931 and then the shortest conductor possible connects it to the transceiver.
With another short conductor, the RF voltmeter is connected to the transceiver and body of the operator (see Figure 5). The RF voltmeter should be battery operated. A homebrew RF probe may be used with a commercial digital voltmeter. The diagram of an RF probe connected to a commercial digital voltmeter is shown in Figure 6.
Tuning procedure for an “artificial ground” such as the MFJ-931 is as follows:
1. Set the variable inductance to minimum
2. Set the variable capacitance to maximum
3. Set the transceiver to the frequency where “stinging by RF” takes place and transmit
4. Adjust the inductor and capacitor for minimum RF energy on the transceiver using an RF voltmeter if available
In many cases, the RF can be eliminated from the transceiver case completely on some frequencies and lowered to a minimal “stinging” level on others.
The setup of this system requires the use ONLY of a RF voltmeter. The current meter used in the “artificial ground” type of device (like the MFJ unit) should not be used. This method effectively eliminates RF burns. Remember that altering the setup of the radio transceiver and the antenna can change the tuning of the artificial ground and cause the RF burn problem of the first kind to reappear. Therefore, when installing a new radio system, or changing the setup, the above methods for eliminating RF burns should be applied again, if necessary.
Conclusion
Now, in conclusion about interferences of the first kind:
1. The radiation field of the antenna couples RF energy with the operator and the transceiver causes the interference of the first kind.
2. In radio amateur practice these interference problems are likely to occur in most HF ranges, but most often in 17-6 meters
3. Interference of the first kind can be eliminated by shielding the location of the operating position
4. Use matched antennas with coaxial cable feed lines
5. Carefully consider sufficient space for placement of transmitting antennas near shack
6. Use lower power for poorly installed antennas
7. Tune out RF potentials between the transceiver and the operator
Solving Problems of RF FEEDBACK - Part 2
Interferences of Second Kind
The presence of RF on the feed line is the most common and widespread interference that occurs when transmitting. However it is the easiest to eliminate. As most may have experienced at times, RF on the feed line that feeds back into the transmitting equipment causes painful burns or stings when the microphone or case of the transmitting equipment is touched. Also, this RF can cause interference with other electronic equipment such as the stereo and TV.
Figure 1 Interference of electromagnetic power radiated transmitting antenna on the antenna feeder
Because of certain factors, there is leakage of the HF current from an antenna on the outer conductor (braid) of the coaxial cable as well as the and from the center conductor wire. As a result, there will be RF voltage on the outer braid of the cable. A typical transceiver/antenna system is presented in Figure 4. You can see the radiation illustrated as “A” flowing on the outside braid of the cable “F” by its length “L” depicting the currents “I”.
Figure 2 Interference on the unsymmetrical disposed feeder of an antenna
The connection of the coaxial cable to the antenna will cause the antenna system to be unbalanced and then further unbalancing of the antenna will be caused by any surrounding metallic objects and this will cause RF to flow on the outside of the cable. In this case the transceiver becomes a load for the RF on the outside of the cable represented by a resistive load R, and the magnitude of this resistance R will depend on many factors.
Figure 3 Interference on the feeder of a vertical asymmetrical antenna
Now the reasons for the RF “V” (Figure 4) voltages on the feed line must be investigated as well as methods to lower this voltage and prevent or minimize this problem. These “leakage” currents may be eliminated with the help of 5 to 10 ferrite toroids placed on the coaxial antenna cable (Figure 5). These ferrite toroids function like an RF choke and will keep the RF from flowing down to the cable causing the level of the RF voltage to be considerably reduced.
Figure 4 The transceiver/antenna feeder system
Another important factor involved is the length of the feed line from the antenna to the transceiver. There will be a null of RF voltage on the end of the coaxial cable if the electrical length “L” of the feed line is a multiple of a half wavelength of the frequency on which the antenna operates. Or, if the combination of devices between the transceiver, antenna tuner and SWR bridge create conditions that approach a voltage null there will be a null of RF voltage on the end of the coaxial cable. This null at the end of the cable will lower the RF voltages on the transceiver even at low powers into the antenna and minimize the chances of getting an RF burn.
Figure 5 RF chokes from ferrite rings
These interferences of the second kind may appear on all amateur ranges, but most likely to occur in the ranges of 160-40 meters. This is because in these ranges a half wavelength will be in the ranges of 80-20 meters. This will place the resonance length of the cable from the transceiver to the antenna in the ranges of the corresponding radio amateur band. These cable lengths can occur when the antenna is installed on the roof of a multi-story apartment house or large private home (Figure 6).
In some cases with an installation of this type the length of the grounding wire from the radio installation can resonate at the transmitting frequency causing the equipment to become “hot” again. More stings and burns result. However, rearranging the installation and changing the length of the ground wire can eliminate this problem.
Figure 6 Antenna stray interferences of the second kind
In the upper radio amateur frequencies of 20-6 meters, the braid of the coaxial cable absorbs reflections from metallic objects that surround the antenna. This voltage causes different phases and voltage amplitudes to appear on the cable (Figure 7). In most cases, this voltage seldom reaches levels that would cause burns or stinging when the transceiver or other equipment is touched. For an object to reflect the radio signal, the object should have dimensions equivalent to the wavelength of the frequency of operation. These dimensions should not be less than a half wavelength of the operating frequency. In the frequency ranges of 20-6 meters, there are many objects that have a physical length of 10-3 meters. Automobiles, metallic fences, metal window frames and other similar metallic components of building all can resonate in these frequency ranges. On the lower bands this is not as likely to occur since the wavelengths are much greater.
Figure 7 Path of interferences on antennas in ranges of 6-20 meters
Other factors that can influence the magnitude of the RF voltage induced on the coaxial feed line are the length of the cable and its surroundings. If the feed line is a resonant length and there is an absence of metallic objects in the vicinity that can re radiate or cause losses in the outer braid of the coax, then there will be significant level of RF voltages induced into the coax causing problems with RF (see Figure 8)
Figure 8 Resonance distribution of voltage on coaxial braid
Should the cable feed line be located in an RF-absorbing environment, and runs through or along steel reinforced walls, or passes through metal conduit, the RF voltage existing on the cable braid will absorbed or eliminated. If the cable is located on the outside of such steel reinforced concrete buildings, a phase shift is caused by the re-radiation from the steel in the walls resulting in a further reduction of RF voltage on the cable braid. Further, the standing waves on the antenna end of the outer braid are cancelled out by the progressive wave from the transceiver end of the cable. This causes the RF to be cancelled out at the transceiver (see Figure 9)
Figure 9 Distribution of directed voltage on cable braid in RF-absorbing environment
Using the information above, it is now time to draw some conclusions as to how to lower the interferences of the second kind:
First, the proper installation of the coaxial cable feed line going to the antenna must be done in such a manner that the currents that can be induced in the feed line are minimized.
Second, the use of toroidal RF chokes on the feed lines should be done as standard practice. These serve as an RF choke on the cables and will prevent RF currents from flowing back to the transceiver (Figure 5)
Third, the feed line length should not be a mulitiple of a half wavelength on the frequencies from 160-40 meters.
Fourth, the installation of the coaxial cable should be in the presence of conducting materials that will aid in the elimination of RF on the outer braid of the cable. Along the wall of a building with reinforcing metal in the walls such as a concrete wall will aid in the RF absorption of the currents that may flow on the outer braid of the coaxial cable. Also, the use of aluminum or copper foil under the coax at the transceiver will eliminate the RF close to the transceiver (Figure 10).
Figure 10 Preferred placement of coaxial cable at a radio installation site
Figure 11 Hooking up to an
artificial ground device
If the above methods will not adequately reduce the RF problem, it will be necessary to use an “artificial ground” to further eliminate the RF on the transceiver. The artificial ground should be connected right at the antenna connection with another clamp over the braid of the coaxial connector (Figure 11).
Some of the more professional connectors have this capability. On some transceivers the ground connection is not very good and this does not allow the RF to be eliminated from the transceiver chassis. The use of the artificial ground allows the tuning of the “antenna system” caused by the outer braid of the coaxial cable to be tuned for minimum RF on the transceiver chassis (Figure 12).
Figure 12 Distribution of voltage and
current in the correctly
tuned system for
“hot” contact elimination
Some radio amateurs believe that the use of a quarter-wave counterpoise cut for the frequency where the “stinging” is most severe will reduce the problem. Instead, this will sometimes make the problem worse. It may reduce the RF on the transceiver, but for maximum reduction of RF on the transceiver, it is necessary to use a counterpoise tuned to the correct frequency. In this case, as pointed out earlier, the entire ground system of the transceiver will be tuned for minimum RF on the transceiver.
Tuning of the ground system should be done on the maximum RF current going to device “artificial ground”. Initially the artificial ground inductance is set at minimum and the artificial ground capacitance is set at maximum. The counterpoise is then connected to the artificial ground and the tuning is done by gradually increasing the variable inductance. Then, the variable capacitor is tuned for maximum indication on the meter of the artificial ground. The counterpoise should be up to 10 meters long on 160 meters and a length of 5-10 meters on the other amateur frequencies. The end of the counterpoise wire will have a high RF voltage and should be insulated accordingly. The short counterpoise can be placed on the floor around the walls of the room where the transceiver is installed. The larger counterpoise of 10-meter length may be minimized in a large room. It would be convenient to use the outer braid of a thick coaxial cable as the counterpoise. However, the counterpoise will radiate very strongly where the artificial ground is located. This will cause a much greater level of TVI and RFI in general when operating.
Interferences of the Third Kind
The most harmless and seldom encountered interferences are the ones that can be easily eliminated with the help of the “artificial ground”. Now, what are these particular interferences of the third kind and how can they be eliminated?
When the transmitter is on the air, the stray capacitance between the coils in the output stages of the transmitter couples a small amount of the HF voltage (current) into the case and chassis of the transmitter. The normal bypass capacitors are not able to provide effective bypassing of the HF voltage and there will be some small amount to be found on the transmitter case. Because of this, when transmitting, there will be enough HF voltage on the chassis to cause “stinging” when touched even if the transmitter is connected to a dummy load. This happens most frequently in the ranges of 15-6 meters.
Under this circumstance the transceiver case, with all of its interconnecting wires and cabling to the antenna of the feed line, become part of the radiating antenna system and becomes hot due to the stray RF leakage currents (Figure 13). If the resonance of the entire system falls into any amateur range then the entire system will radiate and the transceiver will come “hot”. When the “artificial ground” device is connected, it becomes possible to change the entire parameters in the “stray antenna system” (Figure 14). Proper tuning will eliminate the HF currents from the transceiver.
Figure 13 Radiation by the transceiver caused by interferences of the third kind
Figure 14 Elimination of interferences of the third kind
For maximum effectiveness of the “artificial ground” it is necessary to find a point on the body where the maximum removal of RF-energy is achieved. This is the best place to connect the quarter-wave counterpoise. The tuning of the “artificial ground” device and the counterpoises is similar to the method used to eliminate interferences of the second kind.
It should be noted that the elimination of the interferences of the third kind through the use of an artificial ground will result in an increased level of TVI and RFI to the transceiver.
By using RF absorbing paint on the inside of the transceiver case, the interferences of the third kind can be considerably reduced. These coatings are present in the majority of the commercial transmitters.
After reading this article it should be easy for any radio amateur to very simply and easily eliminate these most or all of the RF problems in the shack and to stop the stinging feedback. Unfortunately, this is not necessarily so. In many cases at the installation of the transceiver, there will be numerous conditions that will cause several of these RF feedback interferences take place and cause stinging to occur when the equipment is touched. It takes experience to solve these problems and it is not often easy to do. Many times one countermeasure to eliminate one problem will cause another problem to occur which in turn will need to be resolved. This serves to make the overall solution to the RF feedback problems to become even more complex. Even at professional transmitter sites this is a major concern and the effort of eliminating unwanted RF is always a serious and difficult task and one that requires constant attention at all times.
This is an important task that must be undertaken when the intrusive situation appears at any radio installation. With adequate experience and by careful analysis, the different kinds of RF induced interferences can be eliminated from any radio amateur station with the methods described in this series of articles.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 28th May 2024