RFI, troubles and resolutions

RFI, troubles and solutions

A low pass TVI filter for a HF transmitter ($20) sold by Thomas Distributing. Similar models are sold by WiMo in Germany.

Efficient protections against RFI (II)

When installing a new shack and your antenna, many RFI sources can be avoided if you respect the following simple rules. Of course some cannot always be applied for technical or simply financial reasons.

A perfect isolation is recommended between your radio and your electrical equipment (computers, etc). Often this solution reduces drastically RFI. Without speaking about the problem of external RFI sources described above, several noise sources can be internal to your house and must be identified : there are RFI coming from power lines and the ones coming from your ham equipment.

There are several ways to remove all these noises. Here is some advice published with the courtesy of Denis Kientzel, F6GIB.

The first solution is searching for equipotentials, this is the most efficient way to prevent electrical signals to reach a device offering a sink for the surge. In addition if all your room displays the same electrical potential it offers also an excellent protection against thunders. Of course such a protection request to install a good lightning protection too.

To protect all the ham shack from power line noises, begin by installing an isolation transformer on the power line and a powerline filter on all your station, and ground them separately from the home circuit. The filter will remove power spikes and other impurities from the AC device that powers your equipment. In addition, some powerline filters offer a good protection to your equipment against low energy lightning strikes and natural electromagnetic pulses (NEMP).

Then install the following protection devices : a small powerline filter on each power supply, a low voltage powerline filter on your radio transmitters, and a large braid to get an equipotential on all metal chassis, all being well grounded to an specific grounding system separated from the home ground circuit. I suggest you to read the next page dealing with lightning protection if you need more detail about the assembling of an amateur radio station grounding system.


Side by side, it is easy to see the difference between RG174, RG58 and other RG 213 coax cables. Disadvantage of this last, it is harder to bend. Its challenger is the famous Belden 9913, more expensive but very well protected against RFI and well adapted to very high frequencies. At right, the internal structure showing the insulation and the copper or aluminum mesh of different coax cables.

VHF RFI in dish antennas

Using high power at very high frequency (VHF, SHF), your own equipment can be the source of RFI in your other devices, mainly through the coaxial feed line. I explain. In theory a coaxial shouldn't radiate as its objective is to transport the signal and not to play the role of an antenna, Hi ! However using a balanced antenna (e.g. a dipole) with an unbalanced feed line like a coaxial - a typical design using a Windom antenna - some RF energy will travel on the outer side of the shield braid and interfere with other devices. If that happens, begin by installing a 1:1 balun transformer or a common-mode choke balun near the antenna. It will reduce or eliminate the RF radiation from the coaxial. In this regard you will find on the Internet tens of cheap solutions to easily made such baluns;

Now, if you work with a VHF antenna for your ham activities and a dish antenna to watch at the TV, if they didn't match correctly to their feed line impedance, you can get interference between the two equipments, and your family might "spy" you on the air watching at the TV, Hi ! Usually, a satellite antenna displays a high gain (20-30 dBi) and should not see radiation from the lower frequency often used at low or medium power. If you experiment RFI in your TV for example while transmitting on 2m, there are several possible solutions, common to many RFI problems :

- Install a common-mode choke on the dish coaxial close to the antenna (several ferrite beads, a toroid or better, a model using wire bound on a ferrite core).

- Install a high or low-pass filter on the dish feed line. Characteristics of this filter must be checked with the antenna manufacturer to avoid to cut the downlink IF frequency, the DC signal that usually powers the preamp/converter or even your working frequency...

- At last, in the case of a leakage, the problem is more complex and sometimes intermittent. Try to separate of a few meters the VHF from the dish coaxial. There is no optimum distance, the farther the best.

Other suggestions to eliminate RFI

Generally speaking, to minimise harmonic output and eliminate TVI and other "unsollicited" RFI emissions, the low-pass filter inserted as close as possible to your RTX (and even a serial double low pass for HF bands) is recommended. That can be very useful if you have neighbours living less than 100m from your antenna system and if you work with high power or directive antenna. If you point for example directly your beam to your neighbour's building, or send tunes of 1 kW in a vertical, there are chances to create RFI in some nearby installations.

The second solution is using optocoupling (yes using the light) for controls and switches, and transformers adapted to your RTX impedance for signals. These components are available to any electronic shop. This solution is easy to install and efficient in suppressing most domestic sources of RFI.

Among suppliers, there are Cotubex in Belgium (Brussels), Go Tronics and Lextronic in France where you can purchase some 4N35 optocouplers and 1:1 transformers 600 ohms.

The coaxial cable : where losses give a negative gain

The ordinary 50 ohms coaxial cable RG-58 was invented at World War II time by the US army and knewn a huge popularity after war in the radio amateur community.

Today the "coax" is often used to feed antennas. How it looks like ? It comes with an outer jacket in rubber, black colored, it is quite thick (around 5 mm) and not always easy to manipulate on short lengths. This coaxial is also twice to three times more expensive that the simple electric wire (about 60 € for a 100 m roll).

At least 11 types of coax are available from RG-8/U, RG-213 to Belden 9913.

This last model has a double shield against RFI and offers a very low attenuation (0.7 dB/ft vs 2.35 dB/ft for the RG-58) and a high velocity factor (0.84 vs 0.66). It is very efficient (low loss) but also stiff (you cannot really bend it) and expensive ! It is mainly used by cabling operators and permanent hams installations. You cannot confuse it with another cable: it is silver colored, hard, and is ~20 mm thick.

At last you can already reduce your RFI in carrying out the next actions :

- Move the source of interference as far as you can from your receiver (RTX or antenna) or vice-versa as the noise decreases proportionaly to the square of the distance;

- Avoid placing your antenna in the 15 meters around an aerial high power line (few hundred kV) to prevent corona noises;

- Avoid placing your antenna in the 15 meters around a distribution aerial line (few kV) to prevent EMI and arcing;

- Keep the feeder perpendicular to the antenna (dipole) to minimize the amont of RF energy appearing on the coaxial;

- Install a common-mode choke balun at the end of the feed line, close to the antenna to avoid signals to couple with the ground and to prevent RF radiation from the coaxial. This choke must be waterproofed;

- Place a large common-mode choke around your power lines to reduce RFI or insert a MFJ noise reducer on the feed line;

- Use a double shielded cabling system for all devices installed in the ham shack and nearby rooms or replace your coaxial with a coax RG-213, its AirCom equivalent or better, with a Belden 9913 or a Hardline coaxial cable;

- Use wireless keyboard and mouse and plasma display (however some are also strong RFI emitters).

If you have doubts about this project contact a specialist and simulate together the wiring on paper before investing the least money because a bad grounding for example is worst that no grounding at all. A bad high or low-pass filter can prevent signal or DC to pass through your feed line as well.

Desperately searching a miracle solution

Having myself experimented RFI at several occasions, I wondered if I could ever find a solution to definitely solve this problem. Excepted in changing my location for a quieter spot, I cannot move my devices a few meters away, and working in another room didn't change the problem. Users told me also that installing small chokes on all interfering wires would probably give few results as these coils are generally not powerful enough to cancel interferences coming from emitters much more powerful. Installing another type of antenna should probably not solve the problem either. So at short delay I needed a miracle solution...

RFI cancelers

Two "noise receivers" that detect and amplify the noise and inject it out of phase into the receiver, canceling or greatly diminishing (up to 40 dB) the offending signal. They are most effective on a synchronous (stable in frequency and phase) noise signal, spot and not widespread all over the spectrum, but they have to be evaluated in your peculiar working conditions. Above MFJ-1026 and below Timewave ANC-4. Their price is on par ($180-210).

If you are not living too close of a HV power line, a huge leaking transformer or BPL source - for which I'm sorry to tell you there are no other solution than moving away -, in all other cases if you cannot apply the simple rules listed above to remove RFI, here is probably one of the best solution : install the MFJ-1026 Deluxe Interference & Noise Canceler; this is a true "miracle box". I don't say it will remove all interferences you will hear, but surely most of them.

I installed this unit recently and according my tests this device cancels quite well most interferences, dropping sometimes their intensity from S-9 to as small as S-1. Very effective when installed properly, it works on all HF bands with a bandwidth of about 40 kHz. It works best when noise comes from a specific direction and if you use a secondary antenna, a simple wire or whip, working with the same polarization as your main aerial.

Its secret is to cancel noises in phase opposition in shifting their phase by 180° and reinjecting them in the circuit. The result is amazing and can even amplify weaks Q signals if you use two receive antennas and the Phase control. If you are interested in this rig I suggest you to read my review for more detail. This accessory cost about $180 plus handling/shipping. It is sold in France at GES or in Germany at Difona for 290 € charge included.

Reviews : MFJ-1026 Deluxe Interference & Noise Canceler (on this site)

Timewave ANC-4 Antenna Noise Canceler (on eHam)

Among competitors of this product, Timewave sells a device named Antenna Noise Canceller ANC-4 and WiMo in Germany sold a QRM Eliminator. Both work the same way as the MFJ model. They request an auxiliary receiving antenna, like a longwire or a short vertical. The unwanted signal is received with both antennas resulting in different phases. The WiMo model costs about 160 € charge included.

If you are skeptical about its utility, WiMo accepts to rent you a unit for running your own test - just give him a call.

At last, if you are tired or unable to locate the source of RFI that disturbes your amateur bands, you may always ask for the help of an expert equipped of a spectrum analyzer and dedicated devices (Search under "Government", "Environment", "Pollution" in yellow pages) or question the administration of your district. Using high-tech devices and highly specialized skills their service is expensive. To give you an idea, to locate one source of noise and get a technical report (e.g. 1 spot located one floor up in less than 24 hours) the mean price is about 400 € and probably more if specialists have to search for it in a few places around your home. But advantage, you have all chances to locate the source of noise and to get a solution to suppress it.

For more information

An RFI Problem with a Happy Ending, by TF3JB, eHam.net

The Mitigation of Radio Noise from External Sources at Radio Receiving Sites (6 MB PDF), US Navy

Radiofrequency Meters

Electronics Tutorials

Zapchecker (RFI detector)

PCE EM-30 (magnetometer)

Electrical power products