Eliminating TX RF Feedback Problems
Fri, 19 Nov 1999
Malfunction of transceivers due to stray RF pickup is a common problem, especially when using end-fed antennas, high-impedance feedlines, or a poor ground counterpoise. Problems are especially likely during field operation, since ad-hoc short or random-length antennas are often erected whose characteristics are unknown, and since ground systems are likely to be minimal or non-existent. SSB mode is far worse in this regard, because the microphone audio line and mic circuitry may be very sensitive to RF. You can even get an RF burn from the mic in worst-case situations, not to mention from the chassis or other equipment used in the system.
Unless you get an actual RF burn or a HI CUR warning message, you MAY NOT KNOW that you have a problem with RF feedback. You should monitor your signal with another receiver or get an on-the-air report. Check your SSB signal on every band the first time you set up a new antenna system, especially if using an antenna tuner. If you do have problem with RF feedback--typically on one or two bands--reducing power may cure it. But this is not the best solution.
A better way to eliminating RF pickup is to electrically isolate the rig from the antenna by using a common-mode RF isolation choke (or "current balun") at either the rig or the antenna, as explained below.
NOTE: A traditional voltage balun (various turns ratios, usually wound on a large iron-powder core) will *not* necessarily perform the same function as an RF isolation choke. It may be completely ineffective in eliminating RF on the chassis.
A Real-Life Example from N6KR
I use an end-fed random wire on ALL bands at my home station--a single piece of #26 enamel wire about 40 feet long, tossed into a nearby pine tree and routed directly to the back of my K2 internal antenna tuner. (Never mind why my XYL talked me into this nearly invisible antenna.) This worked fine in CW mode, and I never knew I had an RF problem until I started using SSB. I then discovered that on ONE band--15 meters--significant RF signals were getting into my microphone circuits. Listening on another receiver I found that my signal was distorted at any power level above 2 watts. (This could happen with any rig, not just the K2.) It turned out that my mic was particularly sensitive to RF because it incorporated a DTMF IC.
I first tried a balun, with no luck at all. I then tried various arrangements of antenna and ground feed. I found that the only way I could completely eliminate the problem was by using an RF isolation choke right at the output of the rig. The details of this choke are given later.
Preventive Techniques for RF Pickup
The following steps are strongly recommended, especially if you use an end-fed random wire:
1. Build and use a common-mode RF choke at the rig's antenna output (see below).
2. Use the best possible ground counterpoise, preferably with two or more radials that are resonant (1/4 wavelength) on each band to be used.
3. Locate any antenna high off the ground and away from the rig when possible, using low-loss balanced feedline. If unbalanced (coaxial) feedline is to be used, place a coaxial RF choke at the antenna to remove common-mode RF voltages from the feedline. If NO feedline is used (direct
feed at the rig), an isolation choke at the rig can be used (see below).
4. Do not provide a 5V supply to your mike (at P1, front panel) unless it REALLY needs it. For example, the ICOM HM-23 mic has a DTMF tone generator IC that runs on 5V via pin 2. Since there's no need for DTMF in using the K2, pin 2 can be left unconnected. The 5V supply to the mic element goes only to pin 1 in this mic, via a resistor, and is thus independent of the DTMF IC. Each mic is different--check the mic schematic.
5. In really difficult cases you may need to put miniature 100 uH RF chokes in series with each signal and ground lead coming from the mic (at P1, front panel). But an RF isolation choke at the antenna output is likely to be much more effective.
Building a Low-Loss, RF Isolation Choke
There are many ways to build your own RF isolation choke. (They are also available commercially.) The one described here does a good job of eliminating RF on the chassis. Input on this topic from K2 builders is definitely welcome, and we'll provide an update later that includes more construction details.
Start with a large ferrite binocular core. (A large ferrite toroid will also work.) Wind several turns of small-diameter coax (such as RG-174) through the core. Connect the coax to BNC connectors (or connectors of your choice) at either end. THE TWO JACKS MUST NOT BE GROUNDED TO THE SAME POINT. This means that you must use either a plastic enclosure OR isolated jacks.
You can make a high-power isolation choke using a short length of RG58 coax with 25 or more ferrite toroid cores strung onto it. W2DU and W7EL have experimented extensively with such chokes (this is how the term "current balun" was popularized). Commercial isolation chokes found in QST typically use this construction, and are priced around $25-30.
Using the RF Isolation Choke
Connect one jack of the isolation choke to your K2 and the other to your antenna and RF ground system. DO NOT CONNECT THE SAME GROUND TO THE K2 CHASSIS -- this will defeat the purpose of the RF choke!
The choke will work with random length wires and coax-fed antennas, and with or without a tuner. If you use hi-Z balanced transmission line, you may *also* need a balun with 1:4 impedance ratio (or higher).
If the antenna is far away from the rig and fed with balanced line, you probably won't need the isolation choke at all. Same goes for coax-fed antennas that are well matched.
One important consideration in using the isolation choke is that it will exhibit some loss if you use a very poor ground system. If a poor ground is unavoidable, you may choose to leave the choke out, but do on-air tests to make sure RF feedback isn't a problem (especially if you're using SSB).
Power Supply Considerations when using an RF Isolation Choke
The use of a common-mode RF choke has important implications for your power supply. As long as the (-) lead of your battery or power supply is NOT connected to the *RF* ground system directly or via other equipment, there's no problem--the isolation choke will do its thing.
With most commercial power supplies, the (-) and (+) outputs of the supply are floating (high resistance) with respect to the power supply's chassis, and hence are isolated from house AC ground, water pipes, etc. (This is good news, since some stations use the house AC ground, perhaps unwittingly, as a ground counterpoise.) But if your power supply--homebrew or commercial--has the (-) lead connected to the chassis and this in turn is connected to the RF ground, you may need to revise some of your system wiring in order to achieve the desired isolation. One possibility is to use an low-resistance RF choke between the rig's chassis ground and the rest of your equipment. This will allow the ground to work at DC and 120Hz AC without causing problems for the RF isolation choke. A typical choke of this type might have 10 turns of #22 insulated wire on a large ferrite core.
The situation is the same in a hotel room. If you use the hotel's AC ground as an RF ground, make sure your power supply (-) lead is NOT connected to the same ground. The RF isolation choke will then be effective with any random-length antenna or a short vertical antenna connected directly to the back of the rig.
©1999, Elecraft LLC