Getting Started on RTTY
RFI Page
Radio frequency interference (RFI) can be a problem when running RTTY. You may never have an RFI problem when running SSB or CW or not notice it anyway, but all of a sudden you find RFI creeping into your system when running RTTY. Why? Because RTTY is full keyed down power out at 100% duty cycle. This does not happen during SSB or CW.
When talking about RFI, there are two scenarios. Either your transmitter is causing the RFI to your computer and/or associated cabling or your computer is causing RFI to your radio. The discussion on this page is concerned with the first circumstance of your transmitter causing RFI to your RTTY setup. Common symptoms of your transmit signal causing RFI to your RTTY setup are erratic keyboard & mouse operation and distorted or erratic RTTY transmissions either AFSK or FSK.
You will normally only get RFI with RTTY when your antennas are very close to
your shack and you run a linear amplifier. But as most of us know, RFI can
happen anywhere. The most common devices affected by RFI are the computer
keyboard, mouse, audio cables, FSK cables and network cables (CAT 5).
RFI can be eliminated rather easily in most cases. Ferrite toroids and
rods work wonders in keeping RF out of cables going into your radio and
computer. Before installing toroids or rods, check all your antenna cable
connectors to make sure you don't have a bad connection. Check your antenna
cables to make sure they are in good condition and not leaking RF. Also, use
shielded cable on all your cabling for audio or FSK. And make sure all
equipment is properly grounded. It's best to ground each piece of
equipment separately and to bring the ground wires back to a central bus bar
which is connected directly to a ground rod or other suitable ground.
Ground wires should be kept to the shortest length possible. If shielding
and grounding doesn't get rid of the RFI, the next step is to install either
ferrite or iron powder rods or toroids.
In order for these toroids or rods to eliminate RFI caused by Amateur HF transmissions, they must be made of the correct material. The best source to learn about ferrite & iron ferrite devices is CWS ByteMark. CWS ByteMark is one of the largest distributors of ferrite & iron core toroids, beads, rods and just about anything you can think of made of these materials. Their website is full of all kinds of information concerning which ones work best for RFI and EMI suppression.
The CWS ByteMark site contains so much information, I found it difficult to navigate. And even though they have a link to a page specifically for hams, I found section 2 of their Specifications and Downloads page the most beneficial. In particular the PDF files on EMI/RFI Suppression (which shows a chart showing each material and which frequencies they suppress), Ferrite for RFI, and Ferrite Cores for RFI Suppression should be read.
Ferrite material is recommended for better RFI suppression than iron core material although I have had success with both. And for simplicity sake, ferrite material 73 is probably the best for suppression in the 1 to 40 MHz range. Having said this, I also want to add that I have had success with material not actually known for suppression in the HF range. For example, ferrite rods are normally made of material 33 or 61. Material 33 is used for low frequency antennas and material 61 is known for RF suppression above 200 MHz. I've used rods with great success in eliminating RFI. I don't know the reason for this, they just worked. I like rods better than toroids because it's easier to wind cable around.
Despite being popular items, I have not had much success with round cable suppression cores as shown below.
Instead, I have better success with toroids and rods as shown here.
I'm not sure why this is. The major difference between the three are that with rods and toroids, the cable is wrapped around the ferrite material whereas with cores, the ferrite is around the cable.
I have also found that the bigger the rod or toroid, the better the suppression. When choosing a toroid, be sure you get one that is big enough for your cable, especially if you have some sort of connector already installed on the end.
Cable is wrapped around toroids and rods as shown below.
The cable shown above may be a little "over kill" but it might be necessary to put a ferrite device at each end of a cable to completely eliminate RFI. In most cases, I have found that one toroid or rod will work. Placement of the toroid or rod may be important as well, but not always. For instance, on keyboard cables, I like to put use a rod instead of a toroid and I place it near the input to the computer.
How do you know if you have RFI into your keyboard? You can
tell when you are keyed up and typing away but the characters you type are not
the characters going out on the air. Wrap the keyboard cable around a ferrite
rod and it will eliminate the RFI. The same goes for your mouse. Although I
can't recall ever having RFI problems with my mouse, I suppose it could happen.
Try a short rod.
If you run AFSK, I highly suggest using a toroid on the cable used for transmit
audio from the sound card to the audio input of your radio. RF on the transmit
audio cable will cause all kinds of problems. Even though I operate FSK on
RTTY, I have to use an audio transmit cable when running PSK31. When I first
started PSK31, I had bad RFI problems on this cable, even running low power
(less than 50 watts). A ferrite rod took care of that problem and now I can run
100 watts on any band.
One symptom of RFI on a transmit audio cable can be seen by the RF power output
jumping around erratically. When transmitting RTTY, your RF power output should
be constant or fluctuate only slightly. If you have large fluctuations in
your transmit carrier, you may be experiencing RFI.
When transmitting AFSK, you should have the monitor turned on your radio so you
can hear what the transmit audio sounds like. If the tones sound distorted,
then RFI is probably the culprit.
You could have RFI on the transmit audio cable and not know it. If you have a
second radio, it's always a good idea to listen to what your signal sounds like.
Or meet someone on the air who can tell you if your signal is clean or not.
It is possible to get RF into your computer. Many times, this is caused by RF
getting onto the AC cable and into the power supply. An RF suppression AC power
strip can cure the problem or use a toroid on the AC power cord. RF might be
getting straight into the computer or into the sound card. Not being an expert
on RFI, I can only suggest you check other resources for information on RFI
suppression. There is an RFI reflector on
contesting.com that may assist in
getting answers to your RFI questions.
It's important to recognize and fix RFI problems in order to ensure smooth RTTY
operation.