Whenever two or more transceivers are used in close proximity, there is some level of interference involved. This level can vary from practically no problem at all to actually overloading the receiver.
Don’t get me wrong–modern radios have come a long way. But even the best transmitters produce undesired products along with a main signal. Transmitters produce substantial output at the operating frequency as well as multiples of the operating frequency. The 2nd and 3rd harmonics are usually the worst. For example, a 7 MHz transmitter will produce harmonics at 14 and 21 MHz with enough energy to cause interference to someone trying to listen on these bands. In addition, two forms of random noise are also generated in a transmitter–phase noise and wideband transmitted noise.
RF interference might be compared to a stubborn water leak–hard to find, traveling along several possible paths, and very annoying. So how do we deal with the problem? Here are 10 strategies to help rein in the RF chaos during Field Day.
Bandpass Filters
This solution has been mentioned before in OAB blogs such as Don’t Forget the Band Pass Filters on Field Day by Wayne, KE8JFW. Band Pass Filters are multistage devices designed to specifically limit the RF band pass to a single Amateur band. They eliminate image and harmonic RF interference issues for dedicated monoband operations in multi-radio contesting and Field Day environments.
These passive filters provide the additional isolation needed to allow multiple transceivers to operate, letting your transceiver play well with others. Remember that the single units are band-specific and you should have one for each radio in operation. Bandpass filters will not help much when operating CW, phone, digital, and/or GOTA radios on the same band. This is when the suggestions that follow will become your plan B.
Maximum Antenna Isolation
Obviously, more spacing between antennas will reduce the power picked up by the receiving antenna. Keeping your same-band CW and SSB stations and their antennas at opposite ends of the 1,000 foot circle will make a significant difference. It also doesn’t hurt to keep your individual rigs as far apart as practical, but reasonably close to their own antennas.
Antenna Orientation
If your Field Day layout allows, locate the antennas with all of the elements in line. The ends of the dipoles are the null points, with maximum radiation perpendicular to the antenna elements. Are Yagis part of your antenna arsenal? They should also follow the same plan and be pointed the same direction and not at each other.
Cross polarization of antennas is another solution. You might try using a vertical antenna for one radio, placing it in the null of the horizontal antenna’s pattern and separating both as far apart as possible. Avoid antennas with mixed horizontal and vertical polarizations, including G5RV, Carolina Windom, and sloper/inverted L antennas.
Monoband Antennas
Use monoband antennas to help minimize coupling on undesired bands. They don’t radiate or receive out-of-band signals as efficiently compared to those within their designed frequency range.
Baluns and RF Chokes
Baluns and RF chokes will reduce common mode currents on feedlines. The choke/1:1 balun stops the feedline from conducting common-mode current and becoming an antenna or having RF current show up at the radio or connected devices. They should be installed near the feedpoint.
Receiver Settings
Set receiver controls to minimize interference. Those with an asterisk below are adjustable and should be optimized for operating conditions.
Preamp off AGC on*
Attenuator on Band Pass Tuning on*
Noise Blanker off Noise Reduction on*
RF Gain on* (minimum) Notch Filter on*
High-Performance Transceivers
To manage interference on Field Day with operations on the same band, transmitter purity and low composite transmit noise need to be minimized. Using transceivers that generate a minimum of transmitter phase noise is very important. Transmitters with high phase noise generate broadband noise across large sections of the spectrum that are difficult to be filtered out by nearby receivers.
On the receiver side, transceivers with a bulletproof front end are essential. With the potential for multiple transmitters being on the air at the same time as one station is trying to hear a weak signal, the receiver must be able to tolerate very high “out-of-band” RF levels. Look for receivers with high receiver dynamic range and reciprocal mixing dynamic range (RMDR). What radios fit this description? Look no further than the Rob Sherwood Receiver Test data. Some examples of radios that appear high on the list include Kenwood TS-590SG, Elecraft K3/K3S, Icom 7300, and Yaesu FTDX10.
Individual Station Grounds
All power generators/supplies and rigs need a good earth ground for AC power safety and interference mitigation. When you’re in a hurry to set up, there’s the temptation to ground all the rigs at the FD site together (daisy-chained). Take the time to ground each station individually.
First, bond all your equipment together (radio, tuner, antenna switch, power supply, etc.) using the shortest, heaviest, straightest, lowest impedance material you can. Tinned copper braid works well for this purpose. The easiest configuration is the “star” composed of separate connections from each piece of equipment to a common point. Then run the shortest line possible to your ground rod–your ground rod. It’s okay not to share.
Filter AC Lines
Portable operation is fun but also creates challenges to radio operators in the form of increased receiver noise, RF interference from adjacent sources including nearby transmitters, and RFI from portable generators. AC power strips such as the MFJ-1164B and ECG Products’ EMF-63A have a built-in choke that reduces EMF/RFI noise from 150 KHz to 30 MHz.
Also consider snap-on ferrite split beads on the leads from your logging computer and toroids at the generator output line.
QRP Digital to Minimize Interference
Many digital modes can perform well at low power and can be less prone to inter-station interference on Field Day. Also, many of the digital modes are more immune to interference and are less likely to be affected by the CW ops. With reduced power and adequate antenna separation, you might find that the CW and digital stations can share the same band without serious issues–which can be a bonus when few bands seem to be open on Field Day.
Lessons Learned
Field Day can be hectic. There just aren’t enough hours to prepare and operate. But one more thing should become part of your to-do list: keep a written record of what worked and what didn’t. Embrace your mistakes. Making mistakes simply means you are learning faster.
It’s likely no one will want to meet after the Sunday afternoon tear-down, even if drinks and pizza are involved, but set up a meeting within a few days after Field Day while the experience is still fresh in your mind. Share your notes and brainstorm. Consider it your first planning meeting for next year’s Field Day.
