Unwanted Antenna Coupling
Excuses offered for not worrying about receiver damage from close-spaced antennas:
I never saw or heard of this problem, so it is not a worry.
Fact: Outside of contests not too many of us have same-band antennas, or antennas with good harmonic response, that are connected to receivers and transmitters at the same time. It would be unreasonable to think anyone would commonly damage receivers because very few people have antennas that work well on the same band coupled to a receiver, while a transmitter is running.
Police, public service, and marine installations operate multiple radios without problems, so it should be the same for HF radios.
Fact: Power levels, for the same antenna spacings in feet, decrease decrease dramatically as frequency increases. A 3.5 MHz installation with 1/4 wave verticals has well over 100 times the coupled power of two 2-meter antennas, when both systems have the same physical spacing.
I can just turn the other radio off, or not tune it to the same frequency, and it will be safer.
Fact: Overload damage to nearly all radios is unaffected by the distance off frequency the radio is tuned. HF radios often are minimally affected by having power on or off, or the band they are on.
If you are using close-spaced antennas with multiple radios, especially with very close spacing or higher power, we should worry about damage. We should test the system to get at least some rough idea how significant coupled power is.
Coupled Power Levels, between transmitting and receiving antennas
People sometimes ask the safe minimum antenna spacing to avoid damage to radios. The data below should help prevent damage at Field Day or in other multiple transmitter contest or emergency operating environments. The data below is based on two matched antennas, where the load (the receiver) matches the antenna impedance. The systems have no feed line losses.
Data below does not represent an absolute maximum coupling. Levels can be higher, depending on antenna height and orientation. Levels can also be much less, but the levels below are a very reasonable estimate of maximum power for typical antenna installations.
Some basic rough rules for reasonably wide antenna spacings:
1.) Doubling spacing distance reduces power 2-4 times (at very wide spacings or far field with horizontal antennas, power diminishes quite rapidly).
2.) Outside of very close spacing, doubling frequency, with a fixed physical distance between antennas, reduces coupled power by about four times.
3.) A dipole and a vertical have minimum coupling when the vertical is centered on and directly broadside to the dipole. Coupling to a vertical actually increases off the dipole ends. If we want best isolation, we should not install a dipole or horizontal antenna with horizontal antenna ends toward the vertical.
4.) Two horizontal dipoles have minimal coupling when they are nearly end-to-end, but this varies with height and soil.
5.) Improper feed systems, such as those with common mode currents, can radically increase coupling levels between antennas.
6.) Some receivers are more damage prone than than others. Most receivers I've tested will handle over 20 dBm (100 mW or 1/10th of a watt) for extended periods without damage. Most newer receivers use 1/8th watt resistors in attenuator pads, and have other parts that can be damaged at 1 watt or so. I consider any receive antenna power over .125 watts to be potentially damaging to receivers, and over .5 watts to likely cause damage, but this is just my opinion.
7.) Damage problems are generally from same-band operation, not same frequency operation. Tuning off-frequency does not reduce chances of damage. The exact frequency difference between a transmitter and receiver does not matter much, but the band does. This is because the radio's wide bandpass filters pass the signal on to easily damaged components, which are ahead of the mixer and narrow selectivity.
8.) Turning a receiver off usually will not eliminate chances of overload damage, and often does not even reduce damage from high coupled power levels. If we don't want radio damage, we should disconnect the antenna from the unused receiver, or make sure coupling from the transmitter to the receiver is at safe levels.
9.) It is unlikely that transmitter or amplifier harmonics will damage receivers. Nearly all 1500-watt amplifiers have less than 50 milliwatts on the worse harmonic. While that level can travel thousands of miles, it is far below damaging levels at any distance. The real danger is an intentionally generated signal's fundamental energy getting into early receiver components.
Coupled Power Levels
The following levels are based on EZNEC models. The models use same-band antennas, which is a worse-case condition.
Two 1/4-wave verticals, each with zero ground loss. Transmitter power at antenna = 1000 watts
Vertical-to-dipole, dipole oriented so vertical is nearly in line with the dipole's end
Dipole-to-dipole, broadside to each other, 1/4 wave above earth, with good conductivity soil
* Green cells antenna in farfield with elevation pattern creating a null, making drop in receiver power very abrupt.
Depending on antenna height, soil conductivity, and quality of balance and construction, power levels can increase or decrease substantially. Remember the above is for same-band antennas.
Different Band (Harmonic) Coupling and Damage
Looking at the case of a 80-meter dipole's fundamental signal to a 40-meter dipole. The antennas are broadside, 50 feet apart, and 100 feet high. The 40-meter antenna is terminated in a matched 50-ohm lossless feed line:
For a 40-meter harmonic on the 80-meter transferring to the 40-meter antenna, again assuming perfectly matched lossless feed lines (which will never happen), we have:
Receiver levels would be considerably less than this, because the 40 meter SWR to the 80-meter transmitter would be very high.
40-meter levels, with a kilowatt into the 80-meter antenna, are:
Worse-case fundamental coupling is from 40- to 80-meters, which might be be even stronger if the 80-meter antenna is matched to the receiver for 40 meters.
In all cases, a harmonic filter for transmitters is not even close to being necessary to prevent equipment damage. A receiver input filter, for the band the receiver is on (different than the other transmitter), is normally required.
Same Band Power Coupling Between Two 80-Meter Dipole Antennas
The antenna is a source for the receiver when receiving, and the receiver is the load. While the antenna determines SWR when transmitting, the receiver's input impedance determines feed line SWR when receiving.
We can model antenna coupling using a program like EZNEC. With two broadside 80-meter dipoles, each 67-feet high over medium soil, with the antennas spaced 200 feet apart, we have the following receiver voltage, current, and power levels for matched receiver input, shorted receiver input, and open receiver input:
Actual voltage and current is probably somewhere around the matched value, but the second antenna can deliver up to .83 amperes, or 100-volts peak voltage, to a receiver depending on receiver input impedance. This can cause receiver damage, even though the antennas are 200-feet apart, not very high, and horizontally polarized.
Coupled Power, very close-spaced elements, different bands
Five-foot spacing between elements
|Antenna test null system 10/26/2011||Power at 5 watts||all levels in dBm or dB|
|nflr-76||tx ants||tx ants||tx ants||tx ants||tx ants||tx ants||tx ants||tx ants||tx ants||TX delta||MAX||MIN|
|N vrt to ref||12.67||15.42||10.14||14.32||16.2||12.9||12.8||13.35||15.1||6.06||16.2||10.14|
|NE vrt to ref||9.5||8||12||8.58||4.7||7||7.6||8.7||8.35||7.3||12||4.7|
|E vrt to ref||15.67||21.79||15.3||17.3||13.3||26.25||23.95||25.1||19.9||12.95||26.25||13.3|
|SE vrt to ref||17.1||19.05||22.3||25||20.7||17.5||18.75||19.6||21.6||7.9||25||17.1|
|S vrt to ref||17.35||17.7||18.8||30.7||20.5||16.3||18.2||19.2||21.2||14.4||30.7||16.3|
|SW vrt to ref||15.95||18.9||22.6||24.5||21.1||17.5||18.65||19.5||21.3||8.55||24.5||15.95|
|W vrt to ref||12.5||16.75||23||27.8||17.45||16||18.05||19.35||19.7||15.3||27.8||12.5|
|NW vrt to ref||18.75||19.2||22||33.8||20.6||17.85||20.05||21||23||15.95||33.8||17.85|
|N bev to ref||3.32||4||23.53||13.15||11.6||2.6||5.55||5.7||7.45||20.93||23.53||2.6|
|NE bev to ref||7.49||7.75||16.63||13.9||6.9||6.35||10.85||13.2||12.85||10.28||16.63||6.35|
|E bev to ref||14.84||12.7||19.38||16.85||11.1||13.4||20.55||22.1||18.85||11||22.1||11.1|
|SE bev to ref||14.64||14.7||21.48||22.05||19.1||13.8||16.45||16.9||17.45||8.25||22.05||13.8|
|S bev to ref||14.74||13.9||21.03||19.85||15.3||13.7||17.15||17.9||17.05||7.33||21.03||13.7|
|SW bev to ref||11.34||11.7||13.03||13.95||11.5||11||12.05||12.9||12.15||2.95||13.95||11|
|W bev to ref||26.74||27.7||28.33||30.05||31.6||26.5||27.15||27.6||27.65||5.1||31.6||26.5|
|NW bev to ref||26.74||27.6||29.58||28.25||28.5||26.9||27.85||27.8||28.25||2.84||29.58||26.74|
|NE New||-23.45||-17.15||-22.75||-27.5||-29.45||-25||-28.3||-25.25||-18.36||12.3||-17.15||-29.45||-109 dBm|
|NE Old||-11.93||-11.9||-9.82||-10.5||-25.4||-19.15||-21.1||-16.1||-26.1||16.28||-9.82||-26.1||-112 dBm|
|N bev to ref||10.19||-1.85||1.48||9.65||10.05||13.5||18.45||13.65||0.71||20.3||18.45||-1.85|
|NE bev to ref||-1.33||-7.1||-11.45||-7.35||6||7.65||11.25||4.5||8.45||22.7||11.25||-11.45|