Unwanted Antenna Coupling

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Field day has come and gone, and there are a few cases of damaged radios reported on reflectors. While most radios return from field day unscathed, some do not.

People sometimes ask the safe minimum antenna spacing to avoid damage to radios. Here are some basic simple antennas and the power level that appears in the second antenna. The data below should help prevent damage at field day or in other multiple transmitter environments. This data is based on two matched antennas, where the load (the receiver) matches the antenna impedance and with zero feedline losses. The data below does not represent an optimum or maximum coupling. Levels can be higher, depending on antenna height and orientation. Levels can also be much less, but the levels below are probably a fairly reasonable maximum for typical antenna heights.

Some basic rough rules for reasonably wide antenna spacings:

1.) Double the spacing distance and reduce power 2-4 times (at very wide spacings or far field with horizontal antennas, power will diminish quite rapidly).

2.) Double the frequency at the same physical spacing, and the change will approximately quarter the coupled power.

3.) A dipole and a vertical have minimum coupling when the vertical is centered on and directly broadside to the dipole. Coupling increases off the end of a dipole to a vertical, do not install a dipole with the dipole ends towards a vertical to produce a null!

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 change levels.

6.) Most receivers use 1/8th watt resistors in attenuator pads, and have other parts that can be damaged at 1 watt or so. Some receivers are worse than others and the safe level is your responsibility. Most receivers I've tested will handle over 20 dBm (100 mW or 1/10th of a watt) for extended periods without damage. I would consider anything over .125 watts to have some worry, and over .5 watts to be potentially damaging, but this is just my opinion.

7.) Any damage problem is generally from same-band operation, not same frequency operation. The exact frequency does not matter much so long as the band is the same. This is because the radio's bandpass filters pass the signal better, and because the antennas are on the same band so the antennas couple much better.

8.) Turning a radio off is no guarantee you will eliminate damage, and often does not help in the least with higher coupled power levels. If you don't want any chance of radio damage, disconnect the antenna!

Coupled Power Levels

Two 1/4 wave Verticals, perfect grounds, applied power = 1000 watts

Dipole to a vertical that is broadside-to and centered-on the dipole, perfect grounds, and 1000 watts

Vertical to dipole, dipole oriented so vertical is nearly in line with dipole 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 strength very abrupt.

Depending on antenna height, soil conductivity, and quality of balance and construction power levels can increase or decrease substantially.

Antenna Source Impedances

The antenna is a source for the receiver when receiving. We can model the characteristics using EZnec. Looking at two 80-meter dipoles 67 feet high over medium soil, we have the following impedances for matched load, shorted load, and open load:

We see the second antenna can deliver up to .83 amperes or 100 volts peak voltage. Keep in mind the current and voltage at the receiver could be higher than this because the feedline can act like a transformer. What is important is the power available, because the receiver in not a short or open.

Some excuses offered for not worrying are:

Myth: I never saw this problem, so it cannot exist.

Fact: Few people have same-band antennas, or antennas with good harmonic response, that are active with close spacing. It would be unreasonable to think anyone would commonly see this problem because nobody sees a good sample of problem installations because such installations, where multiple radios run at the same time 9especially on the same band) are uncommon!

Myth: Police, public service, and marine installation operate without problems, so it should be the same for HF radios.

Fact: Power levels, for the same antenna spacing, decrease by an amount much larger than the frequency increase. A 150 MHz installation with 1/4 wave verticals would have much less than 1% of the coupled power of a 3.5 MHz installation at the same spacing!

The Safe Way to Test

The best way to prevent damage is to measure power from one antenna to a dummy load while the other transmitter is on at maximum power. This would give you an idea of the path loss between antennas. If one or both antennas are on rotors, they should be rotated for maximum signal level.

I measured my antennas with a selective level meter. This allows me to make measurements at greatly reduced power levels.

I have some measurements on this page http://www.w8ji.com/coaxial_cable_leakage.htm and I'll try to get my other data for transmitting antennas into an excel spreadsheet someday.

Out of all these antennas, the 15 meter and 40 meter antennas are worse. The 15-meter Yagi couples to the 40-meter Yagis on 15 meters because the 40 meter Yagis are harmonically resonant on 15 meters.

Even with only 45 feet of spacing, there is virtually no coupling from 40 to 20 (or between other bands) because the antennas for other bands do not work well on harmonics.

The antennas, in essence, add selective filtering.

I do have problems from my low 80-meter dipole to a high 80-meter dipole about 250 feet away. Coupling between those two antennas is high enough to damage receivers if they are both on 80 meters at the same time, so I cannot use two high power 80-meter stations at the same time while using the transmitting antennas for receiving.

To avoid problems, I segregate antennas on the same bands. I try to have directional antennas not point through or into other antennas that might interact or couple.

Coupling problems are a low 80 meter dipole on the rotating tower to a high 80 meter dipole on another tower 250 feet away. Levels are high enough to damage receivers with two stations on the same band. Signal levels from the 160 meter vertical antennas, even with 350 feet of spacing, are much too high for receivers connected to a 160 dipole. This is because the 160 dipole is not directly broadside to the 160 verticals.