Unwanted Antenna Coupling

Unwanted Antenna Coupling

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Coaxial cable leakage

Filter and Stubs

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

Dipole-to-vertical that is broadside-to and centered-on the dipole, perfect ground,
and 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.

Link to filters

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

Frequency = 21 MHz
Total applied power = 1000 watts

Load on 1 (20M element)

Termination Impedance = 50 + J 0 ohms

Voltage = 50.37 V
Current = 1.007 A

Total load power = 50.74 watts

The 20 meter element looks like 155.8 +j434.7 ohms on 15 meters. In theory if
we terminate that element with the conjugate, we will have maximum coupling.

Frequency = 21 MHz Total applied power = 1000 watts

Load 1 Voltage = 753.6 V
Current = 1.632 A
Impedance = 155.8 – J 434.7 ohms
Total load power = 414.9 watts
 

With the very same spacing and power, if the 15M termination impedance of the
20 meter element is 155.8 -j434.7, we now have 415 watts coupled power.
Obviously the impedance reflected back to the 20M element is critical, yet no
one to this date considers this. Not antenna manufacturers, not filter
manufacturers, and not articles on filters or stubs.

Near-minimum coupling would occur when the filter/transmission line
combination, on 15 meters but at the 20 meter element feedpoint, would be
closest to a dead short or open with inductive reactance. Since the resistive
part is 155-ohms, a 1 ohm j0 termination on 15 meters would be a 155:1 mismatch.
It would be difficult to obtain 155*155= 24025 ohms on 15 without hurting 20, so
a low resistance with inductive sign appears to be a better target for feeder
and filter (stub) on the 20M element.

With a 20 meter open 1/2 wave stub across the 20M element, and assuming 50 j0
as the 15M load on the 20 meter element, we have:

Frequency = 21 MHz Total applied power = 1000 watts
Load 1 Voltage = 22.09 V
Current = 0.4417 A
Impedance = 50 + J 0 ohms
Total load power = 9.756 watts

With the other solution, which is a 20 meter shorted 1/4wave stub across the
20 meter feedpoint, we have:

Frequency = 21 MHz Total applied power = 1000 watts

Load 1 Voltage = 44.77 V
Current = 0.8954 A
Impedance = 50 + J 0 ohms
Total load power = 40.09 watts

These load power levels are not etched in stone, because they ASSUME the
receiver looks like 50 j0 on 15 meters when the receiver is set to 20 meters.
Actual receiver power will almost certainly be much less than the above cases,
although it could be more with sour lengths of feed line in combination with
certain receiver input impedances.

While I don’t have time to look at combinations in more detail, the purpose
of this is to show that nearly all stub and filter, or antenna coupling analysis
on the Web and in articles, that do not consider source and load impedances, are
incorrect. Electrical distance from filter or stub to the antenna and the radio
system has a large effect on attenuation and coupled power. Unfortunately this
distance varies with the type of filter, radio (or amplifier), or antenna. Here
are some general rules:

• The stub belongs at the antenna element, and the optimum stub type and
  length varies with the antenna characteristics. In general (except for cases
  of odd-multiple resonances), a short is always best, so a high-Q
  series-resonant trap will likely always be better than a stub.
• If a stub is used, it is worth investigating optimum type and optimum
  distance from load or source.
• Coupled power is never what a 50 ohm load shows
• Heat or stress on a filter is never what a 50-ohm load test shows
• Power is largely reflected from filters and stubs, NOT
  absorbed

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 running at maximum power. This us 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, and a wide-range
matching network to tune for maximum power (or a load that really represents the
equipment impedance). This allowed
coupled power measurements at greatly reduced (safe) power levels.

I have some measurements on this page

http://www.w8ji.com/coaxial_cable_leakage.htm and in the text below.

In this group of single-tower antennas, highest coupling occurs from the 15-meter
antenna to the 40-meter antenna system. The 15-meter Yagi couples to the 40-meter Yagis on 15-meters
with -31 dB attenuation, because the 40-meter Yagis below the 15 meter antenna
is harmonically resonant on 15 meters.

With 45-foot spacing, because of impedance and resonance mismatches, there is
negligible measured coupling from 40- to 20-meter antennas. This is also true
for all other bands tested, because
the antennas do not work well on harmonics.

This system also has potential problems from a low 80-meter dipole on this
tower to a SE/NW broadside 80-meter dipole, up 160-feet on a 318-foot tower.
Despite over 250-foot spacing, coupling between these two 80-meter antennas is high enough to
raise concerns of receiver damage. This could happen if both antennas are used on 80-meters at the same time
at high power.

Signal levels from the 160-meter vertical antennas, even with
350-feet of spacing, are also worrisome in my old high 160-meter dipole. This is
mostly because my high 160-meter dipole is not directly broadside to my
160-meter verticals. Fortunately, I almost never use my transmitting antennas to
receive on 160.

160-Meter Antenna Coupling

The table below lists receiver port levels and path loss
between my transmitting and receiving antennas on 160-meters.
 

TX antennas: eight direction 4-square and 200-ft omni, with omni
centered in 4-square

Reference antenna: 70-foot vertical 375 feet SW of TX antenna
center point

Rear Bev: ~1500 feet SW of TX antennas

Rear Verticals: 8-cir array ~1200 feet SW of TX antennas

NE Front New: Pair of broadside 800-foot Beverages, 375 feet
broadside, about 600 ft  NNW of TX antenna

 NE Front Bev old: pair of broadside 1200 ft beverages
300 feet NE of TX antennas