Unwanted Antenna Coupling

Related Pages:
Excuses offered for not worrying about receiver damage from closespaced

Band  400foot spacing  200foot spacing  100foot spacing  50foot spacing  25foot spacing 
160  26 watts  66 watts  207 watts  
80  7.5 watts  29 watts  67.5 watts  223 watts  
40  2 watts  7.5 watts  29 watts  67.5 watts  223 watts 
20  0.5 watts  2 watts  7.5 watts  29 watts  67.5 watts 
10  0.125 watts  0.5 watts  2 watts  7.5 watts  29 watts 
Dipoletovertical that is broadsideto and centeredon the dipole, perfect ground,
and 1000 watts
Band  400foot spacing  200foot spacing  100foot spacing  50foot spacing  25foot spacing 
160  0.13 watts  0.38 watts  0.79 watts  
80  .049 watts  0.13 watts  0.38 watts  0.79 watts  
40  .013 watts  .049 watts  0.13 watts  0.38 watts  0.79 watts 
20  .013 watts  .049 watts  0.13 watts  0.38 watts  
10  .013 watts  .049 watts  0.13 watts 
Verticaltodipole, dipole oriented so vertical is nearly in line with the dipole’s end
Band  400foot spacing  200foot spacing  100foot spacing  50foot spacing  25foot spacing 
160  1.9 watts  4 watts  10.5 watts  
80  .41 watts  1.6 watts  4.1 watts  10.5 watts  
40  .10 watts  .41 watts  1.6 watts  4.1 watts  10.5 watts 
20  .11 watts  .41 watts  1.6 watts  4.1 watts  
10  .11 watts  .41 watts  1.6 watts 
Dipoletodipole, broadside to each other, 1/4 wave above earth, with good
conductivity soil
Band  400foot spacing  200foot spacing  100foot spacing  50foot spacing  25foot spacing 
160  14 watts  76.2 watts  296 watts  490 watts  
80  1.5 watts  14 watts  76.2 watts  296 watts  490 watts 
40  .11 watts  1.5 watts  14 watts  76.2 watts  296 watts 
20  .0075 watts*  .11 watts  1.5 watts  14 watts  76.2 watts 
10  .000486 watts*  .0075 watts*  .11 watts  1.5 watts  14 watts 
* 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 sameband antennas.
Different Band (Harmonic) Coupling and Damage
Looking at the case of a 80meter dipole’s fundamental signal to a 40meter
dipole. The antennas are broadside, 50 feet apart, and 100 feet high. The
40meter antenna is terminated in a matched 50ohm lossless feed line:
Transmitter power = 1000 watts on 80 meters Receiver on 40 meters Voltage = 4.5 V 
For a 40meter harmonic on the 80meter transferring to the 40meter antenna,
again assuming perfectly matched lossless feed lines (which will never happen),
we have:
Harmonic power 100mW
Receiver Voltage = 0.25V

Receiver levels would be considerably less than this, because the 40 meter
SWR to the 80meter transmitter would be very high.
40meter levels, with a kilowatt into the 80meter antenna, are:
Transmitter 1000 watts 7.1 MHz
Receiver 5.3V 
Worsecase fundamental coupling is from 40 to 80meters, which might be be
even stronger if the 80meter 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 80Meter 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 80meter dipoles,
each 67feet 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:
EZNEC+ ver. 5.0
80 meter dipoles spaced 200 ft —— Transmitter data ——– Frequency = 3.5 MHz Voltage = 288.6 V at 4.82 deg.

EZNEC+ ver. 5.0
80 meter dipoles spaced 200 ft ————— Receiver DATA ————— Frequency = 3.5 MHz Voltage = 34.42 V at 39.71 deg. Total transmitter power = 1000 watts 
EZNEC+ ver. 5.0 80 meter dipoles spaced 200 ft —— Transmitter data ——– Frequency = 3.5 MHz Voltage = 288.6 V at 4.82 deg.

EZNEC+ ver. 5.0 80 meter dipoles spaced 200 ft —————Receiver short DATA ————— Frequency = 3.5 MHz

EZNEC+ ver. 5.0
80 meter dipoles spaced 200 ft
Frequency = 3.5 MHz Source 1 Voltage = 288.6 V at 4.82 deg.

EZNEC+ ver. 5.0
80 meter dipoles spaced 200 ft ————— Receiver open DATA ————— Frequency = 3.5 MHz

Actual voltage and current is probably somewhere around the matched value,
but the second antenna can deliver up to .83 amperes, or 100volts peak
voltage, to a receiver depending on receiver input impedance. This can cause
receiver damage, even though the antennas are 200feet apart, not very high, and
horizontally polarized.
Coupled Power, very closespaced elements, different bands
Fivefoot 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.
Nearminimum 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 155ohms, 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 oddmultiple resonances), a short is always best, so a highQ
seriesresonant 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 50ohm 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 widerange
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 singletower antennas, highest coupling occurs from the 15meter
antenna to the 40meter antenna system. The 15meter Yagi couples to the 40meter Yagis on 15meters
with 31 dB attenuation, because the 40meter Yagis below the 15 meter antenna
is harmonically resonant on 15 meters.
With 45foot spacing, because of impedance and resonance mismatches, there is
negligible measured coupling from 40 to 20meter 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 80meter dipole on this
tower to a SE/NW broadside 80meter dipole, up 160feet on a 318foot tower.
Despite over 250foot spacing, coupling between these two 80meter antennas is high enough to
raise concerns of receiver damage. This could happen if both antennas are used on 80meters at the same time
at high power.
Signal levels from the 160meter vertical antennas, even with
350feet of spacing, are also worrisome in my old high 160meter dipole. This is
mostly because my high 160meter dipole is not directly broadside to my
160meter verticals. Fortunately, I almost never use my transmitting antennas to
receive on 160.
160Meter Antenna Coupling
The table below lists receiver port levels and path loss
between my transmitting and receiving antennas on 160meters.
TX antennas: eight direction 4square and 200ft omni, with omni
centered in 4square
Reference antenna: 70foot vertical 375 feet SW of TX antenna
center point
Rear Bev: ~1500 feet SW of TX antennas
Rear Verticals: 8cir array ~1200 feet SW of TX antennas
NE Front New: Pair of broadside 800foot 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
Antenna test null system 10/26/2011  Power at 5 watts  all levels in dBm or dB  
nflr76  tx ants  tx ants  tx ants  tx ants  tx ants  tx ants  tx ants  tx ants  tx ants  TX delta  MAX  MIN  
rear vert  omni  N  NE  E  SE  S  SW  W  NW  
N  26.47  36.37  39.14  34.12  37.5  25.4  22.25  25.15  34.9  16.89  22.25  39.14  
NE  23.3  28.95  41  28.38  26  19.5  17.05  20.5  28.15  23.95  17.05  41  
E  29.47  42.74  44.3  37.1  34.6  38.75  33.4  36.9  39.7  14.83  29.47  44.3  
SE  30.9  40  51.3  44.8  42  30  28.2  31.4  41.4  23.1  28.2  51.3  
S  31.15  38.65  47.8  50.5  41.8  28.8  27.65  31  41  22.85  27.65  50.5  
SW  29.75  39.85  51.6  44.3  42.4  30  28.1  31.3  41.1  23.5  28.1  51.6  
W  26.3  37.7  52  47.6  38.75  28.5  27.5  31.15  39.5  25.7  26.3  52  
NW  32.55  40.15  51  53.6  41.9  30.35  29.5  32.8  42.8  24.1  29.5  53.6  
Reference  13.8  20.95  29  19.8  21.3  12.5  9.45  11.8  19.8  19.55  9.45  29  
Losses  
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  
rear bev  omni  N  NE  E  SE  S  SW  W  NW  
N  16.58  23  44.8  31  31  14.1  15.4  17.3  25.1  30.7  14.1  44.8  
NE  20.75  26.75  37.9  31.75  26.3  17.85  20.7  24.8  30.5  20.05  17.85  37.9  
E  28.1  31.7  40.65  34.7  30.5  24.9  30.4  33.7  36.5  15.75  24.9  40.65  
SE  27.9  33.7  42.75  39.9  38.5  25.3  26.3  28.5  35.1  17.45  25.3  42.75  
S  28  32.9  42.3  37.7  34.7  25.2  27  29.5  34.7  17.1  25.2  42.3  
SW  24.6  30.7  34.3  31.8  30.9  22.5  21.9  24.5  29.8  12.4  21.9  34.3  
W  40  46.7  49.6  47.9  51  38  37  39.2  45.3  14  37  51  
NW  40  46.6  50.85  46.1  47.9  38.4  37.7  39.4  45.9  13.15  37.7  50.85  
Reference  13.26  19  21.27  17.85  19.4  11.5  9.85  11.6  17.65  11.42  9.85  21.27  
Losses  
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  
Frnt Bev  omni  N  NE  E  SE  S  SW  W  NW  
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  
Reference  13.26  19  21.27  17.85  19.4  11.5  9.85  11.6  17.65  11.42  9.85  21.27  
Losses  
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  