Last week I did some RX measurements on my receiver and antenna :

1. The sensitivity of my receiver (Kenwood TS440 - 500Hz CW filter) on
136kHz is 0.5µV (-113dBm) for 10dB S/N. As signal source I used a
Rohde&Schwarz SMS2.
Assuming that an 'exercised ear' easily can read a 0dB S/N CW signal this
means that signals down to -123dBm could be read (if no QRM/QRN)

2. Using a Rohde&Schwarz URV3 millivoltmeter (10kHz-2GHz) I measured the
antenna-voltage. There was almost no difference between the voltage with a
50 Ohm load and the RX input as load, so the RX input is close to 50 Ohm.
With the antenna (13m high and 26m long inverted-L tuned to 136kHz)
directly to the receiver input the voltage measured was almost constant at
35mV during daytime but was going up and down between 50mV and 200mV during
the night.
Insering a low-pass filter (-3dB at 170kHz) between antenna and RX reduced
the voltage to 30mV during daytime and 30 to 120mV at night.

Conclusions :

- When using a 136kHz transmit antenna for receiving (without additional
filtering the signal at the RX input is in the range of 35 to 200mV (-16 to
-1dBm) and will cause severe intermodulation to most receivers, especially
at night.

- Adding a low-pass filter doesn't change the antenna-voltage during
daytime (only -1.3dB) and reduces the antenna-voltage only minimal (-4.5dB)
at night. This means that most of the sources that cause this high
antenna-voltage are below 200kHz.

- The measurements of PA0SE (results distributed to the list last week)
indicate that most 136kHz ham signals are in the range of -75 to -95dBm.
This is 28 to 48 dB above the 0dB S/N level of my RX. This means that a
20dB (or even 30dB) attenuator can be put between antenna and RX without
loosing any sensitivity. Adding the attenuator however will increase the
IM-behavior of the receiver by 40dB (for 20dB attenuation) or even 60dB
(for 30dB attenuation).
Although a broadband 'ohmic attenuator' already will bring a major
improvement, a 'selective' attenuator made by a series of loose-coulped
high-Q band-pass filters will be even better provided the correct ferrite
material is used to prevent bad IM behavior of the filter itself.

73, Rik

Rik Strobbe  ON7YD
[email protected]
Villadreef 14  B-3128 Baal  BELGIUM   (JO20IX)