Hi Minto,
you could overcome the dynamic range limitation of
the measurement by using an attenuator for the signal measurement.
1. tune to the center of the LF band,
with DCF39 outside the passband. Turn up the gain control as much as
needed. Read the noise level in SpecLab.
2. tune to 138.83 kHz. Insert a 40 dB
attenuator but don't change the gain. Read only the signal level (and
ignore the noise).
To obtain DCF39 SNR, simply
subtract noise level (1) from signal level (2), and add 40
dB.
In my location at 320 km from Magdeburg, DCF39 is normally
about 3 mV/m (70 dBuV/m). The best-case band noise has been down
to about 0.06 uV/m in 1 Hz (-25 dBuV/m/sqrtHz). Thus SNR can be up to 95 dB
in 1 Hz here.
DCF39 is said to have 40 kW EMRP. Thus a 1 W ERP
amateur station at the same distance would be 49 dB weaker.
Best 73,.
Markus (DF6NM)
Sent: Monday, August 08, 2011 12:20 AM
Subject: LF: Re: CW sked on 137 friday morning?
Hi Stefan,
Let me try to explain a bit better
what I have done. When I disable the AGC of the FT897 I need to dial-down the
manual AGC in order to avoid overloading the IF AND the AF sections when tuned
to DCF39. (If I don’t DCF39 sounds
horribly distorted). With this setting I measure the LF output of the 879 with
Speclab.
With the same manual AGC setting I tune to say 137.700 and measure
the LF output again with Speclab. But because the manual AGC is dialed down
significantly the band noise ‘drops through’ the noise floor of the FT897. So
this method is unusable to determine the strenght of DCF39 w.r.t. to the
noise.
If I dial up the AGC (or set it on
auto again) the noise floor increases with about 10 dB when I connect the
antenna, I think this should be enough..
By the way, this moment I can not
only see your CQ with Speclab, but with headphones I can even hear
you.
Regards,
Minto
pa3bca
