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Dave, G3YMC, wrote:

Dick's field strength meter looks interesting - well done Dick.  Not sure
whether it would detect much of my QRPP set up at 1km!

The DVM I use with my FSM indicates voltages less than 1000 mV in three
digits.
With no signal tuned in the meter reads 1 mV. This must be receiver noise.
But due to the low cut off frequency of the low pass filter (- 3B at 88 Hz)
it sounds like a low rumble.

For a meaningful result the reading should be 10 mV or so as a minimum. Due
to the uncertainty of the last digit in a DVM this would mean an
(in)accuracy of plus or minus 10% which would be adequate I think.
(Measuring field strength is never a precision job). As  a reading of 1000
mV corresponds to 5 mV/m  in my FSM the field producing a reading of 10 mV
is 0.05 mV/m. Inserting that  and a distance d = 2 km into  equation (1) of
my FSM story yields a radiated power of 0.111 mW (or 0.111 * 1.83 = 0.203 mW
ERP).
I don't know how QRPP your station is Dave but I expect my FSM could cope
with it.



I wonder how this direct conversion receiver works as a general purpose 136
receiver.  I suspect it is probably a little lacking in gain, but perhaps
Dick can comment.  I need to sort out a suitable receiver for portable use,
possibly for an Alderney trip where there is a 15kg air luggage limit.

There are two factors to be considered, sensitivity and selectivity.

A dc-receiver can be made as sensitive as a superhet.
I  have put my FSM between the Helmholtz coils again to find the minimum
signal that could be detected by ear. It corresponded to a field strength of
about 9 microvolt/m. That is not good enough for a receiver but remember
that the low pass filter starts to attenuate  an audiosignal at 88 Hz. The
test signal had to produce a note well above that to be heard in the
headphones and thus the signal had already suffered considerable
attenuation. Also for CW a note between 700 and 1000 Hz is usual and the ear
is much more
sensitive there.
By removing resistor R4 in parallel with the antenna circuit the gain would
increase by a factor of three or so.

So a dc-receiver along the lines of my FSM could have sufficient gain as a
receiver. (When used with the transmitting antenna for reception instead of
the ferrite rod the gain would definitely be plentiful but this would
require extra front-end selectivity to obtain an acceptable  strong signal
behaviour. But that applies to a superhet as well of course).

Now the selectivity matter. For a FSM my system with a three section RC low
pass filter is sufficient because an interfering signal is added
quadratically to the wanted signal. As an example let the signal to be
measured produce an audio signal of 500 mV on the DVM.  Assume that also
present is an interfering signal at a different frequency that would produce
50 mV on the meter if alone. Then the meter does not read
500 + 50 = 550 mV but SQR (500^2 + 50^2) = 502 mV. Though the interfering
signal is only
20log(500/50) = 20 dB weaker that the wanted one the influence on the
measurement  is negliglible.

When used for aural reception the story becomes quite a different one. The
ear hears  signals at different frequencies  separately  so the quadratic
summing trick does not work . In addition the ear/brain-system has an
enormous dynamic range. I don't know how much but  think at least 60 dB or
so.
When my FSM is tuned away from DCF39 with volume  turned up I can still hear
the audio note until it reaches about 2500 Hz.  The signal then must have
been attenuated in the low pass filter by some calculated 68 dB! This
illustrates the problem.

In case of  a dc-receiver for CW the low pass filter must be replaced by an
audio band pass one with a pass band of 700....1000 Hz, say. Below and above
that band attenuation must steeply rise to 80 or more dB. An almost
impossible requirement.
Another drawback of the  dc-receiver is that it is a double sideband
receiver. In the case of our example there exists another passband of 300 Hz
wide on the other side of the oscillator frequency. With a phasing system
the one sideband can be suppressed but the the simplicity of the dc-receiver
is then lost. Also I expect  sideband suppression at best may reach some 50
dB or so and that does not really help.

My conclusion is that a dc-receiver for the 136 kHz band may be used for
occasional listening but is no good for serious work.

The selectivity requirement is met in superhets by cascading filters at
different intermediate frequencies, sometimes augmented by an audiofilter.

My own long wave receiver has an i.f. of 30 kHz and selectable i.f.-filters
with different pass bands, all having 11 high-Q tuned circuits with ferrite
pot cores. The narrowest filter has a 400 Hz wide passband..
Even with this filter I can still hear DCF39 weakly in the filter stop band.
I therefore also use audio filters (35 or 200 Hz wide) between the audio
output of the receiver and the headphones.

Hope this answers Dave's query.

73, Dick, PA0SE

JO22GD
D.W. Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel. +31 71 589 27 34
E-mail: [email protected]
or
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