Dear Roger, LF Group,
Re: using the FT-817 and long wire for receiving the WSPR beacons - I have
an FT-817 also, which I mostly use with a converter for VLF - MF reception
due to its poor sensitivity at low frequencies. I just did a quick
sensitivity check on mine - the following figures are for 10dB SNR using the
500Hz CW filter:
4.5MHz (the output frequency of my converter):
"IPO on" (ie. preamp off) -117dBm
"IPO off" (preamp on) -125dBm
Sensitivity is much the same over the HF range.
502kHz:
IPO on -105dBm
IPO off -108dBm
137kHz:
IPO on -89dBm
IPO off -80dBm
So sensitivity is about 20dB down on 500k, which is more or less passable,
but a bit disastrous at 137k where it is around 30 - 40dB down depending on
whether the preamp is selected or not.
The un-tuned wire will also be a contributing factor - at low frequencies,
it will have a high capacitive impedance with a huge mismatch to the low
impedance RX input. You can expect to lose a further 20dB or more at 500k,
and 30dB or more at 137k, compared to having some sort of antenna tuner.
These figures are not unusual for the situation of using a "random" wire
with typical amateur HF rigs with general coverage RX, so it is not
surprising people often get reasonable results at 500k using this sort of
setup for receive, but 137k usually needs a bit more sorting to get useful
reception
Cheers, Jim Moritz
73 de M0BMU
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