Date: Sat, 17 Nov 2007 10:08:17 +0000
From: John Pumford-Green GM4SLV <gm4slv@sighthound.demon.co.uk>
To: rsgb_lf_group@blacksheep.org
Message-ID: <20071117100817.54c90983@lurcher.twatt.local>
Organization: The Gammy Bird
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Subject: LF: RX artillery
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Hello LF,

While I was making some recordings of various signals on MF and HF to
demonstrate the "sound" of my Inrad 250Hz filter for another list
member's benefit I decided to see what real effect the various
additional external hardware I have built made to the ability to pull
signals from the noise.

I've noticed many times recently that I can often hear people on 500,
especially during the day, and yet be unable to make myself heard. I
always put this down to my low noise level. While this will have some
bearing I also wonder if some of my simple extra hardware is
responsible.

I have an op-amp based tuneable RF pre-amp and a simple op-amp 10Hz
audio filter. Nothing "rocket science".

My previous understanding of noise floor and necessary RX sensitivity
was that, especially on the lower bands, performance was limited by the
external band noise and really sensitive RX front ends were unnecessary
- all you do is amplify the noise....

My IC-718 has a measured MDS of -119dBm (IPO) and -125dBm (pre-amp on)
on 500kHz and similar figures on 300kHz in an SSB filter b/w. 

The old maxim of "if you can hear the noise increase when you connect
the antenna then you've got enough sensitivity" would indicte that this
is more than adequate.

However....

Have a listen to the recording here :--
http://www.sighthound.demon.co.uk/gm4slv/OZN_372_RX_test.mp3

In this test I tuned to 372.420kHZ, the upper sideband ident of the
OZN-372 NDB in Greenland. At 09:30z it's not too strong here. In fact
it was totally inaudible. The noise level was too high to make it
worthwhile adding more gain in front of the RX....or so I thought.

The recording starts with the RX in CW mode, 2.3kHz IF filter and no
internal or external pre-amp. Pretty noisy and no sign of either OZN or
the more local ODR. 

I switch in my 10Hz AF filter. The wideband crud disappears, but still
no OZN.

Next comes the LF353 op-amp tuneable pre-amp. This shouldn't help, as
all I'll do is amplify the noise, right...?

Bingo!

Both OZN and, a few Hz lower, ODR. 

I have a local NDB (TL-376) and the keying harmonics (multiples of
400Hz) can be just be heard (wearing 'phones) - a final switch if the
IC-718 internal 250Hz inrad filter cleans it up.

My queston now is....

Would more people benefit from similar, simple, additions to their
500kHz stations? 

Many people are still trying to receive on wide IF filters and
straightforward amateur rigs (the IC-718 is one such rig). My
experience of being able to hear people perfectly Q5 yet be inaudible
to them might be in part due to the extra hardware, not purely my
superior location? 


-- 
John GM4SLV
IP90gg
Clousta, Shetland