On 05/05/2012 01:09 PM, Ulrich Bangert wrote:
Magnus,
on 26.8.2011 Stan has posted the Stanford Research circuitry of the original
whip antenna belonging to the FS700. My friend Frank and I have both built
this antenna from scratch and it works remarkably well.
I have the schematic in the manual.
However there is one caveat with whatever antenna for the FS700 which I had
to learn the hard way: With several different kinds of active antennas that
I have tried out with the FS700 a lot of them showed low receiver gains
(indicating the antenna gain) and high noise margins (nice!) in the FS700's
status display (only available after a lock). But when I looked to the FS700
some minutes to hours later it showed an error saying "Noise margin< 1dB"
and was in a unlocked condition. Frank experienced absolutely the same 100
km away from me so it could not be a problem of the individual receivers
and/or antennas but must have been a more principle effect.
I have discussed this problem here in the group but there was no satisfying
answer available.
The effect stopped to appear from the very moment when the antenna was put
out of the house in a distance of abt 20 m. While the effect is still not
completely understood there are at least 2 possible reasons:
1) The FS700 has an BNC output on the front which delivers a higly amplified
100 kHz signal so that you can easily view the LORAN waveform on a scope or
look to its spectrum on an analyser. If you have the antenna in the very
near of the receiver (which I had before<5m) then there are chances that
the antenna catches some of the amplified signal which can lead to unwanted
oscillations of the complete system overdriving the receiver completely.
2) Despite the fact that the receiver reported high noise margins with the
close in antennas there are a lot of noise sources (switching power supplies
f.e.) which's aggregated effect on the noise margin may be small in general
but may add up in some moments to make the FS700 unhappy. Moving the antenna
away from the receiver naturally moved it away from these noise sources as
well.
3) May also be a combination of 1) and 2): Say you are normally below the
self-oscillation level then a sudden interfering signal may be large enough
to start the self-oscillation with the same effect as in 1)
For that reason
If I do a quick and dirty attempt, I probably won't go for out-door
operations.
may not be the best idea. At least, if you see this message you know what to
do.
I'll try to cook up an LNA and see if that helps.
Cheers,
Magnus
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