[time-nuts] FS700 antenna

Magnus Danielson magnus at rubidium.dyndns.org
Sat May 5 14:15:33 UTC 2012


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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