System BLUE / capability to handle external RF fields
#1
Hi,

just joined to the forum :-)

I am looking for a future opportunity to setup my monitoring station behind a remote internet connection. I am interested of the system BLUE when it comes available - hopefully later this year.

Do we have any preliminary information (possibly from development team) how good this equipment is for handling nearby RF fields? I got some comments from local amateur radio operators, that the VLF area where BO hardware operates, would be not so easily disturbed...

I mean the frequencies, for example in HF band roughly 3.5 to 30MHz, that is used for the ham radio operators. I ask this because the planned location of the station, would be same as the location of my RF-transmitters. The power levels feed on antennas at HF would be estimated 50...100W max. Maybe in a future I could install some VHF/UHF transmitters also...

thanks,
Timo K, Espoo / Finland
OH7HMS
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#2
Our primary interest is in the area 3-300KHz.
Outside this area we absorb very strongly - more than 100dB/octave (done AFTER Pre-Amps)
On the other hand - near a dipole fed with power of 100W, there are several hundreds of volts/meter in the near field.
WORST CASE - Pre-Amps are saturated while keying your transmitter, that it :-)
Stations: 584, 585, 2017
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#3
(2015-07-03, 08:38)RichoAnd Wrote: Outside this area we absorb very strongly - more than 100dB/octave (done AFTER Pre-Amps)
Just a small remark on this:
If you write it in this manner the OP may expect several hundred dB suppression at the mentioned frequencies. However in practice you may be happy if the isolation is more than 100 dB in general out of the desired band.

And: filters behind pre-amps do not avoid intermodulation products generated by the pre-amps that fall in band.

I understood the receiver of Blue system will not much differ from the Red system. So the schematics is more or less available for further analysis.
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#4
I know Smile thats why I only wrote 100dB Smile

We don't need high sensitivity nor high dynamic range.

Amplifiers differ a lot from RED
H-Field Pre-Amp use a Fully Differential Op-Amp, 100 Ohm twisted pair an trafo on input of main amplifier.
Stations: 584, 585, 2017
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#5
(2015-07-03, 18:45)RichoAnd Wrote: I know Smile  thats why I only wrote 100dB Smile

You wrote 100dB per octave. Tongue
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#6
Sorry Smile
42dB/Oct - 140dB/Decade
Stations: 584, 585, 2017
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#7
(2015-07-03, 08:38)RichoAnd Wrote: Our primary interest is in the area 3-300KHz.
Outside this area we absorb very strongly - more than 100dB/octave (done AFTER Pre-Amps)
On the other hand - near a dipole fed with power of 100W, there are several hundreds of volts/meter in the near field.
WORST CASE - Pre-Amps are saturated while keying your transmitter, that it :-)

thanks for comments...

When it is stated like "pre-amp may be saturated" I understood it means actual voltage amplified (from antenna) is too high so that any lightning indication may not be enough to trigger actual detection ?

Is there any experiences from earlier releses, what kind of antenna maybe less sensitive (loop, ferrite, E-field) to strong nearby RF fields - and still has a decent "gain"?
I refer to chapter 4 at project description:
http://www.blitzortung.org/Documents/TOA...1436034986

What I understood, it cannot fully be predicted whether the basic unit (or amplifier) of the System Blue -release is capable of handling strong nearby RF fields. I quess it needs an actual testing  - case by case - on planned site location...

The antennas that I mentioned would not be very far from planned location the BO receiver equipment. Not more than 20 meters. Basicly the 50...100W levels, are maximum values. On the transmitters there is possibility to setup also low power levels, if disturbances generated to any other electric equipment...

-Timo K.
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#8
(2015-07-04, 18:51)tkuhmone Wrote: When it is stated like "pre-amp may be saturated" I understood it means actual voltage amplified (from antenna) is too high so that any lightning indication may not be enough to trigger actual detection ?
As bonzo said; saturation is leading to intermodulation, which probably would result in the station going into interference mode.
But this isn't a problem.

Small loop antennas are probabably less sensitive to HF-Fields, since they are from an e-field point of view a short-circuit. But near-fields are difficult to foresee, so there will be no other way as trying.
I had no problems with 100W in 20m distance at 14MHz. The bigger problem was interference from the switching power supply of the transceiver.
Stations: 233
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#9
(2015-07-04, 19:32)Steph Wrote: As bonzo said; saturation is leading to intermodulation, which probably would result in the station going into interference mode.

Well, that is not exactly what I wanted to say. Intermodulation happens already below the saturation level. It can produce in-band products from two or more out-of-band signals. And once they are in band they can not be eliminated by filters.

So filters should be appear as early as possible in the signal chain.

BTW: Another issue with the strong field is that it could ecxite a common mode signal in the cables from the antenna probes. Thus the signal(s) is/are not picked up by the antenna itself but by the aerial cabling. Especially if dimensions are multiples of quarter lambdas. Good grounding (= creating a RF reference plane) is crucial.
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#10
Each Amp channel also including two digital gain adjustable Amps, a digital LP-Filter and an anti aliasing filter just before the A/D-converter

Pre-Amp for the H-Field is tested for a long period and with different antennas and operating as intended.

E-Field is a slightly modifyed pre-amp as used in System RED

All analog circuitry is straightforward, but we need to test possible disturbance from the digital circuits surrounding the Amps - IT takes time because everything must sit properly on the final PCBs before ordering hundreds Smile
We need an adaptation from the antenna to the amplifier via a cable - the cable must be properly terminated and therefore the easiest way is with a pre-amp, since the antenna impedance varies greatly.
Stations: 584, 585, 2017
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#11
(2015-07-04, 19:32)Steph Wrote: I had no problems with 100W in 20m distance at 14MHz. The bigger problem was interference from the switching power supply of the transceiver.

Good point here... I have heard similar comments from local ham radio operators, who run also BO equipment. Althoug SMPS would not be main topic of this thread - it needs to be considered as potential interference source.

I have also planned to modify HP server 12V power supply for amateur radio use, and it also used SMPS principle Undecided . If there is interference, remains to be confirmed...
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