LF: RE: RE: active ant on metallic mast

[<victor13@online.nl>]

Hello Stefan,

 

I will sent you the design directly because it is to big a file for blacksheep.It is a design with just 2 fets and 3 transistors.

The shed has ia 230V supply so it isn't isolated.When I increased the hight both qrm decreased and signals increased,getting higher then 7m didn't improve much.Since I am not allowed to build higher then 5 m above the roof I left it this way! (the pole and the coax have their own earthing in the shed,their are also 2 common mode shokes in the coax to the house)

For the lenght of the whip 60cm was ideal for this situation,I could still hear the weakest beacons (var,fny) and  their where no signs of im of the local mw stations.DCF is 500uV (S9+20dB) at this moment,off course on a calibrated meter.

 

good luck with your experiments,

 

73

 

Victor

-----Oorspronkelijk bericht-----
Van: Stefan Schäfer [mailto:owner-rsgb_lf_group@blacksheep.org]Namens Stefan Schäfer
Verzonden: zaterdag 6 februari 2010 23:51
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: AW: RE: active ant on metallic mast

Hi Victor,

 

Where can i get the design of your preamp? Is it a broad band design or also with some band pass filters? Is it with FETs and/or with OP amps? 28V? I assume +-14V, right? ;-)

Is in this shed a 230V supply or is it completely ("electrically") apart from the house?

 

"Less qrm and higher output", does this mean the noise level itself decreased, so not compared to the wanted signal? If hight increases and the influence of surrounding lossy things decreases, the E-Field and thus the input voltage increases, and thus the SNR, if noise level remains the same. And if you then decrease the AF of your RX to get again a proper visible signal in e.g. Argo, the noise gets into the background. But does the noiselevel itself decreases also significantly? If so, there must be a noise source in the near of the shed, isn't it?

 

I think when i will do the test with the active antenna with the 2 UKW variable antennas, i will mount it to the kite and look how it depends when it is 50m above ground, completely isolated. Perhaps not very useful but interesting ;-)

 

You decreased the lenght from 1m to 60cm. Have you also tried 30cm?

A hight of 7m gave the best results you said. So things get worse, when you go to 10m hight?? If so, what do you think is the reason for that? Or is the shed under a 400kV power line? ;-)

 

Victor and Roelof, tnx for your hints and ideas. I like that conversation and i like to try mni unusual things. A receiving loop will give the best signal i assume, but it is not very easy to handle if one is always /p.

 

Congrats to the canadian stns to be back on LF!! Hpe to get a contact to VO1NA next! But since 6 weeks there is almost no wind at weekend, the longest break since 2 years :-( But i will be back with a big signal :-)

 

73, Stefan/DK7FC

 

 

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Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von victor13@online.nl
Gesendet: Sa 06.02.2010 20:46
An: rsgb_lf_group@blacksheep.org
Betreff: LF: RE: active ant on metallic mast

Hello Stefan,Roelof,

 

for about 4 years now I am using an active whip with good results.The antenna is placed on a shed about 10 m from the house.I varied the height between 2 m and 9 m.With a height of 2 m(the roof) I had alot of qrm and a low output,raising it to 5m or higher improved things a lot.Less qrm and higher output,to prevent im i decreased the size of the whip from 1m to 60 cm.At a height of 7m this gives the besr results.no detectable im from local mw stations and a good S/N ratio of weak signals.As an experiment I replaced the steel pole for a glassfibre one,this didn't change any thing.This could be espected because of the coax cable you are always at the same potential as with a steel pole.Ofcourse  using a battery and a glassfibre changes things but for me this is not very practical because my whip draws 120mA @ 28V.

Hope to hear more from your experiments,

 

73

Victor

PA3FNY

-----Oorspronkelijk bericht-----
Van: Stefan Schäfer [mailto:owner-rsgb_lf_group@blacksheep.org]Namens Stefan Schäfer
Verzonden: zaterdag 6 februari 2010 17:47
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: active ant on metallic mast

Hi Roelof, LF,

 

Roelof, why do you think that this is the case? Interesting Question! "Won't work" means "Won't work properly" it assume. There will be a signal but perhaps it is not optimal. And why could this be the case? If the application is in a metallic housing that is mounted to a metallic mast, the capacity of the "ground electrode" against the environment must be very high (can be seen as infinite, i assume), compared to the active element (gate of the FET). Perhaps there will also be a better coupling to qrm sources? I also have to make some measurements. The antennas we use are comparale i think. So we could share (all) share our experiences :-)

 

A special question comes up when a optic fiber cable is used, since in a coax application there remains a high C of the ground electrode, even when a symmetrical transformer is used (since the active element has just a few pF to the far field). That special question is: What will happen, when the case/ground electrode becomes small against the active element (e.g. circuit in SMD, small battery inside)?! Will there be a change in the optimal C of the active element to the far field? I assume, then, one has to define an optimal C ratio of both electrodes. Will it behave as a short dipole? And what about a short vertical (30cm) active antenna that is directly placed on the ground with a almost ideal conducting ground plane, e.g. aluminium foil (out of the household) in a radius of 1m (ignoring the local qrm problem, so e.g. in your garden, apart from the city)?

 

On my new qth i made first steps to receive LF with the active antenna mounted just 2m above ground but hung up on a wet tree. Results were vy bad. I thought that SNR could be better when increasing the active element since signals were weak (DCF39 at S7), but it wasn't. The optimal length was still 30cm. That seems not only to be "sufficient" but rather optimal! So less and more is worse, unaffected by the hight above gnd?

 

I think one has to imagine the E-Field lines that are going through the ambience. The fieldstrength seems just to be very small in a lossy environment (on my hill, where no trees can be found, the antenna was also 2m up and DCF39 was S9+20). So hight above ground seems to essencial, even without local qrm. Each decrease of input signal can be compensated by more gain but SNR decreases, of course.

 

What are your ideas to these thoughts? What do you think will happen if the ground-electrode-C becomes small against the C of the active element? What will be the optimum C for both? Will the electrodes have the same "importance", like as a short dipole? Questions over Questions ;-)

 

Recently i had the idea to test that with a variable antenna of a /p UKW radio, perhaps two for each electrode. That will be interesting to play with ;-)

 

73!! Stefan/DK7FC

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Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von Roelof Bakker
Gesendet: Sa 06.02.2010 13:53
An: rsgb_lf_group@blacksheep.org
Betreff: Re: LF: AW: Re: test signal wanted

Hello Stefan,

Probably a bit late, bit I have been told that an active whip won't work 
well on a metalic mast.
I never tried it myself (still on the to do list) due to lack of a 
suitable mast.

73,
Roelof, pa0rdt

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