LF: RE: active ant on metallic mast

[<victor13@online.nl>]

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

---

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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