Re: LF: AW: 8.97kHz - Near field and radiated signals?

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Thanks to all for the explanations. 
I'm glad others get confused too!!

73s
Roger G3XBM

On 26 May 2010 10:54, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

Chris, Roger,

 

there seems to be a lot of confusion.

I just looked it up in some old tekstbooks, but they are very brief on this matter.

A quick "google" wasn't of much help either.

A clue maybe in one of the tekstboooks that made a difference between the far field, the radiating near field and the reactive near field (defined as "very close to the antenna").

In the radiating near field signal attenuation is given as 40dB/decade (for the H-field), in the reactive near field as 60dB/decade.

 

I guess that the best way to find out is to try it. If G6ALB has a computer you can just hook up your portabe RX to the PC (connect the RX to the soundcard input instead of the earpiece), if possibe with the RX outdoors.

 

If we take the worst case (56 dB):

Assuming  that the bandwith for CW copy with the simple RX is 100Hz and and you have good CW copy (let's say at least 10dB S/N) you will need an extra 46dB , what means reducing bandwith to 0.0025Hz (= QRSS400, or over 6min/dot). So a good point to start with is a 10 min or 15 min on/off cycle (and a xtal controlled TX).

 

73, Rik  ON7YD - OR7T

 

---

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Rik Strobbe [Rik.Strobbe@fys.kuleuven.be]
Verzonden: woensdag 26 mei 2010 11:01

Aan: rsgb_lf_group@blacksheep.org

Onderwerp: RE: LF: AW: 8.97kHz - Near field and radiated signals?

Roger,

 

sorry, I made a mistake.

in the near field the H field drops 40dB/decade and the E-field drops 60dB/decade.

Assuming the ground antenna acts as a loop (= H-field) the signal drop between 0.4km and 3km will be 35dB.

The defintion of near field is vague, depending on the source it goes from 0.1 wavelength to 1 wavelength.

But at 1kHz even 0.1 lambda is 30km, so you are defenitely in the near field.

 

73, Rik  ON7YD - OR7T

 

---

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Roger Lapthorn [rogerlapthorn@gmail.com]
Verzonden: woensdag 26 mei 2010 10:43
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: LF: AW: 8.97kHz - Near field and radiated signals?

Rick (et al),

I am confused by the near field attenuation rates quoted. Some quote 18dB for doubling distance which would mean an additional path loss of 56dB going from 0.4km to 3km. This makes a lot of difference to what may or may not be possible.

Which is the correct figure? Help!

73s
Roger G3XBM

On 26 May 2010 09:31, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

Roger,

 

the near fiels signal drops with the 3rd power of the distance.

So 3km versus 0.4km = 422 times weaker = -26dB

Going from regular CW to QRSS30 will give you a 24dB benefit, so there is a fair chance to be copied at 3km in this mode. Of course the signal needs to be very frequency stable, it has to be better than an free running oscillator.

Moving up in frequency would also help.

And if you can lay your hands on an old stereo amp it would provide an extra 10dB TX power.

 

Even if your goal is regular CW over a 3km path, starting with QRSS will most likely give a visible signal at 3km and allow you to watch any progress.

 

73, Rik  ON7YD - OR7T

 

 - The best DX targets are quite modest: I would like to exceed 1km

 initially with the TX power I currently have (around 4W). If I could reach
 G6ALB in the next village 3kms away I would be delighted. If I got any
 further I would be both amazed and VERY happy!

--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM    GQRP 1678      ISWL G11088

-- 
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM    GQRP 1678      ISWL G11088