Re: LF: VLF in Canada - earth loop

[Markus Vester <markusvester@aol.com>]

Stefan, Joe

 

> I think that 8m above ground should be fine for any tests :-)

well (apart from practical reasons like people tripping over it), you might just as well lay the (insulated) wire on the ground.

 

From receive measurements with your earlier and shorter earth antenna, we deduced a depth of return current of 29 meters at 23.4 kHz, which would scale to about 48 m at 8.3 kHz. Thus a two kilometer long earth antenna would have the same radiation resistance (0.35 milliohm) as a 17 m high top-loaded vertical. But skin depth and efficiency might be greater for very low ground conductance.

 

Best 73,

Markus

 

-----Ursprüngliche Mitteilung-----
Von: Markus Vester <markusvester@aol.com>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 21 Jul 2010 9:07 pm
Betreff: VLF: RE: DLF passive received with earth antenna

...

 

Thus the effective height of the 280 m long earth antenna was
 heff = U/E = 3.97 m at 23.4 kHz,
giving a ground loop area
 A = heff * lambda/2pi = 8090 m^2,
and an effective return current depth
 d = heff /length = 29 m.

 

This depth appears to be much less than half of the baseline length, so it is probably indeed skin effect limited. Scaling the 23.4 kHz results to 9 kHz would give 1.6 times more skin depth, but 1.6 times less effective height (ie. 2.5 m) due to the larger wavelength to loop length ratio. Thus the transmit efficiency at 9 kHz would be about 26 dB less than that of a 100 m kite antenna with similar loss resistance.

...

-----Ursprüngliche Mitteilung----- 
Von: DK7FC <73dk7fc@googlemail.com>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Di, 24 Jun 2014 3:05 pm
Betreff: Re: LF: VLF in Canada

Oh yes, Joe, if you have plenty of forest where you can play VLF games, i would also play with a dipole antenna. That's what i said to Laurence a few months ago. Just install a dipole with say 10 km length, adding 100m each day and see how the performance increases. This will be an interesting project :-)

Maybe you remember my 700m long earth antenna which allowed me to be copied in 45 km distance in 4.5 mHz FFT BW, back in 2011 :-) Someone removed the antenna completely but if you have more space, just give it a try. I think that 8m above ground should be fine for any tests :-)

Nice to think about it :-)

73, GL, Stefan

Am 24.06.2014 12:55, schrieb Markus Vester:

Joe, good to read this!

 

Regarding the loading coil, I would agree with Stefan that a large multiturn air coil is the best option. A laminated iron core would suffer from excessive eddy current losses, and the effect of ferrites is limited by saturation and hysteresis losses.

 

My 1.3 henry coil consumed 2.3 km of 0.4 mm enameled wire, using 7 buckets with 480 turns on each:
df6nm.bplaced.net/VLF/VLF_110304-06/coil_legospacers.jpg.
The advantage is that due to magnetic coupling between layers, you will need less wire for a given inductance. And the inductance is adjustable across a wide range, using spacers. Disadvantages are the high electric field between layers limiting voltage capability, and less effective heat removal from the inner buckets. Stuffing the buckets too tightly into one another is surely not a good idea:
df6nm.bplaced.net/VLF/pictures/arced_coil_140601.jpg

 

An nonresonant earth antenna across high resistivity rock could also radiate moderately well, avoiding coil making and high voltage issues altogether. But to compete with your vertical in terms of radiation resistance, it would need to be really long, on the order of a couple of miles. It will have magnetic loop directional pattern which may be a disadvantage.

 

For a transatlantic detection eg. by Paul Nicholson, a Rubidium or GPS locked signal source would be very beneficial. Using SpecLab with 1pps phase lock to eliminate soundcard output glitches has worked well for DJ8WX, PA1SDB, and myself.

 

Best of luck,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: DK7FC <73dk7fc@googlemail.com>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Di, 24 Jun 2014 12:48 am
Betreff: Re: LF: VLF in Canada

Hi Joe,      Fine to hear that you're give it a try. Please, don't ask Industry    Canada if the 10mW is RF power of ERP!      To get out a real signal on VLF, there is no alternative to a real large    coil without ferrites! Think about the high voltages... But 100m of    antenna wire is a good start. I remember 550 mH for my 100m kite    antenna. For me it was 2000m of 0.4mm diameter wire :-) Nice work is    that. Takes long but is a funny game :-)      73, GL, Stefan      Am 23.06.2014 21:33, schrieb jcraig@mun.ca:   Dear Group,   

I tried again to get an LOA  for VLF.  This time, the response from    Industry Canada was favourable: 10 mW 8.0-8.3 kHz.  Yesterday, after a    few slight rf burns, and  thanks to PA0RDT's miniwhip design and    DL4YHF's Spectrum Lab and advice, VLF sigs on 8.277 kHz were heard at    200m from the 100m wire aerial. This was a reception outside my back    yard!  10 watts from an 1970's keyboard amp (volume control set on 3)    were stepped up with an xfmr to a lossy 500 ohm tuning coil peaked    with a ferrite rod for maximum squeal at L=0.35 H.   

Is it possible for someone who is not especially enthralled by winding   a mile of 0.7mm wire to use an iron core to make a VLF tuning coil?   

73   Joe VO1NA