I seem to remember from the 73kHz days and article in an early LF Handbook
(the blue cover ??) on matching a loop with a pair of capacitors at the feed
point and a program to calculate the values. It doesnt get round the voltage
rating problem but if you can crack that it may solve the ferrite saturation
??
Alan G3NYK
----- Original Message -----
From: "Rik Strobbe"<[email protected]>
To:<[email protected]>
Sent: Monday, March 21, 2011 8:30 AM
Subject: RE: LF: Loop TX antennas at VLF?
Hello Scott,
the discussion is about a 10 by 10 meter loop and 100W RF power.
Using 4 x 1.5mm Cu wire (parallel) the DC resistance of a 10 by 10 meter
loop is about 0.1 Ohm. Ignoring other losses and tuning the loop the current
is about 32A.
I don't have an idea what addional (ground) losses tha loop will suffer from
at 9kHz.
The loop indictance was calculated 40uH (by Jim), so the loop has a
reactance of 2.3 Ohm at 9kHz. This means that the loop voltage is only about
70V, so stray currents to the ground (or other grounded objects near the
loop) will be minimal and these losses can be ignored.
Other losses are due to induced currents (so called Eddy currents), but if I
remind well these losses are proportional to the square of the frequency
what means that at 9kHz these lossses are over 200 times less than on
137kHz.
In an ealier mail I suggested to use coax cable as loop wire.
But to my own surprise Belden (main coax cable manufacturer) gives rather
large DC resistanses:
RG58 = 4.2 Ohm/1000ft for the shield and 3.3 Ohm/1000ft for the conductor.
So 40m (133ft) of RG58 would be 0.56 Ohm for the shield and 0.25 Ohm for
shield and conductor in parallel. See
http://www.emaxit.asia/htx/document/P0002/7807A.pdf
Better would be RG8 with 1.2 Ohmm/1000ft for the shield and 1.9Ohm/1000ft
for the conductur. 40m of RG8 would be 0.16 Ohm for the shield and 0.1 Ohm
for shield and conductor in parallel. See
http://www.hytamerica.com/Download/Private/file/PDF/belden/8237.pdf
But even RG8 would be no better that 4 x 1.5mm Cu wire in parallel.
73, Rik ON7YD
________________________________
Van: [email protected] [[email protected]]
namens Scott Tilley [[email protected]]
Verzonden: maandag 21 maart 2011 3:52
Aan: [email protected]
Onderwerp: Re: LF: Loop TX antennas at VLF?
Hi Rik
I think I missed a message or two. What current level is the consensus for
100W input at 9KHz into a reasonable loop? I missed how this was modeled
and wonder if my original assumptions where off base.
Thanks for any clarification you can offer.
73 Scott
VE7TIL
On 3/19/2011 9:27 AM, Rik Strobbe wrote:
Roger,
the antenna reactance about 2.5 Ohm, so the antenna voltage (and thus
capacitor voltage) will be less than 100V.
I assume that polypropylene caps that work fine on 137 and 500 will also be
OK at 9kHz.
Farnell sells 1uF/275Vac at 0.44 Euro (10 QTY) and 0.1uF/305Vac at 0.32 Euro
(10 QTY), so for less than 10 Euro you should be able to tune the antenna in
0.1uF steps. As the antenna Q is rather low (2.5/0.1 = 25) a 0.1uF step
should be OK for a first try.
73, Rik ON7YD - OR7T
________________________________
Van:
[email protected]<mailto:[email protected]
[[email protected]<mailto:[email protected]
g>] namens Roger Lapthorn
[[email protected]<mailto:[email protected]>]
Verzonden: zaterdag 19 maart 2011 12:47
Aan: [email protected]<mailto:[email protected]>
Onderwerp: Re: LF: Loop TX antennas at VLF?
Rik, et al
Actually I am beginning to think that this small VLF TX loop is not such a
totally daft idea after all. The main issue seems to be with the capacitors
but these seem to be less onerous than winding a very big (and lossy) coil.
Certainly there sounds to be merit in a larger TX loop for /P operation.
Thanks everyone for the constructive feedback on this thread. Most
interesting.
73s
Roger G3XBM
On 19 March 2011 10:42,
<[email protected]<mailto:[email protected]>> wrote:
Rik, Roger, Jim, Mal
There are soil losses to consider with the loop as well. At 185 kHz (Part 15
lowfer band), I ran a 50' X 50' transmitting loop made from mil spec RG-11
(copper braid) and the soil losses were about equal to the wire losses. Not
sure what the soil loss would be at 9 kHz but it would be interesting to
know.
Jay W1VD WD2XNS WE2XGR/2
----- Original Message -----
From: Rik Strobbe<mailto:[email protected]>
To: [email protected]<mailto:[email protected]>
Sent: Saturday, March 19, 2011 6:07 AM
Subject: RE: LF: Loop TX antennas at VLF?
Roger,
as Jim calculated running 100W in a 10 x 10 m loop will give about 0.5uW ERP
(is you use 4 x 1.5mm wire in parallel instead of a single 3mm wire in order
to avoid skinn effect losses). Using more parallel wires of a coax cable
might pump up the ERP to 1 or 2uW.
Looks pretty poor, but will a vertical antenna of a similar size do better ?
At 9kHz the radiation resistance of a 10m high + 10m topload vertical
75uOhm.
The antenna capacitance is 110pF, a reactance of 161kOhm. What means that
you will need a loading coil of 2.84H (yep Henry). Apart from the fact that
it will cost a lot on copper wire the coil losses will be high. You will
need a Q of 160 to reduce the losses to 1kOhm. In addition for such a small
antenna you can excpect several 100 Ohm ground loss, so let's assume a total
loss of 1500 Ohm. 100W TX power will result in about 0.25 A antenna current
and an ERP of about 8uW.
That's 6 to 10dB better than the loop, but instead of some cheap cap's you
will need a monster coil. I ran it one on the online coil calculators and it
came up to a 2.5m high and 1.8m diameter coil with amost 10km of 1.5mm Cu
wire (weight 150kg). And running 0.25A into the antenna will result in 40kV
!
So, despite the vertical could be 10dB better than the loop, the loop seems
much more easy (and cheap) to build.
It might be easier and cheaper to get the extra 10dB by usung mor wire in
the loop and pump up the power.
73, Rik ON7YD - OR7T
________________________________
Van:
[email protected]<mailto:[email protected]
[[email protected]<mailto:[email protected]
g>] namens Roger Lapthorn
[[email protected]<mailto:[email protected]>]
Verzonden: vrijdag 18 maart 2011 23:02
Aan: [email protected]<mailto:[email protected]>
Onderwerp: Re: LF: Loop TX antennas at VLF?
Yep, I guess you're right Scott. And you know better than most. Ah well, it
was an interesting idea to toss around.
73s
Roger G3XBM
On 18 March 2011 21:53, Scott Tilley
<[email protected]<mailto:[email protected]>> wrote:
Hi Roger
The practicality of pumping 35A into a loop is not an easy task! Couple
this with the stability of most capacitors creates a real engineering
challenge for a loop on 9KHz, notE the BW and Q. Not to mention really low
ERP one would get.
This will be an engineering challenge for sure!
73 Scott
VE7TIL
On 3/18/2011 2:39 PM, Roger Lapthorn wrote:
Hi All
Just run Andy's spreadsheet for magnetic loops to see the sort of figures we
get at 8.9kHz. Assuming 100W and a loop diameter of 10m with 3mm wire the
efficiency works out at -87.4dB and the ERP -67.4dBW (0.2uW). There is also
the matter of the low loss 6211.7nF capacitor. With larger loop diameters,
thicker wire (or multiple paralleled wires) and maybe 200W then the ERPs are
starting to get more useful.
The Marconi does seem a better bet, even with all the issues with losses in
the huge loading coil, but a VLF TX loop doesn't look a total "no-hope"
approach. Larger loops, with improved efficiencies, may be easier than kite
or balloon supported ones in a /P location.
And then there is the widely spaced earthed electrode antenna..... but I
won't start a discussion on the merits or otherwise of this as I am about to
go on holiday this weekend and will not be able to respond to emails next
week. We know from work by DK7FC (and VLF professionals) that this does work
as a radiating structure.
73s
Roger G3XBM
On 18 March 2011 14:09, Roger Lapthorn
<[email protected]<mailto:[email protected]>> wrote:
Just wondering if anyone has done the maths to work out what sort of ERP
could be expected at 8.97kHz with, say, 100W to a smallish loop antenna in
the garden?
It would certainly avoid the need for very very large matching coils and may
be easier to engineer than a Marconi. Even an efficiency of -80dB would
allow 1uW ERP and, judging by results from G3XIZ with around 2uW, this could
be useful with long stable carrier transmissions of several hours. Most of
us could run a loop with an area of 100sq m. with thickish wire in our
gardens. A loop might also be more practical for portable operations perhaps
with a triangle with one high support.
Certainly my own results with WSPR at 136 and 500kHz with just a few watts
and quite thin wire and around 80sq m loop area were encouraging. Mind you,
9kHz is very much lower than 136kHz, so the radiation resistance would be
tiny I assume.
73s
Roger G3XBM
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G3XBM GQRP 1678 ISWL G11088
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