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
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]>
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 -----
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
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]>
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]>
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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http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
https://sites.google.com/site/sub9khz/
G3XBM GQRP 1678 ISWL G11088
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
https://sites.google.com/site/sub9khz/
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