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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