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:
But even RG8 would be no better that 4 x 1.5mm Cu wire in parallel.
73, Rik ON7YD
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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