Hi John that is a good point, balanced is always better . I had wondered
whether the cap in series with the loop could be split into two parts to
retain the balance. The article is in the blue backed "Source book" and is
called High Power Matching at VLF by G3OKA ..........split C1 into two parts
?? This was originally intended for driving a73kHz loop from an 8ohm audio
amp.
Alan
G3NYK
----- Original Message -----
From: "John Andrews" <[email protected]>
To: <[email protected]>
Sent: Monday, March 21, 2011 12:56 PM
Subject: Re: LF: Loop TX antennas at VLF?
> Just one caution - particularly for impedance measurements, and probably
> for operating: Treat the loop as balanced. If you use the capacitor
> method that Alan suggests to get to 50 Ohms (or whatever), follow it
> with a 1:1 matching transformer.
>
> From experience at 137 and above, it is very difficult to measure the
> impedance of a "large" (but electrically small) loop with mains-powered
> unbalanced test equipment. My usual setup is an unbalanced RF impedance
> bridge, a battery-powered receiver, and a battery-powered RF generator,
> all set up a meter or so above the ground.
>
> Back to the capacitor-matching approach: It's no more than a classic "L"
> network, where the loop supplies the inductance. The reactance of the
> series capacitor is smaller than necessary for tuning.
>
> John, W1TAG
>
> On 3/21/2011 8:11 AM, Alan Melia wrote:
> > 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
> >
> > --
> > 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/
> > 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/
> > 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/
> > G3XBM GQRP 1678 ISWL G11088
> >
> >
> >
> >
>
|
