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Date: Wed, 23 Mar 2011 02:13:43 +0100
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Subject: Re: VLF: Resonate the antenna, or not?
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Hello Markus, VLF,

Am 22.03.2011 12:49, schrieb Markus Vester:
> Jim's small VLF loop example (0.1 + j 2.2 ohms) had a Q of only 22, 
> ie. 1/Q = 4.5% of the energy is lost per cycle. So the total power 
> comparison would be 6.5% for the amplifier version versus 4.7% for the 
> capacitor - really not that much difference.
> But as Stefan says, the amplifier would have to be big enough to 
> handle all the reactive power (VxA). If fed by a large enough power 
> supply, the very same amplifier could deliver 2.2 kW and raise the 
> radiated power by a factor of Q, ie. 13 dB.
...but there is a difference between 2.2 kW and 2.2 kVA, regarding your 
electrical energy bill, if you run the TX all the time :-)
> BTW There exists a similar criterion for active receive antennas: If 
> you have a small capacitive probe and resonate it with a coil, the 
> coil losses will add in some resistive noise. In a ferrite loopstick, 
> a (nearly lossless) capacitor is typically used to tune out the 
> inductive reactance, but this makes the antenna narrowband.
Yes, and narrowband is not that good in the case of a loop that can 
slightly change its size (due to moving trees in the wind or so) or 
change its capacitance due to thermal drifts.

The focus or goal is to radiate a signal as strong as possible. If  the 
sourroundings are to lossy and a loop is available, with given 
dimensions and wire diameter, the goal is to run a current into it, as 
high as possible. While the limit on most VLF marconi antennas is the 
voltage across the coil and on the wire, the limit will be the current 
of the loop. The power supply is the mains. If suitable caps are choosen 
on the DC side, many (!) kVA reactive power can be handled. The mains 
has just to supply the losses. So the critical parts are the FETs and 
the loop wire diameter. I think 100 A rms is a problem for most loops. 
But i would give it a try. Even the 4x or 5x 1.5 mm^2 could take that 
current for a while, depending on the outside temperature. BTW there 
will be a current limiting when the temperature increases :-)

Who is the one who does the first test? Scott? :-) BTW the distance 
between VE7TIL and KL7UK is just 2084 km! 
http://no.nonsense.ee/qthmap/?qth=CN89DK&from=bp51ip

Scott, what do you estimate is the current limit of your loop wire and 
what is the area? And what is the bearing?

73, Stefan/ EI/DK7FC



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Hello Markus, VLF,<br>
<br>
Am 22.03.2011 12:49, schrieb Markus Vester:
<blockquote   cite=3D"mid:8CDB69E4AD7AA3C-1490-33A81@webmail-m018.sysops.a=
ol.com"   type=3D"cite"><font color=3D"black" face=3D"arial" size=3D"2">
  <div>Jim's small VLF loop example (0.1 + j 2.2 ohms) had a Q of only
22, ie. 1/Q =3D 4.5% of the energy is lost per cycle. So the total power
comparison would be 6.5% for the amplifier version versus 4.7% for the
capacitor - really not=C2=A0that much difference. </div>
  <div>=C2=A0</div>
  <div>But as Stefan says, the amplifier would have to be big enough=C2=A0=
to
handle all the reactive power (VxA). If fed by a large enough power
supply, the very same amplifier could deliver 2.2 kW and raise the
radiated power by a factor of Q, ie. 13 dB.</div>
  </font></blockquote>
...but there is a difference between 2.2 kW and 2.2 kVA, regarding your
electrical energy bill, if you run the TX all the time :-)<br>
<blockquote   cite=3D"mid:8CDB69E4AD7AA3C-1490-33A81@webmail-m018.sysops.a=
ol.com"   type=3D"cite"><font color=3D"black" face=3D"arial" size=3D"2">
  <div>=C2=A0</div>
  <div>=C2=A0</div>
  <div>BTW There exists=C2=A0a similar criterion for active receive
antennas: If you have=C2=A0a=C2=A0small capacitive probe and resonate it=
 with a
coil, the coil losses will add in some resistive noise.=C2=A0In a ferrite
loopstick, a (nearly lossless) capacitor is typically used to tune out
the inductive reactance, but this makes the antenna narrowband. </div>
  </font></blockquote>
Yes, and narrowband is not that good in the case of a loop that can
slightly change its size (due to moving trees in the wind or so) or
change its capacitance due to thermal drifts.<br>
<br>
The focus or goal is to radiate a signal as strong as possible. If=C2=A0=
 the
sourroundings are to lossy and a loop is available, with given
dimensions and wire diameter, the goal is to run a current into it, as
high as possible. While the limit on most VLF marconi antennas is the
voltage across the coil and on the wire, the limit will be the current
of the loop. The power supply is the mains. If suitable caps are
choosen on the DC side, many (!) kVA reactive power can be handled. The
mains has just to supply the losses. So the critical parts are the FETs
and the loop wire diameter. I think 100 A rms is a problem for most
loops. But i would give it a try. Even the 4x or 5x 1.5 mm^2 could take
that current for a while, depending on the outside temperature. BTW
there will be a current limiting when the temperature increases :-)<br>
<br>
Who is the one who does the first test? Scott? :-) BTW the distance
between VE7TIL and KL7UK is just 2084 km!
<a class=3D"moz-txt-link-freetext" href=3D"http://no.nonsense.ee/qthmap/?q=
th=3DCN89DK&amp;from=3Dbp51ip">http://no.nonsense.ee/qthmap/?qth=3DCN89DK&=
amp;from=3Dbp51ip</a><br>
<br>
Scott, what do you estimate is the current limit of your loop wire and
what is the area? And what is the bearing?<br>
<br>
73, Stefan/ EI/DK7FC<br>
<br>
<br>
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