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Re: VLF: Resonate the antenna, or not?

To: [email protected]
Subject: Re: VLF: Resonate the antenna, or not?
From: Stefan Schäfer <[email protected]>
Date: Wed, 23 Mar 2011 02:13:43 +0100
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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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