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Tnx info Markus
I mean use a ferrite core inductor to reduce wire
resistance to resonate the vertical. I have already put 162 turns on a
0.5cm diam barell 1mm pvc insulated wire and only get 14.5 mH
This could be a life time job hi
de mal/g3kev
----- Original Message -----
Sent: Tuesday, March 22, 2011 4:52
PM
Subject: Re: LF: Re: Resonate the
antenna, or not?
Hi Mal,
no, I would not recommend transmitting on a vertical without a
resonant loading coil, it's not really efficient.
A 150 mH coil would not be very difficult to make. For a
straight single layer solenoid, you'd need approximately 950 turns of 0.4 mm
magnet wire on a 0.3 m diameter former. If you really want a larger
one, use about 550 turns of 1 mm wire on a 0.6 m former.
Can be done in one afternoon ;-)
Ferrite or iron can in principle be used reduce the size, but you will
still need a large number of turns and a large core crosssection to avoid
saturation. As ferrites are far from perfect insulators, large
spacings from the wire to the core are required to avoid voltage
breakdown.
Hope to see you soon on Dreamer's Band!
Best 73,
Markus (DF6NM)
-----Ursprüngliche
Mitteilung----- Von: mal hamilton < [email protected]> An:
[email protected]Verschickt: Di., 22. Mrz. 2011,
15:59 Thema: LF: Re: Resonate the antenna, or not?
Markus
Have u discovered another method of resonating a
vertical type antenna other than a large air wound water butt.
I was thinking of a smaller coil with ferrite core. In
my case I need about 150 mH not too large but winding a BIG coil is still a
BIG job hi
de mal/g3kev
----- Original Message -----
Sent: Tuesday, March 22, 2011 11:49
AM
Subject: VLF: Resonate the antenna, or
not?
Hi Stefan,
you are raising the interesting question of how important it is to
resonate the antenna. I think it boils down to a comparison of the amplifier
efficiency versus the Q factor of the matching element.
Without the
matching element, the amplifier (really a switchmode DC-to-AC converter) has
to transfer the reactive power back and forth between the load and the power
supply capacitors. With ideal switches, indeed nothing would be lost. But
with finite efficiency, you will loose a portion of (1 - eta), eg. 2 % per
cycle if your amplifier had 98% efficiency measured into a real
load. On he other hand, if your loop was resonated using a
capacitor with Q-factor 500, you would dissipate only 0.2% per cycle in
it.
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.
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.
If on the other hand you do not compensate the reactance, you
can still noise-match the preamp to the modulus of the source
impedance, at the penalty that it's own noise temperature will
increase by a factor of Q. For very low preamp noise figures (ie.
Tpreamp < Tantenna / Q), adding the tuning element brings
little or no benefit for the system noise figure.
Best wishes,
Markus (DF6NM)
-----Ursprüngliche
Mitteilung----- Von: Stefan Schäfer < [email protected]> An:
[email protected]Verschickt:
Di., 22. Mrz. 2011, 0:41 Thema: Re: LF: Loop TX antennas at
VLF...
Hi Scott,
I have some comments, yes.
I would try not to resonate the PA but drive it with a H bridge PA that
can handle pretty much reactive power. If you think about Rik's and
Jim's calculation and assume 70 V across the loop, while taking 32 A, you (
ah, you have 115 V mains?) you may directly rectify the mains and drive a H
bride with it and feed the loop with that switched rectified voltage. So you
would get about 75 A. That's not a problem for some well choosen FETs and
caps (on the DC side!). Jim and Wolf already suggested to try that.
I know it from my kite antenna: When the wind is poor or if the wire
gets disconnected from the hot end of the coil (this happened 1x), the Z of
the primary winding (unresonated) is the current limiting factor. I am still
switching at 300 VDC and 8970 Hz to a bundle of wire of 50 turns on a water
barrel (say 70 cm diameter on the lower side). There will be heavy reactive
currents but that is no problem for the FETs! The resistive losses in the
whole circuit (measured on the 230V AC side by a true rms multimeter) are
below 25 W in such a situation, including the driver/exciter power.
I
think it wouldn't be a good idea to try this with a standard audio
amp....
My FETs are simple IRFP460, about 2.5 EUR each. There are no ferrite
parts in the whole circuit! But, unfortunately, i think there is
another problem: If you try that with your loop, who will be the next
receiving station? What distance? Even if you run 100A rms into that loop
(BTW, what is the area?) you will radiate probably below 1 mW ERP...
Now reading the above comments from the others ;-) Best 73,
Stefan EI/DK7FC, currently in IO62SI41ISAm 21.03.2011 12:30, schrieb
Scott Tilley: Hi Rik
Thanks for this!
I may be
comparing apples to oranges alittle (maybe alot). The loop I have
would only require a 1.8uF cap and this coupled with a higher Q makes for
a little dicier tuning scenario. I concede this scenario is not as
bad as I first thought when I thought about this briefly when Stephan
enquired about a possible test from here many months ago. A careful review
of your summary Rik was very helpful, thanks to all that
contributed.
The two big , and one minor design issues to
get maximum potential from a loop down at 9KHz will be:
1) Current
handling capability of the caps.
2) Tuning capability.
3) Ferrite
saturation for the matching xfmr... Or should we use
ferrite?
#1 can be resolved, I think, as you have suggested with
the use of LOTS of polypropylene caps...
However, #2 will likely be
the difficult one and be abit of an engineering challenge as the small
loop proposed below may just get away with course tuning but as you get
larger and reduce the Rac the Q goes up and so does the tuning
criteria. This is where I got stuck with my original glimpses of
thought on this. We can't rely on a good old vacuum variable so some
sort of other idea...
Hmmm, how about a special decade type box
made of Polypropylene caps? The box could be designed once the
loop's characteristics after installation have been bracketed
in...
The other downside of polypropylene caps will be there
tendency to drift once you put alot of current into them, so you'll need
lots! This drifting issue could be very problematic as I discovered
on 137 with anything other than good transmitting micas with high current
ratings at the operating frequency.
Based on the results of
others using mains transformers on 9KHz maybe item #3 is not a big problem
after all? Anyone have a comment here?
Looking forward to
more ideas as this is tweaking my interest into building another
tuner.
73 es
TU
Scott
VE7TIL
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