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 IO62SI41IS
Am 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
On 3/21/2011 1:30 AM, Rik Strobbe wrote:
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
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