VLF: Resonate the antenna, or not?

[Markus Vester <markusvester@aol.com>]

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 <schaefer@iup.uni-heidelberg.de>
An: rsgb_lf_group@blacksheep.org
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 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
...