load the tuned circuit

[ppppenguin]

A paradox of the impedance matching for power transfer is that half of the total power is dissipated uselessly in the source. The other half usefully in the load.

Maybe I've misunderstood something, because I'm sure transmitters have been made that give better than 50% efficiency from electricty in to power in the aerial.

[Crackle]

I dont understand how that circuit in post 1 works, the heater circuit is incomplete.
What have I missed?

Mike

[Alistair D]

Jeffrey, you haven't misunderstood the maximum power transfer theory with regard to transmitters. Most transmitters are designed to have a much lower output impedance than the aerial so although max power transfer is not achieved much more of the output power is dissipated in the aerial than the transmitter. This point was mentioned just recently in a post over on UKVRRR.

The fact that 50R is the accepted standard aerial impedance has nothing to do with power transfer but is to do with 

a) It is the characteristic impedance of a half wave dipole.

b) It is purely resistive.

c) 50R is a good compromise between aerial voltage and current.

Mike, there should be a dot on the schematic where the heater wire crosses the wire from the heater battery to the HT battery showing that those connections are joined.

Al

[Crackle]

Yes a dot there would make sense of the circuit.
Thanks
Mike

[Terry]

Mike, it's all part of the evolution of technology!

Where lines crossed they were connected unless the crossover symbol was included in one of the lines. The dot to show that two straight lines were joined came later.

These days, of course, the little crossover thing has vanished and total reliance is placed on the dot for indicating a junction which, to my mind, is a retrograde step because a poor photocopy might not show the dots up very well and even add some extra ones into the bargain!

Freehand drawn circuits are even worse and I would always show the crossover clearly on any quick drawing to make doubly sure that it can't be misrepresented.

[Crackle]

I remember my father, who was an electrical engineer at STC, telling me that they stopped using the loop cross over symbol because sometimes on bad reprints it could be misinterpreted as a dot if too much ink was allowed to get on the paper.

Mike

[reset]

OK
so are you saying if the impedance is not matched it wastes power?

Sometimes Z matching can be confusing because of the rule of thumb being used out of context. To get the maximum power from a battery we would match the battery internal resistance but in the process we would waste power heating the battery and if it is a wet cell boiling it dry. The better rule of thumb is to maintain a 10 to 1 ratio of load to source resistance. With the 10:1 ratio the source would not be as likely to be burned out. With the battery supply we need to know our maximum available but we don't need to load to it. What is the cranking current for your car battery? Don't you want the battery to be able to produce more than is needed?

Check out this little excursion into transformer action. Note the datasheet says the transformer matches 10K to 200 AND 25K to 500. Now which is it? The simple answer is the turns ratio sets the Z tranformation but the transformer has to be suitable for the service. It is obvious I can't wind a transformer with 100 turns primary and 10 turns secondary and expect it to power a tube filament. it is obvious the inductance of the primary must be able to limit the primary current with no load BUT it is less obvious that it must still be able to limit the primary current when loaded. So what they are saying is the primary will produce 25KOhms at the design frequency with a 500Ohm load or 10KOhms at 200Ohms load. You can see as I adjust the load the primary inductance changes.

http://radio.radiotrician.org/2016/02/mu...ormer.html


In the test I ran above the source sees a load reflected back on it because the secondary is drawing power. If I short circuit the secondary the wire resistance limits the load. When you buy headphones they maybe rated 3KOhms. That is to say the winding resistance is 3KOhms. This would imply a large number of turns of fine wire. The magnetic field would be rated in amp turns of electromotive force. So you would have a win-win situation. High Z to prevent loading the system and high amp turns to make a very sensitive headset. Sadly some less than professional folks would wind the coil with German Silver wire. The resistance of the wire would allow fewer turns to give the desired resistance but not the electromotive force.

You can apply the transformer loading to a tuned circuit. As you load the tank you draw power and it in turn loads the antenna. Because the antenna circuit is power limited you have to load it as lightly as possible while still drawing enough power to drive the phones. The tank is complicated because of the reactive loading producing a voltage or current gain(series voltage gain parallel current gain). Q = Xl/R so we get a gain factor based on Q. The good news is that just as we can wind a transformer to produce more inductance than will limit the current we can build tanks to produce high Qs.

WARNING: I'm going a little crazy to make a point.
Suppose I have a tank with a Q of 100 and I need a a 5Khz bandwidth at 500Khz Fo. BW=Fo/Q=500K/100=5Khz. That is good except I can't load the tank without reducing Q and broadening the BW. Now assume the resistance of the tank is 1OHM (just a round number). So Xl=100 and R=1 giving the Q=100. Now use a larger wire and make R=.1Ohm. Now Q=100/.1=1000 but what happened to BW? BW=Fo/Q=500Khz/1000=.5Khz. Unless I'm receiving CW that is to small a bandwidth But when I load the tank and draw power from it my Q goes down and BW increases so the Higher Q allows me to draw power from the tank and still maintain my desired BW. In effect I transfer the resistance of the load back into the tank and I can load .9Ohm reflected load to have the 1Ohm I had in the first example BUT in the latter case I'm using the power that was being dissipated in the coil winding.
.END crazy

[Pat Pending]

Greg.
The point you make about the loading of the tank Q reminds me of the "uncle Clive" circuit that is used in the supergainer IF amplifier that was in that vintage homebrew transistor radio that I was trying to recreate, and failing.

In the original set, the transistor was an OC170, and would have had a hard job to be more than an hFE of, say, 80. In my recreation the transistor I used was a BC559c, with an HFe of 600. With the old Mullard alloy drift transistor the stage gain was much more reliant upon the regenerative feedback, however, when the silicon transistor version was made, the gain was already quite near that required for the stage to drive the audio amp, and as the regenerative feedback is less, then that explains why the stages BW is wide as the feedback level is not there to "Q multiply". Its difficult to prove that that is what's happening as the circuit inherently applies AGC action on strong signals, so that increasing regeneration only appears to narrow the bandwidth on the vintage version, and put the circuit into oscillation on my version.

Cheers Pal!

I've just realized that no one here is likely to know WTH I'm talking about! (This was all posted on another forum). So I'd better add an attachment of the circuit I'm referring to. It's a very nineteen fifties diagram complete with dots and jogs 

Andy

[ppppenguin]

High Hfe (or beta if you prefer) often makes very little difference at RF. A BC559C witha typical hfe of over 500 has an Ft of 300MHz. The actual current gain falls to unity at Ft. Fortunately gm (mutual conductance) doesn't fall in the same way. Or have I got that wrong?

[Pat Pending]

Practical experience with this circuit is that the BC559c gives higher gain with less regeneration than the old OC170 was my point Jeffrey, the beta was just an easy method of illustration, not really claiming how applicable it is to anything in the real world (as we know!), it was just easy to stick them into the socket of my cheapest multimeter. I think the basic gain of the circuit in my post is very dependant on the transistor specs as it has the emitter grounded, (the gain control/reflex bias scheme wouldn't work otherwise, in fact to change to a silicon transistor you also need to change the diodes to silicon types too). The basic circuit was much beloved by Sir Clive Sinclair, in his miniature earphone sets from the sixties, micromatic et al.

Andy