A correction after just re-reading my last post above where I wrote that "the r-f voltage at the base of the whip would rise to 110V, while the field at 1609 m (1 mile) would rise to 0.120 mV/m..."
While it is true that the r-f voltage at the base of the whip would increase to about 110V for those conditions, the distance to the 0.120 mV/m field then would occur at 1 km, not at 1.609 km (1 mile).
Apologies.
My thinking is that the load seen from the base of the radiator referenced to ground is the same regardless of whether a loading coil is used or not. The resulting current is determined by this voltage divided by the Z of the radiator and the ground R.
A tube could drive this load to 100 V despite the terrible mismatch and the power delivered would still be V^2/Z (V is the feed point voltage at the radiator, Z is the radiator and ground Z) just as it is with the loading coil.
What interests me about this is in my situation half the power output of the transmitter is lost in the loading coil. With no loading coil there is no such loss but the deal breaker could be the loss in the tube Rp.
This is mostly thinking out loud.
Neil
A tube could drive this load to 100 V despite the terrible mismatch and the power delivered would still be V^2/Z (V is the feed point voltage at the radiator, Z is the radiator and ground Z) just as it is with the loading coil.
Looking back at my spreadsheet above, that transmitter delivered 2.54 V at 0.034 A (e.g. 86 mW) to the load connected at the output element of the final r-f amplifier. In most cases that power is applied to a loading coil, which is needed to offset the very high capacitive reactance of the 3-meter whip in the antenna system.
When load reactance is zero then the antenna system is resonant, voltage and current are in phase, and maximum power is coupled from the transmitter output amplifer to the antenna system (loading coil+ground resistance+radiation resistance).
The spreadsheet shows that 100 V is present at the base of the whip antenna, for those resonant load conditions.
Wouldn't it be expected that even though a vacuum tube transmitter may be able to supply 100 V of r-f directly to a load, if that load is highly reactive then very little real power would flow into it -- regardless of the available voltage?
If the 2954 ohms of reactance of the 3-m whip shown in the spreadsheet were not offset by the loading coil then the load VSWR would be 1,553:1 -- meaning that 99.87% of the power available at the antenna system input connector would be reflected back to the transmitter.
"When load reactance is zero then the antenna system is resonant, voltage and current are in phase, and maximum power is coupled from the transmitter output amplifer to the antenna system (loading coil+ground resistance+radiation resistance)."
Agreed, but the radiator itself is not in resonance.
"Wouldn't it be expected that even though a vacuum tube transmitter may be able to supply 100 V of r-f directly to a load, if that load is highly reactive then very little real power would flow into it -- regardless of the available voltage?"
Yes, and this is also the situation with the loading coil in place when looking at the input to the radiator. Very little real power is delivered to the radiator due to the high reactance. The loading coil does not change this despite that it provides a resistive load to the transmitter. The V and I phase angle is still close to 90 degrees at the radiator input.
The only thing which produces radiated power is I^2xRr and if the same I can be produced at the radiator input with the same voltage applied at the radiator with or without a loading coil then the I and radiated power will be the same.
Neil
Picking these lines from #19:
Rich :"When load reactance is zero then the antenna system is resonant, voltage and current are in phase, and maximum power is coupled from the transmitter output amplifer to the antenna system (loading coil+ground resistance+radiation resistance)."
Neil: "Agreed, but the radiator itself is not in resonance."
But isn't the whole idea of loading or matching the antenna to obtain a resonant radiator?
"But isn't the whole idea of loading or matching the antenna to obtain a resonant radiator?"
Though the radiator itself is not resonant the antenna system as seen by the transmitter is. This allows maximum real power to be transferred but unfortunately most of it is lost as heat in the coil and ground resistance. The resonant condition, for a given input power, also maximizes the voltage at the base of the radiator and thereby also maximizes the radiated power. The loading coil, in a way, is acting as a transformer to boost the collector (or drain) voltage up to a high voltage.
Digressing a bit, this whole idea of using a tube to generate a high voltage at the radiator is not mine. The KnightKit broadcaster does just this. The antenna is directly coupled through a capacitor from the tube plate and my KK produces over 100 V at the ant. terminal as measured with a scope using a 10X probe. This prompted me to realize that as far as the radiator is concerned it sees the essentially the same voltage whether delivered by the KK or a solid state transmitter with a loading coil.
There is a similarity here in that the plate of the KK output is at the connection between an inductor from B+ and capacitor in series to circuit ground so in a way at resonance the inductor could be considered a loading coil. The nice thing here is that this coil need not be included when measuring the input power to the tube.
Neil
The antenna is directly coupled through a capacitor from the tube plate and my KK produces over 100 V at the ant. terminal as measured with a scope using a 10X probe.
Any idea as to the (unmodulated) d-c input power to the plate of that tube when it was generating over 100 V at the base of a 3-meter radiator without resonating it with a loading coil?
schematic at bottom of page. it should be noted that alot of people changed the bias resistor in the output to obtain a larger input power.
http://www.crompton.com/KnightBroadcaster/
Hi, Rich,
It's been a while since I measured the final power input but I recall it was about 180 mW. This was calculated from the average plate to ground voltage X the cathode current. Except for a crystal in the grid circuit the circuit is "stock" original.
Neil