02-03-2016, 08:26 AM
(This post was last modified: 02-03-2016, 08:29 AM by ppppenguin.)
Presumably this is also why a ferrite rod works so well as it also receives the magnetic component of the wave.
Tranistors can give a lot more gain than valves since, as Gary implies, gm (mutual conductance) is proprtional to emitter current. Gain = gm * collector loadresistance. This is just expressing Gary's rule of thumb in a more usual form. It also requires that the fT of the transistors is much greater than the frequencies of interest which will always be true for modern transistors and medium wave. Transistors are also low noise, perhaps not in absolute terms but much lower than the atmospherics etc found at these frequencies. The downside may be if strong signals are present as you can get overload and intermodulation. I understand that this is why RF power devices were often used at the front end of solid state comms recivers.
Am I right in thinking that if you wanted to use this sort of antenna with a valve front end you'd probably need to use a grounded grid stage to get a low input impedance? Or have I got something horribly wrong.
Tranistors can give a lot more gain than valves since, as Gary implies, gm (mutual conductance) is proprtional to emitter current. Gain = gm * collector loadresistance. This is just expressing Gary's rule of thumb in a more usual form. It also requires that the fT of the transistors is much greater than the frequencies of interest which will always be true for modern transistors and medium wave. Transistors are also low noise, perhaps not in absolute terms but much lower than the atmospherics etc found at these frequencies. The downside may be if strong signals are present as you can get overload and intermodulation. I understand that this is why RF power devices were often used at the front end of solid state comms recivers.
Am I right in thinking that if you wanted to use this sort of antenna with a valve front end you'd probably need to use a grounded grid stage to get a low input impedance? Or have I got something horribly wrong.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
