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alpal
Joined: 30 Jan 2008
Posts: 202
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 Posted: Sat Jul 31, 2010 7:38 am Post subject: |
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| exray wrote: | Alpal is correct but there's something else to consider. The tank circuit of the receiver IS loaded down somewhat by the antenna circuit. That's another one of those inevitable things. If the antenna q were higher (I don't know how you would manage that on a simple random wire antenna other than with an external tuner) then the loading would be less.
Of course we routinely handle this with minimal coupling between the antenna and the tank circuit. When done properly the ideal coupling will give max volume and selectivity pretty much at the same point. |
The thing is Xray that you can only analyse such a situation
with a practical example of a particular Xtal set & it's particular antenna.
It all comes down to mathematics.
Antennas are usually inductive so a series variable capacitor with
the antenna will tune it as well as possible for the frequency of choice.
Then comes the problem of tapping into the circuit.
A lyonodyne set uses a tuggle front end so that the earth is also in series with the first tuned circuit.
A more simple Xtal set will just connect onto a tap about 1/4 of
the way from the ground end of the first tuned circuit.
Solution:
define a circuit and analyse it with mathematics.
My personal opinion - spend more time on getting a decent antenna
& earth rather than worrying too much about your Xtal set.
A single tuned Xtal set will then pull in heaps of stations quite loudly
as long as there is no high power station within 20 miles.
For an earth don't just stick a metal pole in the ground & hope for the best.
Attach as many bare copper wires as possible & lay them radially from the
earth pole with a nice bolted on properly connected center point.
Bury the wires into the wet soil.
For an antenna - as long & high as possible for medium wave.
Use at least 1mm copper wire but preferably much wider & make proper
insulators at each end.
You'll be surprised at how many more stations you'll get if you run
this earth an antenna through variable series capacitors into
even the most simple Xtal set. |
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