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radiation resitance of non-ideal antennas

P

Paul Burridge

Jan 1, 1970
0
Hi chaps,

I'm completely thrown when trying to do any hand calculations or Spice
modelling at 40Mhz by the lack of clue as to what roughly to expect in
terms of load seen by the final transistor when the antenna is at best
a telescopic job with or without a base loading coil and at worst, an
arbitrary length of limp wire. Can anyone come up with any
constructive suggestions as to how to overcome this obstacle to
meaningful circuit development?

p.
 
J

John Jardine

Jan 1, 1970
0
Paul Burridge said:
Hi chaps,

I'm completely thrown when trying to do any hand calculations or Spice
modelling at 40Mhz by the lack of clue as to what roughly to expect in
terms of load seen by the final transistor when the antenna is at best
a telescopic job with or without a base loading coil and at worst, an
arbitrary length of limp wire. Can anyone come up with any
constructive suggestions as to how to overcome this obstacle to
meaningful circuit development?

p.

Reg Edwards has his nice "Botload2" programme on site.
http://www.btinternet.com/~g4fgq.regp/page3.html#S301"
The answers seem not unreasonable for a small bottom loaded whip and the
calculated components could go straight into a Spice.
regards
jhon
 
R

Rene Tschaggelar

Jan 1, 1970
0
Paul said:
Hi chaps,

I'm completely thrown when trying to do any hand calculations or Spice
modelling at 40Mhz by the lack of clue as to what roughly to expect in
terms of load seen by the final transistor when the antenna is at best
a telescopic job with or without a base loading coil and at worst, an
arbitrary length of limp wire. Can anyone come up with any
constructive suggestions as to how to overcome this obstacle to
meaningful circuit development?

Antennas tend to have their tuning circuits.
Usually with several knobs.
A directional coupler measures the return power.
Some fiddling with the tuning buttons let you
minimize the return power.

Rene
 
D

Don Pearce

Jan 1, 1970
0
Antennas tend to have their tuning circuits.
Usually with several knobs.
A directional coupler measures the return power.
Some fiddling with the tuning buttons let you
minimize the return power.

Rene

It is quite easy to match an antenna that way and get a great return
loss. But that doesn't answer the real question - how much of that
power is going into the radiation resistance, and how much is simply
warming up the loss resistance. Maximising the radiation resistance is
the name of the game.

There are plenty of antennas in the annals of the literature that get
this balance way, way wrong.

d

_____________________________

http://www.pearce.uk.com
 
A

Active8

Jan 1, 1970
0
Hi chaps,

I'm completely thrown when trying to do any hand calculations or Spice
modelling at 40Mhz by the lack of clue as to what roughly to expect in
terms of load seen by the final transistor when the antenna is at best
a telescopic job with or without a base loading coil and at worst, an
arbitrary length of limp wire. Can anyone come up with any
constructive suggestions as to how to overcome this obstacle to
meaningful circuit development?

p.
you might search for 4nec2. it might give radiation resistance. it'll at
least give impedance and pattern. i figure a transmitter should be
designed to operate into 50 w and you match the antenna. the radiation
pattern for a given excitation will give you an idea of how much power
you need to achieve a given EIRP.

mike
 
M

Marc H.Popek

Jan 1, 1970
0
Paul,

Baring use of a known antenna, i.e. dipole Rrad about 73 ohmS at resonance.
and barring specificying and obtaining an antenna, packaged and pigtailed
and characterized "into 50 ohms" over a speced cf and bandwidth (which I
will gladly do for you!)

then you must swamp out the so-called antenna port (that will have crappy
old wires or bedspring dangled from it) and feed it with a 'matching" and
swamping (resistive) arrangement .. Pi and tee etc can readily be adapted to
accommodate foolish sorts of terminations.

For an interesting antenna go here>>> FWT.NIAT.NET

Marco Polo
 
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