[BC] Re: Cutting Vs Matching Antenna Vs no code

[Cowboy]

On Saturday 23 July 2005 22:20, Larry Bloomfield wrote:

> As for Antennas, we all know that an antenna is resonate at one 
> frequency only.

 I suppose, if you ignore harmonics.....
 ( and there's no reason a wire won't radiate even harmonics as well as
 odd, or as well as the fundamental )

> If you intend to change from that frequency, you MUST  
> change the electrical length so the transmitter thinks it is the correct 
> length - shorter or longer as the frequency you're putting to it is 
> higher or lower.

 Unless you're using an ATU of some sort, or an LTU if it's at the
 line input end.....

 I'll concede you *might* need to change the transform of the Z, but no,
 it's not true that one *must* change the electrical length.

> I have always taught my students there are two ways of  
> doing that: either lengthen or shorten the physical antenna or employ an 
> antenna tuning unit that fools the transmitter into think that's what 
> you did.

 Well, that's not really what a tuner does, though some would have you believe.
 ( nor a slant wire, gamma match, beta match, hairpin, etc. )

 For begining students, it's easier to grasp that concept, and I've taught
 something similar.

> If you don't, you'll get reflected power and that's not good no  
> matter which method you employ. OK argue with that!

 Since you invited the argument......

 VSWR as far as coupling power into the radiator is irrelevent.
 The Z as seen by the final amplifier, is relevent, though the means by which
 the Z of the antenna is transformed is only partially relevent.
 We're not talking about multiplexed signals here, so "intermod" group delay,
 and the like being ignored in this context.
 ( the various and multiple reflections on a line with a high VSWR, and of
 significant length can destroy stereo imaging, digital data, and the like but
 I didn't see that in the context of the invitation to argue   ;-)

 Any given length of conductor will present some complex Z at the feed point
 at any given frequency. ( which may be 50+j0, or not )
 The physical charachteristics of that conductor alone determin the radiation
 characteristics of that radiator. The Z of that conductor has nothing to do
 with that.
 
 And your counter argument ?
 :-)

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