Posted by
Fred Townsend on
Sep 19, 2012; 8:20pm
URL: http://elecraft.85.s1.nabble.com/KX3-ATU-tp7495809p7562989.html
David, remember becoming 'nit free' leads to becoming 'lice free'. You are
correct that 'tuning' may have something to with bringing the circuit into
resonance but the term 'tuning' has other meanings too. For instance if we
equate tuning as bringing into resonance then we could not use the term
'tuning' applied to non-resonate antennas. You see the term is more
frequently applied to impedance matching and band centering.
Matching at the antenna is quite different than matching at transmitter.
Transmission lines are only pure transmission lines if they are matched at
both ends. If either end is mismatched the transmission line becomes a
circuit element. A transmission line inverts the reactance every quarter
wave. Hence R+jx becomes the complex conjugate R-jx after a quarter wave in
length. This increase the effects of frequency change requiring more
frequent retuning across the band. The higher the VSWR the higher the copper
losses in the coax. You seem to dismiss copper losses as being
insignificant. The higher the mismatch the more significant they become so
those losses can be saved if the match occurs at the antenna.
If you want to run your own little proof experiment take your favorite coax
feed antenna and measure and note the SWR at the transmitter. Make sure
there is a high SWR to measure like your 40 meter doublet running an 80
meters. Then add a long length of same impedance coax in series, say a 100',
and re-measure the SWR at the transmitter. The SWR will usually be much
different because of the additional attention from the longer coax.
73
Fred, AE6QL
-----Original Message-----
From:
[hidden email]
[mailto:
[hidden email]] On Behalf Of David Gilbert
Sent: Wednesday, September 19, 2012 1:10 AM
To:
[hidden email]
Subject: Re: [Elecraft] KX3 ATU
That's pretty much picking nits, in my opinion. It doesn't really matter
whether you physically locate needed complex reactance at the antenna or
present it to the antenna via the transmission line physics of a feedline
... the net result that exists at the antenna is exactly the same
(neglecting transmission line losses, of course) in either case. The only
relevant distinction I can see is that "tuning" more accurately refers to
bringing something to resonance rather than also transforming it to a
different load impedance, but that falls into the category of useless
semantics for me and I can give you all sorts of examples where it would be
next to impossible to distinguish electrically where an antenna ends. The
dividing line between an antenna and the rest of the system is not at all as
definitive as you suggest, and pretending it is seems more likely to give a
false impression of how things really work than not.
73,
Dave AB7E
On 9/18/2012 11:52 PM, Adrian wrote:
> Matt, Of course I don't question Don's technical stature, however
> "tuning the antenna" is a poor choice of explanation, and can give a
> false impression to those weak on the subject. Adding 'system' would
> have been a good move as you indicate. We tune where we measure.
>
> -----Original Message-----
> From: Matt Maguire [mailto:
[hidden email]]
> Sent: Wednesday, 19 September 2012 3:35 PM
> To: vk4tux
> Cc:
[hidden email]
> Subject: Re: [Elecraft] KX3 ATU
>
>
> On 19/09/2012, at 2:48 PM, vk4tux <
[hidden email]> wrote:
>
>> Don, When you say "The ATU will tune the antenna to an acceptable
>> SWR", you are contradicting yourself, because as you rightly said the
>> tuner does not change the antenna feedpoint swr.
> It is clear from the context that Don meant the ATU will tune (ie.
> match) the antenna *system* to the radio, where the antenna system
> consists of the ATU, antenna and connecting feedline considered as a
> single unit. This of course does not mean that the VSWR is 1:1 everywhere
within that system.
>
> The theory predicts that the measures VSWR will be constant along a
> *lossless* feedline. As you point out, practical feedlines do have losses.
> This will include I^2.R copper losses, but there can be losses from
> other sources as well, such as dielectric losses. Often such losses
> will be modelled as some sort of equivalent series resistance.
>
> 73, Matt VK2ACL=
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