[ARC5] Why an UNUN?

[Mike Hanz]

I was referring to looking down the coax line into the output of the 
unun.  Define it as you wish...the principle is the same.

On 10/24/2012 9:52 PM, Bruce Long wrote:
>
> How about:
> A transmitter presents a source impedance?
> For maximum power transfer the load should have the conjugate impedance?
>
> Actually I think you can say the source "outputs " an impedance.  I 
> once measured the "output" impedance of a xtal source by using a 
> directional coupler to inject and external signal into the frequency 
> source output connector and then measured the amplitude and phase 
> reflection coefficient.  A few seconds with a Smith chart gave the 
> source "output" impedance.
>
> I do agree with you however there are a lot of misconceptions about 
> this and the use of consistent if not proper nomenclature woould help 
> immensely
>
> ------------------------------------------------------------------------
> *From:* mac <w7qho at aol.com>
> *To:* ARC-5 List <arc5 at mailman.qth.net>
> *Cc:* Mike Hanz <aaf-radio-1 at aafradio.org>
> *Sent:* Wednesday, October 24, 2012 6:27 PM
> *Subject:* Re: [ARC5] Why an UNUN?
>
> A transmitter does not present a "load" as such to the input end of a
> transmission line.  The transmitter delivers power to the transmission
> line, i.e., "drives" the line.
>
> The commonly used term "output Impedance" is misleading, a transmitter
> does not "output" an impedance.  What we are actually referring to
> here is that impedance value which when connected to the output
> terminals of the transmitter  will reflect a value of impedance  back
> to the plate circuit of the PA tube that will, in turn, cause the tube
> to provide a desired power transfer to an external load.
> Approximately 12,500 ohms reflected to the plate circuit of the 250TH
> PA would be correct in the case of a BC-610.  (About 5K for a command
> xmtr.) Most transmitters include some form of  impedance translation
> network to allow connection to commonly  encountered load impedance
> values, 50 or 75 ohms for ex.  Variable link coupling to the
> transmitter's tank coil is a simple and common example of such a
> network.  The tank coil and the link form a transformer circuit and
> varying the mutual coupling between the tank and the link has the
> effect of varying the turns ratio of the transformer windings and
> hence the impedance transformation ratio.
>
> " Loading" is the descriptive term commonly applied to the process
> whereby the impedance reflected back to the plate of the PA is
> adjusted so as to deliver a desired power output value to an antenna
> or other load.
>
> Note that we normally talk in terms of purely resistive impedance
> values here.  We are not necessarily limited in doing so, however.
> The calculations are just more complex when reactance must also be
> taken into account.
>
> Dennis D.  W7QHO
> Glendale, CA
>
> ****************
> On Oct 24, 2012, at 12:42 PM, Mike Hanz wrote:
>
> > /at its feedpoint/ to the characteristic
> > impedance of the coax (like 50 ohms resistive), you minimize losses in
> > the coax significantly by having a similar pure 50 ohm resistive
> > load at
> > the distant transmitter end.
>
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