[AMRadio] antenna tuners transmision lines and more


Mark Foltarz Foltarz at rocketmail.com
Mon Apr 24 11:26:32 EDT 2006


Hmmm,  TMC TAC1 tuner specifies 80% efficiency. So at least in this case, 20%
is lost.  ftp://bama.edebris.com/bama/tmc/tac1/pages/tac1_04.jpg

Apparently it gets worse as the frequency climbs up beyond 18 Mc

I suspect the same is true for the johnson boxes that also use air coils.

de KA4JVY

Mark



> FALSE STATEMENT #3 --- Tuners waste a lot of power and just make the
> transmitter think the antenna is right.
> TRUE STATEMENT --- A tuner consists of coils and capacitors neither of
> which by mathematical definition consumes energy.  The adjustments of
> the coils and capacitors change the phase as well as the voltage to
> current ratios of input and output.   The slight amount of energy that
> may be consumed by tuners is generally so negligible that it is very
> difficult to measure.  In some cases a tuners components maybe made of
> poor quality material and too small for the job.  These types of
> components will get hot.  Heat is an obvious point of loss.  I had a
> small MFJ tuner that was manufactured some years ago. It was just a
> small external Pi-Net device and I found it to have a measurable
> insertion loss.  It turned out to be the rivets that held the connectors
> on the little chassis.  I soldered braid across the connectors to the
> chassis and then the loss was then immeasurable.
> 
> Modern solid state equipment is designed to work into a 50 ohm non
> reactive load.  Connecting a dummy load of 60 ohms instead of 50 ohms
> will cause the rig to put out less RF current and make the automatic
> drive level circuitry start pulling back on drive prematurely.  If the
> load becomes slightly reactive as well then the RF production will
> decrease rapidly.  A tuner is nearly a must for these rigs.
> 
> In tube type XMTRs the use of toroidal transformers for the output is
> impossible because of the high output Z of tubes.  These rigs used
> instead a Pi-NET or link coupled tuned circuitry to do the job of
> matching the tube to the low impedance output.  This type of circuitry
> could match a relatively wide range of impedances from 25 ohms to
> several hundred ohms as well as compensate for some reactance.  Because
> of this an external tuner may not have been necessary especially if
> confined to one band on one antenna.  A lot of folks put up multiple
> antennas one for each band or used a multiband trapped dipole or some
> other multiband radiator with a single coaxial down line.  The Pi-Net in
> the rig did all the compensation for them.  But with solid state rigs
> and no internal tuning it would be an near necessity to have an external
> tuner if nothing more than a small PI-Net tuner such as the one I had
> from MFJ      
> 
>     
> 
> Having to do with the conservation of energy laws.  Here are some facts.
> 
> 1. High quality capacitors (especially air or vacuum type with good
> aluminum plates) have little or no measurable loss.  They give almost
> 100% of the energy they absorb back to the load or source.  They are
> adjusted with the inductors so as to send the energy to the load and not
> the source.
> 
> 2. Air inductors are also almost lossless except for a small amount due
> to the resistance of the material.  The energy they absorb is stored
> magnetically and almost all given back to the load or source.  They also
> are adjusted with the capacitors so as to send the energy to the load
> and not the source.
> 
> 3. Antenna systems (including tuners) are made of material that is very
> low in resistance to electron flow (or they should be).
> 
> With the above facts in mind, consider the following scenario. 
> 
> 1. A transmitter is connected to an antenna system made with quality
> components
> 
> 2. The finals are not dissipating any more heat than they would if
> connected to a perfect dummy load.
> 
> 3. There is no measurable heat dissipated in any of the components of
> the antenna system.
> 
> 	Then the energy that is produced from the finals must be being
> used by something irregardless of resonance.  The energy must be going
> to out into space because nothing is dissipating any heat that we can
> measure and it makes no difference what length the antenna is because th
> tuner is compensating for the reactance and transforming the current to
> voltage ratios as needed to get the energy out.
> 
>   It is being radiated, hence the term "radiation resistance".
> 
> 	Most folks mistakenly think of the term radiation resistance as
> a fixed value of 73 Ohms.  BUT THIS IS NOT TRUE.  73 Ohms is the
> radiation resistance of a center fed 1/2 wave dipole in free space and
> by the way increasing the size of the wire has very little effect on it.
> A center fed full wave dipole will radiate the same amount of energy but
> has a much higher radiation resistance.  It has no greater or less
> radiation efficiency than does the 1/2 wave dipole (negligible copper
> resistance loss).   It just radiates in a slightly different pattern.
> 
> 	Theoretical, (neglecting copper losses) if all of the energy of
> the radiated signal could be recaptured and measured from each of the
> two antennas the measured amounts would be equal. 
> 
> 
> 
> Here is some question that I have never learned the answer to.
> 
> 	I have never seen a value of radiation resistance assigned to a
> center fed full wave dipole.  Perhaps it is too difficult to measure?
> As Don,K4KYV pointed out, "There has to be some current flow there, else
> there would be no power transferred"
> 
> 	I would also like to know the theoretical feed point resistance
> of a theoretically infinite length dipole and why a rhombic is
> terminated with a 600 ohm resistor instead of, for the sake of argument,
> say a 100 Ohm resistor or some other value.
> 
> 	I understand that Rhombic and long wires (10 wave lengths or
> more) radiate 90% of there energy before the signal reaches the end of
> the wires. And that the terminating resistor is there to lower
> reflections that might make the antennas bi-directional.  So could that
> mean that 600 ohms is about the Radiation resistance of a infinite
> length of wire?
> 
> 	
> Here is a little tidbit that may not be well known.
> 
> 	Don, K4KYV, once explained to me, the reason for the 300 ohm
> feed point of the folded dipole.  It went like this.  
> 
> There or two wires which must divide there current evenly.  
> 
> Consider a 100 watt carrier gong into a 1/2 wave dipole.  
> 
> With the 73 ohm radiation resistance the current at the feed point would
> be about 1.17 Amps and the voltage would be 85.4 Volts
> 
> If another wire is added to make the antenna into a folded dipole then
> each wire would have a current at the center of (1.17 / 2) or .585 Amps.
> 
> But since only one wire is fed then in order to get the 100 watt value
> the voltage must be double to 170.8 volts.
> 
> R = E/I so 170.8 volts divided by .585 Amps equals about 292 Ohms.
> Hence the value rounded to 300 Ohms.
> 
> It is some what like an impedance ratio of 1:4 for a 2 wire folded
> dipole.
> 
> In a 3 wire folded dipole the currents will be divided by thirds per
> wire.  This will work out to near 600 Ohms for a 3 wire folded dipole.
> 
> 
>     
> 
> 
>  
> **********************************************************************
>      
> 
> -----Original Message--Edited for space---
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Gary Schafer
> Sent: Sunday, April 23, 2006 11:49 AM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] antenna tuners
> 
> 
> When using an antenna tuner and we tune the reactance out of the circuit
> so
> we only see a resistive component we can say it is resonant. But what is
> really resonant? The antenna is not, the feed line is not, the tuner is
> not.
> 
> 
> The only thing resonance means in this case is that the capacitive and
> inductive reactances at the tuner are equal. It confuses many people.
> 
> 73
> Gary K4FMX
> 
> 
> 
> 
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