|[AMRadio] antenna tuners transmision lines and more|
bcarling at cfl.rr.com
Sun Apr 23 22:27:06 EDT 2006
Some of these "MYTHS" may actually be "myths about myths."
Actually there are a number of commercially manufactured tube
RF finals that DO indded use toroidal transformers.
Dentron, for one example - they made a number of linear amps
like that, and they were/are not alone.
When we speak of SWR, we are actually measuring VSWR
in most cases, i.e. VOLTAGE standing wave ration - not
current. Although they are related of course.
The power may not be wasted very much in the tuner, BUT
REFLECTED power goes back into the RF final and is disippated
in the famil amplifier device(s) - at least many people have
written articles for decades describing that marticular "myth"
or so-called "FALSE STATEMENT." I am not so sure it is false
> You are correct Gary, it is very confusing to many and I was one
> confused guy for many years. It's not easy to get a grip on things of
> this nature. Invisible radiation and weird parts that have no movement,
> makes it all seem like wizardry and magic. Of course this is what makes
> it fascinating. I'll just add some more to the confusion.
> I am by no way a XPERT on this stuff but I have been told that I
> have a way with words as long as I can get a spell checker working. I
> have been asked to do some of this writing. I feel that I should share
> this with others and I have chosen this place to do it. I don't have a
> lot of opportunity to go get on the air much any more. I get stuck here
> at home watching kids once in a while and so this is when I type up
> these long stories.
> So please excuse the long winded transmission here.
> I hope some one gets something from it.
> Old Wives Tales (Misleading statements)
> FALSE STEAMENT #1 -- A high SWR reading is an indication that a lot of
> power is wasted and not being radiated. -----
> TRUE STATEMENT --- SWR is the ratio of currents measured at physical
> points on a transition line. It is the ratio of the maximum current on
> the line verses the minimum current on the line. These two physical
> points will be 1/4 electrical wavelength apart. They do not necessarily
> have to be at the load end or the source end. IF the load end is
> representative of a pure resistive load then the SWR will be the ratio
> of the load resistance to the line characteristic impedance. If the
> load resistance is non reactive and equal to the line characteristic
> impendence then the SWR is 1:1 and current will be the same at any point
> on the transmission line that you care to measure it except for the
> normal loss due to line characteristics. Even a perfectly matched
> load:line such will have slightly less current and voltage at the load
> end than at the source end although as some one earlier pointed out, "It
> is generally a negligible difference". It would need to be a very long
> line to be significant on 80 or 40 meters.
> FALSE STATEMENT #2 --- There is no need for a tuner if the antenna is
> resonate and the line is matched.
> TRUE STATEMENT -- If the antenna feed point is equal to the line Z and
> the transmitter is made to work into this load then there may be no need
> for a tuner. This is an almost impossible task as some one pointed out
> earlier, and even if it were to be done it would only be true for a very
> small range of frequencies. QSY would be a compromise.
> 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
> 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
> 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
> 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
> The only thing resonance means in this case is that the capacitive and
> inductive reactances at the tuner are equal. It confuses many people.
> Gary K4FMX
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