Hi all,


Months ago, when the C-word was something still something far away in a
distant land, we had a small discussion in our radio-club.
The idea was this: "fundamentals Fridays"(*), a chance to ask questions
on aspects of amateur-radio that we all know that are true, but nobody
seams to be able to explain why exactly they are the way they are.



One of the questions that popped up is this: (although I am not sure of
this is a question about physics or about antennas)


It is "common knowledge" that when using an antenna with an
antenna-tuner, the efficiency of an antenna goes down: the smaller the
size of the antenna compared the wavelength, the less power is emitted
and the more power is "lost in the tuner".

But, why is that?
Where does this "lost" energy go to?




In essence, the goal of an antenna-tuner is to do impedance-matching:
match the impedance of an antenna at a certain frequency to the (50 ohm)
output impedance of the transmitter and the transmission-line. For that,
it uses inductors or capacitors. (although I know that these components
do also have a resistive part, but I think we can ignore this here)


Now, I understand that a capacitor can "store" energy in the electrical
field between the two plates, and an inductor uses electrical fields to
create a current to counter changes in current, ... but why does this
create a "lost" of energy?

In what form is that energy then "lost"? Is it converted to heat? Is it
"emitted"?


I've been reading about the "Radiation resistance" of an antenna (**)
and, although I am not a physicist, I kind-of understand the notion of
the transfer of energy from the momentum of an electron to a photon.

But physical process is at work inside an antenna-tuner?
And to what kind of energy is the "lost power" converted? Heat?

Does an antenna-tuner actually heat up?
(again, ignoring the "resistive" loss of the components of the tuner)




(*) Fundamental Fridays: (c) EEVblog

(**) https://en.wikipedia.org/wiki/Radiation_resistance


73
kristoff - ON1ARF

In article , kristoff
wrote:

In essence, the goal of an antenna-tuner is to do impedance-matching:
match the impedance of an antenna at a certain frequency to the (50 ohm)
output impedance of the transmitter and the transmission-line. For that,
it uses inductors or capacitors. (although I know that these components
do also have a resistive part, but I think we can ignore this here)

Kristoff-

Are you over-thinking this? The power is lost in resistance. For a
great mis-match, currents might be very high in the tuner.

Suppose you have a lousy antenna where 50 percent of your power is lost
in the transmission line and tuner. Anyone listening to you would
suffer a 3 DB reduced signal compared to the ideal antenna. That is one
half S-Unit. They probably would not know the difference.

Also your lousy antenna may have a poor pattern, transmitting your
signal in the wrong direction. The tuner can not fix that.

Fred

HI Fred,


On 11/07/2020 04:57, Fred McKenzie wrote:

In essence, the goal of an antenna-tuner is to do impedance-matching:
match the impedance of an antenna at a certain frequency to the (50 ohm)
output impedance of the transmitter and the transmission-line. For that,
it uses inductors or capacitors. (although I know that these components
do also have a resistive part, but I think we can ignore this here)

Kristoff-

Are you over-thinking this? The power is lost in resistance. For a
great mis-match, currents might be very high in the tuner.

Well, that's the question. (As noted, this is why we called it
"fundamental Fridays" :-) )


The reason I kind-of ignored resistive loss as that component is not
relative to frequency while the efficiency of an antenna+antenna-tuner
system is clearly frequency dependent.


It is however an interesting thought that power-dissipation due to
resistance can be frequency-dependent via its current.

But would this not mean that the efficiency of tuned antenna would be
dependent of the design of the tuner and that a theoretical
antenna-tuner without resistance would have 100 % efficiency.

I have not found this in any documents I have been reading on this.




Suppose you have a lousy antenna where 50 percent of your power is lost
in the transmission line and tuner. Anyone listening to you would
suffer a 3 DB reduced signal compared to the ideal antenna. That is one
half S-Unit. They probably would not know the difference.

OK, but you can just as easy apply this for -say- an antenna for 475 or
137 KHz band where the efficiency of the antenna-system is ... euh ..
less then 50 % (unless you have a very very very big garden :-) )



Also your lousy antenna may have a poor pattern, transmitting your
signal in the wrong direction. The tuner can not fix that.

That's true and if this was the only effect playing here, then an
antenna-tuner would not have a lower efficiency then a fully matched
antenna, just a different radiation-pattern.

But we all learned at the ham-radio academy that a tuned antenna has a
lower efficiency, no?
Or where things wrongly represented at the ham-radio lessons?




I know. We have been chewing on this question for some time too and
so-far have not come up with an answer neither.

Every answer seams to produce as many counter-arguments.
(I guess that what you get from asking "fundamental" questions) :-(







Fred

Kristoff - ON1ARF

kristoff wrote:
The reason I kind-of ignored resistive loss as that component is not
relative to frequency while the efficiency of an antenna+antenna-tuner
system is clearly frequency dependent.

Well, with so many assumptions already cast in stone, there is not
very much to discuss anymore, right?

In article , says...

The reason I kind-of ignored resistive loss as that component is not
relative to frequency while the efficiency of an antenna+antenna-tuner
system is clearly frequency dependent.


It is however an interesting thought that power-dissipation due to
resistance can be frequency-dependent via its current.

But would this not mean that the efficiency of tuned antenna would be
dependent of the design of the tuner and that a theoretical
antenna-tuner without resistance would have 100 % efficiency.




The loss in the tuner is the loss in the components, mostly the
resistance of the coil. It could be some in the capacitors if they are
not a air or vacuum variatables ( switched in capacitors sometimes used)
It could also be in the internal wiring.

Tuners are only optimised over a small band of frequencies and
impedances. If one designs the tuner to be the best at 14 MHz, then the
coil and capacitors will be too large or too small at other frequencies
for optimen power transfer. That is often referred to the Q of the
circuit. YOu start to get larger and larger circulating currents in the
tuner and the losses go up due to the resistance of the coil and wires
in the tuner.

Look at transmitters of the old tube circuits. The plate circuit is
designed for a Q of around 10 to 12. That seems to be the best
compromise between the harmonic reduction and efficency. Go one way and
the losses go down, but the harmonics go up. Go the other way you
reduce the harmonics but the efficeny goes down.

On Fri, 10 Jul 2020 23:27:37 +0200, kristoff
wrote:

Does an antenna-tuner actually heat up?
(again, ignoring the "resistive" loss of the components of the tuner)

Yada, just ignore whatever you don't like.
w.

Helnut,



On 11/07/2020 10:00, Helmut Wabnig wrote:

Does an antenna-tuner actually heat up?
(again, ignoring the "resistive" loss of the components of the tuner)

Yada, just ignore whatever you don't like.


A tuner has a current flowing through it and it has a resistance so it
to heat up. But that's not the question. Resistive loss is independent
of the frequency.


The question is, .. are the other additional physics effects playing
here that will cause it to "ventilate" energy?


73
kristoff - ON1ARF

On 11/07/2020 14:57, Jim H wrote:
===================================
Antenna Matching Units ( I prefer not to use the word "Tuner") do have
fixed resistance in inductors and wiring as has been stated in this thread.

Frank , EI7KS

Jim H wrote:
On Tue, 14 Jul 2020 18:07:01 +0100, in ,
Jeff wrote:

On 14/07/2020 14:16, wicklowham wrote:
On 11/07/2020 14:57, Jim H wrote:
===================================
Antenna Matching Units ( I prefer not to use the word "Tuner") do have
fixed resistance in inductors and wiring as has been stated in this thread.


No! The amount of inductance varies depending on how much of the
inductor is used, thus the resistance of the unit varies with the
setting. Resistance also varies with frequency since the skin effect
is "deeper" (lower resistance) with lower frequency.

And, the antenna resistance to be matched also varies. When the
antenna is too short it will have a lower resistance, and thus any fixed
losses in tuner and feeder will have a higher relative effect.

All in all it is a completely wrong assumption that the losses will
be independent from frequency, even when te resistive values in the
tuner would be the same.

Hi Rob, Jim, Jef, all.



Thanks for continuing on this small thread.
As said, I asked this as a fundamental" question, so I am really
learning a lot here.


(see inline comment)



On 15/07/2020 16:27, Rob wrote:

Antenna Matching Units ( I prefer not to use the word "Tuner") do have
fixed resistance in inductors and wiring as has been stated in this thread.

No! The amount of inductance varies depending on how much of the
inductor is used, thus the resistance of the unit varies with the
setting.

OK. makes sense.


Resistance also varies with frequency since the skin effect
is "deeper" (lower resistance) with lower frequency.

Correct me if I am wrong, but it looks to me that these two elements
(partly) counter each other.

To tune / match an antenna for a lower frequency, you need more inductor
wire (i.e. greater resistor), but the skin-effect will be lower.





BTW. I always had the idea that skin-effect was only important for
frequencies of VHF and higher, not for HF.


Are there figures of how much resistances the skin effect adds to a
wire, in respect to the frequency?

(Just to get an idea of the scale of things)



And, the antenna resistance to be matched also varies. When the
antenna is too short it will have a lower resistance
This is something I do not completely understand.


I guess you are talking the radiation resistance, correct?

As far as I understand it, the radiation resistance is a "virtual"
resistance which is created by the fact that an antenna converts
electrical energy in electromagnetic waves that are radiated, thereby
extracting energy from the wire.
This "loss of energy" in the wire is modelled as a virtual resistance.


So, saying "a short antenna has a lower resistance then a full-size
antenna", is then the same as saying "a short antenna emits less energy
then a full size antenna", which is equivalent to "a short antenna is
less efficient"



So isn't this a circular reasoning?



What exactly is the reason that a short antenna has a lower resistance?



.., and thus any fixed
losses in tuner and feeder will have a higher relative
effect.

So, are there are then two different effects at play he
- the resistance of the tuner/matching unit which changes with frequency
due to the practical way it is build
- the radiation resistance of the antenna that changes with frequency.





All in all it is a completely wrong assumption that the losses will
be independent from frequency, even when te resistive values in the
tuner would be the same.


73
Kristoff - ON1ARF