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Author Topic: Antennas & Transmission Lines (feedlines)  (Read 24817 times)

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It's not as complicated as it may seem...
Seems there have been many debates in the past amongst RF engineers, and Ham and CB operators about "tuning" the antenna system by cutting the coax (i.e. transmission line or feedline) to a specific length, usually 1/2 wavelength or multiple thereof, or by trial and error to minimize VSWR at the transceiver. Well, seems I've started such a debate at my job.

Given a 50 Ohm generator, 50 Ohm coax, but 150 Ohm load, there will be a 3:1 VSWR. But the VSWR measurement from the generator end will vary, depending on the length of the coax. This part is easy to prove by measurement, so all agree.

The contention however is this; Will the length of the coax (same scenario) affect the performance of the antenna, i.e. will changing the coax length, change the total radiated power of the antenna?

My belief is that it will not. The only part that really changes is what the generator sees, due to the reflections. Changing the coax length only changes the measurement point on the standing wave, and that is why it varies.

I have tested my hypothesis using a simulator called "bounce", and it seems to validate my belief. It shows that the dissipated power remains constant no matter the length of the coax, and I am assuming this also translates to a constant radiated power as well.

What are your thoughts guys?
   

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I was once heavily into CB radio's,and still have all my equipment-EG,SWR meters,and auto tuners.

The length and type of coax will have different db losses. Say your useing a cheap coax that may have a 3db loss every 25 meters of length,and you change this coax for an expensive coax that has only a .1db loss every 25 meters of length. You have gained nearly a 3db signal strength to your antenna O0. Big problem is that the recieving station and your reciever wont really see that gain. You need an increase of around 5 to 6db to see any gain.

In saying that,i would have to say(although negligble in most cases)there is a loss as you increase the length of the coax.It also makes sence in that by increasing the length of a wire,you also increase the resistance. If you increase resistance,you increase the amount of energy disipated across that resistance that is avaliable from the source.

But then we have absorbed energy, reflected energy and re/reflected energy flowing through the coax :-\
Mmm-well im not really sure now i think about it poynt,but i do know there is a definite db loss over a given length of coax,but wether that loss is worth worrying about is another story.


End thought.
Current is flowing forward(toward the antenna),then the absorbed energy is transmited out from the antenna as electromagnetic waves. What is not radiated is returned back through the coax(reflected energy). This is then re/reflected at the tuner,and sent back into the coax-->added to the transmitters energy output. It is said that because of this re/reflection,100% of the energy that leaves the transmitter will be radiated out of the antenna-->minus the losses in the coax.


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I use to dabble in 11 meters ,and I always felt that what Poynt is investigating
was important ,that the coax length be a derivative of the wave length in quarters
[at the very least].

I had line of site stations that I would tune to and compare ,I would imagine real time
testing of a faint signal would make quick work of this thought line.



   

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Seems there have been many debates in the past amongst RF engineers, and Ham and CB operators about "tuning" the antenna system by cutting the coax (i.e. transmission line or feedline) to a specific length, usually 1/2 wavelength or multiple thereof, or by trial and error to minimize VSWR at the transceiver. Well, seems I've started such a debate at my job.

Given a 50 Ohm generator, 50 Ohm coax, but 150 Ohm load, there will be a 3:1 VSWR. But the VSWR measurement from the generator end will vary, depending on the length of the coax. This part is easy to prove by measurement, so all agree.

The contention however is this; Will the length of the coax (same scenario) affect the performance of the antenna, i.e. will changing the coax length, change the total radiated power of the antenna?

My belief is that it will not. The only part that really changes is what the generator sees, due to the reflections. Changing the coax length only changes the measurement point on the standing wave, and that is why it varies.

I have tested my hypothesis using a simulator called "bounce", and it seems to validate my belief. It shows that the dissipated power remains constant no matter the length of the coax, and I am assuming this also translates to a constant radiated power as well.

What are your thoughts guys?

Hi Poynt

There are two points to consider, one is the transmitter to transmission line, and the other is transmission line to antenna.

Though in practice an ATU is normally placed at the transmitter end so as the transmitter sees a good match and as so is happy and does not get damaged by high reflective current, it does not solve a mismatch of line to antenna.

If the antenna has a good impedance "such as 50 ohms", any mismatch will happen at that end due to the feed line. Feed lines should always be multiples of 1/2 wave length so as to make sure there are no standing waves generated by the feed line. Other than 1/2 wave lengths can be used to bring a poor "line impedance" into the realms of where it should be eg. 50 ohms, and should only be so when the transmitting cable is of poor quality "not exactly 50 ohms". By todays standards these transmission cables are very good, as so, the length should be multiples of 1/2 wave length O0

All this being said, your point is of an antenna mismatch of 150 ohms, all others being good, therefore an ATU should be placed at the antenna end and not the transmitter end. A simple coil and capacitor bridge would solve the problem if the frequency of transmission is fixed.

Half wave lengths of feed cable must be used, it is obvious that if this was not so, at the point of the connection to the antenna with say a 5/8 wave termination, there will be a mismatch creating a standing wave at that point.

I hope I have clarified the problem :)

regards

Mike 8)



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It's not as complicated as it may seem...
Thanks guys.

Yeah, resistive losses are known, and a secondary effect that is a reality of life. But that is not really what I'm getting at.

My test using the bounce program shows that standing waves are built up on the coax no matter what length is used. I do agree however that the system can be separated somewhat into the transceiver/feedline, and feedline/antenna. In our systems, we do not use ATU's at all.

My findings indicate that the only value added by using specific 1/2 wavelengths of coax is to allow for an accurate measurement of an antenna VSWR measured back at the radio end. That's all. Coax length seems to have absolutely no affect (taking into account known losses) on forward and reflected power, the ratio of the two, nor how much power ultimately radiates from the antenna.

"Tuning" the coax to minimize VSWR is a myth in my opinion, and only masks the problem which really is the antenna mismatch. Yes it can minimize the effects of the reflected power at the radio, but it has no effect on the antenna itself.

As Chet pointed out, I think I will have to set up an experiment whereby I use a spectrum analyzer to monitor the signal strength of the carrier being radiated through a slightly mismatched antenna some distance away. Take three different measurements with three different lengths of feedline, and see if the received signal strength changes at all.

.99
   
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I hope no one minds if I include this link which I think gives a pretty good description for the benefit of those who are perhaps not Hams but have an interest in the subject.

https://www.youtube.com/watch?v=w1eE13UXAKs

consider this ... as a broadcaster you transmit into a resonant antenna, In electrical terms a resonant circuit uses no ' real power' ergo power = VI cos Φ = 0 of course this is a perfect world we consider perfect resonance and perfect conversion to the EM wave.
In the radio world we are familiar with here ends the tale.
However there is a huge BUT ... This 50 ohms you consider is initially a design factor of the transmitter its arbitrary .. It could be anything. There are of course also two distinct resonant points where VI cos Φ = 0 one where the voltage leads the current by 90 deg and another where current leads the voltage by 90 deg (in electrical terms series and parallel resonance) The series resonant wireless system (tends to) transmits electrostatically through the ground whilst the Radio system (tends to) transmit electromagnetically through the air. The far more efficient electrostatic wireless system of Tesla and the very different electrical theory associated  has been buried for over 100 years. still either side of the two resonant conditions is miss match ... spurious emissions of immense power can turn up anywhere on the globe,  obliterate whole bands and probably not make you very popular down you street! :-[ kind regards Duncan


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Poynt

Why have an antenna design which is a mismatch in the first place (150 ohms) when the transmitter is 50 ohms, or am I missing something :-\

regards

mike 8)


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Duncan

ELF (extra low frequency) is primaraly an earth transmission, hence used for submarine communications. It is a bit like in reverse, the air becomes the earth and the ground becomes the arial, it can only be done in select sites on the earth, one of which was close to my home town, the reason being of underground aquatic and plate connection.

regards

Mike 8)


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Hi Mike great talking to you again,  O0 it had slipped my mind that you are one of a relativity few to  have worked practically in this area .certainly where I lived any transmission out of the ordinary would have been jumped on very quickly with dire results (as you know) still it wasn't a frequency I was thinking of be it ELF or UHF but rather transmission method. more Specifically the method considered by EPD here
https://www.youtube.com/watch?v=f5ZWrjcrPl4   from 1M 30 sec on  and further demonstrated here by the old scientist

https://www.youtube.com/watch?v=wjz-5Lqtxow

as EPD tells this is created by making use of and transmitting the series resonant state  O0 of course extrapolating from that many things are altered
kind regards Duncan



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Duncan

It was as a reference to Tesla and EPD who both used the earth as the main means of transmission, not the atmos: and with no losses. The problem is for information transmission it is very limited, morse only. Sub's nowadays still have this, but their main system is via sat's but needs a buoy floating on the water and a wire down to the sub, not always possible in times of war.

I could explain your circuit as I have with video some time ago concerning the 3BGS. The 3BGS will only work for a short time until the chemistry changes in the battery.

I am going to start an open thread on a similar theme where Kirchoff's law does not apply :D

regards

Mike 8)
« Last Edit: 2015-03-14, 17:03:14 by Centraflow »


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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
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@Poynt99
Quote
Yeah, resistive losses are known, and a secondary effect that is a reality of life. But that is not really what I'm getting at.
When I understood what you were really getting at last night I decided not to post and consider it further, it is not an easy question.

Quote
My test using the bounce program shows that standing waves are built up on the coax no matter what length is used. I do agree however that the system can be separated somewhat into the transceiver/feedline, and feedline/antenna. In our systems, we do not use ATU's at all.
My findings indicate that the only value added by using specific 1/2 wavelengths of coax is to allow for an accurate measurement of an antenna VSWR measured back at the radio end. That's all. Coax length seems to have absolutely no affect (taking into account known losses) on forward and reflected power, the ratio of the two, nor how much power ultimately radiates from the antenna.

I would agree and this relates to Tesla's hairpin circuit in that the current on a external load is not unlike a bird on a wire however where the bird is on the wire in the wave and the distance between it's feet may determine whether it lives or not. Since our antenna is a termination it makes sense that where our meter is located effects the measure because that is where it is in the wave more so at resonance.

Quote
"Tuning" the coax to minimize VSWR is a myth in my opinion, and only masks the problem which really is the antenna mismatch. Yes it can minimize the effects of the reflected power at the radio, but it has no effect on the antenna itself.

I would agree and it would seem to me they are tuning their meter and not the transceiver/antenna mismatch. Tesla would tune his load current on the hairpin circuit by moving the load terminals along the conductor which is really no different than extending the conductor. In both cases the power is constant in the circuit as a whole however the load current or measure may change along the conductor. It would seem to me the capacitance of the termination ie. antenna would be the easiest way to tune the system as a whole.

Nice catch Poynt99 and I have always found it strange that we can do and see things all our lives and then in an instant we question them and everything changes. We find we were not really thinking clearly we were simply following others.

AC

« Last Edit: 2015-03-14, 16:45:01 by Allcanadian »


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I look forward to it Mike you certainly have my attention  O0


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In the kingdom of the blind, the one-eyed man is king.
D. Erasmus
   

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Thanks guys.

Yeah, resistive losses are known, and a secondary effect that is a reality of life. But that is not really what I'm getting at.

My test using the bounce program shows that standing waves are built up on the coax no matter what length is used. I do agree however that the system can be separated somewhat into the transceiver/feedline, and feedline/antenna. In our systems, we do not use ATU's at all.

My findings indicate that the only value added by using specific 1/2 wavelengths of coax is to allow for an accurate measurement of an antenna VSWR measured back at the radio end. That's all. Coax length seems to have absolutely no affect (taking into account known losses) on forward and reflected power, the ratio of the two, nor how much power ultimately radiates from the antenna.

"Tuning" the coax to minimize VSWR is a myth in my opinion, and only masks the problem which really is the antenna mismatch. Yes it can minimize the effects of the reflected power at the radio, but it has no effect on the antenna itself.

As Chet pointed out, I think I will have to set up an experiment whereby I use a spectrum analyzer to monitor the signal strength of the carrier being radiated through a slightly mismatched antenna some distance away. Take three different measurements with three different lengths of feedline, and see if the received signal strength changes at all.

.99

It's not possible to change the mismatch of the antenna by changing the length of the feed line. My point is that the antenna is the mismatch in your writings. If the antenna was a perfect 50 ohms and the feed line a perfect 50 ohm impedance, then you will have a perfect 1:1 VSWR at the antenna and at the transmitter but only if the length of the feed line is multiples of 1/2 wave length. The difference will be small, but there is a difference and why a 1:1 is difficult to obtain in practice unless it is a wave guide at microwave frequencies, where every little counts a lot. At HF the difference can't be detected, so what frequency are we talking about?  1cm at 1MHz is not the same as 1cm at 1200MHz ;)

regards

Mike 8)


---------------------------
"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
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As a general rule, the most successful person in life is the person that has the best information.
   

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Aboard Navy ships when radio communication was the only
means of signalling while at sea (before satellites) the standard
transmit/receive antenna was a 35 foot vertical whip.  Navy
transmitters then were designed to operate within the range of
2 MHz to 30 MHz.

In order for the 35' antenna to radiate effectively all all frequencies
within that range and in order to maintain a feedline condition when
transmitting power that was flat (1:1 VSWR) it was necessary to have
installed a Tuner/Coupler device at the base of the antenna.

The Tuner/Coupler contained motor driven L and C components which
were remotely 'tuned' to match the antenna impedance to the impedance
of the transmission line at any frequency within its range.  It effectively
adjusted the electrical length of the antenna to assure a near perfect
match at any frequency.

The idea was to avoid resonant line conditions with standing waves
which would result in excessive transmission line losses and less than
ideal radiated power from the antenna.  In actual practice the
tuner/coupler was very effective and a skilled operator could tune the
antenna to result in a VSWR of not more than 1.5:1 quite easily.
With additional tweaking effort it could be reduced even further.

Here are some photos and tech data
on the stuff we used back then.


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Wow a nice vacuum variable cap in those ATU's O0 cost an arm and two legs :D

Yep motor driven ATU's at the base of the vertical antenna will sort the HF length, 1.5:1 is acceptable, I used to run a sigma 4 vertical on 28MHz it was a half wave length, pretty high, and no ATU, 100w out from a dual final valve transmitter, YASU 101, front end digital the best of both worlds, 5 and 9 from England to Australia with a high sun spot year, like speeking to the person in the same room, those were the days.

regards

Mike 8)


---------------------------
"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   

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Seems there have been many debates in the past amongst RF engineers, and Ham and CB operators about "tuning" the antenna system by cutting the coax (i.e. transmission line or feedline) to a specific length, usually 1/2 wavelength or multiple thereof, or by trial and error to minimize VSWR at the transceiver. Well, seems I've started such a debate at my job.

Given a 50 Ohm generator, 50 Ohm coax, but 150 Ohm load, there will be a 3:1 VSWR. But the VSWR measurement from the generator end will vary, depending on the length of the coax. This part is easy to prove by measurement, so all agree.

The contention however is this; Will the length of the coax (same scenario) affect the performance of the antenna, i.e. will changing the coax length, change the total radiated power of the antenna?

My belief is that it will not. The only part that really changes is what the generator sees, due to the reflections. Changing the coax length only changes the measurement point on the standing wave, and that is why it varies.

I have tested my hypothesis using a simulator called "bounce", and it seems to validate my belief. It shows that the dissipated power remains constant no matter the length of the coax, and I am assuming this also translates to a constant radiated power as well.

What are your thoughts guys?


i agree, the mismatch stays (with the antenna) so no change in radiated power, only the indication on equipment measuring the standing wave .

Trimming the antenna would affect the degree of mismatch therefor the radiated power.
 
Regards itsu
   

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It's not as complicated as it may seem...
Poynt

Why have an antenna design which is a mismatch in the first place (150 ohms) when the transmitter is 50 ohms, or am I missing something :-\

regards

mike 8)

Hi Mike.

It's not by design actually. I just used 150 Ohms because this is a standard mismatched load used for setting reflected power alarms. And in the simulation, it accentuates the problem so it is easier to see and discuss. Otherwise, it does not matter really the degree to which a mismatch occurs, the effect is the same, just differing degrees of VSWR.
   

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It's not as complicated as it may seem...
Duncan

It was as a reference to Tesla and EPD who both used the earth as the main means of transmission, not the atmos: and with no losses. The problem is for information transmission it is very limited, morse only. Sub's nowadays still have this, but their main system is via sat's but needs a buoy floating on the water and a wire down to the sub, not always possible in times of war.

I could explain your circuit as I have with video some time ago concerning the 3BGS. The 3BGS will only work for a short time until the chemistry changes in the battery.

I am going to start an open thread on a similar theme where Kirchoff's law does not apply :D

regards

Mike 8)

Mike,

If you're thinking non-conservative fields, there is a thread on that.
   

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It's not as complicated as it may seem...
It's not possible to change the mismatch of the antenna by changing the length of the feed line. My point is that the antenna is the mismatch in your writings. If the antenna was a perfect 50 ohms and the feed line a perfect 50 ohm impedance, then you will have a perfect 1:1 VSWR at the antenna and at the transmitter but only if the length of the feed line is multiples of 1/2 wave length. The difference will be small, but there is a difference and why a 1:1 is difficult to obtain in practice unless it is a wave guide at microwave frequencies, where every little counts a lot. At HF the difference can't be detected, so what frequency are we talking about?  1cm at 1MHz is not the same as 1cm at 1200MHz ;)

regards

Mike 8)

The results I obtained with bounce show absolutely no difference in the load made by changing the length of coax, regardless if the load is matched or not.

I am working with about 800MHz and about 1 foot to 15 feet lengths of coax.
   

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It's not as complicated as it may seem...

i agree, the mismatch stays (with the antenna) so no change in radiated power, only the indication on equipment measuring the standing wave .

Trimming the antenna would affect the degree of mismatch therefor the radiated power.
 
Regards itsu

I agree Itsu.

So it would seem that the real purpose of cutting the coax to a specific length (based on frequency and VOP), is to a) minimize SWR at the transceiver, and b) allow for an accurate VSWR measurement without having to perform it right at the antenna. It can be performed right at the radio feed.
   

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The results I obtained with bounce show absolutely no difference in the load made by changing the length of coax, regardless if the load is matched or not.

I am working with about 800MHz and about 1 foot to 15 feet lengths of coax.

The load is the load, and agreed the coax does not change the load at all, only cutting of the antenna as Itsu correctly states, will the antenna (load) change, but the coax will change what the transmitter see's and only that, it see's the coax first and load second, but if the coax is perfect in both impedence and length, the transmitter does not see the coax, only the load 8)

800MHz (alarm systems) ;)

regards

mike 8)


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Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   

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Buy me some coffee
@ Poynt

If you are going to go into depth with this reserch,could i ask a favour. When i was in full swing with radio clubs here in Oz,there were guys that insisted that ladder line was the way to go. If you get the time,could you do a comparison between ladder line,and coax of the same length-->if you have any ladder line avaliable. I had coax to start with on my setup,and switched to ladder line after a couple of years. I never noticed any difference !until it rained!. I found that when it rained,and the ladder line got wet,the signal strength went way down ???.

Anyway,i was just curious to know if ladder line was actually better in any way. It's all well and good to read how great it is,but actual facts would be great to know.


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@ Poynt

If you are going to go into depth with this reserch,could i ask a favour. When i was in full swing with radio clubs here in Oz,there were guys that insisted that ladder line was the way to go. If you get the time,could you do a comparison between ladder line,and coax of the same length-->if you have any ladder line avaliable. I had coax to start with on my setup,and switched to ladder line after a couple of years. I never noticed any difference !until it rained!. I found that when it rained,and the ladder line got wet,the signal strength went way down ???.

Anyway,i was just curious to know if ladder line was actually better in any way. It's all well and good to read how great it is,but actual facts would be great to know.

Ladder line feed is 300 ohms, how did you match that to your 27MHz Tx? which are all 50 ohms output as far as I know! You would have needed a balanced to unbalanced transformer as the antenna is balanced and the feed line unbalanced, a total mismatch. Interesting you have brought this up, the balun would have to be placed at the antenna at exactly the multiple of 1 wave length along the feeder (see, wave length feeder length does matter).

Yes rain can cause problems with a twin feeder, as does water down the inside of a coax cable ( the capacitance changes).

regards

Mike 8)


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It's not as complicated as it may seem...
The load is the load, and agreed the coax does not change the load at all, only cutting of the antenna as Itsu correctly states, will the antenna (load) change, but the coax will change what the transmitter see's and only that, it see's the coax first and load second, but if the coax is perfect in both impedence and length, the transmitter does not see the coax, only the load 8)

800MHz (alarm systems) ;)

regards

mike 8)
I think we are in agreement Mike.

However I might correct you slightly on one point. The transmitter sees the coax, then the effects of the load and coax together if the load is mismatched. If the load is perfectly matched, then the transmitter really only sees the coax, because the effect of the load is nil.

Again, the gist of my argument is about what effect, if any, the coax length has on the power transferred to the load, whether it be a matched load or mismatched load. It turns out that no matter what impedance or VSWR the transmitter sees (affected by coax length), the load will always dissipate the same power, assuming the transmitter doesn't shut itself down or throttle back its power.

 :)
   

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I think we are in agreement Mike.

However I might correct you slightly on one point. The transmitter sees the coax, then the effects of the load and coax together if the load is mismatched. If the load is perfectly matched, then the transmitter really only sees the coax, because the effect of the load is nil.

Again, the gist of my argument is about what effect, if any, the coax length has on the power transferred to the load, whether it be a matched load or mismatched load. It turns out that no matter what impedance or VSWR the transmitter sees (affected by coax length), the load will always dissipate the same power, assuming the transmitter doesn't shut itself down or throttle back its power.

 :)

We are both head cases, semantics enters our typing, I personally agree with you, we are both right in the end, semantics apart, what a world of egg heads we live in ;D

I think if we were talking face to face we would have agreed in 30sec. I have a problem to express myself for two reasons, one is English has become a second language after living and speaking daily Spanish for over 25yrs, my age hasn't helped,64, second I have had over a dozen very small stokes which has affected small parts of my memory (names of things,spelling in English, can't remember if I have seen something before when I really have etc etc.) small detail, but frustrating, called short term memory by my doctor ;D

The biggest problem I have is when people do not take me seriously, what frustration for me, drives me mad, probably I am :D. I have given some things (not complete) to people on this forum, and they have not seen the potential of what has been given, you have had access to this ;) they were things that came by accident and not overall design (that's how life is most of the time).

LENR, I think Steven saw through this straight away, I have had tests done and the explanations, NONE, apart from NR. It's not my field, but I CAN hold my own to a point, and it is repeatable time after time if people do not change things to suit themselves.

Sorry about the rant on a thread that does not warrant this, but I think the thread is now dead and done anyway. Just drunk half a bottle of wine for lunch and had to get this off my chest :D  Look, anyone, at the photo below and explain what you see. The tube is 316 SS food quality, time in reactor around 30min, look very carefully, amplify it, it is 10mp, and explain the colouring and the pitting in relation to the untouched part (out of reactor), input power 12v @ 2amps @ 40% duty "why the colours and why the pitting" look carefully at the pitting "the form of the edges", I know, but asking if someone else knows ;) There is more to this than meets the eye ^-^

Poynt if you want to speak to me directly let me know, I use skype as you know, and my handle is as here on the forum.

Again sorry for the rant :)

regards

Mike 8)


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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   
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