Smith charts are a useful visualisation tool, even if the final answer is calculated by a computer. I remember a Prof once telling me that most of his research students don't reallly know how to design an antenna - they just fiddle with the dimensions until the simulator gives them the results they want, but they don't know what to change to bring about a desired result. This is what passes for "design" nowadays.
I still know how to use a slide rule. If you understand slide rules then you understand logarithms and so understand exponentials and decibels too. I can sometimes estimate in my head more quickly than some youngsters can use a calculator (and sometimes I am more accurate than them too!).
I still know how to use a slide rule. If you understand slide rules then you understand logarithms and so understand exponentials and decibels too. I can sometimes estimate in my head more quickly than some youngsters can use a calculator (and sometimes I am more accurate than them too!).
Actually, any RF antenna's gain patterns (and all other parameters) for transmit and receive are identical. So if it wouldn't transmit well then it also wouldn't receive well.
Yes some of the freeware programs don't work too well, and some do. I work in an RF lab so we have access to Agilent ADS. It can do far more than I can.
A resonant antenna (the physical length is related to the wavelength) will always work better than one that is is of random length. On receive it will be a question of efficiency, or receive range. On transmit there are two problems. An incorrect antenna will not properly radiate the signal, and it will return some of that signal to the transmitter. In extreme cases the transmitter can be damaged.
Agreed, and the good software uses the Smith chart as a display choice.
I still have one around here somewhere. Haven't seen it in a while though.
OK, don't trust me. It doesn't matter as long as my boss does! Actually I am not saying that it is dead. There is a network analyzer sitting on my desk at work, and it does have those funny looking circles on the screen. And my desk is in an RF lab inside a Motorola plant. I have worked there for 37 years, so I have that chart stuck inside my head. I am saying that I don't actually use a paper chart to design a matching network any more. It is too cumbersome when the network must span a decade of frequency range. For this, you need a computer.
Ah yes...shooting microwaves down the hall in school...great times! Little Klystron tubes...sending music down the hall for our classmates to pick up...little horns.
I wanted to do a microwave driven on-demand water heater for our final grade in 'Process Controls'........after working on it for two weeks we found it to be dead....'Plan B'....
Then I heard about Phased array radar.....Hmmm? Four conduits....some out of phase, a resultant 'beam' off axis????Scan it like an electron gun in a TV....Oh the possibilities!
------------
-----------------------
----------------------------------
--------------------------------------------
Just last week my Father-in-law was working on the roof.took down that "ugly thing" on a pole. A folded Dipole, Eight directors, Two reflectors......
I just sighed when I told him I could use it for my Harmon Kardon tuner....& he said "Its for TV, not radio".........I've learned not to get 'technical' with anyone in my family.just in one ear & out the other. "Thats nice Honey" the refrain.
So where did I put the 75-300 matching transformer..I'd better do it soon, the scrape-man has his eye on it.aluminum ya know.
____________________________________________________Rick......
I wanted to do a microwave driven on-demand water heater for our final grade in 'Process Controls'........after working on it for two weeks we found it to be dead....'Plan B'....
Then I heard about Phased array radar.....Hmmm? Four conduits....some out of phase, a resultant 'beam' off axis????Scan it like an electron gun in a TV....Oh the possibilities!
------------
-----------------------
----------------------------------
--------------------------------------------
Just last week my Father-in-law was working on the roof.took down that "ugly thing" on a pole. A folded Dipole, Eight directors, Two reflectors......
I just sighed when I told him I could use it for my Harmon Kardon tuner....& he said "Its for TV, not radio".........I've learned not to get 'technical' with anyone in my family.just in one ear & out the other. "Thats nice Honey" the refrain.
So where did I put the 75-300 matching transformer..I'd better do it soon, the scrape-man has his eye on it.aluminum ya know.
____________________________________________________Rick......
hmm "Smith charts are obsolete" means only if on paper now? I don't think it makes a difference, whether the chart is on a computer screen, instrument screen, or plotted on a piece of paper, this statement is plain silly. FWIW I'd take those "funny looking circles" over any computer program, especially for broadband designs. It's simply unbelievable that someone who call himself an RF designer would ever say "Smith charts are obsolete". Also puzzling, why would a plot of impedances shown on a Smith chart be cumbersome over a decade or more if that is the requirement?
Both of these observations are correct to a degree, but betray a lack of the sophistication that is acquired by actually operating or studying radio.
Random wire antennas are common, used with an antenna tuner. Just because an antenna is resonant does not mean that its beam suits one's purposes.
Reception is essentially different from transmission, insofar as it is limited by atmospheric noise and thermal noise in the receiver i.e signal to noise ratio. Given an adequate SNR and sufficient quiet gain in the receiver an antenna can work perfectly satisfactorily on receive but be useless for transmit on the same frequency.
If anybody needs a Smith chart program I have a DOS based freeware one which, while the graphics are crude, works perfectly satisfactorily. PM me for a copy.
w
Last edited:
In my case I could go into great detail about the theory of operating, studying, and designing radio equipment. In this thread however I attempted to explain that in three sentences to someone who doesn't knoe what VSWR is.
Yes, I am using a tuner on my HF rig. I also have several different wire antennas switchable by remote control (the tuner is on the roof) for different radiation patterns.
OK, you guys win. I made the comment based on the fact that I have not touched a paper Smith chart in ten years. Yes, the test equipment and the simulator plot on Smith charts, and I see them in use almost daily. The chart was devised as a graphical method of displaying a complex impedance and for graphically deriving a simple matching network between two points on the chart.
Plotting a range of impedances over a range of frequencies is simple. Using a chart to derive a match from a constant impedancs (say 50 ohms) to the impedance range presented by a typical active device over a decade of frequency might sound like fun to someone. Multiple itterations are required.
If the device manufacturer is willing to give you a device model in the format used by ADS or Microwave Office (many are on their website) and you have that software on your computer (we do) why would you not just enter a schematic into ADS, take a guess as to the starting values (yes this takes experience), set up the optimizer, and let it rip. Todays simulation tools can nail impedance matching (you input the return loss target), and give good results on things like noise figure of an LNA or phase noise of a VCO. Yes, I know how to use noise, power, and stability circles and even how to plot those funny little circles too, but the computer will try 10,000 different parts combinations and tell you which will work the best while I go have lunch.
Yes, so did I. Which is why it's particularly irritating to have you intervene with what appears to be a contradiction which is not even accurate.
w
Generally only true at HF and below. From VHF up you usually need a reasonable antenna for receiving too, except where the range is very small compared to what the equipment is intended for. At microwave where receiver noise dominates you need to scrape in as much signal from the antenna as possible.
Gosh, again, I'm astounded at your response, you seem to be still waffling around here. Smith charts and phase noise of VCO designs? This is getting off topic now, I know quite a bit about VCO's, PN, pulling specs et al. The bottom line is the software won't give you the optimum matching topology, this still requires human brain power and looking at a Smith chart. It's not about winning, it's about being accurate (or in the ballpark) when you make bold blanket statements.
I can see that my input is quite unwelcome even if it is in my chosen field. So I'm out of here and won't be back.
I will say that I work in an advanced development group designing soon to be state of the art two way radio equipment. I am the oldest engineer in the group and was the last to cave in and adopt the new ways of doing things. The state of the art in computer RF and microwave simulation tools has gotten that good. Yes, you still need to have enough common sense to know what type of matching topology is needed, but the computer will simulate a network (including the PC board layout) and give you the component values close enough to build a working design requiring minimal tweaking. I mentioned VCO's because that is one of the last places the simulator could be trusted. Today we are simulating VCO's that run at 2 GHZ with a 5% bandwidth that make -121dbc @ 25KHz phase noise. ADS will get the phase noise right within 1 db. Broadband RF power amps are the last place where the simulator isn't yet reliable.
Looks like you guys picked the wrong hombres to mess with <grin>.
Yeah, we were purposely trying to keep it unsophisticated so that everyone else here might be able to understand <grin>. As far as "actually operating or studying radio", in real life I am the head engineer for a family of electronic warfare systems. Does that count? <grin> I live and breathe broadband high power RF transmitting and receiving systems and antennas.
Being "common" does not make them great. A "resonant" antenna is capable of MUCH better performance. Beam properties are an entirely separate issue, whether the antenna is properly sized or not. BOTH should be optimized, especially if frequencies of interest are known in advance.
Nevertheless, reception and transmission properties and performance are _exactly_ identical, as far as the antenna is concerned. The antenna that works best for transmission WILL work best for reception. One that works "satisfactorily" for reception but is useless for transmission IS, by definition, grossly suboptimal for both.
But yeah, "given adequate SNR", almost anything would work to some degree.
Yeah, we were purposely trying to keep it unsophisticated so that everyone else here might be able to understand <grin>. As far as "actually operating or studying radio", in real life I am the head engineer for a family of electronic warfare systems. Does that count? <grin> I live and breathe broadband high power RF transmitting and receiving systems and antennas.
Being "common" does not make them great. A "resonant" antenna is capable of MUCH better performance. Beam properties are an entirely separate issue, whether the antenna is properly sized or not. BOTH should be optimized, especially if frequencies of interest are known in advance.
Nevertheless, reception and transmission properties and performance are _exactly_ identical, as far as the antenna is concerned. The antenna that works best for transmission WILL work best for reception. One that works "satisfactorily" for reception but is useless for transmission IS, by definition, grossly suboptimal for both.
But yeah, "given adequate SNR", almost anything would work to some degree.
Last edited:
Hello All,
None of you all make me feel all warm and fuzzy.
Tubes and antennas have been hobby all my life. My father is 90 years old tomorrow. My dad was a United Stated Army Signal Corps radio repairman in WWII France. Thinking about it, electronics was not forced on me it just happened. I was building crystal sets when I was ten and tube sets not long after that. Matching and tuning antennas to the radio frequency is key to the Dx. Dxing the Beach Boys on Boston AM radio in California was a thrill. The big sky skip. My dad told me that I would never be a radio engineer; I am color blind and cannot splice cables or read resistor color codes.
I am now the greybeard in a mechanical engineering office. Questions about pressure regulators? I think of oscillation as cycles per minute.
DT
All just for Fun!
None of you all make me feel all warm and fuzzy.
Tubes and antennas have been hobby all my life. My father is 90 years old tomorrow. My dad was a United Stated Army Signal Corps radio repairman in WWII France. Thinking about it, electronics was not forced on me it just happened. I was building crystal sets when I was ten and tube sets not long after that. Matching and tuning antennas to the radio frequency is key to the Dx. Dxing the Beach Boys on Boston AM radio in California was a thrill. The big sky skip. My dad told me that I would never be a radio engineer; I am color blind and cannot splice cables or read resistor color codes.
I am now the greybeard in a mechanical engineering office. Questions about pressure regulators? I think of oscillation as cycles per minute.
DT
All just for Fun!
I just read this and about fell out of my chair! Non-sense! Am not an RF amplifier guy (except for VLN preamps for antennas) First off cheap receivers are seldom 50 or 75 ohms in. They are whatever the input stage is and in the case of a GasFET that could be 800 ohms. Never seen 800 ohms in a transmitter but have in receiving systems, especially cheap ones and very common to find just about anything in the cheap receiver. This means the antenna starts changing its look to match Z if a match is even tried. A short antenna means high Z output. Remember the purpose of the transmit antenna is to have an average Z of whatever ohms. One end of the stick is say 50 ohms and what is the other end? Will let you all research that answer. Hint, it is not 50! Second, receiving antennas often are magnetic field to voltage converters such as found in an AM and other low frequency radios or voltage (electric field) detectors being the more common for TV and RC applications. Then there are antennas for receiving which convert both the magnetic and electric portion of the wave to an electrical signal. These make lousy transmit antennas and the highest sensitivity receiving antennas going for their size. And they are going and going with 10 of thousands put into service. Such lousy transmit antennas that in practice only about 12% of the energy ever gets out even when the Z is perfectly matched.
As a note- resonance is not required in a transmit antenna- just a match. If resonance aids in making a match then just fine, if not, fine also. Same is true for receiving.
As for our friend with the RC- maybe a little aluminum foil on the top of either the transmit or receive antenna might improve range. This increases the effective length of the antenna which may be an advantage or a disadvantage. Just have to try it and see.
But then I am the antenna guy buy (one) trade and practice and use everything "RF" scaled down to audio and yes, that works extremely well. Terminated interconnects at 82 ohms and synthetic aperture loud speakers. Those work real well also.
Last edited:
We seem to be getting well off topic. The question was about using a microwave antenna at VHF frequencies. Talk of AM loops and atmospheric noise has nothing to do with this, or terminated interconnects for audio (only useful for very very long cables!). Cries of "My RF knowledge is bigger than yours" sounds like playground boasting, even when true. (And don't say "He started it" - more playground talk!) Scientists and engineers (whether DIY or professional) should not be trying to establish the truth or otherwise of a statement by who said it - we should leave that to the artists, philosophers and politicians. When a wise man says something which is true, other wise men will usually recognise it while others will reject it - that is their choice. "Better to remain silent and be thought a fool, than to open one's mouth and confirm it!"
No. My original statement is precisely true. Antenna receive and transmit properties are exactly identical. See http://en.wikipedia.org/wiki/Antenna_(radio) , in the first paragraph of the section entitled "Transmission and Reception". (But how what you connect to the antenna messes that up is your business, and is unrelated to the validity of the stated facts.)
Very misleading. Nothing is "required", if optimal performance is not a concern. Having impedances matched mainly just minimizes what gets reflected back, i.e. it minimizes the wasting of power that never even gets a chance to get to the antenna to be transmitted. But what is not reflected back still completely depends on the antenna's resonant frequency (and bandwidth), its radiation pattern, its efficiency, and its polarization, for effective (or not) radiation.
And the exact reciprocal is true for reception.
Said another way, impedance matching (low VSWR) is necessary, but not sufficient, for getting the best antenna operation. If what you said was true then a perfectly-matched dummy load would be a wonderful antenna!
As for our friend with the RC- maybe a little aluminum foil on the top of either the transmit or receive antenna might improve range. This increases the effective length of the antenna which may be an advantage or a disadvantage. Just have to try it and see.
But then I am the antenna guy buy (one) trade and practice and use everything "RF" scaled down to audio and yes, that works extremely well. Terminated interconnects at 82 ohms and synthetic aperture loud speakers. Those work real well also.
Aluminum foil?! Give us a break!
As we suggested to him originally, first and foremost he should simply use the correct LENGTH for the antenna's conductive element, itself, based on his frequency of interest!
In free space, one wavelength at 260 MHz would be approximately v/f = 300x10^6 / 260x10^6 = 1.154 meters.
Other factors could (and will, to some degree) change the antenna's electrical length or compromise performance in other ways. But, for the original poster's situation, trying an antenna of the proper length is the best first step.
Frankly, it is difficult (for me) to understand why some of you have attempted to disagree with or even just minimize or downgrade the importance of first trying the simplest and very-likely best solution for the original poster's problem.
- Tom
Last edited:
Sorry, but these are both just plain wrong. These are only true in weak signal conditions.
w
Wak,
If we are only talking about the antenna (and I was), then my statement is obviously correct, and for all signal conditions. Please see my post just prior to yours, with link to cited reference that corroborates what I said. There's no way around the physics of it.
Regards,
Tom
Did suggest using a simple method to change antenna tuning for our thread starter. The aluminum foil trick is like a top hat antenna being less elegant though equally valid. How does one know how to tune the length of a monopole antenna without knowing the load at the receiver? That means without knowing the load it is impossible to determine the "correct" length. This is why trial and trial again to "discover" what works best as he has already done. Kudos to wise testing!
gootee- I see the link but it is somewhat wrong information, sorry. It is popular belief though. Resonance is a handy matching method but a transformer can produce a match and is not necessarily (hardly ever) resonant. You know there are unidirectional couplers? Well this can be true in antenna also. The energy only wants to go one way as my receive antenna, "in" is +5dB over reference dipole and "out" is -5dB under reference dipole.
I know we are off subject here- how about us antenna guys start a thread to beat antennas to death? Let me help anyone truly interested in antenna. Antennas by Lamont V. Blake has everything in it but three antenna designs ever seen here. Having read about 148 books on antenna this is the only one really needed.
gootee- I see the link but it is somewhat wrong information, sorry. It is popular belief though. Resonance is a handy matching method but a transformer can produce a match and is not necessarily (hardly ever) resonant. You know there are unidirectional couplers? Well this can be true in antenna also. The energy only wants to go one way as my receive antenna, "in" is +5dB over reference dipole and "out" is -5dB under reference dipole.
I know we are off subject here- how about us antenna guys start a thread to beat antennas to death? Let me help anyone truly interested in antenna. Antennas by Lamont V. Blake has everything in it but three antenna designs ever seen here. Having read about 148 books on antenna this is the only one really needed.
Concur.
I don't think that "popular" or "belief" should have anything to do with it. Science and engineering are not supposed to be faith-based endeavors. I didn't want to get too deeply into this stuff, here. My statement was about ONLY the antenna (with nothing connected to it). It gets too complicated (for this forum) very fast when we start to also consider what we might connect to the antenna. But yes, I totally agree that the impedance presented by the feed-line can have large effects on some of the parameters of the antenna itself, including its resonant frequencies, electrical length, etc. But the matching impedance and the resonance are still two different things, physically, i.e. with some independence from each other. And the antenna's resonance is not just for matching purposes. Sorry, I usually only think in terms of getting the absolute best propagation effectiveness, which is definitely not only about matching.
Yes, I know about directional couplers, and how they are designed. In measuring your antenna, you no doubt had to connect something to it. I have to assume that whatever you connected caused the 10x directionality effect. But I also believe that there is infinitely more that I don't know than I do know. So without more information I can't say that what you have described is wrong. In fact, it's interesting to me.
Thanks for the book reference. I don't think I have that one. And I still have a lot of learning that I want to do. I'm going to order it right now, if I can find it.
Like speakers for audio systems, I usually think of antennas as the "Achilles Heel" of RF transmitting and receiving systems. Help is always very welcome, and needed, for my work, but unfortunately, for my work I wouldn't be able to discuss frequencies or power levels or modulation types or even intended receiver types, etc, which would make discussions fairly difficult. But general antenna discussions would be cool. How about "the best ways to make small-as-possible broadband-as-possible medium-power transmitting antennas"?
Cheers,
Tom
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.