|
RESONANT ANTENNAS
Howdy,
What's all this stuff about resonant antennas? Some great antenna designs do not use a resonant length. 73 de Jack, K9CUN |
What's all this stuff about resonant antennas? Some great antenna designs
do not use a resonant length. All antenna 'systems' are resonant because they all present a resistive load to the transmitter. QED. |
Very broadband antenna systems, such as log-periodics, are still resonant.
They are a collection of different-frequency resonant elements. Another way of looking at it, a collection is broadband because as a whole it has a very low resonant Q. Resonant circuits have an effective Q or a collection of Q values even when the impedance-frequency response is flat-topped. (As inside a double-tuned 455 KHz IF transformer can.) At sufficiently high and sufficiently low frequencies the reactive component of the input impedance always predominates. The definition of resonance as being adjusted to present a resistive load to the transmitter should not infringed. Of course, it is quite possible to operate a transmitter with a non-resonant load, ie., the load impedance having a reactive component in addition to the essential resistive load. But if only for economic reasons this condition is nearly always avoided. ---- Reg. ======================================= "Dave Shrader" There's a world of difference between a resonant antenna and a resonant antenna system!! BTW, is a Log Periodic Antenna, example Tennadyne T8, resonant across the frequency range of 13.5 MHz to 30 MHz?? No tuning required on any frequency between 13.5 and 30 MHz, and VSWR 1.7:1 across the range! Deacon Dave, W1MCE + + + Reg Edwards wrote: What's all this stuff about resonant antennas? Some great antenna designs do not use a resonant length. All antenna 'systems' are resonant because they all present a resistive load to the transmitter. QED. |
Reg Edwards wrote:
All antenna 'systems' are resonant because they all present a resistive load to the transmitter. QED. Actually, when the transmitter circuitry folds back, it means that the antenna system is not resonant. When I change bands with my screwdriver and start tuning, my antenna system is certainly not resonant. -- 73, Cecil http://www.qsl.net/w5dxp "One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike ..." Albert Einstein -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
Richard Harrison wrote:
. . . If the antenna is operated off-resonance, it still works but with less vigor due to diminished current opposed by inherent reactance. . . . This is true only if no effort has been made to match the antenna to the transmitter. If it's matched, it will have the same current and "vigor" as a resonant antenna. Assuming negligible loss, if 100 watts is applied to resonant and non-resonant antennas by any means, 100 watts will be radiated from each. Roy Lewallen, W7EL |
What's all this stuff about resonant antennas? Some great antenna designs do
not use a resonant length. 73 de Jack, K9CUN Hi Jack, When you posed this same query back in June of 1999, (myth od the resonant antenna) you got 193 responses. You trying to beat your own record? That was about the time I happened upon this Newsgroup, I really enjoyed that particular thread. I have gone back and re-read it a couple of times. 73 Gary N4AST |
Roy, W7EL wrote:
"This is true only if no effort has been made to match the antenna to the transmitter." It is true with every transmitter which occupies more than zero bandwidth. Reactance is zero at one point in the frequency spectrum. Off-resonance, an antenna system accepts less current than it does exactly on resonance. But, the difference is usually less than 1 db. I wrote that if the antenna is operated off-resonance (excited by a frequency other than its resonant frequency) it works but with less vigor etc. If the antenna`s power factor has been externally corrected for some frequency other than its natural resonant frequency, then it is resonanat at a new frequency. Its vigor will not be subdued by inherent reactance at the new resonant frequency. When I said an antenna operated off-resonance works with less vigor, I tried for a statement true with a solid rod without connections and in free-space, a receiving antenna, and a transmitting antenna, all operated at a frequency other than their resonant frequencies. All are transmitting antennas because they all radiate when excited, no matter how the excitation is delivered. I think I succeeded in saying it correctly but failed in saying it well if it was misunderstood. Best regards, Richard Harrison, KB5WZI |
Actually, when the transmitter circuitry folds back, it means that the
antenna system is not resonant. ================================== Wrong ! The antenna 'System' IS resonant, by definition, if it has a purely resistive input impedance. If that interfering nuisance of your fold-back circuit springs into action then it means the pure input resistance is something other than 50 ohms. But it is still resonant. Actually, in YOUR case, the antenna is NEVER resonant. You make sure the antenna is NOT resonant by making the whole system resonant by varying the length of your transmission line. --- Reg, |
Reg Edwards wrote:
Actually, when the transmitter circuitry folds back, it means that the antenna system is not resonant. ================================== Wrong ! The antenna 'System' IS resonant, by definition, if it has a purely resistive input impedance. If that interfering nuisance of your fold-back circuit springs into action then it means the pure input resistance is something other than 50 ohms. But it is still resonant. Reg, the chances of a foldback being caused by a resistive antenna is about 1 in 360. Actually less than that because the transmitter will not fold back between 25 ohms and 100 ohms. -- 73, Cecil http://www.qsl.net/w5dxp "One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike ..." Albert Einstein -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
If that interfering nuisance of your fold-back
circuit springs into action then it means the pure input resistance is something other than 50 ohms. Hi Reg, That foldback circuit is an interfering nuisance agreed, but it comes in handy at times. My homebrew mobile antenna is difficult to keep tuned, much less in one piece doing 100 km/hr down the US Interstate system. My HF transceiver folds back if things are not right with the antenna system. The fold back circuits tell me I need to do something with the mobile antenna, before I smoke the final semiconductors in my rig. It doesn't tell me what I need to do, I use other stuff for that. I used a couple of your programs in the design of this beast, Thanks! 73 Gary N4AST |
On Mon, 14 Jul 2003 22:01:59 +0000 (UTC), "Reg Edwards"
wrote: Actually, when the transmitter circuitry folds back, it means that the antenna system is not resonant. ================================== Wrong ! The antenna 'System' IS resonant, by definition, if it has a purely resistive input impedance. If that interfering nuisance of your fold-back circuit springs into action then it means the pure input resistance is something other than 50 ohms. But it is still resonant. Actually, in YOUR case, the antenna is NEVER resonant. You make sure the antenna is NOT resonant by making the whole system resonant by varying the length of your transmission line. --- Hi Troll, let's talk about measuring antenna impedance. I have got a MFJ 269 which clearly shows that none of my antennas is purely resistive, or resonant. w. |
Reg wrote,
Richard, thanks for the reminder. Yes, the Beverage and other long-wire terminated antennas, although having lots of L and C, exhibit (ideally) no signs of resonance yet have purely constant vs frequency resistive feedpoint impedances. They are all transmission lines which radiate because the spacing between conductors is an appreciable fraction, or more, of a wavelength, one of the conductors being whatever the local environment consists of. Their equivalent lumped circuit networks come under a class of 'constant-resistance' networks commonly found in design of filters and equalisers. The most simple example of a constant-resistance network is a capacitor in series with a resistor, both in parallel with an inductor in series with a resistor. When all 4 components have the same value in ohms (R) then the input resistance is a constant resistance R from DC to infinity. ---- Reg, G4FGQ Since the reactive components change reactance with frequency, Reg's network may be a little hard to realize in practice. Try making the inductance equal to R^2*C Reg. You might have better luck. You also might want to review Everitt's take on this subject, starting on page 284 of the second edition of his book, _Communication Engineering_. His ideas are quite enlightening. 73, Tom Donaly, KA6RUH |
The most simple example of a constant-resistance network is a capacitor
in series with a resistor, both in parallel with an inductor in series with a resistor. When all 4 components have the same value in ohms (R) then the input resistance is a constant resistance R from DC to infinity. ---- Reg, G4FGQ Since the reactive components change reactance with frequency, Reg's network may be a little hard to realize in practice. Try making the inductance equal to R^2*C Reg. You might have better luck. You also might want to review Everitt's take on this subject, starting on page 284 of the second edition of his book, _Communication Engineering_. His ideas are quite enlightening. 73, Tom Donaly, KA6RUH ========================================= Tom, sorry to be so disappointing. My Little formula for calculating L and C for the constant resistance network is quite correct. When designing constant-resistance networks it is convenient to have a design-frequency. It can be the frequency at which I said Xc = Xl = R ohms. So we can now calculate both L and C without prior knowledge of either of them. After a little arithmetic it will be quite enlightening to discover , as you say, that L = C*R^2, but which is a less-convenient starting point. In addition to a design frequency there can also be a design time constant. --- Reg, G4FGQ |
Reg wrote,
Tom, sorry to be so disappointing. My Little formula for calculating L and C for the constant resistance network is quite correct. Yes, of course, but misleading because it implies that all four components have to have the "same value in ohms (R)." In fact, the input resistance will be the same no matter what the value of the reactive components as long as they obey the requirement that L/C = R^2. When designing constant-resistance networks it is convenient to have a design-frequency. Convenient, but not necessary to show that constant resistance networks exist. It can be the frequency at which I said Xc = Xl = R ohms. It can, indeed, or any other frequency for that matter. So we can now calculate both L and C without prior knowledge of either of them. After a little arithmetic it will be quite enlightening to discover , as you say, that L = C*R^2, but which is a less-convenient starting point. Perfectly true, but what are you ultimately after? In addition to a design frequency there can also be a design time constant. Indeed. --- Reg, G4FGQ Tom Donaly, KA6RUH |
Perfectly true, but what are you ultimately after?
======================= Beyond getting back to the subject matter - nothing! |
Jack, K9CUN wrote:
"I have referred to my various engineering texts on antennas and transmission lines and can not find any discussion of antenna "vigor"." I did not copy my statement. Vigor is defined in my "American College Dictionary as: "1. active strength or force---". A rod in free space becomes excited and accepts energy, which it must re-radiate, when it is swept by a passing wave of its resonant frequency. Its first resonance is near a 1/2-wavelength. At frequencies slightly off-resonance, little current flows in the rod due to the opposition of its reactance. You may have seen a mechanical analogy in the vibrating reed frequency meter. Best regards, Richard Harrison, KB5WZI |
Is that as in: "Oomph, oomph, omphpapa"? Oh! My poor tuba!!
Deacon Dave :-), W1MCE Roy Lewallen wrote: The correct technical term for this is "oomph". Roy Lewallen, W7EL JDer8745 wrote: Someone sed: "I wrote that if the antenna is operated off-resonance (excited by a frequency other than its resonant frequency) it works but with less vigor etc." ------------------------------------ I have referred to my various engineering texts on antennas ans transmission lines and can not find any discussion of antenna "vigor". Jack K9CUN |
I have referred to my various engineering texts on antennas ans transmission lines and can not find any discussion of antenna "vigor". Jack K9CUN It is Viagora, it makes all antennas resonanted and transmission lines SWRless. Add some Fractals, CFAs, EH? Bada BUm |
Vigor is defined in my "American College
Dictionary as: "1. active strength or force---" -------------------- What is the "strength" of an antenna? What is the "force" of an antenna? Is it the same as the "oomph"? 73 de Jack, K9CUN |
|
Units of force do not include Watts.
Units of force are such things as Newtons, dynes, poundals, pounds, etc. Jack (who exerts a force of many pounds on this chair) 73 |
Reg, G4FGQ wrote:
"All antenna "systems" are resonant because they present a resistive load to the transmitter. QED." =========================== But...is an antenna "system" (undefined) the same as an antenna? Methinks an antenna is just one part of an antenna "system" and that it doesn't need to be resonant. 73 de Jack, K9CUN |
JDer8745 wrote: Reg, G4FGQ wrote: "All antenna "systems" are resonant because they present a resistive load to the transmitter. QED." =========================== But...is an antenna "system" (undefined) the same as an antenna? [SNIP] NOPE!! Methinks an antenna is just one part of an antenna "system" and that it doesn't need to be resonant. 73 de Jack, K9CUN |
Methinks an antenna is just one part of an antenna "system" and that it
doesn't need to be resonant. and you are basically correct. an antenna is 'resonant' at only specific frequencies, even a small bit away from those frequencies it goes out of resonance but performance is virtually unchanged... if this weren't true the venerable 1/2 wave dipole and 1/4 wave verticals would only work on a single frequency... so we know from experience that you don't have to have an exactly resonant antenna. note above i said 'frequencies'. this is to account of course for the resonances at multiples of the lowest resonant frequency. likewise you can operate an antenna well away from it's resonances and it will still 'work'... theoretically an infinitely small dipole will radiate a field only a couple db weaker than a 1/2 wave dipole in free space... with the difference being that the 1/2 wave dipole changes the shape of the doughnut a bit, thus creating stronger fields in some directions and weaker ones in other directions. keep raising the frequency so that an antenna is longer and longer as measured in wavelengths and the pattern of these fields changes, but the total radiated power remains the same... so you can say that any antenna 'works' at any frequency and be correct. The kicker comes when you start considering the whole system. while any antenna will radiate whatever power you get into it (minus a bit for resistance of the elements that gets lost as heat), the problem can be getting that power to go into it in the first place. this is where the 'resonant' antenna does help out. at resonance an antenna presents a purely resistive impedance to the feed line, generally this is a relatively easy load to push power into.... except of course in extreme cases of very low or very high impedances. as such it simplifies the requirements for the feedline and transmitter. if a transmitter doesn't have to handle highly reactive loads, or extremely high or low impedances, it can be made much simpler and from easier to build or buy parts. if you have to design a transmitter to power a highly reactive load you have to be able to handle higher voltages or currents... higher voltages mean wider capacitor spacings, more insulation, and higher dielectric losses.. higher currents mean thicker conductors or expensive plating to lower resistance, and higher resistive losses.. both of those losses mean that in order to create the same radiated field intensity you need to generate more power in the transmitter to get it through the feed system to the antenna where it can be radiate. so while the antenna will radiate whatever you can give it, just getting it there in the first place can be a chore. |
"Reg Edwards" wrote in message ...
What's all this stuff about resonant antennas? Some great antenna designs do not use a resonant length. All antenna 'systems' are resonant because they all present a resistive load to the transmitter. QED. I suppose I am being picky Reg but shouldn't resonance be defined as "totally" resistive load. An antenna can only be 'resonant' at one point or frequency because movement from this point collects reactance. Fortunately it still has a 'resistive' component load which is the PRIME requisite for radiation, whereas 'resonance' is not. With the above being fully understood by newcomers a lot of the mystery about 'antennas' and 'antenna systems' falls aside. Cheers Art |
Yuri Blanarovich wrote:
Here is your answer Cecil. Resistor that radiates. If, as Jim says, an RF wave is not destroyed by being dissipated in a resistor dummy load, it has to radiate - yes? no? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
Reg Edwards wrote: It is only necessary that the load presented to the transmitter should be a pure (or near enough) resistance of the required value. Which is usually 50 ohms. My SGC-500 amp says it will handle an SWR of 6:1 just fine. If that is true, it means it will handle any impedance on or inside a 6:1 circle on a 50 ohm Smith Chart. The great majority of those impedances are not resistive. -- 73, Cecil, W5DXP ================================ Dear Talking SGC-500, Why do think I took the precaution of qualifying my statement with "or near enough" ? By the way, you would be most unhappy if your Tuned-tank or Pi-tank output circuit didn't present your plate(s) with a near-enough purely resistive load of the correct value. And would you mind asking your slave driver to stop cheating with the aid of his 19th Century Smith Chart, please. ---- Reg, G4FGQ |
I was taught that the Smith CHart is a twentieth [20th] century 'tool'.
BTW, it's not cheating! When I took my final exam in transmission lines Professor James Kirwin allowed, and even provided, Smith Charts for student use. DD, W1MCE Reg Edwards wrote: [SNIPPED] ... And would you mind asking your slave driver to stop cheating with the aid of his 19th Century Smith Chart, please. ---- Reg, G4FGQ |
Oh Yuri!! All resistors in any circuit radiate EM energy! DD, W1MCE ....and cause SWR. Can you picture little, unmatched radiating resistor in the circuit? Live and learn eh? I thought they ate the RF without passing any to our precious environment. What me dummy! But I think we should start new thread or threat on how much resistor, capacitor or asamatteroffact anything radiates, how the SWR affects this un/desired property, what the photons and black holes do about it and how to magnify this effect so we can beat the record on number of postings. Back to my no SWR feedlines. Bada Fun BUm |
Yuri Blanarovich wrote:
Oh Yuri!! All resistors in any circuit radiate EM energy! ...and cause SWR. Can you picture little, unmatched radiating resistor in the circuit? Live and learn eh? I thought they ate the RF without passing any to our precious environment. They do eat RF without passing more than a negligible amount to our environment. But they also convert the energy from RF energy to radiated heat which is just as much of an EM wave as the RF wave was. The old RF wave is destroyed but since the energy in that RF wave cannot be destroyed, it must be converted. In this case it is converted to an infrared wavelength. The way to keep a resistor from radiating heat is to enclose it in a heat sink in which case the heat sink probably radiates heat as well as conducts heat. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
I thought they ate the RF without passing any to our precious environment.
That was cheeque in tongue. Enough! Bye! |
Transmission lines with high SWR are amongst the most useful of radio
components. We couldn't do without them. |
Yuri, I'm pulling your leg a little bit, but just a little bit.
Hot resistors radiate EM energy in the IR region of the spectrum grin. DD, W1MCE Yuri Blanarovich wrote: Oh Yuri!! All resistors in any circuit radiate EM energy! DD, W1MCE ...and cause SWR. Can you picture little, unmatched radiating resistor in the circuit? Live and learn eh? I thought they ate the RF without passing any to our precious environment. What me dummy! But I think we should start new thread or threat on how much resistor, capacitor or asamatteroffact anything radiates, how the SWR affects this un/desired property, what the photons and black holes do about it and how to magnify this effect so we can beat the record on number of postings. Back to my no SWR feedlines. Bada Fun BUm |
Cecil, you are sharp!!
DD W5DXP wrote: Yuri Blanarovich wrote: Oh Yuri!! All resistors in any circuit radiate EM energy! ...and cause SWR. Can you picture little, unmatched radiating resistor in the circuit? Live and learn eh? I thought they ate the RF without passing any to our precious environment. They do eat RF without passing more than a negligible amount to our environment. But they also convert the energy from RF energy to radiated heat which is just as much of an EM wave as the RF wave was. The old RF wave is destroyed but since the energy in that RF wave cannot be destroyed, it must be converted. In this case it is converted to an infrared wavelength. The way to keep a resistor from radiating heat is to enclose it in a heat sink in which case the heat sink probably radiates heat as well as conducts heat. |
Dave Shrader wrote:
Cecil, you are sharp!! :-) Why do you think I draw so much flak? :-) I agree with Albert. -- 73, Cecil http://www.qsl.net/w5dxp "One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike ..." Albert Einstein -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
Jim Kelley wrote:
If, as Jim says, an RF wave is not destroyed by being dissipated in a resistor dummy load, it has to radiate - yes? no? It should have been fairly obvious to most that's NOT what I was saying. You said "Waves cannot be destroyed". I quoted Hecht saying that waves can be created and destroyed. Please tell us again that, "RF waves are not destroyed by a dummy load." When you come to understand that EM waves can be destroyed but the energy in those waves cannot be destroyed you will begin to understand EM physics. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
W5DXP wrote: You said "Waves cannot be destroyed". A google seach says you're wrong. The only hits for 'cannot be destroyed' are by an author named Cecil Moore. jk |
Jim Kelley wrote:
W5DXP wrote: You said "Waves cannot be destroyed". A google seach says you're wrong. The only hits for 'cannot be destroyed' are by an author named Cecil Moore. Try "cease to exist". Here's what you said. Read it and weep. Waves cannot just "cease to exist" for the very same reason that energy cannot cease to exist. "Ceasing to exist" and "being destroyed" are identical events. Waves that are destroyed cease to exist. Waves that cease to exist must, of necessity, be destroyed. So where did you learn your ethics? At the rattlesnake farm? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
No it doesn't need to be matched to the line!
They often aren't. Jack K9CUN |
|
| All times are GMT +1. The time now is 08:48 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com