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DOUBLE RESONANCE IN DIPOLE...THE CAUSE?????
Hi,
I recently tuned up a VHF dipole, and i got a double-dip, double resonance for the swr, and also the minimum swr was around 1.3:1 I did some modifications, and the double resonance was was gone, plus the swr was down to less than 1.1:1 I'm certain that one of two things (or a combo of both) did the trick, but i wanted to see if any of you seasoned antenna boys could guess what the problem was. Thanks for your input! Slick |
On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote:
guess what the problem was. Why do you think it was a problem? 73's Richard Clark, KB7QHC |
Dr. Slick wrote:
.. I'm certain that one of two things (or a combo of both) did the trick, but i wanted to see if any of you seasoned antenna boys could guess what the problem was. If it has two elements (sure, verticals can have two elements) then a difference in resonance between the two elements could easily have been the cause. -- 73, Cecil, W5DXP |
I'm certain that one of two things (or a combo of both) did the trick, but i wanted to see if any of you seasoned antenna boys could guess what the problem was. _________________________________________________ ________ There is no problem. Dipoles and other antennas always show multiple resonances. Just take an SWR analyzer and sweep a wide frequency range. You'll find resonances all over the place. -- Bill W6WRT Dipoles are essentially harmonic devices, so they experience harmonic resonances. If you find resonances that are not harmonic on a dipole, then there are loading objects that are producing them. What exactly is your dipole? What is in proximity to it? How do you know that the coax is choked properly? 73, Chip N1IR |
"Dr. Slick" wrote in message om... Hi, I recently tuned up a VHF dipole, and i got a double-dip, double resonance for the swr, and also the minimum swr was around 1.3:1 I did some modifications, and the double resonance was was gone, plus the swr was down to less than 1.1:1 I'm certain that one of two things (or a combo of both) did the trick, but i wanted to see if any of you seasoned antenna boys could guess what the problem was. Thanks for your input! Slick What happened to the total bandwidth after you tuned it? Tam/WB2TT |
"Dr. Slick" wrote in message om... Hi, I recently tuned up a VHF dipole, and i got a double-dip, double resonance for the swr, and also the minimum swr was around 1.3:1 At what two frequencies? What construction, feed type...? What method of measurement and equipment type/numbers? First, let's determine if there even is a problem with the antenna... I did some modifications, and the double resonance was was gone, plus the swr was down to less than 1.1:1 What mods? 73, -- Steve N, K,9;d, c. i My email has no u's. I'm certain that one of two things (or a combo of both) did the trick, but i wanted to see if any of you seasoned antenna boys could guess what the problem was. Thanks for your input! Slick |
"Dr. Slick" wrote in message om... Hi, I recently tuned up a VHF dipole, and i got a double-dip, double resonance for the swr, and also the minimum swr was around 1.3:1 but i wanted to see if any of you seasoned antenna boys could guess what the problem was. twarnt no problem - if the dipole was 'clean' the double-dip may be due to how close you were to it, the coax length, a nearby object, - your presence near the antenna, the way you held your mouth while taking the measurements, the measurement methods themselves, a passing cloaked Klingon bird of prey, an ionized whif of exhaled air, or something none of us have ever seen. (many causes - most just accept it is as - in Walter Cronkite's words "That's the way it is.") --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.788 / Virus Database: 533 - Release Date: 11/1/2004 |
Richard Clark wrote in message . ..
On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote: guess what the problem was. Why do you think it was a problem? A double dip is a bad sign, and your return loss is not optimum. S. |
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On 10 Nov 2004 11:42:21 -0800, (Dr. Slick) wrote:
I didn't need a choke with this one = the other factor |
Dr. Slick wrote:
A properly tuned and positioned dipole will be resonant at only one frequency. Double-dips are a bad sign, and the return loss suffers. If something is wrong with a dip on 40m and a dip on 15m, someone should warn all the hams who are using their 40m dipoles on 15m. :-) -- 73, Cecil, W5DXP |
Cecil Moore wrote:
Dr. Slick wrote: A properly tuned and positioned dipole will be resonant at only one frequency. Double-dips are a bad sign, and the return loss suffers. If something is wrong with a dip on 40m and a dip on 15m, someone should warn all the hams who are using their 40m dipoles on 15m. :-) Forgot to add that dipoles are resonant near all odd half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ... They are also antiresonant (purely resistive) on all integral wavelengths, 1.0, 2.0, 3.0, ... Using a Smith Chart, if one plots the feedpoint impedances of a dipole VS frequency and connects the dots, that locus of points will describe a (very rough) spiral. (Traversing once around that rough spiral from resonance to resonance is one full wavelength, not 1/2 wavelength.) -- 73, Cecil, W5DXP |
Cecil Moore wrote:
Cecil Moore wrote: Dr. Slick wrote: A properly tuned and positioned dipole will be resonant at only one frequency. Double-dips are a bad sign, and the return loss suffers. If something is wrong with a dip on 40m and a dip on 15m, someone should warn all the hams who are using their 40m dipoles on 15m. :-) Forgot to add that dipoles are resonant near all odd half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ... They are also antiresonant (purely resistive) on all integral wavelengths, 1.0, 2.0, 3.0, ... Using a Smith Chart, if one plots the feedpoint impedances of a dipole VS frequency and connects the dots, that locus of points will describe a (very rough) spiral. (Traversing once around that rough spiral from resonance to resonance is one full wavelength, not 1/2 wavelength.) -- 73, Cecil, W5DXP Explaining an antenna concept so simple to Dr. Slick after the statement that he made seems to me akin to reading Shakespeare to a cow. Irv VE6BP -- -------------------------------------- Diagnosed Type II Diabetes March 5 2001 Beating it with diet and exercise! 297/215/210 (to be revised lower) 58"/43"(!)/44" (already lower too!) -------------------------------------- Visit my HomePage at http://members.shaw.ca/finkirv/index.html Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/index.htm Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/index.htm -------------------- Irv Finkleman, Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP Calgary, Alberta, Canada |
Irv Finkleman wrote:
Explaining an antenna concept so simple to Dr. Slick after the statement that he made seems to me akin to reading Shakespeare to a cow. Awwwwhhhhhh Irv, everyone has a senior moment now and then. -- 73, Cecil, W5DXP |
Cecil Moore wrote in message ...
Cecil Moore wrote: Dr. Slick wrote: A properly tuned and positioned dipole will be resonant at only one frequency. Double-dips are a bad sign, and the return loss suffers. If something is wrong with a dip on 40m and a dip on 15m, someone should warn all the hams who are using their 40m dipoles on 15m. :-) Forgot to add that dipoles are resonant near all odd half-wavelengths, i.e. 0.5WL, 1.5WL, 2.5WL, 3.5WL, ... They are also antiresonant (purely resistive) on all integral wavelengths, 1.0, 2.0, 3.0, ... Using a Smith Chart, if one plots the feedpoint impedances of a dipole VS frequency and connects the dots, that locus of points will describe a (very rough) spiral. (Traversing once around that rough spiral from resonance to resonance is one full wavelength, not 1/2 wavelength.) Granted, i'll give you the harmonics, but a double-dip at for example, 88.1 and 93.7 MHz would indicate a BIG problem! :) Slick |
Richard Clark wrote in message . ..
On 10 Nov 2004 11:36:02 -0800, (Dr. Slick) wrote: Richard Clark wrote in message . .. On 8 Nov 2004 17:13:32 -0800, (Dr. Slick) wrote: guess what the problem was. Why do you think it was a problem? A double dip is a bad sign, and your return loss is not optimum. What makes a double dip a bad sign? Most antennas have many. What does any dip have to do with non-optimal return loss? By definition a dip in SWR indicates better return loss. A double dip in a very narrow band, like 88-108 MHz for example, is a real indication of something wrong. It has always meant that the maximum return loss suffers. S. |
Richard Clark wrote in message . ..
On 10 Nov 2004 11:42:21 -0800, (Dr. Slick) wrote: I didn't need a choke with this one = the other factor Incorrect. I simply didn't need it for this dipole. Anyone else? S. |
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Cecil Moore wrote:
Irv Finkleman wrote: Explaining an antenna concept so simple to Dr. Slick after the statement that he made seems to me akin to reading Shakespeare to a cow. Awwwwhhhhhh Irv, everyone has a senior moment now and then. -- 73, Cecil, W5DXP The devil made me do it. Aside from senior moments I also have second childhood flashbacks! Irv -- -------------------------------------- Diagnosed Type II Diabetes March 5 2001 Beating it with diet and exercise! 297/215/210 (to be revised lower) 58"/43"(!)/44" (already lower too!) -------------------------------------- Visit my HomePage at http://members.shaw.ca/finkirv/index.html Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/index.htm Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/index.htm -------------------- Irv Finkleman, Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP Calgary, Alberta, Canada |
Cecil, W5DXP wrote:
"Awwwwhhhhhh Irv, everyone has a senior moment now and then." That`s my experience. Arnold B. Bailey in "TV and Other Receiving Antennas" is more thorough than most. Bailey writes on page 291: "We have already seen that such conditions (efficient operation) can exist for many resonant lengths of the rod. The rod if divided into two sections and connected to a load at its center, will exhibit resonance when the total length of the rod is any multiple of one half-wave. ----only two resonant lengths will give a simple directivity pattern --- if the rod operates at its fourth resonant frequency, no signal is picked up from directions broadside to the antenna in contrast to operation at its third, second, or first resonance. Only at first resonance is the directivity pattern as indicated by Figs. 6-20 and 6-21. At all other resonances above the first, the pattern is going through a progressive change which will later be more explicitly shown in a quantitative manner." On page 348, Bailey gives radiation resistances and drivepoint resistances for dipole resonances 1 through 10, and shows their current distribution patterns. Best regards, Richard Harrison, KB5WZI |
Dr. Slick wrote:
"I`ll give you the harmonics, but a double-dip at for example, 88.1 and 93.7 MHz would indicate a BIG problem!" Does the dipper dip on one of these frequencies in te absence of an antenna? Is the receiver connected to the antenna, and can you move one of the dips with the receiver tuning? Does augmenting the antenna cable move one or both dips? Does relocating the antenna change the dips? There seem to be two coupled resonant circuits. Best regards, Richard Harrison, KB5WZI |
Irv Finkleman wrote in message ...
Dr. Slick wrote: A properly tuned and positioned dipole will be resonant at only one frequency. Double-dips are a bad sign, and the return loss suffers. Explaining an antenna concept so simple to Dr. Slick after the statement that he made seems to me akin to reading Shakespeare to a cow. Irv VE6BP What if the resonant frequencies are 88.1 and about 92.0 MHz? Not exactly harmonically related, are they! Think about it. I think you need to change your diapers, Irv. Slick |
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Richard Clark wrote in message . ..
As for "double dipping" none of your posts to date have any facts to test your complaint. If we are to assume these two dips occur within the same band, that is actually to your benefit as it could only enlarge the matching prospects. However, your paucity of details leaves this as speculation on both sides. If the two dips occur within and without the band, then you have offered nothing to distinguish this from the natural order of things. Simply put, ALL dipoles have many dips throughout the spectrum. In this regard there is nothing special about your "double resonance." Double dips (or even triple and more) certainly at harmonics of the fundamental, certainly. 88.1 and 92 aren't exactly harmonically related! As for the disparaging comment of "maximum return loss suffers," that too is in conflict with expectation. There is nothing inherently sufferable about having more than your share of "dips." Additional resonances does not detract from any other resonance's capacity to perform within its region of match. A second resonance doesn't necessarily rob another and it could be argued that it is actually a boon if you wish to enlarge the bandwidth of an antenna (which by your only specification of 88-108 would be a positive feature). A broadband antenna usually doesn't have as good a match as a dedicated antenna. This is why when i had two dips, the min. SWR was NOT as good as when i had only one resonant (not incuding harmonics) freq. Now, as to HOW you could achieve TWO SWR dips within the FM broadcast band with a "garden variety dipole," then that is revealed by your comments about not needing (and by inference not having) your driveline choked. Simply put, it sounds distinctly like your transmission line length (combined with velocity factor) added a resonant circuit in parallel with the dipole to offer this second dip. You munged things around with the antenna, but changed lines and the second dip went away (as a function of a different line length, or its becoming balanced or choked). You would have to have stumbled onto an unique antenna design to have forced these two dips into this FM band and this is negated by your own description of a "garden variety dipole." On the other hand, transmission line common modality is as common as rain in Seattle. Again, didn't need a choke for this one. Someone infered the first problem, not high above off the ground. Anyone else? S. |
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Dr. Slick wrote:
What if the resonant frequencies are 88.1 and about 92.0 MHz? Log Periodic? -- 73, Cecil, W5DXP |
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(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com... Granted, i'll give you the harmonics, but a double-dip at for example, 88.1 and 93.7 MHz would indicate a BIG problem! So are you measuring _just_ the dipole, or the dipole through a feedline? Through about 20 feet of RG-58. Do you have any idea at all how many coupled resonators you are measuring? See, e.g., King, Mimno and Wing, to educate yourself about antenna systems seen as a set of coupled resonators. Disabuse yourself of the notion that you actually understand what your measurements mean at this point. I understand it quite well... two non-harmonically related resonances in a very narrow band (88-108 Mhz) is a bad sign if you didn't design you antenna for broadband. S. |
Cecil Moore wrote in message ...
Dr. Slick wrote: What if the resonant frequencies are 88.1 and about 92.0 MHz? Log Periodic? Or perhaps a discone of some sorts, sure. But we are talking about a simple dipole here. S. |
Richard Clark wrote in message . ..
A broadband antenna usually doesn't have as good a match as a dedicated antenna. You have too little exposure to the world of antennas to paint them with that broad brush. A simple example is a discone antenna which is a variant of the biconical antenna, which is the genesis of ALL dipoles. Either the discone or the biconical display a very wide bandwidth (octaves) and are eminently matchable by definition. I'll bet you money that a well designed, dedicated, tunable antenna can achieve a better SWR than any discone. There are broadband folded dipoles for VHF, but you don't get something for nothing! They may be adequate across the band, but nothing beats a dedicated antenna. Again, didn't need a choke for this one. You have yet to show that it is not your problem, much less the lack of need (which is a strict requirement for tuning). "Not needing" it can be accomplished through one of two means: Luck, or a hardwired solution (the customer, like with CB whips, cannot vary the length without causing a major shift in dynamics). Either way, the two are probably the same solution, an even halfwave multiple length line. The longer the line, the more tenuous the solution. Neither. We have varied the coax length, certainly not keeping to multiples of 1/2 wavelength, with results remaining the same. We just don't need it with our design. Someone infered the first problem, not high above off the ground. So what was the second thing i did? S. |
Richard Clark wrote in message . ..
What do you grow in your garden? http://www.marijuanaaddiction.info/i...d%20plants.jpg S. |
"Dr. Slick" wrote in message om... Cecil Moore wrote in message ... Dr. Slick wrote: What if the resonant frequencies are 88.1 and about 92.0 MHz? Log Periodic? Or perhaps a discone of some sorts, sure. But we are talking about a simple dipole here. All this sounds like the characteristics of a dummy load...... |
Dr. Slick wrote:
There are broadband folded dipoles for VHF, but you don't get something for nothing! They may be adequate across the band, but nothing beats a dedicated antenna. Here's something for virtually nothing. Take a commercial FM folded dipole, vary the feedline length, and you have a tunable folded dipole peaking at your frequency of choice. The tuning can be done by mounting two telescoping antenna sections side-by-side. -- 73, Cecil, W5DXP |
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Nilla Wafer wrote:
"Were the two halves of the dipole of equal length?" The adjacent close-spaced wire ends have much mutual resistance and in most cases there os a considerable conductance between them too. In these cases, they resonate as a unit. But, imbalance enhaces radiation from the feedline which can produce a separate resonance. Best regards, Richard Harrison, KB5WZI |
Dr. Slick wrote:
"They (terminated folded dipoles) may be adequate across the band, but nothing beats a dedicated antenna." A rhombic will clean your clock. Best regards, Richard Harrison, KB5WZI |
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Richard Clark wrote:
However, out stellar designer has managed to negate that through a "marijuana garden variety dipole" with radials. ;-) Radials? I missed that. How are the radials coupled to the dipole? -- 73, Cecil, W5DXP |
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