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language and antenna gain
All begin reading the advice of a reseller on a Italian ham magazine, that
claim this (translated): "The MFJ 1796 antenna only at XXX euro !!! 2,6,10,15,20,40 meters Superior Half-Wave configuration !!" Oh well, i'm really a rookie on antenna theory, but - thinking into myself - someone at MFJ can please explain me what the hell make on this antenna to shrink an half wave antenna on 40 meters into a stick shorter than quarter wave?? Before posting the question on this NG to learn more, a look on MFJ site and i read on the description: "Full size halfwave on 2/6 Meters." Ah right, someone here in Italy forget to specify *where* is 1/2 wl designed :) But the funny thing is not what i tell until now... the funny (or sadly) thing is that 7 local hams over 10, asking them if on 40 meters perform well the MFJ 1796 or a common quarter wave antenna, reply "the 1796 for sure ! Is a half wave designed antenna" someone adding also " you know antenna theory, not ??" Of the remaining 3 hams, 2 of this 3 replied with approximately my same doubt ("what?? an half what ??") A special mention to the last one that replied the dipole is the only antenna that he consider. He said that vertical antenna is the best system to deliver half or more applied power to the earth, no matter how is designed. So he don't ever read the advice and don't reply to the original question :) .... Well, marketing and language misuse can add gain on an antenna, now i know this :) Sorry for my english, i hope that the sense of the story is clear to readers. 73, -.-. --.- |
language and antenna gain
On Jun 27, 2:09*pm, "-.-. --.-" wrote:
All begin reading the advice of a reseller on a Italian ham magazine, that claim this (translated): "The MFJ 1796 antenna only at XXX euro !!! 2,6,10,15,20,40 meters Superior Half-Wave configuration !!" Oh well, i'm really a rookie on antenna theory, but - thinking into myself - someone at MFJ can please explain me what the hell make on this antenna to shrink an half wave antenna on 40 meters into a stick shorter than quarter wave?? Before posting the question on this NG to learn more, a look on MFJ site and i read on the description: "Full size halfwave on 2/6 Meters." Ah right, someone here in Italy forget to specify *where* is 1/2 wl designed :) But the funny thing is not what i tell until now... the funny (or sadly) thing is that 7 local hams over 10, asking them if on 40 meters perform well the MFJ 1796 or a common quarter wave antenna, reply "the 1796 for sure ! Is a half wave designed antenna" someone adding also " you know antenna theory, not ??" Of the remaining 3 hams, 2 of this 3 replied with approximately my same doubt ("what?? an half what ??") A special mention to the last one that replied the dipole is the only antenna that he consider. He said that vertical antenna is the best system to deliver half or more applied power to the earth, no matter how is designed. So he don't ever read the advice and don't reply to the original question :) ... Well, marketing and language misuse can add gain on an antenna, now i know this :) Sorry for my english, i hope that the sense of the story is clear to readers. 73, -.-. --.- marketing language is worse than your english! |
language and antenna gain
"K1TTT" ha scritto nel messaggio ... marketing language is worse than your english! Hmm .. then i guess is a total mess :) -.-. --.- |
language and antenna gain
"-.-. --.-" wrote in message ... All begin reading the advice of a reseller on a Italian ham magazine, that claim this (translated): "The MFJ 1796 antenna only at XXX euro !!! 2,6,10,15,20,40 meters Superior Half-Wave configuration !!" Oh well, i'm really a rookie on antenna theory, but - thinking into myself - someone at MFJ can please explain me what the hell make on this antenna to shrink an half wave antenna on 40 meters into a stick shorter than quarter wave?? It is easy to put anyting on paper. Antenna gain advertisement was so bad that many years ago the ARRL would not let anyone put gain numbers in their advertisements. Most things made by MFJ are not very well thought of in the US. |
language and antenna gain
On Jun 27, 4:51*pm, "Ralph Mowery" wrote:
"-.-. --.-" wrote in message ... All begin reading the advice of a reseller on a Italian ham magazine, that claim this (translated): "The MFJ 1796 antenna only at XXX euro !!! 2,6,10,15,20,40 meters Superior Half-Wave configuration !!" Oh well, i'm really a rookie on antenna theory, but - thinking into myself - someone at MFJ can please explain me what the hell make on this antenna to shrink an half wave antenna on 40 meters into a stick shorter than quarter wave?? It is easy to put anyting on paper. *Antenna gain advertisement was so bad that many years ago the ARRL would not let anyone put gain numbers in their advertisements. Most things made by MFJ are not very well thought of in the US. That's why it's referred to as "Mighty Fine Junk" remember the V31BB Mighty Fine Junk network? Too damn funny. |
language and antenna gain
On Sun, 27 Jun 2010 14:32:32 -0700 (PDT), BillyBobMarley
wrote: On Jun 27, 4:51*pm, "Ralph Mowery" wrote: "-.-. --.-" wrote in message ... All begin reading the advice of a reseller on a Italian ham magazine, that claim this (translated): "The MFJ 1796 antenna only at XXX euro !!! 2,6,10,15,20,40 meters Superior Half-Wave configuration !!" Oh well, i'm really a rookie on antenna theory, but - thinking into myself - someone at MFJ can please explain me what the hell make on this antenna to shrink an half wave antenna on 40 meters into a stick shorter than quarter wave?? It is easy to put anyting on paper. *Antenna gain advertisement was so bad that many years ago the ARRL would not let anyone put gain numbers in their advertisements. Most things made by MFJ are not very well thought of in the US. That's why it's referred to as "Mighty Fine Junk" remember the V31BB Mighty Fine Junk network? Too damn funny. The MFJ products that I have purchased and use work well for me. Judging from their popularity and growth, I am not the only one who can make their products work. John Ferrell W8CCW |
Quote:
The line "full size halfwave on 2/6 meters" indicates that on those two bands the antenna is not loaded, but actually physically a halfwave antenna. The manual indicates that as initially built to dimension, before adjustment, the antenna is centered on 51 MHz on 6M, and 146 MHz on 2M, so my assumption s it is acutally a halfwave on those two freqs. |
language and antenna gain
"Token" ha scritto nel messaggio ... The antenna is electrically a half wave on 10M to 40M, the electrical length of an element is not directly related to it's physical length. That is how they make a "halfwave" in a stick shorter than a physical quarter wave. Ok... could you please explain me how i can build an, e.g., electrical half wave for the 20 meters in a 3 meters stick ?? I repeat, i'm a great newbie on antenna theory and related arguments, so the question i pose to you now is not ironic but really posted to increase my knowledge, if is possible. Thanks in advance also to other people that may contribute to this question. -.-. --.- |
language and antenna gain
-.-. --.- wrote:
"Token" ha scritto nel messaggio ... The antenna is electrically a half wave on 10M to 40M, the electrical length of an element is not directly related to it's physical length. That is how they make a "halfwave" in a stick shorter than a physical quarter wave. Ok... could you please explain me how i can build an, e.g., electrical half wave for the 20 meters in a 3 meters stick ?? I repeat, i'm a great newbie on antenna theory and related arguments, so the question i pose to you now is not ironic but really posted to increase my knowledge, if is possible. Thanks in advance also to other people that may contribute to this question. -.-. --.- "Electrical half wave" doesn't have much meaning when applied to an antenna, so there isn't a good answer to your question. There's no way to make a short dipole behave exactly like a half wavelength dipole in all respects. But if you mean you want to make a short antenna resonant (one of the properties of a physically nearly half wavelength dipole), you can make it or an antenna of any physical size or shape resonant by putting an impedance transforming (matching) network at the feedpoint. Presto, a resonant antenna. If you want the same feedpoint resistance at resonance as a physical half wavelength antenna, you can get that too by adjusting the network component values. The pattern of a dipole of any length shorter than a half wavelength will in practice be indistinguishable from that of a half wavelength dipole. What will happen is that you'll have increased loss due to the larger currents and voltages in the shorter antenna and the matching network, the amount depending on the design. And if the losses are kept reasonable, the bandwidth of the shorter antenna will be narrower than the bandwidth of a half wavelength antenna. Roy Lewallen, W7EL |
language and antenna gain
The antenna is electrically a half wave on 10M to 40M, the electrical
length of an element is not directly related to it's physical length. That is how they make a "halfwave" in a stick shorter than a physical quarter wave. Ok... could you please explain me how i can build an, e.g., electrical half wave for the 20 meters in a 3 meters stick ?? I repeat, i'm a great newbie on antenna theory and related arguments, so the question i pose to you now is not ironic but really posted to increase my knowledge, if is possible. The short explanation is "reactive loading". The simplest way to achieve what you're looking for, is to take a three-meter stick, and wind it with a spiral of wire. The wire should be insulated, to prevent adjacent turns from shorting together. You would feed the antenna in the center, just as if it were a full-sized half-wave dipole. The exact number of turns required (and thus the total length of the wire you'd need) can probably be calculated, but I lack the detailed information to know just what the calcs are. I'm sure that there are examples shown on the Web, and/or in the ARRL Antenna Compendium books. You can see one example of this approach at http://www.w0ch.net/travel_antenna/travant.htm There are a bunch of design alternatives, divided roughly into (1) Wind the wire evenly along the whole length of each half of the shortened dipole. (2) Run the wire straight along the pole for part of the way from the center to the end, and wind turns over the rest. The "travel antenna" is of the latter sort - it puts most of the added inductance (the coiled turns) down near the feedpoint. He designed it as a shortened quarter-wave, but you could take two of these and stick them back-to-back and have a shortened half-wave. If you measure the resonant frequency of this shortened half-wave and find that it resonates at too low a frequency, then you've got too many turns... remove some and run the wire straight along a portion of the pole (or space all of the turns further apart). If it resonates at too high a frequency, you need more turns (more inductance). The behavior of this sort of shortened dipole will be similar to that of a full-length dipole, with several differences: - Slightly less directional gain - Higher electrical losses in the dipole - Lower radiation resistance The latter two factors result in a loss of electrical efficiency... more of your transmitter power turns into heat in the antenna itself, and less is radiated. The feedpoint impedance is likely to be different than a full-sized half-wave, too... it may be lower (due to the lower radiation resistance) or higher (due to the additional loss resistance) or nearly the same (if these two factors cancel out). The approach I've described uses inductive loading - you add inductance in series with the antenna in order to resonate it. Another approach is capacitive loading - you add additional capacitive coupling at the ends of the antenna. This can be done by adding a circular metal "hat" at each end, or a set of radial wires sticking out at a 90-degree angle. The MFJ antenna under discussion actually uses both techniques - it has an inductive loading coil, and a "capacity hat" of wire spokes, at each end of the antenna (actually, one per band that it's supposed to tune). The combination of added inductance, and added capacitive loading, creates the necessary resonance on each band. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
language and antenna gain
A helically-wound dipole or monopole that is physically short in terms
of a free-space wavelength can be made electrically resonant at its input terminals as a result of the inductance of the helical form of the radiating conductor. But that does not mean that it has all of the electrical characteristics of a linear conductor that is inherently resonant, without the need for inductive loading. The radiation resistance of such a helically-wound radiator can be much lower than a naturally resonant radiator, which can mean that the percentage of transmitter power radiated by the antenna SYSTEM can be much lower than when a naturally resonant radiator is used. The link below leads to a page developing this point from Kraus' Antennas For All Applications, 3rd Edition. http://i62.photobucket.com/albums/h8...ndVertical.gif RF |
language and antenna gain
"Richard Fry" ha scritto nel messaggio ... But that does not mean that it has all of the electrical characteristics of a linear conductor that is inherently resonant, without the need for inductive loading. Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain very well, but the sense of my latest question is this: if a half wave end-fed *monopole* antenna have the following primary characteristics (if IIRC): - High Z at the feedpoint (voltage maximum and current node); - very small counterpoise lenght compared to the resonant wavelenght of the antenna (typical 0.1-0.2 lambda) can i mantain the same characteristics shortening the antenna in any way ?? Thanks for read and explain to those who want clarify my doubts. -.-. --.- |
language and antenna gain
On Thu, 1 Jul 2010 15:16:10 +0200, "-.-. --.-" wrote:
"Richard Fry" ha scritto nel messaggio ... But that does not mean that it has all of the electrical characteristics of a linear conductor that is inherently resonant, without the need for inductive loading. Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain very well, but the sense of my latest question is this: if a half wave end-fed *monopole* antenna have the following primary characteristics (if IIRC): - High Z at the feedpoint (voltage maximum and current node); - very small counterpoise lenght compared to the resonant wavelenght of the antenna (typical 0.1-0.2 lambda) can i mantain the same characteristics shortening the antenna in any way ?? Thanks for read and explain to those who want clarify my doubts. -.-. --.- Google EZNEC and find Roy's site. Download & install the demo version- it is free! Go through some of the simple examples like dipoles, 1/4 verticals etc. Bring your questions back to this group and we will all benefit. Antennas are pleasantly addictive! I consider myself a perpetual student... John Ferrell W8CCW |
language and antenna gain
On Jul 1, 8:16*am, "-.-. --.-" wrote:
if a half wave end-fed *monopole* antenna have the following primary characteristics (if IIRC): - High Z at the feedpoint (voltage maximum and current node); - very small counterpoise lenght compared to the resonant wavelenght of the antenna (typical 0.1-0.2 lambda) can i mantain the same characteristics shortening the antenna in any way ?? With proper design you can maintain the resonance characteristic with that short antenna, but not its characteristics of SWR bandwidth, exact radiation pattern (directivity), or radiation resistance. For a single frequency the only important difference will be radiation resistance, unless the short radiator is used with a virtually perfect (zero loss) ground plane. A typical r-f ground loss even in a set of 120 each, 1/4-wave-long buried radials is on the order of two ohms. So referencing the example in the link to Kraus that I posted earlier, the radiation efficiency of that helically-loaded monopole system with a two ohm r-f ground would be about 0.6/2.6 = 23%, approximately. The loss of a radial system using 0.1-0.2 lambda conductors would be significantly higher, so the antenna system radiation efficiency then would be significantly less than 23%. A naturally-resonant, unloaded monopole about 1/4-wave high has a radiation resistance of around 34 ohms. When it is used with a two ohm r-f ground, the radiation efficiency of the antenna system is about 34/36 = 94%. RF |
language and antenna gain
Richard Fry wrote:
A helically-wound dipole or monopole that is physically short in terms of a free-space wavelength can be made electrically resonant at its input terminals as a result of the inductance of the helical form of the radiating conductor. But that does not mean that it has all of the electrical characteristics of a linear conductor that is inherently resonant, without the need for inductive loading. The radiation resistance of such a helically-wound radiator can be much lower than a naturally resonant radiator, which can mean that the percentage of transmitter power radiated by the antenna SYSTEM can be much lower than when a naturally resonant radiator is used. . . . But a helically-wound radiator *is* a naturally resonant radiator if wound to make it so. It just isn't straight. Roy Lewallen, W7EL |
language and antenna gain
Richard Fry wrote:
. . . A typical r-f ground loss even in a set of 120 each, 1/4-wave-long buried radials is on the order of two ohms. So referencing the example in the link to Kraus that I posted earlier, the radiation efficiency of that helically-loaded monopole system with a two ohm r-f ground would be about 0.6/2.6 = 23%, approximately. The loss of a radial system using 0.1-0.2 lambda conductors would be significantly higher, so the antenna system radiation efficiency then would be significantly less than 23%. A naturally-resonant, unloaded monopole about 1/4-wave high has a radiation resistance of around 34 ohms. When it is used with a two ohm r-f ground, the radiation efficiency of the antenna system is about 34/36 = 94%. RF Although 2 ohms is a reasonable approximation for 120 radial ground system resistance, it varies not only with ground quality and frequency, but also antenna height. For example, an NEC-4 simulation for vertical radiators at 3.7 MHz with 120 radials, each a free space half wavelength long buried 0.1 meter in average soil, shows 3.25 ohms ground system resistance when the radiator is 0.24 wavelength high (nearly resonant). When the radiator is shortened to 0.12 wavelength, the ground system resistance increases to 4.30 ohms. And with a 0.06 high radiator, the ground system resistance nearly doubles to 8.06 ohms. This decreases the efficiency of the very short radiator by about an additional 3 dB beyond what it would be if the ground system resistance were fixed at 3.25 ohms. I believe the ground system resistance increase with short radiators is due to concentration of the field very close to the antenna, resulting in much higher ground currents in that region. It would probably be useful to use a larger number of radials, which could be shorter, when the radiator is very short. Roy Lewallen, W7EL |
language and antenna gain
-.-. --.- wrote:
"Richard Fry" ha scritto nel messaggio ... But that does not mean that it has all of the electrical characteristics of a linear conductor that is inherently resonant, without the need for inductive loading. Yes, replying to Richard but also to Roy and Dave.. maybe i can't explain very well, but the sense of my latest question is this: if a half wave end-fed *monopole* antenna have the following primary characteristics (if IIRC): - High Z at the feedpoint (voltage maximum and current node); - very small counterpoise lenght compared to the resonant wavelenght of the antenna (typical 0.1-0.2 lambda) can i mantain the same characteristics shortening the antenna in any way ?? Thanks for read and explain to those who want clarify my doubts. -.-. --.- You can add a top hat to a vertical that's shorter than a half wavelength, and bring it to anti-resonance (high input resistance with no reactance). The impedance won't be as high as if the antenna were a half wavelength high, and it will have narrower bandwidth, so it won't be a perfect imitation. Or you can use a combination of loading inductance and top hat to get a somewhat poorer imitation. Roy Lewallen, W7EL |
language and antenna gain
On Jul 1, 11:53*am, Roy Lewallen wrote:
But a helically-wound radiator *is* a naturally resonant radiator if wound to make it so. It just isn't straight. That is true, but a resonant, helically-wound, electrically short radiator does *not* have the radiation resistance of a straight, self- resonant radiator of about 1/4 wavelength -- which can make a large difference between the two forms in the amount of available energy radiated vs. that lost in the r-f ground system. RF |
language and antenna gain
Richard Fry wrote:
On Jul 1, 11:53 am, Roy Lewallen wrote: But a helically-wound radiator *is* a naturally resonant radiator if wound to make it so. It just isn't straight. That is true, but a resonant, helically-wound, electrically short radiator does *not* have the radiation resistance of a straight, self- resonant radiator of about 1/4 wavelength -- which can make a large difference between the two forms in the amount of available energy radiated vs. that lost in the r-f ground system. RF Guess I just don't understand how a resonant, helically-wound antenna is "electrically short". Suppose you helically wound an eighth-wave vertical in such a way that it was resonant. Its physical length is an eighth wavelength. What would its "electrical length" be? That is, how "electrically short" would it be? How do you determine what the "electrical length" of a vertical or dipole is? Does a loading coil at the base alter the "electrical length"? Does an L network at the base? Roy Lewallen, W7EL |
language and antenna gain
On Jul 1, 3:18*pm, Roy Lewallen wrote:
Guess I just don't understand how a resonant, helically-wound antenna is "electrically short". Suppose you helically wound an eighth-wave vertical in such a way that it was resonant. Its physical length is an eighth wavelength. What would its "electrical length" be? Its overall height in free space wavelengths. This is the definition used by the FCC for the unloaded monopoles used in AM broadcasting Do you expect your 1/8 WL high, self-resonant helical to have the same electrical length and feedpoint parameters as a self-resonant, straight radiator about 1/4 wave high in free space wavelengths? According to Kraus and other authors, your example above still has about the same radiation resistance as a 1/8 WL straight conductor -- not that of a self-resonant, straight conductor about 1/4 WL high. An 1/8 WL high helical may be resonant, but it will not perform the same in a practical antenna system as a straight, self-resonant vertical whose physical height in free space wavelengths is about 1/4 wave. RF |
language and antenna gain
Richard Fry wrote:
On Jul 1, 3:18 pm, Roy Lewallen wrote: Guess I just don't understand how a resonant, helically-wound antenna is "electrically short". Suppose you helically wound an eighth-wave vertical in such a way that it was resonant. Its physical length is an eighth wavelength. What would its "electrical length" be? Its overall height in free space wavelengths. This is the definition used by the FCC for the unloaded monopoles used in AM broadcasting Oh, then the "electrical length" is the same as "physical length". Why not just say "physical length" then? Do you expect your 1/8 WL high, self-resonant helical to have the same electrical length and feedpoint parameters as a self-resonant, straight radiator about 1/4 wave high in free space wavelengths? You just said that the height of the antenna is the electrical length. So no, I wouldn't expect two antennas of different heights to have the same electrical length, using your definition. According to Kraus and other authors, your example above still has about the same radiation resistance as a 1/8 WL straight conductor -- not that of a self-resonant, straight conductor about 1/4 WL high. It'll be a little higher than a straight 1/8 wave conductor due to a bit more favorable current distribution (see the article referenced in my next paragraph for some actual measurements). But it'll surely not be as high as that of a quarter wavelength conductor. Which of course is the reason it's so important to minimize loss if you want good efficiency. An 1/8 WL high helical may be resonant, but it will not perform the same in a practical antenna system as a straight, self-resonant vertical whose physical height in free space wavelengths is about 1/4 wave. Depends. If the ground system is very good and you're willing to keep to a narrow bandwidth without retuning, you wouldn't be able to tell any practical difference between the two. For good experimental evidence see "The W2FMI Ground-Mounted Short Vertical" by Jerry Sevick, W2FMI in March 1973 QST. Roy Lewallen, W7EL |
language and antenna gain
On Thu, 01 Jul 2010 14:27:56 -0700, Roy Lewallen
wrote: Richard Fry wrote: On Jul 1, 3:18 pm, Roy Lewallen wrote: Guess I just don't understand how a resonant, helically-wound antenna is "electrically short". Suppose you helically wound an eighth-wave vertical in such a way that it was resonant. Its physical length is an eighth wavelength. What would its "electrical length" be? {SNIP} Depends. If the ground system is very good and you're willing to keep to a narrow bandwidth without retuning, you wouldn't be able to tell any practical difference between the two. For good experimental evidence see "The W2FMI Ground-Mounted Short Vertical" by Jerry Sevick, W2FMI in March 1973 QST. Roy Lewallen, W7EL Somewhere back in time the word "aperature" was used to describe antennas, especially stacked arrays. I think one of the prominate authors that used the expression was Bill Orr. Would that be relevant to this discussion? I have not heard the term used in years. John, W8CCW Perpetual Student of many subjects. |
language and antenna gain
John Ferrell wrote:
Somewhere back in time the word "aperature" was used to describe antennas, especially stacked arrays. I think one of the prominate authors that used the expression was Bill Orr. Would that be relevant to this discussion? I have not heard the term used in years. John, W8CCW Perpetual Student of many subjects. Here are postings (about 35) I've made in the past in this newsgroup about this topic: http://tinyurl.com/rraa-aperture. Enjoy. Roy Lewallen, W7EL |
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