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#1
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1/4 vs 1/2 wavelength antenna
Hi
I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. All technical notes I have read recommend a 1/4 wave whip over ground plane as offering the best performance, statements like: "Best range is achieved with either a straight piece of wire, rod or PCB track @ 1/4 wavelength over a ground plane", I understand many factors effect performance however I have found that a "bent" 1/2 wavelength length of wire offers better performance. If I use a 1/4 wavelength I need (due to case requirements) to have two 90 degree bends in it (feed - up, across, up). If I use a 1/2 wavelength I need to run it once around the (plastic) case (feed - up, around the case, up). I hope this makes some sense, anyway I have found the 1/2 wave is less effected by polarisation and offers generally better performance. However while more ground plane may help a 1/4 wave it seems to hinder the 1/2 wave, I guess because it shields the loop around the case? Regards |
#2
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"Nug" wrote in message
m... Hi I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. All technical notes I have read recommend a 1/4 wave whip over ground plane as offering the best performance, statements like: "Best range is achieved with either a straight piece of wire, rod or PCB track @ 1/4 wavelength over a ground plane", I understand many factors effect performance however I have found that a "bent" 1/2 wavelength length of wire offers better performance. If I use a 1/4 wavelength I need (due to case requirements) to have two 90 degree bends in it (feed - up, across, up). If I use a 1/2 wavelength I need to run it once around the (plastic) case (feed - up, around the case, up). I hope this makes some sense, anyway I have found the 1/2 wave is less effected by polarisation and offers generally better performance. However while more ground plane may help a 1/4 wave it seems to hinder the 1/2 wave, I guess because it shields the loop around the case? A 1/4 wavelength antenna really needs to be straight and at right angles to the ground plane. That is probably why the 1/2 wavelength antenna works better in your case. Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
#3
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I would make the grandiose statement that since you are bending the wire
it no longer exhibits the performance of a "standard" 1/4 or 1/2 wave antenna. I would suggest that if you indeed made a 1/4 wave GP that protruded from the box surface (with a suitable counterpoise) it would outperform the 1/2 wave bent one.. Assuming you have to put the antenna inside the box or wrapped around it I suggest you look into tuning it with some C and/or L. In that case you would construct the antenna to fit your case parameters and adjust the matching for best radiation. Keep in mind that the C/L tuning components could be lengths of coax and open feeder/wire. (because of the high operating freq) Cheers Bob VK2YQA (Sydney Australia) Nug wrote: Hi I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. All technical notes I have read recommend a 1/4 wave whip over ground plane as offering the best performance, statements like: "Best range is achieved with either a straight piece of wire, rod or PCB track @ 1/4 wavelength over a ground plane", I understand many factors effect performance however I have found that a "bent" 1/2 wavelength length of wire offers better performance. If I use a 1/4 wavelength I need (due to case requirements) to have two 90 degree bends in it (feed - up, across, up). If I use a 1/2 wavelength I need to run it once around the (plastic) case (feed - up, around the case, up). I hope this makes some sense, anyway I have found the 1/2 wave is less effected by polarisation and offers generally better performance. However while more ground plane may help a 1/4 wave it seems to hinder the 1/2 wave, I guess because it shields the loop around the case? Regards |
#4
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On Sun, 20 Feb 2005 22:09:15 -0800, Nug wrote:
Hi I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. All technical notes I have read recommend a 1/4 wave whip over ground plane as offering the best performance, statements like: "Best range is achieved with either a straight piece of wire, rod or PCB track @ 1/4 wavelength over a ground plane", I understand many factors effect performance however I have found that a "bent" 1/2 wavelength length of wire offers better performance. If I use a 1/4 wavelength I need (due to case requirements) to have two 90 degree bends in it (feed - up, across, up). If I use a 1/2 wavelength I need to run it once around the (plastic) case (feed - up, around the case, up). I hope this makes some sense, anyway I have found the 1/2 wave is less effected by polarisation and offers generally better performance. However while more ground plane may help a 1/4 wave it seems to hinder the 1/2 wave, I guess because it shields the loop around the case? A 1/4 wave antenna will match to a low impedance, unbalanced. A 1/2 wave dipole will match to a low impedance, balanced. A 1/2 wave piece of wire fed at the end will match to a high impedance. What kind of circuit are you using for your output? Thanks, Rich |
#5
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In article ,
Nug wrote: Hi I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. [.. 1/4 wave and 1/2 wave ...] An antenna looks like an LC tuned circuit loaded by the radiation resistance. Your output stage has some impedance that correctly matches to it (there are exceptions we will ignore) and it is this impedance you want the antenna system to have. When the correct matching is done, the antenna works as an impedance mathcing network that matches the output stages impedance to the radiation resistance. The normal (90 degrees to) 1/4 wave whip over a ground plane is one half of a dipole that is 1/2 wave length. The ground plane operates like a mirror. The electrostatic lines of force follow the same path with the mirroring as they would if the other 1/2 of the dipole was there. This lets you use a smaller (1/4 wave) antenna to get the same effect as the 1/2 wave. In your case, you are not using a whip antenna. If I've read what you wrote correctly, the antenna spends more of its length parallel to the surface of the PCB than it does running 90 degrees away from it. You have some circuit with a ground plane and a limitted sized box to work with, so the mechanical shape is constained by the box and not the ideal electronics. Since the box is small: If you have the equipment to do so, I suggest you measure (estimate) the impedance of the longest single loop of wire that will fit within the case. ie: connect to both ends. You have to have the electronics PCB in the case when you do this. If you are very lucky, its impedance will not be too hard to match to the output stage. -- -- forging knowledge |
#6
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Nug wrote:
"I am building an rf transmitter for a short range data link at 433 MHz and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance." Your wavelength is about 0.69 meter or about 2.3 feet. In antennas everything depends on wavelength. If you use a transmitter housing as the ground plane for your antenna, it needs to be a sizeable part of a wavelength or the salient part of your antenna must be longer to compensate for the small ground plane. If you had an infinite ground plane, a 1/2-wave wire perpendicular to it would produce up to 50% more volts per meter field strength than a 1/4-wave wire perpendicular to the ground plane. It`s not something for nothing. Total radiation is the same in both cases. More of the radiation is perpendicular to the wire in the 1/2-wave and less goes off at some other angle to the wire. 50% more field in some particular direction is realy not very significant in most cases, and there are other consequences of using a 1/2-wave wire instead of a 1/4-wave wire. An end-driven 1/2-wave wire presents a very high impedance. It is equivalent to a parallel-resonant circuit. It would match a direct connection to a parallel resonant tank circuit perhaps. An end-driven 1/4-wave wire presents a very low impedance when worked against a ground plane, maybe about 30 ohms. How well you are able to radiate a signal from a wire is likely to depend on how well it is matched to the transmitter and less about the bends in the wire. In any case the complete antenna must be resonant to eliminate reactance which opposes the signal`s entry into the wire. For a small transmitter operating at a very short wavelength, the size of the antenna is not onerous and it would be possible to use a center-fed 1/2-wave antenna. Each half would be just a little over a half foot in length. Drivepoint impedance is in the 70-ohm range. Another possibility is a full-wave loop, about 2.3 feet in perimeter with a drivepoint impedance of about 120 ohms. Performance of all the suggestions is probably about the same. You can find the best by trying them. Best regards, Richard Harrison, KB5WZI |
#7
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Thanks for all your responses, good stuff here.
Rich, I am using TX modules from Liapac which specify a 50 ohm load. So putting it all together (I think) by bending the antenna (both 1/4 and 1/2) I am messing with its impedance and subsequent matching? So I would be better off with a straight 1/4 over ground plane if I had the choice. From reading I've done I think that a bend from vertical to horizontal (closer to ground) would be decreasing the antenna impedance, does that sound correct? Would I benefit from adjusting (extending) the antenna length slightly (I don't think so because it would no longer be at wavelength, but I am not sure?). Ken said ... "If I've read what you wrote correctly, the antenna spends more of its length parallel to the surface of the PCB than it does running 90 degrees away from it"... Actually it's about half and half (feed up 15mm , loop around the case, up - {clear of case} 130mm). Note the horizontal loop does not cross over itself. Sort of like this (~~ is horizontal loop) side view. | | | ~~~~ | feed I understand that more of the radiation is perpendicular to the wire in the 1/2-wave (than 1/4 wave), broadly speaking how will the bend('s) in the aerial effect radiation pattern (I understand this is tough to answer)? I can't fit a full loop inside the case. I have no rf test equipment so can only use trial and error, thanks again to all who responded. As I said it actually works fine it's just I don't like not understanding the reason its working, and would like to make any small changes that may improve performance. Thanks Again |
#8
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Ken Smith wrote:
In article , Nug wrote: Hi I am building an rf transmitter for a short range data link at 433MHZ and am almost done, but I would like to understand better exactly what I am seeing with regard to antenna performance. [.. 1/4 wave and 1/2 wave ...] An antenna looks like an LC tuned circuit loaded by the radiation resistance. Your output stage has some impedance that correctly matches to it (there are exceptions we will ignore) and it is this impedance you want the antenna system to have. When the correct matching is done, the antenna works as an impedance mathcing network that matches the output stages impedance to the radiation resistance. RF transmitters are not impedance matched to antennae in the sense of maximum transfer of power. "Maximum transfer of power" is a small signal (ideal linear parameters) issue, not a large signal issue. That is, the antenna/load are not conjugately matched. What is said, is that a TX'er will deliver some given power into, for example, 50 ohms. This says nothing about the output impedance of the PA. Power amplifiers are concerned with DC input power to RF output power efficiency, thus they are load-line "matched," not impedance matched. The concept of "output impedance" breaks down for large signal devices. For example, what is the output impedance of a class C or D amp taken when the transistor is on or off? I suppose one could consider the time-averaged impedance, but I'm not sure of the utility (to be fair, the time-averaged reactive output component is tuned out as best possible). The vague output impedance is a problem even for large signal class A devices. Again, RF PA's should be load-line matched. Output-Z is irrelevent. |
#9
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On Wed, 23 Feb 2005 19:08:20 GMT, gwhite wrote:
RF transmitters are not .... Sorry OM, This was all nonsense. 73's Richard Clark, KB7QHC |
#10
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G. White wrote:
"Output Z is irrelevant." This is an old argument in this newsgroup. I became convinced long ago that there are cases in which impedance is very important. "Transmission Lines, Antennas, and Wave Guides" by King, Mimno, and Wing make a clear and concise case for the princuple of conjugates in impedance matching on page 43: "If a dissipationless network is inserted between a constant-voltage generator of impedance Zg, and a load of impedance ZR such that maximum power is delivered to the load, at every pair of terminals the impedance looking in opposite directions are conjugates of each other. To secure maximum power output from a generator whose emf and internal impedance are constant the load must have an impedance equal to the conjugate of the generator`s internal impedance." Radio transmitters don`t produce significant harmonics. It`s the law. They are linear power sources. We can and do tune them for all the power they will produce under their particular operating conditions of drive and d-c power supply. They operate at more than 50% efficiency which means that they don`t take power 100% of the time, but are switched-off during part of the r-f cycle. Output impedance is thus an average over the entire cycle. It`s OK. We have no harmonics. Gaps are filled by the tank circuit and other filters. The radio is a proper source. The impedance added by off-time is called "dissipationless resistance" because no power is lost in the radio while it is switched-off. Best regards, Richard Harrison, KB5WZI |
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