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#11
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![]() "Roy Lewallen" wrote in message ... There are a lot of factors that can potentially affect measurements. The first and most obvious is impedance matching, which has to be done to extract the maximum possible power from the antennas (assuming that this is what you'll be doing when actually using the antenna) and delivering a known amount of power to each. Another is to make sure the feedlines aren't radiating or picking up signals, by using proper baluns, which you've mentioned. Move the feedlines around and change their lengths, making sure the results don't change. If they do, your baluns aren't adequate. If you want quantitative measurements, you'll have to carefully calibrate your signal strength meter at the power level involved. Don't make assumptions about its linearity. Better yet would be to carefully match the antennas to 50 ohms, then insert a step attenuator in series with the Yagi in the 50 ohm environment and increase the attenuation until it reads the same as the dipole. That way, the FS meter linearity is immaterial (although the attenuator accuracy is important). Then there's the possibility of signals radiated off the back of the dipole being reflected from near or distant objects which would affect the dipole's measured field strength more than it would the Yagi's. The different widths of the forward lobes can also cause unequal reflections. Although reflections can affect the forward gain up to several dB, their impact on nulls or front/back measurements is likely to be greater. I'd do a couple of things. One is to build an NBS reference Yagi which is easy to construct and has a well known and documented gain. It's also easy to model. Modern modeling programs do very well with full size Yagis. Measure this on your range and verify that the measurements agree with its known properties. Another check would be to rotate the dipole and see if its pattern is what it should be. And rotate the NBS Yagi and verify that its pattern matches modeled results. Only after doing those tests would I have reasonable confidence in other measurements made with similar types of antennas. Roy Lewallen, W7EL I thought that NIST prefers a dipole antenna up to 1 GHz, and then a waveguide-fed horn beyond that. I would think that a pyramidal horn would be more predictable that a Yagi, and not unreasonably large, even for 400 MHz. Plus, the construction is simpler, and the design is more robust over time (i.e., it's easy to bend a Yagi element slightly, not enough to notice it, but enough to shift performance). Ed WB6WSN |
#12
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Mike,
Here is something to watch out for. I tried to measure the pattern for a 432 antenna. I had coax going from the shack to the transmit antenna, and another coax going to the receive antenna. I measured complete garbage for the pattern, until I realized that the receive antenna was picking up a signal any time it was aimed at the transmitter COAX. I fixed the problem by moving the signal source to be collocated with the transmit antenna. One way to get accurate gain readings is to use the receive S meter and a calibrated attenuator. For instance, if with the reference antenna you need 15db of attenuation to get an S9 reading, and with the target antenna you need 27 db of attenuation to get S9, the gain of the target antenna is 27 - 15 = 12 db. Some S meters are more accurate for low signals; so, you might want to use something like S3 for a reference. Tam/WB2TT "Peter" wrote in message ... Hi all. This may seem like a fairly basic question. But here we go! I want to performance test a 436MHz high gain antenna. My plan is to construct a simple dipole with a 1:1 balun for 436MHz as a reference antenna and construct another dipole with a 1:1 balun to receive the test signal, measure it with a diode detector and a milli-amp meter (field strength meter) at the shack. Do the calculation and have the antenna gain. This seems to me to be fairly straight forward, but has anyone carried out similar measurements and concur with the approach or are there are there traps and pit falls that I need to be aware of. Or is their simply a better way? Cheers -- Peter Miles VK3YSF Melbourne, Australia |
#13
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Mike,
Here is something to watch out for. I tried to measure the pattern for a 432 antenna. I had coax going from the shack to the transmit antenna, and another coax going to the receive antenna. I measured complete garbage for the pattern, until I realized that the receive antenna was picking up a signal any time it was aimed at the transmitter COAX. I fixed the problem by moving the signal source to be collocated with the transmit antenna. One way to get accurate gain readings is to use the receive S meter and a calibrated attenuator. For instance, if with the reference antenna you need 15db of attenuation to get an S9 reading, and with the target antenna you need 27 db of attenuation to get S9, the gain of the target antenna is 27 - 15 = 12 db. Some S meters are more accurate for low signals; so, you might want to use something like S3 for a reference. Tam/WB2TT "Peter" wrote in message ... Hi all. This may seem like a fairly basic question. But here we go! I want to performance test a 436MHz high gain antenna. My plan is to construct a simple dipole with a 1:1 balun for 436MHz as a reference antenna and construct another dipole with a 1:1 balun to receive the test signal, measure it with a diode detector and a milli-amp meter (field strength meter) at the shack. Do the calculation and have the antenna gain. This seems to me to be fairly straight forward, but has anyone carried out similar measurements and concur with the approach or are there are there traps and pit falls that I need to be aware of. Or is their simply a better way? Cheers -- Peter Miles VK3YSF Melbourne, Australia |
#14
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Perhaps. If I were going to measure a Yagi against a dipole, I'd first
try measuring a Yagi with known gain against a dipole. You might prefer to measure a dipole relative to a dipole. I can't see what you'd learn from it, but each to his own. Roy Lewallen, W7EL Ed Price wrote: I thought that NIST prefers a dipole antenna up to 1 GHz, and then a waveguide-fed horn beyond that. I would think that a pyramidal horn would be more predictable that a Yagi, and not unreasonably large, even for 400 MHz. Plus, the construction is simpler, and the design is more robust over time (i.e., it's easy to bend a Yagi element slightly, not enough to notice it, but enough to shift performance). Ed WB6WSN |
#15
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Perhaps. If I were going to measure a Yagi against a dipole, I'd first
try measuring a Yagi with known gain against a dipole. You might prefer to measure a dipole relative to a dipole. I can't see what you'd learn from it, but each to his own. Roy Lewallen, W7EL Ed Price wrote: I thought that NIST prefers a dipole antenna up to 1 GHz, and then a waveguide-fed horn beyond that. I would think that a pyramidal horn would be more predictable that a Yagi, and not unreasonably large, even for 400 MHz. Plus, the construction is simpler, and the design is more robust over time (i.e., it's easy to bend a Yagi element slightly, not enough to notice it, but enough to shift performance). Ed WB6WSN |
#16
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![]() "Roy Lewallen" wrote in message ... There are a lot of factors that can potentially ............... Roy, Does one have to worry about elevation? Generally the gain at 0 degrees elevation is very low. Does this apply when the two antennas are facing each other in plain line of sight? should he raise the transmit antenna to say, 15 degrees above the receiving antennas? Tam/WB2TT |
#17
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![]() "Roy Lewallen" wrote in message ... There are a lot of factors that can potentially ............... Roy, Does one have to worry about elevation? Generally the gain at 0 degrees elevation is very low. Does this apply when the two antennas are facing each other in plain line of sight? should he raise the transmit antenna to say, 15 degrees above the receiving antennas? Tam/WB2TT |
#18
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The gain at low elevation angles is low only if you're distant from the
antenna, and ground reflection is taking place -- EZNEC's far field analysis assumptions don't apply to a typical antenna measurement setup. Low far field gain at low angles is due to interference between the direct radiation and radiation reflected from the ground. This can and does occur when measuring antennas which are relatively close, but not to the extent you see in idealized far field modeling results, where the antennas are effectively an infinite distance apart. I don't have a great deal of real experience in measuring antennas, but believe that the common practice is to try to put the antennas high and relatively close together. It shouldn't be difficult to get a pretty good idea of the amount of signal cancellation you'd get for a given height and spacing, by setting up and solving a pretty simple geometry/trigonometry problem. Or you could model the two antennas over ground at their proposed positions, with a source in the transmitting antenna and a matched load in the receiving antenna, and look at the power delivered to a load as the height and spacing are changed. Fortunately, dipoles and moderate size Yagis have similar elevation patterns for at least moderate angles relative to straight-on. That means that the percentage of signal directed toward the ground and therefore arriving at the receive antenna as a reflection will be about the same for a reference dipole as for a test Yagi. So whatever attenuation occurs, it should be about the same for the two antennas. This wouldn't hold, though, for a very long Yagi or for high-angle reflections from antennas which are high and close together. It's one of the factors that make really good antenna measurements tricky. Roy Lewallen, W7EL Tarmo Tammaru wrote: "Roy Lewallen" wrote in message ... There are a lot of factors that can potentially ............... Roy, Does one have to worry about elevation? Generally the gain at 0 degrees elevation is very low. Does this apply when the two antennas are facing each other in plain line of sight? should he raise the transmit antenna to say, 15 degrees above the receiving antennas? Tam/WB2TT |
#19
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The gain at low elevation angles is low only if you're distant from the
antenna, and ground reflection is taking place -- EZNEC's far field analysis assumptions don't apply to a typical antenna measurement setup. Low far field gain at low angles is due to interference between the direct radiation and radiation reflected from the ground. This can and does occur when measuring antennas which are relatively close, but not to the extent you see in idealized far field modeling results, where the antennas are effectively an infinite distance apart. I don't have a great deal of real experience in measuring antennas, but believe that the common practice is to try to put the antennas high and relatively close together. It shouldn't be difficult to get a pretty good idea of the amount of signal cancellation you'd get for a given height and spacing, by setting up and solving a pretty simple geometry/trigonometry problem. Or you could model the two antennas over ground at their proposed positions, with a source in the transmitting antenna and a matched load in the receiving antenna, and look at the power delivered to a load as the height and spacing are changed. Fortunately, dipoles and moderate size Yagis have similar elevation patterns for at least moderate angles relative to straight-on. That means that the percentage of signal directed toward the ground and therefore arriving at the receive antenna as a reflection will be about the same for a reference dipole as for a test Yagi. So whatever attenuation occurs, it should be about the same for the two antennas. This wouldn't hold, though, for a very long Yagi or for high-angle reflections from antennas which are high and close together. It's one of the factors that make really good antenna measurements tricky. Roy Lewallen, W7EL Tarmo Tammaru wrote: "Roy Lewallen" wrote in message ... There are a lot of factors that can potentially ............... Roy, Does one have to worry about elevation? Generally the gain at 0 degrees elevation is very low. Does this apply when the two antennas are facing each other in plain line of sight? should he raise the transmit antenna to say, 15 degrees above the receiving antennas? Tam/WB2TT |
#20
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Thanks
Tam |
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