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Near field vs Far field measurements at 2M
Steve Reinhardt wrote:
Well, as I mentioned, this is less about an absolute antenna gain than a figure of merit. Using a dipole as the first, reference transmitting antenna is part of the plan. I may be crazy, but I'm not entirely stupid ;-) (Well, maybe. Time will tell...) You'll get strong reflections from the ground between the antennas. Ah, yet another thing I have to consider. Since at least two of the tested antennas will be primarily vertical, I was planning to make the test antennas all vertical. So, I can talk myself into believing the ground reflections are part of the real world installations, or I can chuck it all and rely solely upon modeling. One is probably smarter, the other more viscerally stimulating. I leave to the reader to sort out which is which. The tricky thing is that the reflections will have a different effect on your reference antenna observations than on your Antenna Under Test (AUT) measurements (consider comparing a dipole to dipole, where you get a definite ground reflection, vs high gain dish to high gain dish).. The way to handle this, if you've got time and inclination, is to scan one of the antennas in height. That is, set up your AUT (or reference dipole), then move the measurement probe vertically over a distance of several wavelengths, making measurements at several points. From this, you can calculate the effect of the ground reflection. If you google: Ground Reflection Range Antenna Gain Measurement, you might turn up something useful. It's also described in Kraus's book. You might also want to look up the "three antenna method" which allows you to get absolute measurements. Jim, W6RMK |
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