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Tom,
Thanks for the suggestion. I have used RFsim and come up with dimensions for a 915 MHz version. I would like to put a coupling strip each side of the centre to pick up forward and reverse power at the same time. The description in RFsim don't mention this - Do you know if I need to make any alterations in dimensions to accommodate this or should I have a second 1/4 in series with another coupler that picks up reverse power ? I would like to use the AD8302 Gain/Phase device to measure the incident and reflected voltages. This thing will has an input range of -60dBm to 0dBm which will be great if I feed the directional coupler with 0dBm and the coupling is -20dB. It provides measurement for +/- 30dB and Phase orange over 180 degrees. It also will operate to 2.7GHz so should be ok for the 915Mhz. I thought I would make this gain/phase detector in a separate enclosure so I can unplug it from the directional coupler and use it for Amp gain/phase etc. With additional of a PIC micro and LCD display it could be programmed to provide various measurement types. There are construction details in the ARRL Antenna book for a fixed gain antenna that looks good for a reference. Hopefully this will give me a suitable arrangement for setting up 900MHz band antennae using the sig gen power levels. Regards David K7ITM wrote: I would expect the outer is a Teflon or Teflon-like material, with probably low loss and fairly low relative dielectric constant, but it's still going to require that you shorten that lower section quite a bit to get the quarter-wave stub to reflect a high impedance. In addition, the dielectric between the outer of the coax and the inner of the sleeve is pretty thin, and the impedance of the resulting coaxial arrangement is pretty low. That means that it won't ever reflect a very high impedance. You can use a tapered sleeve that bells out at the bottom, to good effect. That will also lower the feedpoint impedance and match better to 50 ohms. Think: ground plane with drooping radials. But you can also wind the coax just below the antenna into a small coil (I'd make the axis of the coil coincide with the axis of the antenna) that's self-resonant near your operating frequency, and it will very effectively choke off antenna current (current on the outside of the coax) at that point. Put one such coil an inch or so below the bottom of the sleeve, and another about a quarter wave further down the line. SWR meter: Get RFSim99--do a Google search for it. Build a coupler, per toolsdesigncoupler. I'd suggest a microstrip version, if you can make a little PC board reasonably accurately. Design it for 50 ohms. Terminate each coupled port in 50 ohms (e.g. 49.9 ohm 0805 SMT part). Using vanishingly short leads, connect a simple diode detector to each of those two loads. Use a calibrated attenuator to calibrate at least the relative response of those detectors. Use those two outputs to calculate SWR. You can read the diode detector outputs with a DVM that has good resolution (10uV sensitivity preferred; 1uV is even better). I'd recommend about a 20dB coupler for the power level you're using, though even a 30dB coupler would work. Try terminating the through line in a 49.9 ohm (or a parallel pair of 100 ohm) 0805 parts to check that you see essentially no reflected, and try a 100 ohm load and a 25 ohm load to check that you get the expected reflected. -- To have the coupling right according to the RFSim99 directions, it needs to be 1/4 wave long, but it's a pretty broad peak. Coupling drops to zero at 1/2 wave, and at DC. So you could make one for 900MHz, and it would work OK at 450MHz, you'd just get lower coupling. The ratio for SWR would still be OK. The 450MHz version on FR4 board (fiberglass-epoxy) would be roughly four inches long, if my mental arithmetic is right, and half that for 900MHz. Cheers, Tom David wrote: Tom, Thanks for the information. The inner coax is the smaller RG174 coax. The sleeve is made of earth braid pulled from RG58 cable. The dielectric between the sleeve and inner cable is therefore the outer sheath of the RG174 cable (Not sure what this is, the RG174 I have is Teflon inner dielectric and stranded conductor. The utter sheath is a very strong heat resistant material - I therefore have no ideal of the dielectric constant to calculate Vp for correct electrical length). If I use a copper tube and strip off the sheath from the inner coax, I can calculate correct length as it will have an air dielectric. Do you know where everyone is getting the dielectric constants for various materials ? I noted people using small metal tubes as sleeves and quoting these magic numbers even for copper tubes of certain diameters. From my discussion with Telonic, they say the Rho_Tector was designed as an in-house tool for measuring inputs of amps and filters. They suggest SWR meter would probably be best for antenna adjustments. Do you happen to know where I might find details for a low power SWR meter for 915 MHz ? I need one that will operate with only 20mW applied power. The only SWR meter I have has min. FSD of 3W Thanks Regards David K7ITM wrote: Hi David, You wrote, "The decoupling is via a 1/4 wave sleeve that provides high impedance for RF returning along outer coax and also as the second 1/2 of the dipole. " EXACTLY how is this built? The details of construction make a BIG difference in performance! (There's a lot of BAD info about it out there...) It's not a bad idea to ALSO put some additional decoupling further down the feedline. If your spectrum analyzer/field strength meter is far enough away from the antenna you are testing, then it should provide a reasonable indication of relative antenna radiation performance. The SWR indication, if properly calibrated and given that you are apparently exciting the antenna with a source whose output impedance matches your feedline, should also be a good indication of power actually absorbed by the antenna. That is, lowest SWR represents maximum power absorbed by the antenna. Presumably that power is being radiated as RF, mostly, and not dissipated as heat. But where the RF radiation goes depends on the pattern of currents excited on the conductors that compose the antenna, and nearby conductors as well (such as the feedline). What you probably want is standard resonant half-wave dipole currents on your vertical dipole, and no (very little) antenna current on the feedline and on support structures. By the way, whether the antenna is resonant or not is of little real importance, so long as you can efficiently feed power to it and the antenna currents are in the right places and not the wrong places. But it happens that with your antenna, if things are working properly (properly decoupled feedline, etc), you probably will see lowest SWR at half-wave resonance. If you have no other matching going on, the lowest SWR will probably be about 1.5:1 with 50 ohm feedline. You could add parts to get a better match if you wished. And as you can probably tell from all that, I'm suspecting that your decoupling sleeve, with associated dielectrics in that area, probably isn't doing a very good job... Also...Joe noted that your coax feedline may well be a length that accounts for the SWR peaks and valleys. (I think it may be about twice as long as Joe wrote...but same idea.) Do you see the peaks and valleys when you terminate the line in the precision 2:1 load? If you do NOT, then it's a further indication that the feedline has antenna currents on it, because the flat 2:1 is an indication that your transmission line is matched to the calibration impedance of the SWR bridge, and if that's the case, the SWR bridge should be giving at reasonably accurate estimate of the actual line SWR. If you DO see the SWR ripples vs frequency with just the precision load, either the load isn't "flat" or the line is not the same impedance as the SWR bridge is calibrated to, and the differing impedances is by far the most probable explanation if the line length is right. Cheers, Tom |
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