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*Subject*: Re: R: R: [amsat-bb] phase shift with coax*From*: "John P. Toscano" <tosca005@xxxxxxxxxx>*Date*: Wed, 11 Jul 2001 01:21:44 -0500

Domenicl (i8cvs) wrote: > If the impedance of each antenna is 50 ohm and if i use two 50 ohm > feedlines of any lenght but one 1/4 wavelenght longer than the other > i get only the phase shift for circular polarization but not the > impedance matching. I agree with you. But this was not what I was suggesting be done. > Infact connecting both lines in parallel to a "T" connector i get an > impedance of 25 ohm. Again, we agree. > In order to match 25 ohm to a 50 ohm feed line i need a transformer > made by a line 1/4 electrical wavelengt long wich impedance is SQR > ( 25 x 50 )= 35 ohm and i have to made it with tubing or using two > 75 ohm lines each 1/4 electrical wavelenght long connected in > parallel. I agree, and this is what the dimensions I quoted do. If D is the inner width of the outer, square aluminum tubing, and if d is the outside diameter of the inner, round brass tubing, then the impedance of the "transmission line" consisting of the round tubing centered inside the square tubing is: Z = 138 log (1.08 D / d) For the figures I quoted ("standard" available sizes, which we will show to be close, but not perfect to give us the desired impedance), we get: Z = 138 log (1.08 x 0.875 / 0.500) = 138 log(1.89) Z = 138 x 0.27646 = 38.15 ohms. Pretty close to 35 ohms. Note that with the cable method, you are transforming 50 ohms at the antenna to 100 ohms at the two ends of the T connector, so that the parallel connection of two 100 ohm feedpoints becomes 50 ohms again. And that requires that the 1/4 wavelength cables be Sqrt(50 x 100) = Sqrt(5000) = 70.71 ohms, which is pretty close to 75 ohms, but again not exactly right. > This is more complicated than to match the impedance using two 75 > ohm lines each 1/4 electrical wavelenght long as used with the > original phasing harness on the A144-20 T antennas. In the final analysis, it's more work to do it with the power divider (unless you just buy one!) because you have to fabricate the divider, and still have to fabricate the two 50 ohm cables that differ by 1/4 wavelength, versus fabricating (at a minimum) two 1/4 wavelength 75 ohm cables and one 1/4 wavelength 50 ohm cable. The advantage is presumably lower loss and better impedance matching because of the inherent low loss of the air dialectric transmission line vs. the low-velocity-factor, higher loss polyethylene dialectric of the RG-59, the fact that you might not know the REAL velocity factor of the piece of RG-59 you buy, making the 1/4 wavelength sections the wrong length, and the unknown impedance and loss characteristics of the T connector (some may be good, but I've heard stories of some T connectors working really poorly). But at these frequencies, I doubt there would be a noticeable difference unless the T connector was one of the real bad ones, assuming everything else was done correctly. A very reputable US manufacturer of high-performance VHF/UHF/Microwave antennas, M Squared, recommends the power divider approach that I described when building stacked arrays of antennas. But they also use 75 ohm coax with high-quality, weatherproof F connectors on the coaxial baluns of their folded dipole driven elements, on all four bands of their Yagi's that I own (50, 144, 222, 432 MHz). When stacking their HO-Loop antennas, they offer both options, the 75 ohm coax method and the 2-port power divider method. When I bought my pair of SQLoop antennas from them years ago (the predecessor of the current HO-Loop), they said that they most commonly sold stacked pairs with the coax cable impedance transformer and closer than optimal spacing because it was less expensive, and gave acceptable performance. They offered me the option of building my stacked pair with the 2-port power divider, LMR-400 phasing cables, and an optimal stacking distance, and that's what I went with. At the time, I had no Yagi nor any way to rotate one, so I wanted the extra little bit of performance on 2M SSB that the more expensive solution provided. There's no way I could claim that the difference was measurable, since I didn't buy one of each configuration. But it was a neat way to get a horizontally polaraized omnidirectional antenna system that served me well before I got a tower and rotatable directional antennas installed. So it's all a matter of personal choice. (I really hate to assemble RF connectors onto coaxial cables, but I could be talked into either method myself, depending on circumstances. How's that for indecisive?) 73 de KB0ZEV ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**Re: R: phase shift with coax***From:*Stan Vandiver

**Re: R: phase shift with coax***From:*John P. Toscano

**R: R: phase shift with coax***From:*i8cvs

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