Cross Polarized Yagi - VHFUHF Forum at eHam.net

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Cross Polarized Yagi	Reply
by KD5NVC on December 15, 2004	Mail this to a friend!
I am looking into a single antenna that will provide both vertical (FM) and horizontal (SSB) polarization. There are a few that look very good such as Cushcraft and Gulf Alpha. The question here is, in the ARRL antenna handbook, I noted in section 18 at the end of a paragraph that you may see a 20DB loss with a cross polarity yagi. Does anyone know why? I have looked at stacking, cross polarization and circular polarization yagis and quads for both FM and SSB work but none are very clear on which works best for this type of use. Any thoughts or ideas would be welcome. 73

RE: Cross Polarized Yagi	Reply
by WB2WIK on December 15, 2004	Mail this to a friend!
That's not really what it says. The "20 dB cross polarity loss" occurs when you use a beam antenna like a yagi of opposing polarity to that used by the other station (the one you're trying to contact). If you're horizontal and he's vertical, 20 dB additional path loss might be about the average you'll experience on the typical tropo path between stations. That's not a good thing, and is the very reason that it pays to use only vertical polarization for FM work and horizontal polarization for everything else. The "crossed yagis" as you describe allow you to use either a vertically polarized yagi, or a horizontally polarized one, sharing a common boom. That's all it is. Normally, you'd use two separate feedlines for this: Use the vertical section of the beam for FM, and the horizontal section of the beam for SSB-CW etc. WB2WIK/6

RE: Cross Polarized Yagi	Reply
by KD5NVC on December 15, 2004	Mail this to a friend!
I thought that may be the case but is was a little vague in print. Thanks for clarification.. 73 Glenn KD5NVC

RE: Cross Polarized Yagi	Reply
by WB6BYU on December 16, 2004	Mail this to a friend!
The theoretical loss due to cross-polarization is infinite, and sometimes it can be much higher than 20dB. But in practical use there maybe some polarization rotation, and reflections can change the polarization of a signal and fill in what would otherwise be a null, or one antenna may not be exactly straight, so 20dB is a plausable rough guess for the average effect. But that only applies to having mis-matched polarization, not to using a beam with both polarizations available. (On the other hand if both antennas were fed for circular polarization, the theoretical loss would be 3dB when receivering any linear polarization: horizontal, vertical, or slant.)

RE: Cross Polarized Yagi	Reply
by KD5NVC on December 20, 2004	Mail this to a friend!
Thanks to all for the comments, i apparently mis-read that statement on the 20db loss. 73 KD5NVC

RE: Cross Polarized Yagi	Reply
by KI3BU on December 23, 2004	Mail this to a friend!
I have been using a Cushcraft 148/20 for over 6 years and have found it to be a very good performer. No problems at all. The beamwidth is pretty narrow on both Vertical and Horizontal and gain seems pretty decent. I like to do 2M SSB as well as FM simplex so this antenna really fits the bill. It won't outperform a long boom yagi, but it will give a nice signal on SSB/CW. I really like the ability to switch polarities back and forth to obtain the best signal and/or lowest noise, especially on FM simplex. Good Luck and have fun! 73! Jim ~ KI3BU

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