VHF/UHF antenna polarization and testing

Discussion in 'Amateur Radio News' started by KJ4YZI, Apr 20, 2017.

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  1. KJ4YZI

    KJ4YZI Ham Member QRZ Page

    In this part2, i show the configurations i tried with this Cushcraft 13b2 VHF yagi, and the differences between horizontal and vertical polarization, as well as a couple modifications to get the antenna operating perfectly.
     

    Attached Files:

    IX1FIT and KC8YXA like this.
  2. W1YW

    W1YW Ham Member QRZ Page

    People usually model masts along with the antenna to see effects in SWR and pattern. Clearly your metal mast-system is a resonant structure. It is smack in the nearfield when your antenna is vert pol. Horizontal it is cross pol and the mutual coupling (and thus effect) is low.

    Again your antenna is NOT wideband.

    73
    Chip W1YW
     
    Last edited: Apr 20, 2017
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  3. K0RGR

    K0RGR Premium Subscriber QRZ Page

    That was interesting - I was watching to see the difference between both being vertical and both being horizontal, as others I know have tested that, and consistently found a 1-3 S unit advantage for horizontals on both ends on long distance paths. It looked like vertical to vertical, you were hearing him almost full scale, but on horizontal to horizontal, he was pinning the meter even at lower power levels.

    It's too bad we can't convince repeater owners to use circular antennas instead of verticals. Lots of people hold their handhelds at weird angles, and circular antennas would help with that, but I also believe it would allow more distant users to work the repeater using a horizontal yagi. As you pointed out, horizontals are less fussy than verticals.
     
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  4. KB0HAW

    KB0HAW Ham Member QRZ Page

    You don't have to convince me that circular would be more effective. What you need to do is convince commercial antenna companies to make them for the 145-160 MHz band. There are several great circular antennas available for 88-108 MHz, as most FM broadcasters use them, but no one makes commercial grade circulars for 145-160 MHz, or 430-470 MHz that I am aware of. Those are all vertical.

    As a repeater operator, I am not willing to try to home-brew something, and expect it not to fail when it is up in the air 800-1000 feet. I want something tried, true, and tested so I only have to install it once, and never mess with it again.

    Is there anyone on this message board from ERI? Have you guys ever thought of making a commercial 2-way version of your famous "Roto-Tiller" lineup? a 3 or 4-bay ERI would RULE on 2 Meters, and they wouldn't even need to handle 20KW like your regular antennas.

    Hello... Jampro? ...Shively? ...Nicom? ...anyone?
     
    Last edited: Apr 20, 2017
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  5. W2NAP

    W2NAP Ham Member QRZ Page

    you can contact the companies directly and see, IIRC a LA repeater used a CP antenna long ago and it worked well, but the cost is a major drawback. we can get ex-comm antennas somewhat cheap and even buy new for 1 or 2K. CP antenna would cost quite a bit more I am sure.
     
  6. W1YW

    W1YW Ham Member QRZ Page

    Circular to linear throws away 3 dB. That's equivalent to decreasing the effective aperture by over 40% (1.44 x 1.44 =2=>3 dB)

    You'd have to convince everyone to go CP (thus CP to CP). In general, CP gain antennas are less efficient and more costly, thus requiring bigger apertures for the same link.
     
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  7. W1YW

    W1YW Ham Member QRZ Page

    Those antennas are not cheap, and are stacked (for gain)as collinears . They are not intended to be uni directional.:-(
     
  8. WA3YRE

    WA3YRE Ham Member QRZ Page

    The main reason repeaters are vertical is because in general mobiles are vertical. Much ham practice in vhf/uhf comes from landmobile where vertical is easy, compact and omnidirectional on mobile units. Remember boys and girls that vhf-fm repeaters were around long before folks had handheld radios. I still have about a dozen tube type vhf high band Motorola's with dual 6146 finals from the late 1950s early 1960s.

    While AM/SSB operators have long used halos and other horizontal antennas mobile in the vhf/uhf ham bands the idea of a cop car or taxi cab doing that is pretty far fetched just from the stand point of durability.

    I do wonder how much gain one can get out of an omni CP antenna without becoming rather complex from a physical point of view with lots of places for ice loading in winter? I suspect that a vertical phased array will have a lower wind/ice footprint than a CP array of the same gain.

    To get rid of the coupling to the mast when vertical one can use a PVC mast section, but there will still be some coupling with the coax which one will have to deal with. One way is to take the coax out at an angle from the side of the vertical beam with a boom and then take the coax at a angle to the mast below the antenna.
     
  9. W1YW

    W1YW Ham Member QRZ Page

    90% of your repeater inputs are from duckies (coated short whips antennas). Duckies are mostly ellipitical or unpolarized. This results from the HT's 'ground' itself being part of the antenna, and not being constrained as a purely vertical structure.

    For mobiles, there is fair amount of reflective multipath which tends to depolarize the incoming signal.

    Its really only on high, high-gain VHF or UHF antennas that you maintain polarization integrity, without the degradation (a good thing) of the transmitted polarization by the propagation path.
     
  10. W1YW

    W1YW Ham Member QRZ Page

    If are willing to give up a few dB of gain, and thus pick up both pols, just rotate the antenna to 45 degrees rather than vertical.
     

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