Antenna Polarisation and you

Discussion in 'Amateur Radio News' started by VK6FLAB, Sep 21, 2018.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    foundations-of-amateur-radio_300.jpg
    Foundations of Amateur Radio

    Antenna Polarisation and you

    The first time I came across the concept of antenna polarisation was a decade before I became a radio amateur. To connect to the internet while driving around Australia I became the proud owner of a portable satellite dish. Portable in the broadest sense of the word, 150 kilos with a dish that's 2.4m high, 1.8m wide, steel base, electronics, power and patience to erect and point.

    The dish has a receiver and transmitter component that needs to be aligned, just so, in order to be able to have two-way communications using 5 Watts into geosynchronous orbit. The transmit and the receive are exactly 90 degrees offset from each other. One is called horizontal polarisation, the other vertical.

    The first thing to observe is that if you're using the wrong polarisation, it doesn't really work well. We'll get into what is right in a moment. Depending on where you you ask, the definition of not working well can be as bad as 40 dB loss.

    Just let that sink in for a moment.

    If you want to punch through with more power, you'll need to bring 10 kilowatt with you for the receiving station with the opposite polarisation to hear 1 Watt.

    If you're using a VHF or UHF FM radio in your car, you're likely to have a vertical antenna. The combination of a repeater on a hill and a radio in a car adds up to much more than the the two alone. The line is blurred today because repeaters are very popular and home-base stations are becoming smaller and smaller by the week, so vertical antennas for VHF and UHF at home are today just as common as they are on cars.

    It wasn't always that way. In fact, in HF, it's almost never that way and if you're a fan of Tropospheric Ducting or long distance VHF, then you'll also shy away from vertical antennas.

    Let me explain.

    If you want to erect a HF antenna and you want it to rotate and you want it to be high enough off the ground, you'll build the simplest mast you can get away with. Imagine a HF Yagi. It's got several elements, long to short along a boom, rotator somewhere in the middle. If you mount this Yagi horizontally, your mast will be around half a wave length in height.

    If you mount the same Yagi vertically, aside from the height discussion - should it be mounted higher or not - now your mast becomes another interfering element within your Yagi. The steel wires that keep your mast standing will also interfere with the Yagi elements and your elements will be closer to the ground where they can potentially cause harmful radiation.

    So from a mechanical perspective, putting a Yagi on a mast vertically is not trivial.

    From a radiation perspective you may theoretically get some gain, but adding an element or two will make up for any potential gain that a vertical arrangement interacting with Earth might assist with.

    There's another reason. The ionosphere. It sounds like a smooth billiard ball, it's drawn as a uniform layer around the earth, but in reality, clouds and their appearance are much more likely to represent the actual surface shapes that the ionosphere presents to your radio waves.

    A signal coming in one way is unlikely to come out at the other end in the same way and vice versa.

    That's HF. On VHF and UHF a horizontal signal and a vertical signal when they're used with line of sight are pretty similar, but once you get beyond that, a horizontal signal will travel further, how exactly is a story for another day. If you're doing point to point VHF or UHF contesting, horizontal is the way to go.

    What about a single HF vertical?

    It's excellent for a portable station, it is simple to set up, works in all directions, but it means you'll be able to hear all the local man-made noise as well, so find a quiet spot near the beach if you can.

    So what's the right way? Almost always horizontal, except on cars or when you're on a DXpedition on a beach sipping pina collada and getting caught in the rain.

    I'm Onno VK6FLAB

    To listen to the podcast, visit the website: http://podcasts.itmaze.com.au/foundations/ and scroll to the bottom for the latest episode. You can also use your podcast tool of choice and search for my callsign, VK6FLAB.

    All podcast transcipts are collated and edited in an annual volume which you can find by searching for my callsign on your local Amazon store, or visit my author page: http://amazon.com/author/owh.

    Foundations of Amateur Radio Volume 7 is out now - with chapters on digital modes, coax connector loss, waterfalls, station performance and more.

    Feel free to get in touch directly via email: onno@itmaze.com.au, or follow on twitter: @VK6FLAB (http://twitter.com/vk6flab/)

    If you'd like to join a weekly net for new and returning amateurs, check out the details at http://ftroop.vk6.net, the net runs every week on Saturday, from 00:00 to 01:00 UTC on Echolink, IRLP, AllStar Link and 2m FM via various repeaters.
     

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    K8PG likes this.
  2. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    What, no Antenna is bi-polar?

    KA2RRK
     
  3. AA5CT

    AA5CT Ham Member QRZ Page

    What is the cosine of 90 degrees? Pretty close to the number zero (0) as I recall ...
     
    KA2RRK likes this.
  4. KG5VK

    KG5VK Ham Member QRZ Page

    Is it not Polarization ?
    Overall pretty sound advice
    of Vertical versus Horizontal
    On HF the Big difference is the Take Off angle (TO)
    A vertical gives us a lower TO, versus a Horizontal antenna
    I use both Verticals and Horizontal Antennas
    depending on where I want to work and which bands

    Cheers!
    Steve
    KG5VK
     
    KA2RRK likes this.

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