Omni Directional - Horizontal Polarization - HF

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by PU2NIT, Dec 23, 2009.

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  1. PU2NIT

    PU2NIT Ham Member QRZ Page

    Hello, Zed friends!

    I'm really, really, sorry if this has already been discussed here, but I've been looking a lot, with no satisfatory results.

    I'm starting to think about Antennas to work some HF bands. Since I live in the City, and I'll put them on the Roof, I have limited space.

    The Radio to be used with is a FT-857D.

    I have a C Class License. Here in Brazil its just like the Technician's, the begginers permission. I'll be Upgrading to B soon (equivalent to General's).

    With the C License, speaking of HF, I can use 160m, 80m, part of the 40m, part of the 15, 12m, and 10m Bands.

    I'm more interested in 40m, so I can contact All over Brazil (it's the most used HF band here), and 10m for DX. Are these bands a good choice?

    I don't have Money to buy a rotator. So, no Directional Antennas for me.

    I thought about a G5RV. But as a Dipole, I'd have to orient it North/South or East/West.

    So, I thought about using 2 G5RVs, each one to a different direction, connected to a selector, so I could choose which one to use.

    I thought about a CobWebb. But then, it takes out my idea of the 40m band. Is it possible to Add the 7Mhz to a Cobwebb? I guess it would be possible, but it would be huge.

    And my last thought, and I guess the easyer one, is to use 2 different antennas. One for the 40m, and one for the 10m

    I just can't find any projects that have these antennas with Omni Directional Horizontal Polarization. Can I just Fold a dipole to make it 90º, so each half has a different direction??

    I thank you a lot!!
    And I'm sorry if I'm making dumb questions!!

    Kind Regards

    Felipe
    PU2OVA
     
  2. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    Felipe,

    Not dumb questions at all, and actually quite well presented. Your choices of 40 meters and 10 meters are reasonable, so that's where I'll go with the following discussion. Because 10 meters is unpredictable right now, I'll also make some suggestions that would let you operate any band between 40 and 10 meters.

    In practice, the average dipole installation has a radiation pattern that is nearly omnidirectional instead of the characteristic free space pattern usually associated with dipoles. A dipole antenna needs to be about 1/2 wavelength above ground before anything like two distinct lobes begin to appear. With your roof top installation, a single dipole just might give you satisfactory omni-directional coverage, although large nearby objects can skew the pattern at any height.

    Considering the options you presented: A pair of switchable dipoles installed at right angles to one another would give you some diversity. That is desirable and would be my choice if I could install them very high. A single dipole with legs at right angles to one another is also workable (such was my first dipole in 1960). With either configuration, I would recommend using balanced feed line and a tuner instead of coaxial transmission line. You would then have a multi-band antenna system.

    Food for thought: Instead of a dipole (or a G5RV), consider a full wave horizontal loop for your antenna. A square of about 10 meters per side will cover all HF bands 40 -10 meters when balanced feed line and antenna tuner are used. Shapes other than a square will also work; just try to enclose as much area as possible with the loop. On 40 meters, the take off angle would be high, so you could expect good omnidirectional coverage of Brazil. The take off angle would decrease as frequency increases, so the horizontal loop also has potential for DX contacts on higher HF bands.

    Horizontal loops are most often fed with balanced line, which means the actual wire length can be a few percent more or less than a full wave on the lowest frequency of operation. A horizontal loop can also be fed with coax, but that does involve some pruning to attain resonance, and also reduces the possibility for all-band operation.

    Good luck with your antenna installation. I'm looking forward to hearing you on the air. 73

    Gary, K9ZMD/6
     
    Last edited: Dec 23, 2009
  3. PU2NIT

    PU2NIT Ham Member QRZ Page

    Hey, Gary,

    Thanks for the complete and kind answer.

    Many questions I had are answered now!

    You made me VERY interested in Loops. I'll try to think about it as my first option.

    I'll have to homebrew a Tuner. But I think I can do this.

    As I've been reading, the most area used in the Loop, the better, right? So, it's possible to say that I can even do a Triangle, a Square, an Octagon, or a Circle, but the Efficiency will increase with the Area. Am I Correct?

    If I make a 4 sided Loop, with 10m on each side, I'll have 100m² inside it. It's a lot of space. Will it be possible to use it in the 80m as a Half Wave lenght, or make a 20m antenna and use it in the 40m like a Half Wave Lenght as well?

    I'm sorry if I'm making things a bit messy, but as I was reading, I found a Magnetic Loop Calculator, by KI6GD. Its says that for a 40m antenna, I'd have to use a 13 meter loop circunference, with 93% of efficiency (if in a Circle).

    Can I trust these numbers? Why isn't it Full Wave?

    Thanks for your patience

    Kind Regards

    Felipe
     
  4. KL7AJ

    KL7AJ Ham Member QRZ Page


    Hi Felipe:

    I'm so glad you wrote, because you have an opportunity to prove an antenna I'm very fond of. The crossed dipole, fed with 90 degree phasing. This antenna has two remarkable properties. Horizontally, it is truly non-directional, and for High Angle signals it is circularly polarized, which is just what you want!

    It's a bit tricky to make it work on multiple bands, but for a single band, it's very easy. Just make two dipoles at right angles, and feed them with transmission lines where one line is 1/4 wave longer than the other.

    I think you'll love this antenna.

    Eric
     
  5. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    Felipe, there is another thread in this forum similar to yours, so you might have already seen this page about a Loop Skywire, the name used in the ARRL Antenna Book when describing how to build a full wave horizontal loop.

    The other thread discusses using the 40 meter horizontal loop on 80 meters. The short answer to your own question is, "Not as originally configured", but in that other thread you can learn what is needed to make a 40 meter loop work like an 80 meter half wave antenna. In the same way, a 20 meter horizontal loop can be made to work as a 40 meter half wave antenna. That is good news if space on your roof is too small for a 40 meter loop.

    Home brewing a tuner is a good project. For balanced feed line, a link coupled tuner is best. Here is a page that describes an inexpensive approach to building one. Google will find many others for you.

    You are correct that a horizontal loop works best when constructed to enclose the greatest possible area. A circle is optimum, but a square is more practical to construct. As you suggested, other shapes will also work, but problems develop when the enclosed area becomes too small.

    A magnetic loop for HF is more complex than a full wave horizontal loop. They have a very narrow, tunable bandwidth, and efficiency is dependent on good construction and excellent materials. I have seen one demonstrated, but I never tried to build one.

    :rolleyes: And this started out to be just a short posting! :D 73

    Gary, K9ZMD/6
     
  6. AE5JU

    AE5JU Ham Member QRZ Page

    Rather than 10 meters, I would suggest you consider 15 meters. With just 100 watts SSB, and 25 watts PSK31, I have made a good number of contacts with Brazil and Argentina with my multidipole. 15 meters has given some outstanding DX. But also 20 meters. These bands work only in the daytime and go "dead" at night. 40 meters has also been very good during the evenings for PSK31 and SSB.

    A 40 meter halfwave dipole, with a little help from a tuner, will work well on 15 meters, too.

    You might also consider the "Carolina Windom".

    http://www.hamuniverse.com/k4iwlnewwindom.html

    "NEW Carolina Windom shown cut for 40, 20, 15 and 10 meters.
    It will operate on 80, 30, and 17 meters but will require a tuner for these bands."

    and the "Super Loop"

    http://www.bloomington.in.us/~wh2t/Super Loop Antenna.htm

    "Antenna covers all bands 80-10 meters + 30, 17, 12 meter WARC Bands"
     
    Last edited: Dec 24, 2009
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Felipe,

    A 40m CobWebb is about 16ft x 16ft - you would need four 12ft fibreglass fishing poles to form the support structure.

    73,
    Steve G3TXQ
     
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