Multi-Band Yagi vs. Log Periodic - What's the Difference?

Discussion in 'General Technical Questions and Answers' started by W3ALX, Oct 2, 2008.

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  1. M3KXZ

    M3KXZ Ham Member QRZ Page

    MOXON RECTANGLE - definitely worth considering nested Moxon rectangles for multiband use. Much lower profile than a full sized Yagi, can be constructed very simply from a pair of crossed fibreglass or bamboo poles and wire, and performance is excellent. Can see more at www.moxonantennaproject.com
     
  2. N8YX

    N8YX Ham Member QRZ Page

    Like this example (Mosley)?

    They offer one which covers 3-30MHz with a 10KW power rating.

    I'm planning to put their 6-30MHz version up sometime...

    [​IMG]
     
  3. W3ALX

    W3ALX Guest

    Sounds like the trade off is smaller size and efficiency for yagi's vs. wider bandwidth but larger array for LPDA's. So with regard to LPDA's, would it be accurate to say that they are preferred by stations looking to use more modes than just phone on bands 10-20 mtr?
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Maybe, you'd have to take a poll, I guess.:)

    I think it's really LPDAs are used by people who wanted to try using an LPDA. They do have very wide matching and gain bandwidth, at the sacrifice of gain. For people who have a reason to transmit outside the ham bands (like MARS operators, for instance), LPDAs will allow them to do that without having to use antenna tuners and such. And for "nulling" interference or noise, which is one of the primary advantages of any kind of beam antenna on HF, LPDAs do well at this: A 30-40 dB side null is common (of course most beam designs can do this, and the LPDA is no exception).

    WB2WIK/6
     
  5. AI3V

    AI3V Ham Member QRZ Page

    The LPDA is for a station that may have to transmit (or receive) ANYWHERE between its lower and upper freq limit.

    A slight disadvantage to the LPDA is that it does not provide any Preselection for your receiver, the typical yagi has about -20 db gain off frequency, so there is a greater chance that out-of-band signals on the LPDA will cause intermodulation in your receiver.

    In the same note, the Yagi will attenuate harmonics and spurious emissions from your tx somewhat, the LPDA will radiate them just fine.

    Rege
     
  6. K7KBN

    K7KBN Ham Member QRZ Page

    "Mode" doesn't have anything to do with an antenna's performance. An antenna that's adjusted for CW will perform equally well on CW, AM, FM, SSB, or any other mode on that frequency. An LPDA will allow greater ease of moving from the bottom of the CW band up to the top end of the phone band on, say, 20 meters. The tradeoff is generally less gain for the "typical" LPDA, but much greater bandwidth.
     
  7. W3ALX

    W3ALX Guest

    Thanks for the replies Guys! Very Helpful!

    K7KBN : "Mode" doesn't have anything to do with an antenna's performance. An antenna that's adjusted for CW will perform equally well on CW, AM, FM, SSB, or any other mode on that frequency. An LPDA will allow greater ease of moving from the bottom of the CW band up to the top end of the phone band on, say, 20 meters. The tradeoff is generally less gain for the "typical" LPDA, but much greater bandwidth.

    Thanks, wasn't sure as to the bandwidth limits of yagi vs. LPDA. Thought the limits were much tighter.

    It was suggested that a Tennadyne LPDA is a respectable representation. If a T6/T8 were being considered, what would be a equal model from your favorite yagi company?

    Please define interlaced. Is that where the feed line along the boom crosses over between elements?

    Thanks!
     
  8. W4SUL

    W4SUL Ham Member QRZ Page

    241 confirmed for DXCC using a Tennadyne T-8. Yes, it is big and heavy, but the antenna works great! The T-8 doesn't have the greatest front to back, but front to side is fantastic. I've had many antennas over the years, but have never been anything but thrilled with my log periodic. Mine is up 46 feet on a self supporting tower.

    Sparrow
     
  9. W8JI

    W8JI Ham Member QRZ Page


    Maybe better than some other small antennas, but it is a 3 dBd or less gain antenna at very best.

    A good trap tribander like a TH6DXX or TH7 or a good log like the Tennadyne will blow it away, which was the context of my statement.
     
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Tom, I could nit-pick over the 3dBd but wont!

    My suggestion of the Hexbeam was in response to the original poster's comment:
    TH7? Is that the antenna that has twice the turn radius of the Hexbeam, weighs three times as much and covers two fewer bands :)

    Don't get me wrong - I'm just saying the Hexbeam is worth consideration for the increasing number of Hams who don't have space for more-conventional Yagis. I'd love something bigger, but being on a smallish plot in the centre of a UK conservation area makes it imposssible.

    Steve
     
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