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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. W3ALX

    W3ALX Guest

    Hey Guys, I'm a new ham with my general ticket. I would like some help in trying to understand the difference between a multi-band yagi and log periodic. What would be the advantages/disadvantages of either. I have a Yaesu FT1000 Field, AL 80B, and a Buckmaster 7 band dipole. I would like to make plans for a beam which would be mounted on a 45 feet Rohn 25 tower.

  2. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    A multi band yagi covers several bands ( usually 3 or 4 ) with limited bandwidth on each one. A log periodic is a broadband array that covers a large segment of spectrum seamlessly. Log periodics are generally quite large (understatement) and heavy at frequencies below 14 Mhz. For amateur use a multi-band beam would be cheaper and easier to deal with.
  3. NA0AA

    NA0AA Ham Member QRZ Page

    The above is a good description of the functional difference.

    LPDA's usually are designed with large bandwidths to allow frequency agility - for that, they sacrifice gain and directionality - in an LPDA, typically only 3 of the elements are active at any given frequency.

    The design is also a bit more interesting in that they are actually dipole array - each row is opposite polarity/phasing and the boom includes a phasing system of some sort.

    There are some monster LPDA's out there, but they are rare and very expensive. I don't know of any amateur that has the really big stuff but many military installations have pretty big HF LPDA's.

    I think CD makes an LPDA for something like 10 or 6 meters up to 1.2 gig - I guess if you can have just one?

    Their sheer weight and wind load make them a real challenge as you can imagine.
  4. K7MH

    K7MH Ham Member QRZ Page

    In 37 years I have never personally (at least) known of a ham who used a log periodic array nor have I seen one in use at a ham station, just military installations.
    They probably are used by only a very few hams. You won't see many of them in the ham catalogs that's for sure!
    Single band or??
    Last edited: Oct 2, 2008
  5. W3ALX

    W3ALX Guest

    Multi-band. I have read about numerous beams from hex to opti, trap-no trap, yagi to LP. Wow, a lot of things to think about? Good thing I'm not considering a vertical :confused:. For now, I'm leaning toward a multi-band beam to go along with the dipole. I'm sure that the xyl would appreciate a low profile something! Seems like when it comes to making a choice, budget and array profile (size) are the most important considerations?
  6. K7MH

    K7MH Ham Member QRZ Page

    Is it something you want to scratchbuild or homebrew yourself? It kinda sounded like that in the original post. A monoband yagi is one thing to homebrew, a tribander is another story.

    Just tell her not to look up!
    Last edited: Oct 2, 2008
  7. W8JI

    W8JI Ham Member QRZ Page

    All the chatter about traps and everything else is largely overrated. Good antennas are good antennas, regardless if they use traps or not. As a matter of fact some systems that eliminate traps are much worse than they would be if they had traps!!!

    Hygain makes some very good Yagi's like the TH-6, as has Cushcraft and Bencher. Force 12 has some good antennas, as does M-squared. Any of them, for the same boom length, will work about the same.

    A quad will be no better, and some of the other things like Log Periodics will probably have less gain.

    If you use a Hex Beam it will be worse, despite all the hyperbole.

    73 Tom
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    That's a very sweeping statement. The Hexbeam might lose out to a full sized multiband array, but it "knocks the socks" off some of the beams of similar size. I know - I've had a Cushcraft MA5B and a Butternut HF5B alongside the Hex at different times.

  9. K9STH

    K9STH Ham Member QRZ Page

    Then there are the interlaced multi-band yagis. Those do not use traps but are basically several single band antennas on the same boom.

    Glen, K9STH
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    LPDAs are not rare in amateur circles, and I work stations almost every day on HF using them (generally above 14 MHz, like 14 through 30 MHz). I have one, myself, happily rotating at about 55' above ground on my tower. It's been up eight years.

    Tennadyne and others make reasonably affordable and installable HF LPDAs.

    They have a lot more elements per boom length than Yagis and the longest cell (element) is generally longer than the equivalent longest element (reflector) on a Yagi for the same frequency. Also LPDAs don't have as much gain as Yagis do for any given boom length. But they are relatively simple antennas to construct, don't have any traps or linear loading, or multiple insulators or other stuff that could potentially fail with weather/insects/time/exposure so once "up" they usually last a very long time.

    The big LPDAs the military uses are big, heavy and expensive for most hams. I've seen some installation using towers for the booms, and having 24 or more elements on booms over fifty feet long. Not so many hams could install something like that. But the "amateur" LPDAs are smaller, lighter and less expensive than those (albeit lower performance as well).

    But they are pretty popular.

  11. M3KXZ

    M3KXZ XML Subscriber 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
  12. 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...

  13. 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?
  14. WB2WIK

    WB2WIK Premium 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).

  15. 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.

  16. 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.
  17. 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?

  18. W4SUL

    W4SUL Premium Subscriber 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.

  19. 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.
  20. 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.

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