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. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    >kb3rrg;1357110]Thanks for the replies Guys! Very Helpful!
    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?<

    ::You'd have to pick a 5-band beam, since the Tennadynes cover five bands. Most 5-band Yagis are bigger and heavier than the Tennadynes, and would have a bit more gain but are also more complicated to put together and have more possible points of failure.

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

    ::No. "Interlaced" only means the beam has elements for various bands arranged on a common boom. Many designs are like that, it's very common. A simple example that's easy to picture: If you took a six meter 3 element beam and then laid out a two meter ten element beam on the same boom, with the 2m elements in between and parallel to the 6m elements, you'd have a 2-band interlaced beam.

  2. W3ALX

    W3ALX Guest

    OK Guys, this is good, but please help me with this.

    What is the difference with the forward gain and F/B ratio between an LPDA and yagi of similar boom, element class? Is it just a numbers thing or is there a real world RS/RST/S meter discernible difference?


    Bruce KB3RRG
  3. NA0AA

    NA0AA Ham Member QRZ Page

    As I noted, it's difficult to compare because they don't work the same.

    Typically, 3 cells in a LPDA are engaged at any given frequency.

    A logical person might conclude that performance would be equal to a 3-element yagi-uda array. Looking up the information on the websites is an excercise left for the student.

    If you took a Yagi with the same boom length as the LPDA and the same number of elements, on ONE specific band, the Yagi would beat it hollow....

    Honestly, it's hardly and apples to apples comparison.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Depends on the design of the LPDA and the Yagi. Boom length is "king" in both cases. Doubling boom length generally increases forward gain about 3 dB. A 24' boom has about 3 dB more gain than a 12' boom; a 48' boom has about 3 dB more gain than a 24' boom. This is just an average case, there are some variables.

    Remember any kind of horizontal dipole installed at least 1/2WL above ground has several dB gain "over a dipole" in free space. So, a "one element" beam (rotary dipole) generally has 3-6 dBd gain, all by itself, even without any parasitic elements. If a beam has 6 dBd free space gain, it could have as much as 12 dBd gain in real life, real world situations...and yes, you'd see that on an S meter.

    As such, you might install a 6 dBd gain LPDA, which has in real world terms anywhere from 9-12 dBd realized gain, or a 10 dBd gain Yagi, which has in real world terms anywhere from 13-16 dBd realized gain, and you might be very happy with either one compared with a "too low" dipole or inverted vee, or a vertical.

    In all cases, going from an omni antenna or a "low" dipole to any sort of beam that works and is elevated high enough to work can be absolutely staggering.

    Primary advantages of "any kind" of beam include the ability to steer it to a useful direction and to get it above the "clutter" that precludes many lower antennas from working to their best advantage; additionally, the ability to reduce received noise because the beam focuses in just one direction, thus reducing received noise from all other directions, is an amazing advantage.

    The difference in "working conditions" using a medium sized LPDA and a positively HUGE Yagi might only be 6 dB. The difference between a low dipole and the medium sized LPDA can be much more than that, especially if the little LPDA is well elevated.

    And a lot of the advantage you see is on "receive," which in truth is more important than "transmit," because for transmitting you can always add more power; but for receiving, you can't.

  5. KF6NFW

    KF6NFW Ham Member QRZ Page

    I recall the Placer Co. ARES team having a Log. as part of the response box. I believe it actualy belonged to a team member. One of the reasons it was chose was the ease of setup in a timely manner, and the fact that it was also a little bit more forgiving on the band. I believe it was a dual band for 2m/70cm.
    It was ideal for such operations in mountainous terrain such as Northern Ca. is, and provided excellent F/B for getting out of the canyons, such as those of Foresthill, and Nevada city.

    I havebeen thinking of building/buying one for my own disaster kit as it takes up little space, and packs easily into just about anything you want, including a pvc tube, to be slung over shoulder for hike in.
  6. K0YLW

    K0YLW Ham Member QRZ Page

    I have recently installed a 10 element log periodic and am very pleased. It has a 24 foot boom and the longest element is 38 feet.
    I also have a dipole that is about 200 feet long and the center is about 45 feet and the ends are at 15 feet.
    The log it a huge step up from the dipole.
  7. NA0AA

    NA0AA Ham Member QRZ Page

    Before you build it, be sure you price it, it might be more cost effective to simply buy one: you are looking at the ELK LPDA for 2 M/70 CM - it's a nifty little portable antenna, as you note easy to break down - however to use these as a vertical you must have a non-condutive mast. I use a 3' piece of PCA as an extension on my regular mast.

    I often use mine in public service events where I might be at the fringe of repeater coverage or similar - very helpful.
  8. KF6NFW

    KF6NFW Ham Member QRZ Page

    I am not familiar with "PCA" care to convert to plain English for a lame person?:D

    Actualy I was just talking to the wife about that, as we went and looked at materials today to build, and the cost far outways the cost to order with S/H, plus, it would be operational sooner then if I build as I only get very small breaks in the day to build, or do anything for me, as I am a stay at home dad with a 8 month old.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::I think you meant PVC, not PCA, but anyway...

    The ELK 2m/70cm antenna sucks on 70cm. I tried it, it's very far below a simple 3 element Yagi on 70cm. It works okay on 2m. On 70cm, it has no dedicated elements at all: It uses the "colinear effect," using the 2m elements on 70cm and that doesn't really work because they didn't add the requisite phase shift network at the 1/4-WL points on the elements to do that properly. These guys are not RF engineers.

  10. KF6NFW

    KF6NFW Ham Member QRZ Page

    so Steve, of all the different Logs on the market which would you recommend for 2/70?

    I am considering just buying 2 yagis, 1 for each band, but I am really trying to reduce the weight factor. all though I like the looks of multi antennas on the lot.
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