Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Anyone familiar with Blaw-Knox Antennas?

  1. Jan 14, 2013 #1
    I grew up not far from WLW and it's giant diamond antenna mast. picture These Blaw-Knox antennas were extremely popular in the 1930s. I always wondered why the diamond shape. I found the original patent here, but it really doesn't say specifically why the diamond shape was chosen. It looks like the modern antennas which are largest on the base and taper up came several years later, so I realize that this was probably a predecessor. Still, I cannot imagine why they chose to make the tower a diamond. They should have at least been able to borrow from existing structures and I see no towers like this anywhere. I can't think of any structural or electromagnetic reason why this shape would be chosen over a tapered design. Don't get me wrong, I love the look, but I have to know why!
  2. jcsd
  3. Jan 15, 2013 #2
    Most of what you can see is support scaffolding.

    Item 40 in the patent is the actual aerial, which looks like some kind of whip.

    I always thought Blaw-Knox made ashphalt paving machines
    My grandfather worked for them.

    You live and learn.
  4. Jan 15, 2013 #3
    The tower itself actually is the radiator! The relatively long wavelength of AM radio makes for some very interesting size requirements. If you read through the patent, he actually talks about ways to improve the conductivity of the joints between each beam. Item 40 is just a rod which can be used to make small adjustments to the resonant frequency and beam pattern of the tower. Back then (and even now to an extent) it was very difficult to calculate the properties of such a large antenna precisely. Its really fascinating stuff! FM radio does work in the manner you described. A relatively small antenna is mounted on a tower, sometimes an AM tower to avoid building another one.

    Blaw-Knox is one of the largest manufacturers of paving equipment. At one time, they were also a giant in the steel structures industry thought. Naturally, they entered the developing market for AM radiators. Always cool to find out you have generational ties to a company!

    I have a feeling that the shape probably has more to do with structure than radiation properties... anyone?
  5. Jan 15, 2013 #4
  6. Jan 15, 2013 #5
    It seems obvious that the antenna needs some width against bending, and that the width can be smaller at the tip, were the leverage is smaller. The patent mentions the idea of reducing contact area with the ground as much as possible to reduce losses. So I think that is what they did. They wanted to make all structural members thick enough to be stiff and the contact points especially with the ground as small as possible. Wether that helps I don't know.
  7. Jan 15, 2013 #6


    User Avatar
    Science Advisor
    Homework Helper

    The basic structural idea seems to be to design the top half as a free-standing tower, on top of a "platform" supported by guy ropes with the bottom central "leg" in compression resisting the guy rope tension. http://en.wikipedia.org/wiki/Blaw-Knox_Tower

    For a tall mast, that would have the advantage of reducing the length of the guy ropes compared with attaching some of them at the top, but the disadvantage was when this happened ...
  8. Jan 15, 2013 #7
    As usrdln stated, the entire structure is the antenna.

    It is not a vertical quarter wave "whip" antenna which has lower radiation resistance, making it harder to drive, and is less efficient because it depends on significant current flowing in ground radials (counterpoise).

    The Blaw-Knox is essentially a vertical dipole. Ground radials are often included, but do not act as counterpoise but simply shield induced ground currents from lossy earth.

    The reason for the shape is that, since it is a dipole, its mouning point is swinging with high voltage and is insulated from ground with a large ceramic or porcelain insulator.

    The mounting point is designed to have degrees of freedom for motion (rocking and rotation). This requires it to be a single physical point.

    It widens in the center simply to provide structrual strength.

    See attached photo.

    Attached Files:

  9. Jan 15, 2013 #8
    Thanks for the replies everyone!

    0xDEADBEEF, the patent definitely does stress the advantages of the single contact point with the ground - but I can't help but feel that it is a bit exaggerated. I can't imagine that a tower with three points contacting the ground would leak much more current, it seems like it would just require 3 insulators instead of one.

    AlephZero, the patent also mentions being able to build the top half first and use it as a platform to construct the base. You still have to use a crane to install the top on the base though. To me, it seems like they would be just as well off to build a tower which is wider at the base and tapers upward. It could be constructed in sections and they could lift the sections into position. The wider waist does make a convenient place to mount the guy ropes and perhaps they felt that they could save material in the lower half of the tower by keeping it in compression and tapering it to a point. It seems like the guy ropes would have been such a small cost in comparison, I have to doubt that they made the diamond shape just to save on guy wires. It may have been a combination of all these factors though.

    the_emi_guy, it sounds like you may have this figured out. I guess that I don't understand why the designers wanted to let it sway though - it seems like that would be a very bad thing. And is the power being dissipated enough to make this huge radial lurch around? It sounds like this antenna is actually rotating slightly as it is being driven? Thanks for the picture. It looks like maybe the insulator on that tower has been updated from the original? WLW's looks a bit different.
    Lighting Strike
  10. Jan 15, 2013 #9


    User Avatar

    Staff: Mentor

    So is it center fed and there are insulators at the midpoint joints? Or is it somehow fed as a dipole from the ground?
  11. Jan 15, 2013 #10
    berkemen, I was under the impression that it was actually a monopole. Perhaps you could think of it as a dipole, however, if the earth acted as the second part - sort of an application of method of image charges. I think that this is a valid way of thinking about it.
  12. Jan 15, 2013 #11
    Ceramic or porcelain mounting points are much more fragile than a typical tower mounting point. They are designed so that the only stress that they see is compression from the weight of the tower (where they can be strong).

    A typical 4-legged tower mounting point (steel into a buried concrete footer) is designed to handle both the compression force from the tower's weight, but also lots of sheer and torsion stress. Sheer and torsion stresses come from, among other things, expansion of the metal due to temperature variation and wind loading.
  13. Jan 15, 2013 #12
    They are end fed.
  14. Jan 15, 2013 #13
    So, essentially, the reason that the tower was made with a single contact point was so that the two balls shown here could simply rotate on top of each other, freeing the ceramic insulators from the torsion stress? How is it that modern towers get around this issue? Is it just better insulators that can take the torsion?
  15. Jan 15, 2013 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    so if its endfed, it must be a 1/4 wave with the ground being the counterpoise to give a full halfwave antenna

  16. Jan 15, 2013 #15
    I do not think that WLW uses a counterpoise. Nothing is visible above ground except the large coax cable feeding the antenna. I believe that it is properly grounded. See Fig. 8 of the patent I posted above (message 1).
  17. Jan 15, 2013 #16


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    counterpoises are usually always very large and buried earth mats, you wont see anything above ground of the counterpoise

  18. Jan 16, 2013 #17
    Not sure why you are saying this. It is an end fed half wave antenna (EFHWA).
  19. Jan 16, 2013 #18


    User Avatar

    Staff: Mentor

    I hadn't heard of those. Do you have a link to more information?
  20. Jan 16, 2013 #19


    User Avatar

    Staff: Mentor

    Hmm, I did some reading on the web, and am still a bit confused. The articles did stress the need for a good counterpoise to help manage common-mode reflections at the feedpoint.

    I'd be interested in reading a comparison of a center-fed dipole versis the end-fed dipole+counterpoise, if anybody finds one.
  21. Jan 16, 2013 #20
    I have attached photo of feedline to a typical series fed tower. Feedline comes out of tuning shack to the right and attaches to antenna just above insulator. This particular tower is not a "Blaw Knox" type, but idea is same.

    Fence surrounds it because there are thousands of volts on the feedline and the entire bottom of the antenna.

    I also attached technical documentation for 314 meter high Blaw-Knox which includes schematic of tuning shack, impedance data, Smith charts etc.

    Note that the antenna height is half wavelength at 540KHz. Also note that the antenna feedpoint impedance is about 300 ohms magnitude. If it were a quarter wave monopole the feed would be more like 36 ohms, and the required current an order of magnitude higher.

    Driving this dipole at its midpoint (in textbook fashion) would be really impractical. You would have to install multiple fragile insulators halfway up the structure and relocate the tuning shack way up there.

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Anyone familiar with Blaw-Knox Antennas?
  1. Antenna Duplexer (Replies: 7)

  2. Antenna Basics (Replies: 1)