Circular Log Periodic Antenna

On another thread, a discussion of small home built antennas for UHF TV broke out. (h/t to Jim2) The traditional TV antenna is the Log Periodic. It has dipole like elements spaced on a log scale for broad frequency range and good gain. The problem with it is that at the longer wave lenghts, the antenna can become large. For example, a VHF low channel number can have each element about 1.2 M long, or about 7 foot tip to tip at the wide end of the wedge.

Now it turns out that drooping the ends of a dipole does not weaken it much, nor does shaping it in other ways, even into a semi-circle. As a sidebar, I wondered if these could be made fractal elements, the answer is “yes” and it makes a smaller antenna, but without much change of performance. At the extreme of this drooping, you could end up with a circle. (There are “loop” antennas that are in fact made of circular loops). I got to wondering: “Can you make a log periodic loop antenna?”.

Well, turns out I’m only 17 years late in inventing it ;-)

This is a Masters Thesis from Virginia Tech from 1999:

https://theses.lib.vt.edu/theses/available/etd-080699-180221/unrestricted/ETD_LPLA.pdf

LPLA is log periodic loop array

LPDA is log periodic dipole array

GR is Ground Reflector – a wire or metal sheet at the back

6.1 Summary of Results

Compared to the LPDAs, the LPLAs provide the advantage of size reduction in transverse dimensions by a factor of 2/π.

The radiation characteristics of the loop element in the axial direction are preserved in both LPLAs with and without GR, whose fields in the broadside direction are cancelled due to 180 alternating phase shift between the adjacent loop elements. The Gθ far-field pattern is dominant in the E-plane, while the Gφ pattern is dominant in the H-plane. Hence, the LPLAs have linear polarization and unidirectional patterns.

Improvement of several decibels in gain can be achieved by using ground reflectors in the apex region of LPLAs. For example, the gain (or directivity) of a 6-turn 30° LPLA-GR can be about 12 dBi. Also, the smaller is the subtended angle (α) and the closer to unity is the scale factor (τ), the larger will be the directivity of the antenna.

The ground reflector seems to increase the bandwidth. For example, the 6-turn 30° LPLA with GR yields 47.62 % gain bandwidth.

LPLAs with GR are expected to find applications as feeds for reflector antennas, as mobile communication antennas, and as detectors in EMC scattering range.

Those interested ought to read the whole paper.

The key bits are that this looks somewhat like a conical birdcage, yet it works very well. It is a 12 dB gain antenna (that is quite good) and that is using only 6 ‘turns’ or circles of wire. It is also smaller than the regular log periodic dipole antenna at 2/Pi or about 2/3 the size. That’s a big improvement. You can form it, as the author did, by gluing wire onto a conical form. Heavy paper cones are VERY easy to make.

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About E.M.Smith

A technical managerial sort interested in things from Stonehenge to computer science. My present "hot buttons' are the mythology of Climate Change and ancient metrology; but things change...
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27 Responses to Circular Log Periodic Antenna

  1. pearce m. schaudies says:

    Hi Chiefio. Back in the days when I was active ham, I used 5/8 wave vertical whip, fat tube for a little broad band. Gave doughnut pattern, 4 db gain.

    Regards,
    Pearce M. Schaudies.
    Minister of Future

  2. philjourdan says:

    Mass is not the only place with a great “Tech” school.

  3. LG says:

    So,
    it turned out that a bunch of us are secret RF Geeks !!!
    :D

  4. Serioso says:

    @LG
    I’m an RF Geek too! I once made a six-element Yagi that increased the range of a spread-spectrum 2.4 Ghz kitchen phone to a couple of miles (over salt water). [This was before cell phones were in wide use.] Worked like a charm!

  5. E.M.Smith says:

    Well, at least on the utility of wires bent in odd shapes, we can agree ;-)

  6. E.M.Smith says:

    @Pearce:

    I made what I thought was a new antenna design in about 1983. The “Fat Vertical Helix”. Took a 6 inch PVC pipe (the thin stuff used for leach fields) and wound about 80 M of wire on it in a helix. Well, started with about 20 M…

    Later found out that the (then a bit new) “Rubber Ducky” was the same basic idea, just at very high frequencies… So imagine a Rubbery Ducky 1/2 foot in diameter and 6 feet tall in the corner, being used for SWL’ing…

  7. jim2 says:

    Since it’s a cone, it should be self-supporting if made from the right material. It would be interesting to compare it to the hoverman. Hint hint!

  8. jim2 says:

    Here’s yo daddy’s antenna:

  9. E.M.Smith says:

    @Jim2:

    I hope to build one for UHF sometime next month… for now I’m just swamped. 2 fences to build, a roof to do, a back yard to tame, etc. etc. Heck, I haven’t even had time to just run a new 50 foot of coax down from the already in place old TV antenna… (the end came off the coax… then we got satellite… then a decade passed ;-)

    But “someday” I’ll have one.

  10. E.M.Smith says:

    Well,

    Looks like yet another of my inventions stolen from me before I invented it! :-)

    I thought “If a log periodic conical works, wouldn’t a log spiral? As an analog to the circular spiral.”

    http://www.ece.illinois.edu/about/history/antenna/photos.asp#40

    http://www.ece.illinois.edu/about/history/antenna/

    http://www.ets-lindgren.com/3102

    Not conical, but flat:

    https://en.wikipedia.org/wiki/Spiral_antenna

    http://www.antenna-theory.com/antennas/travelling/spiral.php

    I gotta get me some traffic cones…

    8-‘)

  11. Larry Ledwick says:

    Close cousin of course would be the helical antenna.

    https://en.wikipedia.org/wiki/Helical_antenna

    If the axis or width of the attic was suitable you could create a long helix pointed at a weak TV signal station and get a simple to build gain antenna for that specific channel. The support for the helix could be a cage of small wood spars of the right diameter.

    With a feed point resonance near 100-200 ohms it would be relatively easy to match.

    Most common place to see them is in missile launch telemetry where the source antenna is constantly changing orientation.

  12. E.M.Smith says:

    This one is just unexpected. Made from speaker wire (both wires left together for most of it) in a spiral, but with rectangular turns. This on is attic sized, but a UHF sized one ought to fit on a small board.

    http://www.iw5edi.com/ham-radio/1681/spiral-antenna

  13. Larry Ledwick says:

    Hmmm interesting idea, but his diagram and word description are making my head spin, I can’t make sense out of where he feeds it?

    He says he feeds it at #2 but he shows no feed line connection to it (at least as how my brain is interpreting his diagram) I “think” I understand what he is doing but I have absolutely no clue why he has the thin lines coming down from the upper left and right corners to the center of the diagram and then down to the feed point unless that is supposed to represent the ridge of a hip roof?? over the wire spiral.

    Totally different design with a similar name:
    http://www.kn9b.us/spiral-dipole

  14. jim2 says:

    I have one of these for shortwave receiving.

    http://www.davidmoisan.org/radio/carpetloop/carpetloop.html

    Works pretty well. I’m refurbishing it with a new 360 pF variable cap.

  15. E.M.Smith says:

    @Larry:

    Note that he says to use 450 ohm feeder and that the antenna is a balanced design. I think that is the old “ladder line”, so also a balanced line. That means you just solder the right side to the right spiral and the left side to the left spiral.

    The two ‘slanted lines’ are the second conductors of the speaker wire. He separates them for the last bunch of feet (in the write up describes it) and runs them down into his shack. Then can either connect them together to make a really large loop antenna or leave them dangling to get a spiral dipole out of it.

    I’d likely use 300 ohm twin lead (since I’m not dealing with transmitted power and voltage I don’t need a 2 inch ladder line gap…) for both the feedline and the antenna. Speaker wire is nice, and would carry the power of his transmitter, but I’m not needing to carry transmit power and the 300 ohm ought to assure a nicer match, I think….

    So picture your usual 300 ohm “folded dipole” made of TV flat feedline. Now make it really really big and spiral it in the attic ;-) Final step is to make the non-fed point opposite the feed point switchable from open to joined (i.e. his ‘slanted lines’ that go into his shack).

    Well, at least that is how I’m seeing it.

    @Jim2:

    Hmmm…. Interesting idea. I’ve not really used my SW radios other than for travel for a few years now. (House with dimmers, under a power line, no outdoor antenna – though I could build one – due to kids; but the kids are now gone ;-) The loss of the incandescent bulb is making the dimmers less useful and the new ones have more of a fully off position – basically they have a little slider to set a CFL friendly dimming level then the switch just does on/off – and I think I only have one old style dimmer left. I could see making one of these in the living room…

    FWIW, my “invention” of the pudgy wound helical was installed on the roof some decades back. It worked OK, but I’ve not tested the connections in about a decade. Not really looking forward to running around on the roof until well after the rain stops. Would be nice to have an antenna I could play with without roof hopping…

  16. jim2 says:

    Yep. I have a small led grow light that I have to turn off to listen to some bands. Extremely noisy to say the least. And apparently the gubmint has decreed that wall warts be some sort of switching power supply to “save the planet.” Pox on them.

  17. E.M.Smith says:

    Oh yeah, all the switching power supplies in all the CFL and LED bulbs play Hobb too…

    Only a Pox? I’d rather a Pox + Plague + a visit from The Dark Lord… ;-)

  18. E.M.Smith says:

    The “antenna theory” link above was not working. It has now been fixed (removed an s from https – when first pasted in the http was gone and I assumed https in error).

    I recommend reading it. Has a very nice coax based archamedian spiral antenna AND an explanation of why it works under “infinite balun” that is the first time I’ve really understood those “inside to outside coax” things…

    My old TV Antenna coax is now destined for one of these since the end with missing connector becomes a feature… Trim end. Measure in about 4 M. Slice shield around and trim back on the feed side leaving excess on the far side. Cut center wire and connect from feed side to shield of far side. Wind into spiral putting center / shield crossover at the center, feed point at the perimeter. Enjoy and employ. Low wavelength approx circumference of outer circle, high cutoff about circumference of inner circle. Ought to easily cover 2 m to 20 cm if wound carefully.

    Yes, I need to do actual calculations prior to first cut rather than just SWAG. (Scientific Wild Ass Guess..,)

    http://www.antenna-theory.com/antennas/travelling/spiral.php

    http://www.antenna-theory.com/definitions/infinite.php

  19. philjourdan says:

    @LG – and the rest of us are richer for it!

  20. E.M.Smith says:

    Interesting article on why 50 ohm cables for so many uses that also explains why TV coax is 75 ohm instead of 50…

    http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm

    I’ll stop wishing for a 50 ohm TV antenna now ;-)

  21. Steve C says:

    The coolest (privately owned) log-periodic I ever saw was in a picture in the Radio Society of GB’s magazine a few years ago. Its owner, the late Jimmy, GI3GGY, had saved a 6.6 – 30MHz LP beam as the US military departed some years previously, and had mounted it on (IIRC) an 80 foot, 10 inch diameter tubular mast. For a structure that must have been over 60 feet across at the big end, and of a similar length, it looked really graceful in the photo, and when Jimmy read the RSGB News on 7 MHz on a Sunday morning, he came in like a local all over the country. Very impressive.

    As I recall, he also used the mast as a vertical on the lower bands with, I assume, impressive top loading if required – although, sad to say, the whole assembly succumbed to a storm a few years before Jimmy became a ‘silent key’ in 2012. Sic transit gloria mundi, sigh …

    I do like the circular LP, though. Circular Yagis are seen occasionally, but the LP version is new to me. I think the main weakness of LPs for me is that the gain is never quite as much as you’d hoped, compared with similar sized (but of course narrower band) Yagis – but remember that’s a licenced amateur talking, narrowband is rarely a problem for me if I can wind up the gain by doing it.

    Still, that’s antennas for you. What you want is “small, broadband and efficient”. What you get is “perm any two of those” …

  22. E.M.Smith says:

    I think I saw one of those in a photo… giant things… I’d need a bigger lot for it to fit on my property…

  23. E.M.Smith says:

    Some times nature has a strange sense of humor… This antenna:
    http://www.jpier.org/PIERL/pierl63/12.16070802.pdf

    is a slot antenna – a sheet of metal with ‘slots’ cut out.

    It looks for all the world like a swastika with an iron cross inside each arm…

    I wonder how many other ancient symbols would act as fractal antennas…

  24. Larry Ledwick says:
  25. jim2 says:

    Finally put together enough time slices to build another hoverman, this time with copper tape. The tape was harder to shape around the angles than I anticipated. Also, it is stress from having been rolled into a roll, so it doesn’t want to be put into a straight line, not even the edge of the flat. Also, without warning it can twist and fold into a degenerate protein, irreparable. I had to toss a few prions into the trash before I figured out how to handle it. (Was it charged with static electricity somehow? Plastic backing.)

    This version has the extra, unattached elements on top and bottom.

    Anyway, I had laid out the pattern in pencil. So the best way I found was to cut it to the length of a segment, pull it straight several times between fingers, separate a small section from the backing, line it up and stick the small section to start, then peel. Used a razor blade and tweezers to clean up excess. Got much cleaner angles and otherwise better dimensional integrity that way. Then lightly sanded and soldered the joints. Only takes a few seconds at 30 watts and the solder lays down flat and pretty.

    Will add 300:75 balun probably tomorrow and try it up in the attic without a reflector.

  26. E.M.Smith says:

    Might try some aluminum foil over cardnoard as a passive reflector ground plane…

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