• Another Home Brew antenna?



    This is a very quick post because I just came across another antenna design which is easy to build, looks like it may also be omni directional with some gain and picks up VHF and UHF stations. I may give this one a try. It was designed by another ham radio person and based on a fractal pattern, which I've also wanted to do for HF (Amateur radio High Frequency) use.



    A dream antenna?

    I found it on Instructables, or I should say that the notice I get from Instructables had this one as one of the featured articles.

    Here's the link to the "How-to-build-it" article:

    Fractal Magic DIY HDTV Antenna, by tigers58 - Fractal Magic DIY HDTV Antenna

    He also starts out using 300 ohm twin lead and states that's he's not sure that the transformer is needed. I may test that idea out as well.

    Good Viewing Until Next Time,
    Phil K

    All my blog articles are listed at: Karras' Corner
    or
    Karras' Corner Article Links on my KE3FL web site.

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    Comments 17 Comments
    1. PhilK's Avatar
      PhilK -
      I've had some time to build the antenna and then to run a few quick tests on it as well.

      The setup was the antenna with about two feet of 300 Ohm twinlead, then a connector to about three feet of 75 Ohm coax. The connector either had a transformer in it or not.

      This was then fed into the HVG-950Q tuner stick running the WinTV Ver 7 software and the Digital Signal Monitor software both from Hauppauge:

      USB WinTV-HVR-950Q stick:
      Hauppauge Computer Works : WinTV-HVR-950Q Product Description



      Digital Signal Monitor Software, Free:
      Digital Signal Monitor - Hauppauge



      1. It seems to be better than the small vertical antenna that came with the WinTV HVR-950Q USB stick. I'll have to test longer to see if I can get a good average of the improvement.

      2. It also seems to be better to not use a 300-75 Ohm impedance transformer.

      For #2 the points are preliminary. It seems clear that I'll have to do the tests again and over a longer time base for each individual point for a better approximation of the changes due to not using the transformer. In one case I may also need to use an attenuator to lower the signal since it topped out at a SNR of 27 for both the min and the max values.

      In any event, here are the preliminary results:

      Without the transformer:
      Best: +10 - could be more since this topped out at 27 both min & max
      Worst: -4
      Average: +2
      # Stations that improved: 8 out of 10
      # Stations that worsened: 2 out of 10

      I'm still working on the best way to use the tuner and digital signal monitor so hopefully the test results will be clearer. I also plan on testing a few snap on RF blockers to keep the coax from becoming part of the antenna system which may or may not help in clarifying the test results.

      Until next time...
    1. PhilK's Avatar
      PhilK -
      All the readings I've taken with this antenna comparing it to my standard bowtie antenna show that the two are neck and neck with each one being a bit better than the other at certain frequencies.

      That being the case I would suggest trying to build a simpler Turnstile-Bowtie antenna. I'll have to get a photo of one I built back in 2002. I wrote an article about it at the time which was published in Popular Communications, January 2003, p 6-10, "Phil's Easy Way To Improve Your UHF TV Reception"
    1. PhilK's Avatar
      PhilK -
      After picking up the Digiair Pro ATSC Signal Analyzer, from SolidSignal.com I tested the Fractal antenna and a simple bowtie antenna of the typical 8" element length. That's 16" bent in half with a V shape with the ends being 3" apart as one side of the bowtie.

      The bottom line for this test was simply that the Bow Tie antenna was better, there was unfortunately nothing "magic" about the Fractal antenna. I heard back from the author and he felt that removing the "instructable" was called for since these results were so clearly contradictory to his hopes.

      NOTE: That the Digair channels are the REAL Channel numbers meaning the FCC channel numbers/frequencies so you can look up the actual frequencies,

      The results for the stations tested are:



      As of Aug 25, 2013 it looks like the author put the Instructable back up. I have the new link to it above in the original article. I updated that link on 05/22/2014. -PhilK
    1. VOS's Avatar
      VOS -
      Since you have such an interest in building Home Brew antennas, I have to wonder why you haven't built some reference antennas to measure the gain of your projects?

      It didn't take much effort to do this and get results within a dB of the published gain for the antenna under test, with the channels in this area.
    1. PhilK's Avatar
      PhilK -
      VOS,

      Yes, this has been done now. I have three antennas, the standard 8" (16") bow tie, a 12" (24") version and a 16" (32") version which I've tested to see how the longer elements compare to the standard 8" element version. Those results will be published eventually. The 8" version is my standard and it was the one used above to compare with the Fractal antenna.

      I also selected a location to do my tests from and a standard height so I'll be able to compare different antennas at different times/days, though I'll probably take the standard antenna with me to make sure everything is still the same, and if not then I'll have to record the new readings to do a valid comparison.

      I'm presently in the process of evaluating the results of the 8, 12, 16 inch bow tie elements along with the 8" quad design. Next I will build a dual section (four elements, making two bow tie antennas connected together) antenna with the three sizes of elements. If I get one which is noticeably better than the others then I may turn that into a turnstile design and go with it for my home antenna.

      To let you know what's coming I will say that so far it looks like the 16" element version compares about the same as the quad 8" element version.
    1. VOS's Avatar
      VOS -
      You're not clear if you'll be able to give gain in dBd or dBi, or if you're only going to be able to say "better than, or worse than".
      I was able to measure dBd and compare mine to published gain from a manufacture, for validation within 1 dB.
    1. PhilK's Avatar
      PhilK -
      The Digiair measures in dBuV. I tested my "good" attenuators and noticed no glaring problems, so if I use a 6dB attenuator in drops the signal level measured by the Digiair by 6 dBuV.

      To be better I'd have to test every attenuator against a constant source which I don't have.

      I also do not have either repeatability or reproducibility numbers on the Digiair. There is no way I'll be able to test for reproducibility since I only have one unit, but I can make a stab at the repeatability by using a clear day, a clear shot at one or more DTV signals, and simply repeating the test for a set amount of time. I do have plans to run such a test to at least get some kind of repeatability numbers. They won't be perfect, but they will be real-world numbers. Perfect would be to test a unit (a number of units) against a known dBuV source, with a constant output, over a known distance and take hundreds of measurements. I think we can assume that the transmitters do not change their outputs, at least not significantly, we know that atmospheric conditions can and do effect UHF signals so even on a day-to-day basis the signal levels received will vary. While I don't know the exact distance, I can at least place my antenna in the same spot +/- a foot or two over a distance of about 26 miles, so I think we can assume that will be pretty much the same. I can also measure the humidity and temperature so we have that, and if I ever get the chance I'll try to measure on a significantly different kind of day to see what effect it has on the numbers.

      I will see about building a "standard" antenna. Since almost all designs I've seen on the web and many manufactured units use about 8" I'll standardize on that length for the "standard" bow tie antenna.

      So, to answer your question the units will be dBb - dB vs. standard 8" bow tie antenna.

      I will not be taking hundreds of data points I'm planning on a minimum of 10 and a maximum of 25, I'll see how I feel about doing all this once I get into it.

      Once I have even this level of repeatability number I should be able to then test each of the attenuators, or at least some number of them, and see how they fare.

      Suggestions and ideas are welcome.
    1. VOS's Avatar
      VOS -
      I took a much simpler approach by going back to basics.
      I don't need to know the signal strength. I just need to have one.
      The simplicity of dBd is that it's referenced to a dipole.
      Building a dipole for a given frequency is easy.
      I built seven: 2 for my VHF channels and 5 to cover the my UHF.
      I used a crude USB tuner stick and pads to keep it within it's linear range.
      Measured the delta of the SNR between my dipoles and a Winegard.
      Compared my result with Winegard's and found I was within 1 dB.

      This is as "home brew" as I could be.

      If you want to have a "standard antenna", you might want to follow these steps to characterize it.
    1. PhilK's Avatar
      PhilK -
      VOS,

      Thanks again for your input. I may be missing something, or a number of some things, but from my experience the signals here vary too much for me to say any number is within 1 dB of anything but the repeatability of the measurement.

      As can be seen on my repeatability measurements, located at: Solid Signal Blog - Bow Tie Element Length Evaluation using the Digiair meter I see stations with repeatability within minutes of each reading being +/- 1 dBuV, while other stations are better, or worse. To be completely rigorous I'd have to characterize each station I'm going to use and keep the repeatability measurements for each of these stations.

      At present using the 8" quad bow tie I found at least 34 stations, channel 10 was just measurable at 30.4 dBuV and would probably not be usable for tests unless an antenna improved the signal level and brought it up significantly.

      Your basing everything on a dipole is very good and if you could publish your data then we could all see it. I do not intend making dipoles for each channel bandwidth since you have already done it for your channels. If I find that I cannot reconcile my findings with yours I may cut my own dipoles to see if I can duplicate your numbers and if not, then figure out why not. I am standardizing on the 8" bow tie since it is, by theory, a wideband antenna. Also, when comparing to manufactured antennas I'd much rather have one standard antenna to compare against rather than 34 or more dipoles.

      My test location is not at home, it is up on Mt. Airy ridge in a school parking lot which makes putting the base and mast up at the same location very easy. It also has a much cleaner path towards Baltimore and Frederick than my home location, the path to Washington is still somewhat blocked but seems to still be better than my home location.

      All channel numbers correspond to the FCC mandated channel frequency bandwidths as published before the DTV conversion. From now on I will call the identification "channel numbers" IDs so a channel transmitting on channel 48 may have an ID of "4" as does our Washington DC NBC Channel 4 TV station which is now transmitting on the channel 48 FCC frequencies. So, again Ch # is the real channel corresponding to the FCC published frequency bandwidths, ID, if used, is the number still used by a station that changed its frequencies but not its original channel number as its identification number.

      I believe this is the only way to match up our frequencies since using the stations' channel ID numbers will not always insure that we are talking about the same frequencies.

      Here are the channels the Digiair receives in this test location:

      Quad 8" using the
      300 to 75 Ohm
      transformer
      Ch......dBuV
      07......46.2
      08......36.0
      09......51.2
      10......30.4 - Min
      11......65.8
      12......36.6
      13......64.7
      15......47.9
      18......32.8
      21......33.1
      23......35.0
      24......43.0
      27......38.3
      28......58.5
      29......65.6
      30......33.3
      33......44.6
      34......49.2
      35......51.2
      36......56.9
      37......39.7
      38......79.6
      39......45.3
      40......80.6
      41......81.1 - Max
      42......70.9
      43......37.7
      44......31.2
      45......37.9
      46......78.2
      47......43.5
      48......61.3
      49......31.3
      50......57.6
      Average: 51.4
      Spread: 47.8
    1. VOS's Avatar
      VOS -
      Quote Originally Posted by PhilK View Post
      VOS,
      Thanks again for your input. I may be missing something, or a number of some things, but from my experience the signals here vary too much for me to say any number is within 1 dB of anything but the repeatability of the measurement...
      Your basing everything on a dipole is very good and if you could publish your data then we could all see it.
      Phil,
      This project turned into just a feasibility study, as my interest faded after proving I could "match" the gain results of the antenna manufacturer, by simply building dipoles.
      I've followed your articles, but they've seemed to lack a reference (point). Sort of like dB to dBm, where the latter is power, but the former is "unit less".
      Antenna gain is either in dBd, or dBi, and you've explained how to compare/convert.

      Building dipoles is something your readers can do [if they're interested] and have repeatable results to a common reference.
      From this the various antenna designs should be able to be compared and repeated by others, by either using power meters, or as I did, using a USB tuner stick and measuring the SNR. This method required "pads" to attenuate the signal to keep the software within it's linear range.

      In my younger days, testing was my interest, but after 30 years, my interest has waned to the test setup & methodology for valid results.
    1. PhilK's Avatar
      PhilK -
      Vos,

      I understand perfectly about waning interest. I'll try to put together an adjustable length dipole from two adjustable length radio antennas so I can compare the 8" bow tie to a dipole to see what we have.

      Some, if not all, of my basic assumptions about a bow tie antenna have been proven wrong by my research and these experiments. I like the idea of having a reference point to something we know, and I can certainly make one or two adjustable dipole antennas which should allow me to compare against. I'll first do the calculations to see what is needed as far as length is concerned, I've done this before and sort of remember that the longest wavelength for channel 7 is around 68 inches meaning a dipole should be around 34 inches so that means I need adjustable elements that can extend to 17 inches each. Then they have to compress to at least 8" and possibly 5" but I'll have to look at the channel 50 frequencies again.

      I'm glad to hear you're watching the blog and any time you want to chime in, please do. All ideas and corrections are welcome. Our discussion really motivated me to test my assumptions and the present and future articles will show the fruits of that endeavor.
    1. VOS's Avatar
      VOS -
      Another "tidbit" copper is better than steel.
      My first round was with coat hangers and measured gain was 1dB better than published for UHF and 1.5 dB for VHF.
      A "spot check" using copper matched the published gain much closer, leading me to believe copper should be used.
    1. PhilK's Avatar
      PhilK -
      My guess would be that they used aluminum not copper for any antenna they're testing but I don't really know for sure. The antenna manufacturers definitely use Al but scientists doing the tests? You're probably right that they use copper. I don't doubt that using copper is better but I'll have to use what I have which will be whatever they make those extendable antennas out of, though it might be good to do the tests with one copper antenna for one channel to see what the difference is, if any.

      Thanks for the heads up.
    1. VOS's Avatar
      VOS -
      Copper & aluminum may be close, but using steel looks to lower the gain of the dipole by 1 + dB, so comparing to "the other" antenna makes it appear to have more gain than it does.

      What I didn't get to was making a long dipole and then cut it shorter as I moved up the frequency range.
      This might work for you if you're trying to characterize an antenna that will become your reference for other antennas.
    1. PhilK's Avatar
      PhilK -
      Quote Originally Posted by VOS View Post
      Copper & aluminum may be close, but using steel looks to lower the gain of the dipole by 1 + dB, so comparing to "the other" antenna makes it appear to have more gain than it does.

      What I didn't get to was making a long dipole and then cut it shorter as I moved up the frequency range.

      This might work for you if you're trying to characterize an antenna that will become your reference for other antennas.
      Yes, that would definitely work and I could then compare the extendable antennas to that first to see how they compare and go from there. If they're close then I will simply be able to use them as the reference without any adjustments to a "real" dipole made of copper. otherwise I guess an adjustment will always need to be mentioned.
    1. PhilK's Avatar
      PhilK -
      I hope to build this over the next few days. I've also ordered a few smaller extendable antennas so I can make a version to get to the top frequencies of the UHF TV band. By the way, a dipole antenna, if cut to the correct length for a given frequency, has a free-space impedance of about 73 ohms. This is common amateur radio knowledge and every article I've read has confirmed that. No doubt as we add elements, as in a Yagi-Uda design or in a Log-periodic design, this changes. I've stated elsewhere what I've read about the design impedance of both of these. By the way, I tested out the 16" bow tie elements in a vertical arrangement so I could use them on the amateur radio, ham, 2-meter band. It worked very well and the impedances I measured with my antenna analyzer were between 40 and 60 ohms. Hams try to design antennas to match a nominal 50 Ohms since that is what most manufacturers design the antenna inputs of ham radios to use.

      The only antennas I know of with an inherent 300 ohm impedance are the folded dipole and loop antennas. This impedance is only for the frequency of design, which means the over-all length of the wire, which is a full wavelength long for the frequency of interest/design. As far as I know, no TV antenna is based on a folded dipole or loop antenna. The loop is a very common antenna for shortwave listeners (SWLs) to use for a number of reasons and it is also used by hams in a number of arrangements as well. I did once build a full-wave loop antenna for a neighbor and friend and it worked quite well for her back in the pre-DTV days. I had to design it for channel 2 frequencies at the time since that was the longest wavelength to be received. As hams and SWLs have found, this antenna receives very well from all directions and for all frequencies at and smaller than the design frequency. So, her TV reception with the antenna tacked to the ceiling of her den brought in stations from transmitters to her east and south from the closest two cities.
    1. EdwardKayle's Avatar
      EdwardKayle -
      I am damn sure about it....