Choosing element diameter??

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2USN, Mar 25, 2009.

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  1. KC2USN

    KC2USN Ham Member QRZ Page

    Why can or cant I use an element diameter of my choice for a given antenna formula for uhf? What if I have plans for an antenna that specs out 12ga wire but I want to use 1/4 in tubing?? Does that change my lengths and why? I can't seem to grasp or find out the answer.
    Thanks
     
  2. W7BEC

    W7BEC Ham Member QRZ Page

    The 2008 Handbook (and other editions, I'm sure) have a very good section on this subject:
    So yes, the wire diameter you choose will affect the resonant length of the antenna for a given frequency. I'm not going to give out the answer, tho, because I really really think you should have a copy of the handbook (the formula for length that takes into account conductor diameter is on page 22.4). They're on Amazon for under $30 used.

    Here's some additional "light" reading on the subject: http://www.smeter.net/antennas/wire-cage-dipole.php and http://k4mtr.org/antennas.html.
     
  3. AC0GR

    AC0GR Ham Member QRZ Page

    Heck, get a gently used one on eBay for under $10... its not like the laws of physics have changed much in the last couple decades. :)
     
  4. M0DSZ

    M0DSZ Ham Member QRZ Page

    You should persevere with home construction. To determine dimensional change using quarter-inch tubing have a go at the free version of EZNEC from the internet, or MMANA-GAL, or ask on QRZ if someone could do it for you. I'm always averse to buying anything I could make instead.
     
  5. WB3BEL

    WB3BEL Ham Member QRZ Page

    You have asked a good question and others have given you some sound advice.

    The simple answer is that the effective length of the element depends on the element diameter.

    Yes, if you know what you are doing, you can take an antenna design that uses a one element diameter and convert it to a design which uses another diameter.

    Some antennas are more sensitive to changes in element diameter than others. Antennas that are narrowband usually are more sensitive to diameter.

    So for example a log periodic antenna is relatively unsensitive to element diameter. A high gain UHF yagi with many elements is very sensitive to element diameter.

    For multi-element antennas that use the radiated inter-element coupling like yagis and quads etc, the problem is more complex. You can not just use a formula like you might find in the ARRL handbook. This is because the array performance depends on the RF field coupling between elements. The amplitude and phase of these coupling factors is also a function of the element diameters, lengths and spacing. So the process of taking one design and changing it is rather complicated. For simple antennas it is pretty easy to do. For complex antennas it becomes more challenging. If you have elements with stepped diameter it becomes even more complex. This is where antenna simulation software becomes very useful.

    But I am guessing based on some of your other threads that you are asking with regard to antennas for amateur satellite operation. I know you were asking about quadrafilar helix antenna . I think that changing the diameter by as much as you are asking for a UHF QHA would need to be significantly compensated. I am not sure why you have focussed on the QHA.

    I think that for performance to start on the satellites the eggbeater style antenna is simpler and give reasonable performance. Going beyond that, antenna designs based on yagi or crossed yagi provide improvements in gain and signal to noise ratio. You can also build good designs based on classic helix configurations.

    The one advantage to QHA is that it lends itself to miniaturization although at the expense of performance. Additionally it integrates the antenna and impedance transformation into a single structure. But I think that the antenna performance itself is inferior to a simple helix with a separate impedance transformer. So if the important thing is miniaturization; for example a handheld Satphone application, a QHA might be good. But in a ham installation where performance is more important than size another antenna might be better suited.

    Anyway hopefully you have a starting point and you can follow up if you have further questions.

    Regards,
    Harry WB3BEL
     
  6. KC2USN

    KC2USN Ham Member QRZ Page

    Thanks for all the above advice. I did look in the 21st antenna handbook for the diameter info but I was looking under antenna construction materials and my take on it was that anything goes.
    Harry, my reason for the QHA is because I purchased and read the newest satelite handbook from the ARRL. They featured a QHA with plans. I did some research on the ARRL website and found out that was a reprint from the mid nineties QST mag and had some erroneous info in the article and still isn't corrected in the 2008 satelite handbook. Anyway after realizing much of the info I had just commited to memory was sort of outdated I started looking for a newer QHA plan leading me to where I am now. The QHA intrigued me but perhaps I'll take your advice and others and build the eggbeater first, but I am not admitting defeat yet.
    VHF and UHF are new to me and to be honest I was a bit unprepared knowledge wise when I set out with my satelite goal. I have a new used radio (FT 726) that I haven't even plugged in yet to see if it works as was advertised, of course I don't have an antenna yet either.The world of N connectors, SWR meters that only work at 30mhz or below all threw me and so did the coax loss factor at higher freqs as I only played with HF in the mid 80s and now the antenna dilema.
    I do listen to all advice given by others and appreciate it. I am sure I will have more questions soon.
    Can't wait to do a Moon Bounce.
    Thanks again
    Dave
     
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