How long do buried radials last

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AC6S, Jun 8, 2013.

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  1. AC6S

    AC6S Ham Member QRZ Page

    Sure there are local conditions regarding soil, but I am curious as I am in the process of getting ready to move my antenna and go with a buried ground radial system. Two years? More?

    I picked up 1000 feet of bare braided 18 gauge wire and am going to use that instead of using my 14 gauge THWN sheilded, which will want to roll up on me when laying it out. Trying to make my life easier with the bare wire. But, I may still come back run about 500' of the 14 gauge as well, depending on how ambitious I get. I plan on running them at 33' each (roughly 30 total for 1000') for a 26' antenna.
     
  2. NL7W

    NL7W Ham Member QRZ Page

    I believe you'll find they'll last many years if they are not stressed and broken by foot traffic or similar. You could always just slit the sod or ground and place them just below.

    You're on target. Lay down the 30 each 30+ foot radials, and you'll be fine. I have installed the same number and length of radials under my 4BTV and it performs admirably for a vertical above mediocre ground. I used green, insulated and stranded 14 AWG for my radials as it was an easy find at Lowes. I used DX Engineering's metal lawn staples to hold them down till the grass grew around them.

    If you're installing a Hustler "BTV" type vertical, you'll notice the VSWR at resonance to be around 1.5:1 on all bands with those radials, with the exception of 10 meters where it stay fairly low. The reason: with this many radials and more, the feedpoint impedance is lowered to around 30 to 35 ohms by the reduction in earth losses. This is good, because the radials increase radiation efficiency. Forget about VSWR in this case.

    Adding more radials will help reduce earth losses, but the law of diminishing returns becomes a factor. It might not be worth it.


     
    Last edited: Jun 8, 2013
  3. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Unless you have very corrosive soil radials should last years.
     
  4. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Copper will last many years where aluminum will disappear in short order.
    Using insulated wire is okay and may make them last even longer.

    The ground de-tunes the radials so it's difficult to obtain and retain any
    type of resonance. Soil changes in water content and other factors cause
    the shift.

    You do not need a lot of radials to be effective but departing from a 1/4
    wave does make the radials much more critical. More is better, always.

    You'd be surprised at what will work and work well.
    The difference between 20 ohm ground loss and 2 ohm ground loss with
    a 1/4 wave radiator is about 2.5dbw. The difference in wire on the ground
    is much higher.

    A 20 ohm loss requires 8-9 radials with a total wire length of 125 feet for
    40 meters.
    For 2 ohm loss the total radials are 75 using 2372 feet of wire each radial
    is over 31 feet in length.

    You can see this in the following publications; http://www.ncjweb.com/k3lcmaxgainradials.pdf.
    And here on the vertical portion; http://arrl-ohio.org/convention-2012/Wire Antenna Basics_ARRL_Ohio_8-11-12.pdf.

    A 26 foot radiator on 40 meters is about 50% with a ground loss of 20 ohms.
    The same radiator with a 2 ohm loss is 88% efficient. In db terms it's 2.5 db.

    YMMV and trying different numbers of radials would be a fun experiment.
    Problem is, you have to make other types of measurements to find if
    you're doing that much better or not. Propagation will alter the readings
    and I have yet to find any amateurs that claimed using more radials did nothing
    to their signal.

    Either they wish to verify their results as being a positive one due to the amount
    of work needed or they simply went from zero to max is a days time.

    Hope this helps
    73
    Gary
     
  5. AC6S

    AC6S Ham Member QRZ Page

    Thanks again for the info. Reading through the antenna instructions on ground/ buried radials, ARRL Antennas for Small Spaces Appendix on Radials, and Butternut's Tech Notes "Why Radials" led me to the numbers and lengths I chose. It should give me comparable results to my roof mounted radial system of 4 radials per band, cut to 1/4 WL.

    I read a good section on ground loss and how it affects antenna efficiency, but I have not fully digested it. The first question that comes to my mind is how is someone supposed to know or find out what their ground loss is in terms of ohms?
     
  6. KA0GKT

    KA0GKT Ham Member QRZ Page

    A number of factors come into play, but you can start with a ground conductivity map. For an area like your back yard, you can use three or more short ground rods, an RF source and an RF volt meter. You can do it with a low voltage filament transformer, or with a Megger.

    Drive the ground rods into the soil one meter apart and take some readings with the megger; read the resistance between the rods and divide one by the number of MegOhms. (1/megohms) This will result in the conductivity of the ground in micro-Siemens per meter.

    Or, you can just cut the length of your radials to around 10% longer than a quarter wave at the lowest frequency of expected operation and leave it at that.

    Knowing the ground conductivity of your city lot is cool, but there is little you can do to make it appreciably better, besides, unless you are doing scholarly experimentation the purpose here is to get on-the-air. Do what is necessary and have some fun. If having fun to you includes measuring the ground conductivity, go for it!
     
  7. AC6S

    AC6S Ham Member QRZ Page

    Well, I have a Megger, but I am not willing to go buy 3 ground rods and drive them in rocky soil. LOL. Thanks for the knowledge.
     
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