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Antenna lead question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE5KOG, Jun 17, 2008.

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  1. KE5KOG

    KE5KOG Ham Member QRZ Page

    OK, On here a few days ago I found a link to this antenna.

    http://www.arrl.org/tis/info/pdf/0004033.pdf

    It struck me as a fun little project for my son and I to take on. We will not use it as a mobile antenna but more as an antenna for the back yard below the fence line. Or better yet an antenna that we could take with us from place to place.

    I did pick up a few extra parts to make things easier though... One, I picked up a SO239 so I could put a connector on the end of it and then run coaxe up to the tuner. However, I also found at a radio shack about 100' of 300 Ohm twin lead.

    Would it be better to run the twin lead to the tuner and no use the coaxe?

    This is our first vertical HF homebrew antenna.

    Tim
     
  2. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    Use 50 ohm coax and no tuner. You will need radials or a large conductive mass like a car body for a counter poise. The counter poise acts like "the other half" of the antenna. The vertical portion is incomplete without the counterpoise.

    One member built this antenna and posted his photos here: http://forums.qrz.com/showthread.php?t=165409

    73,

    Terry, K7FE
     
  3. WA9CWX

    WA9CWX Ham Member QRZ Page

    There is a simple answer to this question.

    Grab a sandwich, some coffee, your engineering texts, and an advanced math tutorial, and sit back.

    This will take a while. :D
     
  4. WA9CWX

    WA9CWX Ham Member QRZ Page

    FE answered your questions in the simplest, most direct way possible.

    I would probably shut down the computer and follow what he said. :)
     
  5. KE5KOG

    KE5KOG Ham Member QRZ Page

    Thanks for the info... 2 more quesitons now...

    1. I knew that I would have to add the radials, am I correct that the Radials will attach at the same point as the ground in the diagram?

    2. Just for future reference... Why do you suggest not using a tuner?

    Tim
     
  6. KE5KOG

    KE5KOG Ham Member QRZ Page

    Yeah... Sounds like a good place to start....

    +++ no carrier
     
  7. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    1. Yes. The vehicle ground is what they meant. You must replace that "vehicle ground" with another form of counterpoise. You may use 1/4 wave radials for the band of interest if you wish. Connecting to a chain link or chicken wire fence, metal roof, etc. will work also. The problem that you will find is that each different counterpoise will have a different effect on the antenna's tuning. You are intending a portable operation, so you might be better off to make a counter poise that you take with you. That would eliminate the retuning at each location.

    When an antenna is mounted on a vehicle, one normally spends a few days bonding metal parts together and then matching the antenna to "that" particular vehicle. It is a "set and forget" operation.

    2. Tuners cause a great deal of problems for the newcomer. They might be adjusted so far off that they cannot make contacts or have a VSWR issue. KEEP TUNERS AWAY FROM RESONATE ANTENNAS. Your mobile antenna is a resonate antenna........one band at a time as you change the loading coil tap.

    A resonate antenna should be matched to your feed line impedance...........at the antenna's base, not 50 feet down the coax line. If the antenna is not matched at the base, your coax losses will waste RF energy, irregardless of what your VSWR is at your tuner/transceiver. Those losses may be greater that your antenna eventually radiates. Tuners also have their own losses, while normally low, can be quite high with improper use or poor design.

    Mobile antennas like you are building may be easily matched with a "shunt coil" from the antenna base to ground (counterpoise and coax shield). K.I.S.S. Simplicity works best.

    73,
    Terry, K7FE
     
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