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Author Topic: Half wave vertical feeding  (Read 9404 times)
N7QF
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Posts: 12




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« on: June 14, 2000, 06:52:04 PM »

What is the preferred method of feeding a half wave vertical for HF ?
(for 50 ohm line)
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INTHEMAIL
Member

Posts: 2




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« Reply #1 on: June 15, 2000, 01:57:01 AM »

The normal method of half wave vertical feed is through a 1:1 current balun connected in the middle and shunted by either a hair pin match or a solenoid coil. Both methods require a bit of the old cut and measure methodology.

Keep the coax coming out perpendicular to the antenna and out the side at the mid point - else if its run down the length, you will pick up current on the coax by coupling and it will require some feritte coax beads to get it shut down.

Pete
KG6BOG/AE
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N7QF
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Posts: 12




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« Reply #2 on: June 15, 2000, 11:01:28 AM »

Dear KG6BOG/AE,

I was referring to a half wave vertical "radiator",
not a half wave vertical dipole.
TNX
N7QF
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W0YR
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Posts: 4




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« Reply #3 on: June 16, 2000, 06:58:19 AM »

I use an L network.  Connect a coil in series with the center conductor.  Connect a variable capacitor of about 150 - 200 uuf with at least 1/8' spacing between the input to the coil and ground.   Use a ground rod; you do not need radials.  Set the capacitor to about mid range and tap up and down the coil for the best match (shooting for 1:1 VSWR).  Then adjust the capacitor for the lowest VSWR.  They are killer antennas.  When I was in Russia I used one with great results and I presently have a 40 meter 1/2 wave vertical which is matched in the manner I have just described and it is tremendous.  Hope this helps.  Mike W0YR  
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K4DPK
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« Reply #4 on: June 22, 2000, 02:17:50 AM »

Radiators that are half-waves or multiples of half-waves can be voltage fed at the end.  This can be done very simply using a parallel-resonant LC circuit, with the bottom of the network grounded to a ground rod and the top of the network connected to the end of the radiator.  The coax braid is attached to ground, and the center conductor is tapped up from the bottom of the coil to find the proper impedance match.  Either the L or C element may be variable in order to find resonance.  Since the unit will be exposed to moisture (outdoors), it is best at all but the lowest power levels to use a fixed capacitor and variable coil.

The capacitor should have ratings of approx. 2 picofarads per meter of wavelength, e.g. 80 pf on 40 meters, etc. and voltage ratings commensurate with power level.  Several KV is needed for legal limit, because it is a high impedance system.  Determine the LC constant by dividing 25,330 by the frequency in mhz, squared.  For 7.15 mhz, LC=496.  Then L=496/80=6.2 microhenries.  Wind a coil of 7 or more microhenries so you have room for adjustment.

Start with the coax tapped at a couple turns from the bottom, and with a very low level signal exciting the system, begin shorting turns on the top of the coil until max field strength or lowest SWR is found.  Adjust the coax tap for further improvement.  Tap in one turn increments, then slide around that turn to optimize, on both the top and bottom of the network.  There will be interaction between these adjustments.

High voltages are present near the ground in this type antenna system.  That is why there is no need for radials.  It is also the reason why great care must be taken where children might contact the wire.  I use 40 KV insulation for the lower parts of vertical elements on this kind of array.

73
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