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Author Topic: velocity factor antenna??  (Read 1625 times)
KD5FOY
Member

Posts: 38




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« on: February 11, 2007, 05:59:57 PM »

given that the length of a half wave dipole is
calculated by 468 / freq in mhz when velocity factor
is 1, ie 468 / 1.9  is about 246 ft.

i'm sitting here wondering why folks with small
city lots don't use (468 / freq in mhz) * velocity factor,
to construct a much shorter antnenna, particularly on 80m & 160m?

as an example, rg59 coax typically has a velocity
factor of .66, so plugging to the formula,
468 / 1.9  is about 246 ft, * .66 is about 162 ft.

why couldn't 162 ft of rg59 be cut in two, attached
to a 50 ohm, have the remaining ends of the rg59 shorted
together, and hoisted into the air?

so what am i missing? there are no free lunches:-)

larry
kd5foy
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N3OX
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Posts: 8853


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« Reply #1 on: February 11, 2007, 06:32:23 PM »

You're missing the fact that coaxial cable has essentially three conductors!  The *shield* does not allow RF to pass through it.  That means that the *outside* of the shield and the *inside* of the shield are two surfaces on which RF current can flow independently.  The *only* way that RF can communicate between the inside and the outside of the cable is at the ends!

The velocity factor of coaxial cable is the velocity of electromagnetic field propagating down the **inside** of the coaxial cable.  

The outside of the coaxial cable is out in the air except for the thin jacket.  It's just another insulated wire as far as the RF propagating through the air is concerned!

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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Posts: 8853


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« Reply #2 on: February 11, 2007, 06:33:12 PM »

"It's just another insulated wire as far as the RF propagating through the air is concerned! "

And has a V.F. of maybe 0.95, I should add.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
Member

Posts: 13038




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« Reply #3 on: February 12, 2007, 10:24:47 AM »

There are actually antennas that have been designed based on this
concept, but they really don't work that way.  The velocity factor applies
to signals INSIDE the coax, but not those flowing on the outside of the
shield.

As N3OX mentioned, there IS a velocity factor that applies to antenna
wires, and it is a function of the insulation on the outside of the wire.
People have made shortened antennas by increasing the velocity factor,
but this often has involved at least a foot thick coating of plastic,
ferrite, or even water.  For general ham use there are simpler methods
of shortening the span of an antenna such as folding the ends, capacity
hats or inductive loading.
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KD5FOY
Member

Posts: 38




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« Reply #4 on: February 13, 2007, 07:06:54 AM »


guys

tnx. sage advise and apprecitated.

larry
kd5foy
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