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Author Topic: Antenna building  (Read 1225 times)
2ST123
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Posts: 56




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« on: March 10, 2007, 06:38:27 AM »

When it comes to building an antenna , what makes a 5/8 wave antenna a 5/8 and a 1/4 wave antenna 1/4 ? Is it the diamater of the wire used? I am wanting to build a antenna for my scanner using copper wire.. We monitor the scanner 24 hours a day and sometimes the farther stations are a little hard to hear.I want to build it spacifically to receive the police/fire band..Using the calculators i have found , it looks as if it would be rather short..


                              Roy
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AA4PB
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Posts: 12843




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« Reply #1 on: March 10, 2007, 09:24:42 AM »

It is the length of the wire in relation to the frequency that makes it a 5/8 or 1/4 wavelength. Taking into account the velocity factor in wire, 1 wavelength = 936 / Frequency (in MHz). A 1/4 wave is 1/4th of that length.
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WB6BYU
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Posts: 13248




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« Reply #2 on: March 10, 2007, 12:26:19 PM »

At a frequency of, for example, 150 MHz, the wavelength is given by
300/f = 2 meters.  (Hence we talk about the "2 meter band" from 144
to 148 MHz, since we are rounding the numbers off.)  So a 1/4 wave
antenna would be 1/4 of that, or 50cm long.  A 5/8 wave antenna would
be 5/8 of that, or 125cm.

Note that a 1/4 wave antenna with a suitable ground plane is a resonant
antenna, while a 5/8 wave antenna is not - it needs some sort of
matching network at the base, usually a coil.  Also, when used with
wire radials they must be horizontal, not sloping, as the current in
them will be out of phase with the majority of the current in the radiator
and will tend to cancel the radiation at useful angles.

Another thing to remember is that the 5/8 wave antenna only has gain
over a quarter wave when the bases must be at ground level, and then
only because the radiating portion is higher in the air.  It works just
about as well as a dipole or ground plane if the tops of the antennas are
at the same height.

In your case, overall antenna height above ground (rather than antenna
length) will contribute far more to your reception than the gain of the
antenna itself.  Putting an antenna up on the roof and connecting it to
the scanner with coax cable will do far more to improve reception than
using a 5/8 wave whip on the back of the scanner.  (Granted, I'm making
some assumptions here about your setup, but the general conclusion
usually holds:  antenna height contributes more than using a higher
gain antenna at the same height.)
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2ST123
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Posts: 56




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« Reply #3 on: March 12, 2007, 05:51:31 AM »

Right now i am using a old astroplane,I am having trouble picking up 3 or 4 counties away..I also have a fiberglass frances whip,can this be cut down to work?and if so , what lenght for 155.835 mhz..


                      Roy
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N3BIF
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Posts: 1190




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« Reply #4 on: March 12, 2007, 09:26:31 AM »

936/155.835= 1 wave length

1 wave length/4 = 1/4 wave antenna for 155.835


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WX9DX
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Posts: 64




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« Reply #5 on: April 02, 2007, 11:32:20 PM »

 have enough tower for two Rohn 25G towers to be used for 1/4 wave phased antennas out in my front field. Anyone ever done this??? It's lots of Money and I'd like to get it right the first time. I did price the bottom insulators at approx $385 for 3 and I'd rather use a Gamma system to match them rather than loose the physical strength of the tower using insulators on the bottom of the towers. Also another concern would be the exact height of the tower since I'm using such a large face radiator. Approx how much might I loose in height using Rohn 25G tower? ETC ETC ETC. I'd built phased verticals for other bands but this is my first time with Rohn tower and on 75 meters.

Jimmy:) WX9DX
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