From: "Phil Allison"
Subject: Re: Single wire FM antenna
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Wed, 8 Jan 2003 11:11:00 +1100
NNTP-Posting-Date: Wed, 08 Jan 2003 11:00:46 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Jeff Verive" wrote in message
> We have lost the intent of the original poster, particularly in why a 5/8,
> 1/2, 1/4, or 1/8 wave (or any other fraction) is superior.
> Basically, think of a receiving antenna as a sine-wave AC voltage source.
> At 1/2 wave, one end of the antenna will be 180 degrees out of phase with
> the other, which for a sine wave transmission is the peak to peak value of
> the signal. Half-wave dipole antennas (made of two 1/4 wave antennas
> feeding a differential RF input) are quite common for stationary
> but for mobile receivers the lower half of the antenna is considered to be
> fixed at a virtual ground. The receiver then feeds the RF front end with
> the top 1/4 wave portion of the dipole. This is not necessarily better
> any other construction, just a historical glimpse at the transition to
> single "whip" antenna design.
> A 1/4 wave whip is not going to generate the same voltage as a dipole (in
> the far field, where the distance from transceiver to receiver is much
> greater than the wavelength, this is less of a problem), but at low
> frequencies the 1/4 wave whip is ~ half the length of the 1/2 wave whip,
> making it easier to handle (ever tried to talk on a set with a 1/2 wave
> 3-meter band radio?). In addition, the 1/4 wave antenna will have a
> different impedance (and non-zero reactance, to boot), so the receiver
> circuitry will need to be tuned to null out the reactance. At 5/8 wave,
> reactance is lower, and the directional gain is higher, but this is going
> be offset considerably by the bend the antenna makes as a result of wind
> resistance. So a 5/8 or even 3/4 wave antenna may be better in a moving
> receiver with a curved antenna, where the effective antenna length is
> approximately equal to the end to end distance of the curved antenna.
> At 75MHz, I calculate a full wave antenna to be 1.33 meters (52.5 inches),
> 5/8 wave ~= 32.8 inches,
> 1/2 wave ~= 26.2 inches,
> 1/4 wave ~= 13.1 inches, and
> 1/8 wave ~= 6.5 inches.
> As one poster put it, trial and error might have to be the truest test.
** Excuse me - what do think the speed of light is ?