From: "Jeff Verive"
Subject: Re: Single wire FM antenna
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 7 Jan 2003 16:26:34 -0600
NNTP-Posting-Date: Tue, 07 Jan 2003 16:26:34 CST
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 receivers,
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 than
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, the
reactance is lower, and the directional gain is higher, but this is going to
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.
"Kijoma" wrote in message
> best not start the bomb discussions again :)
> "mook johnson" wrote in message
> > The controller is from and RC can but the application is different.
> > I'm mainly concerned with the receivers ability to "pickup" the
> > signal at various antenna lengths.
> > Mostly interested in the length of antenna wire that extends from the
> > receiver and how it affects reception.
> > "mook johnson" wrote in message
> > news:riiQ9.82742$Nz5.firstname.lastname@example.org...
> > > I have an FM receiver (75MHz) that has an antenna tuned to
> > 1/8
> > > of the wavelength of the carrier.
> > >
> > > From what I've read, 5/8 would be better.
> > >
> > > This receiver only needs to work over a short range (~1000 feet max)
> > a
> > > fairly strong transmitter nearby (~1 watt).
> > >
> > > This is for an R/C car controller
> > >
> > > Could someone explain the performance differences between 1/8,1/2 and
> > > wavelength single line antennas?
> > >
> > >