On Sun, 26 Apr 2009 13:42:28 -0500, MikeWhy wrote:
"Lew Hodgett" wrote in message
...
wrote:

was told that this dimension was related to the bandwidth one
expected to receive. If so, wouldn't there be a difference in required
length for UHF vs VHS, vs FM, say?

I'll let any Hams on list answer your question; however, 60" covers
everthing including FM in my area.

SFWIW, Radio Shack sells a factory made unit which is 60".

The "accepted" (on FCC tests) simplified equation for a half-wave dipole
wire length is:
feet = 468/MHz.

That is the "accepted" formula because it yields the proper length for
any half-wave long wire antenna, including a folded dipole.

A folded dipole simply folds each leg of the dipole back to the
center. Effectively, the 5' long folded dipole has 10' of wire. Its
resonant frequency is thus 46.8 MHz according to the formula.
Interestingly, this works out to about a full wave for the 100 MHz FM
band.

Not correct. The resonant frequency of a half-wave dipole, folded or
otherwise, at 60 inches long is 93.6MHz. That is a little below the
center of the FM broadcast band (98MHz).

The effect of making the half-wave antenna in this manner is twofold;
(1) it raises the feedpoint impedance to 300 ohms, as compared to a
single-wire half-wave dipole at about 75 ohms; and (2) it increases the
useful bandwidth of the antenna somewhat over that of a single-wire
half-wave dipole.

As with any antenna, the actual resonant frequency, feed point impedance
and bandwidth will all be somewhat dependent upon the environment around
the antenna.

The simplified formula differs from the theoretical value in a vacuum
by the velocity factor of the wire, in this case apparently about 95%
(from 492/MHz in a vacuum).

Yes. In free space (vacuum), the wavelength of a radio wave is found by
the equation 300/(Frequency in MHz). For example, 50MHz has a wavelength
of 6 meters in free space. Converting this to feet, where 1 meter is
3.28 feet, gives 984/(Frequency in MHz). Divide by two for a half
wavelength and you get 492/(Frequency in MHz). So you see, the equation
used to compute the length of a half-wave antenna takes into account
fact that we are computing the length of a physical antenna, rather than
free space wavelength. It is resonably accurate so long as the diameter
of the conductor is very small compared to the operating wavelength.

I wouldn't worry much about it. The antenna's resonant frequency is
not nearly so important for receive-only operations as it is for
transmitters. A mismatch on a transmitter presents a very high
impedance, causing the feedline to also radiate, and plays all kinds
of havoc to equipment in the vicinity.

Very oversimplified, but probably appropriately so for this discussion.
However, I would not go so far as to say that the frequency an antenna
is "cut" to isn't important for receiving operations. The antenna will
not perform as well if it is mistuned, and that effect can be dramatic,
depending upon how far off resonance the antenna is.

A dipole is also somewhat directional, with about 2 dB of gain in its
broadside direction compared to a point radiator. This implies the
same 2 dB attenuation in its side lobes, off the ends.

I think you're referring to an isotropic radiator? The gain of a dipole
(folded or not) is around 3dbi IIRC. This is the ideal gain broadside to
(perpendicular to) the antenna. The actual gain will be very much a
function of height above ground, the conductivity of the ground, and the
proximity and type of of surrounding opjects.

The gain "off the ends" can be very low, much worse than 2 or 3db below
the maximum.

Given a choice, I would face the antenna toward the signal and the
ends toward the local RF noise. However, if reception is so marginal
that this is enough to make or break the chain, consider it broken
and get a tuned, multi-element, directional antenna. The same goes
fiddling with the wire length.

Often the proper orientation of any antenna with a small to moderate
amount of directivity, such as a dipole, is a compromise. We can't
always arrange for an interfering source to be 90 degrees away from the
direction of the transmitter we are trying to receive signals from.

--
Art Greenberg
artg at eclipse dot net