On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:

On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote:

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole.

Hi Buck,

Modeling would suggest otherwise.

The difference between a 1/4 wave center fed and off-center fed (at
10% from the end) is about a 12% increase in real resistance, and
.25dB gain improvement, both favoring the off-center fed.

Are you comparing short antennas to long antennas? If so, it wouldn't
be a surprise, would it?

Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

73's
Richard Clark, KB7QHC

The original comment was that a multiband OCF antenna is a
'compromise' antenna and wouldn't be as good as dedicated 1/2 wave
dipole cut to frequency (or so I understood it.)

If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the
OCF would lose, but if it were compared to a 20 meter dipole, it would
have gain in the direction of various lobes.

Yes, the comparison is between different length antennas, and you are
right, generally speaking, the longer, the better. (no doubt someone
can find an exception to the rule, but that isn't the point of this
discussion.)

The reason for my statement of 'properly fed' is that I know that the
impedance changes radically from band to band at the feed point.
Buxcomm is selling a popular OCF with a 6:1 balun to coax. I don't
think that is good as the impedance will be pretty low at the coax at
times. However, to each his own.

Feeding an OCF directly (no balun) with coax probably isn't a great
idea either. I used one that way for a long time, and with great
results (75 ohm indoor cable-tv coax) and switched to Radio Shack
low-loss 300 ohm tv twin-lead which appeared to have better results.
The best results seemed to come from a friend's setup which used 600
ohm twin lead (the good wire-man stuff) all the way to the tuner.

My favorite OCF design is still the clothesline antenna... a loop of
wire to make a 40 meter dipole at 300 ohms, a 4:1 balun with coax
attached and a motor at the pulley on one end. Tune the antenna by
moving the balun towards or away from the center of the dipole. No
tuner needed and it worked adequately in the man's attic, according to
the article I read. It is an old design, but interesting still today.



--
73 for now
Buck, N4PGW
www.lumpuckeroo.com
N4PGW