[OKDXA] Dipole Antenna Question

[Nelson Derks]

> Yes for a change I am glad I asked.

Since it's going to be a cold and wet weekend, here's a web page you may
want to spend some quality time with:

http://www.cebik.com/radio.html

> I have read about that type of tuneing, but not in that much detail.
> The broad banded dipole is a wonderful idea.
> Their are allot of antenna makers who claim that their product will do
broad
> band and don't.
> The B&W adds a resister to the top of their folded dipole and sey its like
a
> squashed rhombic. bull sh!#.

And here's the analysis of a broadband folded dipole like the B&W:

http://www.cebik.com/wbfd.html

...which introduces the related concepts of efficiency and gain.

The most efficient antennas tend to be full-size resonant wires as they can
couple nearly 100% of the transmit power into the Ether. Throw in some
tricks, like a loading coil, to make a big antenna more practical in size
and the efficiency will be reduced. Same for multi-band trap verticals, or
the terminated folded dipole which relies on a resistor to absorb reflected
power that would otherwise trip a newer rig into SWR shutdown. The B&W
design trades efficiency for bandwidth utility, and that's not necessarily a
bad idea if your choice is between something like the B&W or a mono-band
wire. Better to have a modest signal on several bands than a good signal on
one band if that band just happens to be closed tonight. The B&W looks poor
on paper when you realize it has no gain (and negative efficiency) over a
Dipole, but every multi-band wire is a compromise of efficiency and gain.

Which brings us to gain... There is no such thing as free power, but some
folks think of it that way. Take a VHF Yagi with a rating of 10dB gain. Put
50 watts in and it's like 500 watts out. Helluva' deal, but that's in one
direction only. As long as you can rotate the antenna, and don't mind
working the rotor box to catch a weak CQ off the back of the beam during a
contest, you have a good thing going. On HF the antennas are often too large
to rotate, so you pick your favorite beam headings and build to match. In my
case, the back yard has a longest run that's true North-South. If it were
East-West, my log would look much different or, I'd become a master of
compact antenna design. You do your best with what you have to work with,
and on a city lot with a bad orientation the B&W could be a very practical
choice...

- AC5UP