RadioBanter - View Single Post

Roy Lewallen · March 28th 09, 06:09 PM posted to rec.radio.amateur.antenna

wrote:
I am doing some design with vertical antennas for the low bands
40/80/160M. I require a somewhat large reactance on the element to
help load it on 160M, (40&80M) are working somewhat satisfactory now
with the design i am using. The required reactance that i need and the
size it would be using a coil (with the materials that i have right
now) may not last a winter storm in VO1 land when loaded with ice or
snow in 90km/h wind, so my question is, has anyone had experience
using a toroid wound with the correct amount of reactance installed
on the element to to achieve resonance instead of a coil and how it
has been for them.

In most applications you'd be concerned about the inductor Q, since
higher Q means less loss for the same inductance. You probably won't be
able to get as high Q with a toroid as you will with a good air core
inductor.

But Tom, W8JI pointed out that with mobile antennas on the low frequency
bands, the ground loss is so much greater than loading inductor loss,
the latter isn't usually an important factor. It'll be difficult to get
really low ground loss with a fixed antenna, too, so a toroid should
work just about as well in practice unless you have a very good ground
system.

You don't, though, want the Q to be outrageously low. And that can
happen if water gets between the turns. (That's also true of an air
wound inductor.) So I recommend putting it into a container or coating
it to keep that from happening.

I built a toroid-loaded quarter wavelength (half length) 40 meter dipole
for Field Day, and measured the gain relative to a full size dipole. The
loss due to the inductors was less than a dB.

Expect a very narrow bandwidth if the loss is low.

Roy Lewallen, W7EL