Buck wrote:
On Mon, 21 Feb 2005 03:55:43 -0800, Roy Lewallen
wrote:


It's not clear to me how you correlate how warm something gets in the
microwave oven with how much loss it'll incur at the end of an HF
antenna. Maybe someone can explain this.

Roy Lewallen, W7EL


The microwave works with RF to heat the 'food'. If an object absorbs
RF, it does so as heat, therefore, if you subject it to intense RF by
using a microwave oven, it will heat up. If it heats up, it is
affecting the antenna's radiation.

No, that's not correct. If you heat up an object in the microwave oven,
it indicates that an object of that material, size, and shape absorbs
some fraction (how large a fraction, you have no idea) of energy when an
intense microwave field is applied. It says nothing about what that
object will do when put at the end of an HF antenna, where it isn't
exposed to an intense microwave field.

Depending on how it is connected to the antenna and type of antenna,
it may not make a noticeable difference on the signal, but in many
cases, especially where the wire is running along with or inside of a
material such as PVC, the material may have a detrimental impact on
the signal or SWR.

Why yes, it might, or it might not. This can be said of any material.

Not being aware of this a long time ago, I built a two meter half-wave
vertical dipole by dropping a line of wire into a piece of CPVC and
capping it off. My mobile 5/8 wave whip on the car, twelve feet below
it, using the same rig has much much better reception and signal than
the dipole. I plan to replace that antenna with a similar dipole using
1/2 inch copper tubing.

When I nuke the cpvc for one minute, it gets warm to the touch. I
don't expect to melt it using my 706, but I do have a couple of
repeaters I can't hear from that antenna that I would like to reach.

That's an interesting anecdode, but it hardly supports the contention
that there's a direct correlation between heating in a microwave oven
and detraction of performance of an HF antenna when used as an end
insulator.

Roy Lewallen, W7EL