Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?

On Tue, 08 Feb 2005 00:06:25 +0800, Dan Jacobson
wrote:

Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?


I have a multi-band dipole, similar to the fan-dipole. A couple of
weeks ago the rain coated my antenna and bridged the gap between
center and shield on my coax. My antenna went from matching 80, 40
and 20 to matching 12 and 17. This was short lived, but it lasted
long enough for my friend and I to get a good laugh.

Typically, without the ice, the weather shifts the center of resonance
on my antennas. Sometimes it kills the use of a band or in case of 80
meters, it shifts from the SSB to CW (or visa versa).

I threw this antenna up in a hurry and added to it off and on over
time. It was supposed to be temporary so I didn't use a balun or
weatherproof the so-called cobra-head, which is really only a piece of
PVC with three holes drilled in it and the center to one wire and the
coax to the other.

I also have a properly built 20 meter antenna where I ran the coax
into a PVC t-connector and the wires connect at points several inches
away so there is no water bridge and the coax stays dry.


--
73 for now
Buck
N4PGW

On Tue, 08 Feb 2005 00:06:25 +0800, Dan Jacobson
wrote:

Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?

Hi Dan,

Water is an insulator, and a very good dielectric. It would make for
a very good method for building high valued capacitors (displacing
inferior oil), but for one remarkably aggressive characteristic.

Water will disassociate many things (act as a solvent or simply as a
transport). As such it quickly degrades its dielectric characteristic
when in contact with nature. If you were to insert two probes to
measure its initial conductivity (presuming you start with distilled,
de-ionized water), you would find it easily in the megOhms if you
could measure that high in the first place. However, the simple
immersion of the probes (depending on their material and time
immersed) will lead to increasing that conductivity - such is the
aggressive nature of water. Nature is rarely that pristine, so to
dwell on de-ionized rain offers its own comedy of failed expectation.

So, that is the long way of saying you are on the greasy skids of
seeking an all-inclusive answer except to simply offer: don't trust
one. Decades of experience, however, yield practical answers. Look
at insulator materials and construction geometries. Largely, these
two are well tailored to shed water and increase surface area for a
fixed length. If you are worried about salt water, visit a marine
supplier who sells electrical/RF equipment (hard to imagine they would
by shy of examples). These rather simple items reveal how few find
water worrisome after their installation.

Now, if you want the warm snuggly feeling of data to two or three
places, the theory to back it up, and allusions to wave mechanics (be
they water or RF). Time and space will enlarge in this forum to allow
that easily.

73's
Richard Clark, KB7QHC

The short answer is that water won't cause any problem, and even salt
water probably won't (except corrosion).

When water bridges across a driven element or to the boom (not by any
means the same as the Earth), you've put a high value resistor and
capacitor across the driven element or from the element to the boom.
Pure water is a good insulator but acts also like a lossy capacitor at
RF; rain water on an antenna will have some dissolved minerals so that
reduces the shunt R. The feedpoint impedance of a beam is quite low, so
all you've done is put a high value of impedance across a low value. The
change is therefore very small. This holds for half wave dipoles,
quarter wave verticals, and most other common antennas, too.

A few antenna types have high impedances at the feedpoint, like an
electrically short whip, or a full wavelength dipole. Although I don't
know of any rigorous tests, I don't think even these are commonly
bothered much by water.

Roy Lewallen, W7EL

Dan Jacobson wrote:
Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?

The most common problem is if water gets into the traps (if your horizontal
antenna has them)

Some traps have drain holes and they should be oriented so the drain hole is
downwards

A friend had them upwards and the SWR would go way out of sight when it
rained, so he was off the air until the traps dried out.

We changed them, no problems even in a downpour


--
Caveat Lector (Reader Beware)
Help The New Hams
Someone Helped You
Or did You Forget That ?



"Roy Lewallen" wrote in message
...
The short answer is that water won't cause any problem, and even salt
water probably won't (except corrosion).

When water bridges across a driven element or to the boom (not by any
means the same as the Earth), you've put a high value resistor and
capacitor across the driven element or from the element to the boom. Pure
water is a good insulator but acts also like a lossy capacitor at RF; rain
water on an antenna will have some dissolved minerals so that reduces the
shunt R. The feedpoint impedance of a beam is quite low, so all you've
done is put a high value of impedance across a low value. The change is
therefore very small. This holds for half wave dipoles, quarter wave
verticals, and most other common antennas, too.

A few antenna types have high impedances at the feedpoint, like an
electrically short whip, or a full wavelength dipole. Although I don't
know of any rigorous tests, I don't think even these are commonly bothered
much by water.

Roy Lewallen, W7EL

Dan Jacobson wrote:
Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?

I've noted rain sometimes shift antenna tuning a little bit. But, no big
problem.

Dan,
It depends on how 'clean' the water is, but in general,
there shouldn't be much (if any) changes that can't be
compensated for (retune). the biggest problem is that it
makes you sound like your under water...(-)(o)
'Doc

I've had 2 Comet base loaded 2 meter mobile antennas degrade after a
year or so apparently due to condensation inside the so called sealed
base coil. I've now drilled a small weep hole near the bottom in the
second one and it seems to be drying out and working again.

There is no such thing as a sealed container outside. You need a
small hole to let the water out. Water will always get in by
condensation.

Mark