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eHam Forums => Elmers => Topic started by: 2E0FGO on May 17, 2019, 10:07:47 AM



Title: Operating in a light drizzle
Post by: 2E0FGO on May 17, 2019, 10:07:47 AM
Hi,

I have a home-made inverted V cut for 20m operation with about 20m of RG8mini coax down to my Icom 7200 running at 50w.  I get the centre of the V up at about 9.5m using a fibreglass pole.  I run this as a portable outfit and until today I've always used a local recreation ground and had reasonable results - breaking through pileups with 50w is a challenge.

This afternoon I tried operating from a new location in a field just south of Chelmsford, England.  As I erected the antenna there was a light drizzle.  Once it was up I checked the SWR with my AA-55 Zoom and got 1.09 at 14151 kHz and less than 1.5 across the band.  By the time I got to connect my 7200, the drizzle had stopped.  I then tried making calls.  The first contact, on the Italy Switzerland border couldn't get my call sign, and after several attempts we gave up.  I then managed a QSO with an operator further south in Italy.  He gave me 57 but I had to repeat everything several times and so I suspect he was being generous.  I then spent an hour calling CQ and responding to calls and got nothing.

When I packed everything away I noticed that there was moisture on the antenna feedpoint (one of those plastic T pieces with a built in SO-259).  Also, the paracords that runs from the end of the antenna wires to stakes into the ground were pretty wet.

* Given the conditions, did I get the expected result?
* Is it possible to operate in the rain?

Thanks and regards...Paul


Title: RE: Operating in a light drizzle
Post by: WB6BYU on May 17, 2019, 10:54:17 AM
Yes, it is possible to operate in the rain - I've done it many times, especially when portable.
I operated one Field Day with the rig in a plastic garbage bag and a fiberglass hardhat over
the tuner.

But you do have to make sure your antenna is up for it.  My dipole insulators have the coax
directly attached:  the coax is coated with some sort of sealant to keep rain from soaking
into the braid (and the the center insulator for foam coax like RG-8X:  the foam acts like
a sponge.)  But water in the coax is more of a long term problem (corrosion) than immediate
high SWR, as the most common symptom is increased losses, which lower the SWR.

At the other end I just tie a loop in the end of the antenna wire and tie a synthetic rope
to that.  Works great with insulated wire.  With bare wire you may need some extra
insulation if your rope gets waterlogged.  (That hasn't been a problem with plastic baling
twine.)  But rainwater shouldn't be very conductive (although there is some ionization due
to carbon dioxide picked up on the way down).

Rain can change the velocity factor of solid-dielectic twinlead, which shifts the impedance
at the rig end and can affect the SWR.  You can reduce that with a coating of automotive
wax.


The first question would be, did you actually check the SWR on the antenna while you were
in the field and it didn't seem to be working well?  That's easy to do if your rig has a meter.
I've never had such an issue with my backpack dipole kit, even though I've used it through
a number of major rain storms.


Title: RE: Operating in a light drizzle
Post by: WA3SKN on May 17, 2019, 03:21:17 PM
Yes, you can operate in the rain.
But the first question is did you check the SWR after everything got wet?

-Mike.


Title: RE: Operating in a light drizzle
Post by: AA4PB on May 17, 2019, 07:27:00 PM
Many transmitters (without a tuner) will start reducing the output power when the SWR reaches 2:1 or more. The ends of a dipole are very high impedance and water droplets or a wet insulator can lower the resonant frequency of the dipole, causing the SWR to increase at the original operating frequency. I think the poor Tx signal is due to the power reduction rather than additional loss in the antenna, so a tuner should correct the problem by providing a low SWR at the transmitter. I experience this problem quite often with my 6M Yagi in rain, especially when the Wx is cold and the cold metal starts to ice up a little.


Title: RE: Operating in a light drizzle
Post by: K0ZN on May 17, 2019, 09:20:45 PM
It is not at all impossible that HF propagation conditions changed on you. It CAN happen very quickly and sometimes dramatically.

I would strongly recommend a "real" (glass, ceramic or hard plastic end insulator... with ceramic or glass preferred as they shed and bead water better; 98% of all commercial/marine/military antennas use ceramic or glass end insulators; there is a reason for that!)  between the wire and the rope. The end of the dipole is the high voltage point and a wet support rope, while being an "insulator" material, gets dirty and that mixed with water can be somewhat conductive/lossy.

If the ground under your antenna goes from dry to wet (or visa versa) that will impact the antenna's feedpoint resistance and/or SWR by some
minor to modest amount depending upon soil conditions.

Bottom Line: rain is not a big issue, but it is normal to see some minor changes in SWR in a properly constructed antenna.

73,  K0ZN


Title: RE: Operating in a light drizzle
Post by: KE2KB on May 19, 2019, 09:20:49 AM
You probably wouldn't notice the change in SWR due to a wet antenna while using an antenna analyzer to measure it. The AA puts out such a weak signal, you will never get enough voltage at the ends of the antenna (or anywhere else) to cause a problem with a slight amount of conductance.
The only method to read true SWR while transmitting is an in-line meter; either built into the radio, or in the transmission line.


Title: RE: Operating in a light drizzle
Post by: 2E0FGO on May 20, 2019, 12:36:57 PM
Thanks all.  I'll try some of the suggestions.  I have some plastic insulators.  I'll try those and maybe pick up some ceramic or glass ones.  I'll try checking the SWR once the kit is wet too.


Title: RE: Operating in a light drizzle
Post by: W1VT on May 20, 2019, 01:04:24 PM
On the 10GHz microwave band rain can create band openings around obstructions like trees and mountains,

http://www.wa1mba.org/10grain.htm
http://www.wa1mba.org/scatter.wav
what it sounds like

I've experience several "band openings" created by rain.   ;D


Title: RE: Operating in a light drizzle
Post by: G4AON on May 21, 2019, 09:04:14 AM
Glass insulators are better than plastic ones for long term deployment, with one exception... if you have a high antenna and something snaps, glass insulators may cause more damage if they fall onto something.

However, for portable antennas and 50W, I would just use those rectangular “corner” blocks you find in hardware stores, the type with three screw holes to mount wooden panels at 90 degrees. I use them on ground plane radials, neat and cheap.

73 Dave