[RFI] G5RV opens/closes garage door

Blackburn Blackburn" <blackburn@qnet.com
Thu, 5 Nov 1998 21:31:44 -0800

Most problems such as this are due to common mode RF coupling into
susceptible equipment, i.e., conductors such as power lines, pushbutton
wiring, etc., act as antennas and conduct RF current into the equipment. 
Unless the susceptible equipment is very close to the feedline, high SWR is
not likely to be a contributor, particularly for a balance antenna/feedline
combination like a G5RV (by the way, the original G5RV design was designed
for 20 meters, and was never intended to be used on other bands without a
matching device at the transmitter, but that's a whole different
discussion).  Harmonic content of your transmitted signal is also not
likely to be a contributor unless there is a relatively sharp resonance
somewhere in the opener or lamp wiring that happens to fall on the
harmonic.  Since you don't indicate the problem occurs only on one band,
and more than one device is affected, it's probably not related to a
resonance in the susceptible installations. 

There are basically two approaches to solving such a problem, both aimed at
reducing the RF power induced into the susceptible electronics.  The first,
which is inherently repulsive to most hams, is to reduce transmitted power.
 The second is to break the coupling path between your antenna and the
susceptible electronics.  One way, of course, is to relocate your antenna
to increase the distance between the antenna and the offending items,
thereby reducing the field level at the susceptible devices. 
Unfortunately, for most hams on city lots this is not much of an option.

This leaves us with attacking the coupling path at the susceptible devices.
 The idea here is to keep RF out of the electronics, either by providing a
low impedance path around the susceptible electronics, or by introducing a
high impedance in series with the RF path, or both.  I am presuming you are
not interested in making internal modifications to the opener or lamp,
since they don't belong to you.  

In my case, only the opener equipped with a remote keyless entry was
affected.  First, the combination of pushbutton and remote entry wiring was
acting as a bent dipole antenna with the opener electronics at the
feedpoint.  Bypassing the pushbutton and remote entry wiring with 0.1 uF
disk capacitors connected between each wire and between each wire and the
opener chassis eliminated problems from this path.  Keep the leads as short
as possible to avoid potential resonance effects on the higher bands. The
remote keyless entry electronics were also being directly affected by RF. 
This path was broken by a common mode choke (abt 8 - 10 turns of the
twisted pair wire wrapped through a large ferrite bead) where the wiring
entered the keypad.

The approach of identifying potential RF coupling paths and then either
breaking the path or bypassing the induced RF around the susceptible
electronics is applicable to almost any RF susceptibility problem not
associated with coupling into an intentional receiver front end.  In the
case of a garage door opener, the first thing I would try is common mode
chokes on the button wiring and the power line, since these mods can be
made without affecting the operation or warrantee of the opener.  If the
opener is equipped with a photocell safety device, you will have to treat
this wiring also.  Common mode chokes with reasonable effectiveness at HF
frequencies can be constructed from split cores available from MFJ, AES,
Ham Radio Outlet, etc., or from large ferrite beads sold by companies like
Amidon (I don't remember if Radio Shacks sells cores or not).  Wrap at
least 8 - 10 turns of the wiring through the core....it takes a (relatively
speaking) lot of inductance to have any effect at HF, and a single turn
passed through the bead just doesn't cut it unless you want to stack up a
whole bunch of them in series.  Common mode chocks for power lines can be
made by wrapping the power line around a ferrite rod (fill the length of
the rod with close spaced turns) and taping or ty-wrapping in place.

If this isn't sufficient, you'll have to look at adding some bypassing. 
I've never heard of a garage door opener being affected by capacitance on
the pushbutton lines (you're essentially dealing with DC at this point),
but you never know.  Proceed with caution if it isn't your opener.

A similar approach might help with the clap lamp.  Start with the power

Be aware, though, that external fixes don't always work.  Any unshielded
conductor immersed in an RF field will pick up some energy, which depending
on the circuit may or may not be sufficient to cause an undesired response.
 Without modifying the circuit itself, or enclosing the whole thing in a
shield, the best you can do is try to keep as much RF as possible out of
the device and hope.

Hope this helps.  

73,  Keith   AC6SN

> From: Frank <ftbrady@earthlink.net>
> To: rfi@contesting.com
> Subject: [RFI] G5RV opens/closes garage door
> Date: Monday, November 02, 1998 4:38 AM
> My brand new G5RV installation opens and closes neighbors remote
> controlled garage door.  Every dit/dah sends the door in the opposite
> direction.  It also assaults another neighbors hand-clap-controlled
> lamp.  The garage is about 35' from the ladder line stub.  The
> hand-clap-lamp is about 50' in the opposite direction.  Needless to say,
> my hamming debut in this neighborhood was a flop.
> This G5RV has the highest SWR I've seen (5 to 1 on 40 meters & 3.5 to 1
> on 20 meters), but I've heard that's just the way they 'work' in some
> installations.  This G5RV (from RadioWorks) has a current balun between
> the 30' 450 ohm ladder line stub and the 70' coax feedline to the rig.
> Questions:
> Is the G5RV part of the problem - with it's high SWR?  The ladder line
> is in the clear, drops straight down from the 102' flattop, and the
> balun is about 4 feet off the ground.
> Can anyone help me with these two problems or tell me how to access
> archives if the solution has been discussed already?
> Thanks much!
> Frank T. Brady
> W0ECS - in Appleton WI indefinitely
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