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Author Topic: Antenna directly over the roof - RF  (Read 2773 times)

Posts: 112

« on: January 18, 2013, 08:45:40 AM »

I'm pretty limited on where I can put an antenna at the new QTH, as the house takes up quite a bit of the lot.

Last night, I finally got some time to put up a simple 20M dipole over the roof, just to see how things would go.

My shack is on the 2nd floor, which was an attic converted to some extra bedrooms and a bath, and the space I've claimed as my ham shack.  The dipole was directly over the shack, and laying on the composite shingles.  Yes, I expected some RF, but, I got more than I anticipated.  At about 20 watts and above, the RF turns on the inducer fan for my HVAC unit upstairs, which is in the mechanical closet next to my shack.  I also noticed that it was turning on some lights downstairs with the touch-on controls.

My initial thought was to make a cobweb antenna, and place it just above the roof on the 2nd floor (mainly because of it being the highest point on the lot), but, does it look like that I'm going to be running into the same RF problems by having an antenna directly over the roof?

If I put the cobweb antenna further down the roof line, it will get it further away from the HVAC and shack, but, it's still going to be directly over some kind of electronic equipment and/or bedrooms.

My only other option is going to be putting it directly over the roof of the storage room, but, it's only about 15ft above ground, and will be set lower than half the house.

Any magical distance above the roof that will cut out the RF into the house?

Posts: 21764

« Reply #1 on: January 18, 2013, 08:51:39 AM »

What prevents simply going up "higher?"

Distance solves a lot of problems.

Even with a dipole or a Cobweb, or whatever, a few masts to support it 20 feet above the highest part of the roof would likely work wonders in RFI minimization or elimination, while allowing the antenna to work better.

Posts: 17483

« Reply #2 on: January 18, 2013, 08:55:25 AM »

A good start would be to understand how the RF is affecting the controls.

First, are you using a balun on the dipole?  Does the feedline run off at right
angles to the antenna?  If the feedline isn't adequately decoupled from the
antenna, then you get RF flowing back down the outside of the coax and
into anything connected to it - often including the ground side of the AC
line.  Sometimes that route causes the worst interference, and many AC
line filters don't filter the ground lead.  You can check that by operating
the radio off a battery without any connecting to the AC wiring.

Another common source of RF interference is control lines that act as an
antenna.  For example, if you have a thermostat connected to HVAC via
a long wire.  In that case, adding a bypass capacitor and/or ferrite filter
to the wire may be sufficient to eliminate the interference.  (I've seen
this effect both with garage door openers with a wired remote switch,
and stereo systems where the speaker wires were resonant on 40m.)

Touch lamps are always a problem because of the way they work:  they
have an oscillator and look for a change in the RF level to indicate that
someone has touched the lamp base.  Unfortunately they can't tell the
difference between that and the RF change when you key up the rig.  The
best solution in that case is to relocate the lamp to a local landfill.

Posts: 9930

« Reply #3 on: January 18, 2013, 10:51:48 AM »

My sprinklers were comming on in time to my CW keying. a small (.003 mfd ?) capacitor across the 18 v input at the controllor fixed it.

Posts: 112

« Reply #4 on: January 18, 2013, 09:09:33 PM »

I decided that while I had the nice weather outside this afternoon, and that I already had the wire cut, I'd string a 20M droopy loop on the roof to see what the difference would be.  I fed the loop with ladder line, inside to a 4:1 balun, then through the wall to 3' of coax, then to a tuner.

Oddly enough, on 20M and up, I have absolutely no RF in the house, even at 100watts tx.  No rf in the radio, no HVAC issues, no wife coming upstairs telling me the blender is freaking out...

This is a MUCH better improvement so far.  I'll continue to play with the loops.
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