Antennas, metal objects and HF reception
 

I was reading the instructions that came with an LF Engineering H-800
active antenna earlier today. They caution against mounting the antenna
in the vicinity of metal objects. At the same time, they say it's fine
to mount the antenna on a metal mast. What's the explanation of this?
If metal objects will interfere with reception, why doesn't this hold
true of metal masts?

And more generally, I'm curious about the extent to which relatively
small metal objects interfere with reception. In my (admittedly
limited) experience, so long as an outdoor antenna is at least a few
feet away from metal objects (e.g., gutters, ventilation pipes, etc.),
their effects on reception are minimal. Is this correct? I don't know
the physics, and other may have had experiences that differ from mine.
So you tell me: Are metal objects bigger problems than I think?

Steve

God knows what they're talking about..

Mess around with it !

See where it gets the best reception / least interference

then get a long peice of closet pole

coat it with weather resistant paint

& mount it in that place, on the closet pole

& then

take your wife out somewhere nice for dinner

~H~

I wasn't actually looking for a place to mount the H-800. I was just
walking around with it, trying to find quiet zones on my roof.

Steve

sdanie...@ wrote:

I was reading the instructions that came with an LF Engineering H-800
active antenna earlier today. They caution against mounting the antenna
in the vicinity of metal objects. At the same time, they say it's fine
to mount the antenna on a metal mast. What's the explanation of this?
If metal objects will interfere with reception, why doesn't this hold
true of metal masts?

And more generally, I'm curious about the extent to which relatively
small metal objects interfere with reception. In my (admittedly
limited) experience, so long as an outdoor antenna is at least a few
feet away from metal objects (e.g., gutters, ventilation pipes, etc.),
their effects on reception are minimal. Is this correct? I don't know
the physics, and other may have had experiences that differ from mine.
So you tell me: Are metal objects bigger problems than I think?

Steve
---------------------------
That depends....

An active antenna mounted on a small metal mast, or even a Rohn 25
TV style tower will experience minimal effect from the mast.
But mounted down near aluminium siding, or near a metal awning, near
metal roof will experience some effect. Well grounded metal should be
OK. But metal that is more or less floating, like aluminium siding,
will pick
up noise(any unwanted signal) and reradiate it into the antenna.

The best rule of thumb is far from any lines, electric, telephone, or
cable.
Even being over conductive pipes like water or NG can introduce noise.
I have my Amrad mounted up high on a shingled roof with a rat wire,
AKA hardware cloth, ground underneath. Works great.

Same thing for loop antennas or even wire antennas.

High and clear is the best.

Some may swear the "Snake" antenna is a "great thing". While I guess
it may have a place, it is about as far from a acceptable antenna as I
can think of.

High and clear.

With a good ground.

If I had to choose between a less then ideal antenna, say ~30'
unmatched
direct feed without coax with a good ground, and a great antenna with a

"so so" ground, I would go with the marginal antenna and good ground.

Even with a great active antenna like the AmRad, you still need the
best
ground you can get. Improving your ground will give a better return for
your time and money then anything else.

Terry

Thanks. Part of what I'm trying to do is figure out where to locate a
random wire on the roof of my building. It won't be my only antenna,
since I have a Wellbrook loop that I'm happy with. However, I like to
experiment with wire antennas and a wire would be a nice complement to
the loop.

The metal objects I'm primarily concerned with are ventilation pipes
that come up through the roof of the building. Any random wire up there
is going to get within at least four or five feet of at least one or
two of these pipes. I've done some trial experiments with 100 ft
lengths of wire, and didn't notice and detrimental effects of this, but
I'm always curious about what others have found.

Another concern is the paint that coats the surface of the roof. It's
the shiny aluminum paint that's used to keep buildings cool in the
summer. I don't know how much of an effect that can have. I lose a lot
of signal and gain a lot of noise if I let a length of wire drop down
directly onto the roof, but this is probably due more to the wiring and
appliances inside the building than the aluminum paint on the roof. The
noise of course drops way down when the wire is up 7 or 8 feet in the
air.

I have the L-400B LW antenna and what they mean about the metal mast is
that as long as the mast is parallel to the first few inches of the
antenna at the bottom where you would clamp it you will be OK. But
don't let the middle to upper part of the antenna be close to metal
objects. I found that even 20' proximity to lush trees had a big
impact on the L-400B reception on LW. As long as I kept it in the
clear of metal or wet objects it performed well. If no near objects
were present, mounting any higher than about 10' made no difference.

Frank
Tucson

There have never been but two 007's worth a da.n before.Sean Connery and
George Lazenby.roger moore always looked just like a sissy stuck up
P...Y!
cuhulin

Steve,