Back Into Hobby: Some Basic Questions, Please
 

Hi,

Have finally gotten back into hobby, now that I am retired.

Have been reading the ARRL books, a bit, again, and realize I'm still quite
confused over the antenna pattern diagrams they show for the various types.

a. What is plotted in these diagrams; the H field, the E field, or the
resultant of the two ?

b. When they talk about field strength, what do they mean ? Again, the H
field, or... ?

c. Have a receive-only antenna running around the 4 sides of my attic. Open
at the end.
Should I tie the end to the beginning, making a horizontal loop up
there, or not ?
Why ?

d. Do listening only, from 0 to 30 MHz, and from all compass points.

Would a straight run of wire along one side (thus foresaking the
lengths provided by the other 3 sides)
gain me anything, or is the square loop configuration probably the best
overall compromise
for pulling stuff in ?

e. Tried to find some but couldn't.
Any pix/diagrams on the web showing the lobe patterns as a function of
frequency for:

a single random-length wire
a 4 sided loop, open at the end, like I now am using
a 4 sided loop, closed at the end

Thanks,
Bob

Hi Bob

Might be worth downloading and playing with 4NEC2 from
http://www.si-list.org/swindex2.html. This will allow you to model
antennas and check radiation patterns (incl E vs H) with and without
ground and other objects influence. Its GPL.

Also have a look at http://www.cebik.com for a stack of info on modelled
antennnas in the amateur service.

Cheers Bob VK2YQA

Robert11 wrote:
Hi,

Have finally gotten back into hobby, now that I am retired.
--snip--

On Fri, 11 Feb 2005 13:44:48 -0500, "Robert11"
wrote:

a. What is plotted in these diagrams; the H field, the E field, or the
resultant of the two ?

Presumably E Field.

b. When they talk about field strength, what do they mean ? Again, the H
field, or... ?

That should be expressed in the units consistent with H or E. If no
units are expressed, then it is usually a comparison with dB, dBi, or
dBd.

c. Have a receive-only antenna running around the 4 sides of my attic. Open
at the end. Should I tie the end to the beginning, making a horizontal loop up
there, or not ? Why ?

Yes(No). It is (is not) important.

d. Do listening only, from 0 to 30 MHz, and from all compass points.

-Oh- Well that changes everything!

Would a straight run of wire along one side (thus foresaking the
lengths provided by the other 3 sides) gain me anything, or is the square loop
configuration probably the best overall compromise for pulling stuff in ?

This changes even more!

e. Tried to find some but couldn't.
Any pix/diagrams on the web showing the lobe patterns as a function of
frequency for:

a single random-length wire
a 4 sided loop, open at the end, like I now am using
a 4 sided loop, closed at the end

Hi Bob,

The same geometry at all the different frequencies you propose do not
yield the same lobe patterns. That is your first lesson.

You express the desire to receive from all 360° which will certainly
happen for many of the designs you describe above, but only through
happenstance and being in the attic (which means close to ground, as
in earth, not wood/stone/mortar).

What you want to concentrate on is keeping noise out of your antenna
(this will probably be answered in the course of the thread).

The long and short of it is without spectacular care, those lobes are
rarely observed unless you have your antenna in the air, and more than
a quarter wavelength above ground - difficult to do with your
from 0 to 30 MHz,
....Quarterwave of 0MHz is pretty high up.

A simple dipole is handy, so is a loop, and so is an open loop, as is
a vertical, or phased elements - but only with respect to their size
at any particular frequency.

Your basic question is far more complex to answer than any complex
question which often resolves to a simple yes/no.

73's
Richard Clark, KB7QHC

Robert11 wrote:
Hi,

Have finally gotten back into hobby, now that I am retired.

Have been reading the ARRL books, a bit, again, and realize I'm still quite
confused over the antenna pattern diagrams they show for the various types.

a. What is plotted in these diagrams; the H field, the E field, or the
resultant of the two ?

Either one. Once you get more than a fraction of a wavelength from an
antenna, the E and H fields are in a fixed ratio (E/H = 377 ohms). So
plots of the two are identical, except for the 377 ohm scale factor. The
pattern is more often expressed in dB relative to some reference (most
often to an isotropic source, expressed as dBi). This numerical value is
then the E field, H field, or power density far from the antenna
relative to the reference. The pattern shape and dBi value of all three
are the same.

b. When they talk about field strength, what do they mean ? Again, the H
field, or... ?

Field strength technically means either the E field (in volts/meter) or
H field (in amps/meter) but, as I said above, once you know one you can
easily calculate the other unless you're very close to the antenna.
Pattern plots are nearly always for the far field, where E and H have a
fixed ratio and the pattern shape is the same regardless of distance. In
the near field (which isn't of any interest for your application), the
pattern is different for each distance, and the ratio of E to H isn't
fixed. Therefore, near field data are usually given in tabular form.


c. Have a receive-only antenna running around the 4 sides of my attic. Open
at the end.
Should I tie the end to the beginning, making a horizontal loop up
there, or not ?
Why ?

Tying the end might change the pattern enough to make a difference in
your ability to receive, depending on the directions of the signals you
want to hear and of incoming noise. There's no single answer for
everyone. You'll simply have to grit your teeth and experiment.

d. Do listening only, from 0 to 30 MHz, and from all compass points.

Would a straight run of wire along one side (thus foresaking the
lengths provided by the other 3 sides)
gain me anything, or is the square loop configuration probably the best
overall compromise
for pulling stuff in ?

See the above answer.


e. Tried to find some but couldn't.
Any pix/diagrams on the web showing the lobe patterns as a function of
frequency for:

a single random-length wire
a 4 sided loop, open at the end, like I now am using
a 4 sided loop, closed at the end

There are a number of free antenna modeling programs that will help you
answer your questions. Just keep in mind that for HF receiving, gain in
the desired direction is of no consequence by itself. The only thing
that will help you receive better is to have a pattern that has more
gain in the direction(s) of the desired signals than in the direction(s)
which noise comes from. If the signal and noise come from the same
direction, there's nothing you can do to improve your ability to hear
signals. In that case, more gain will make both the signal and the noise
louder in the same proportion. You can get the same effect a whole lot
easier by turning up the volume control.

Roy Lewallen, W7EL