Re: Newbie Questions...

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From: John J. McDonough (wb8rcr@arrl.net)
Date: Sat Jun 23 2001 - 09:35:14 CDT


Dennis

I noticed I didn't answer the antenna part of your question, and
surprisingly, nobody else did, either.

Like the rig, there is no absolute answer to the antenna thing. A lot of
folks seem to look for a silver bullet. The antenna depends on what
resources you have in space, time, and money, as well as what you want to do
(surprise, surprise).

First of all, there is almost nothing better than a resonant dipole that's
high enough and in the clear. Other than a multielement antenna of some
type, anything else is a compromise.

In the clear means away from conducting structures. Between some aluminum
sided houses would be a problem. In the attic of a frame house without
insulation in the roof isn't so bad. Trees are noticeable, but not all that
bad.

High enough, though, is another of those "it depends" kinds of questions. A
dipole works "best" when it's electrically distant from the ground, say a
half-wavelength or more. On 20, this is a pretty reasonable thing to do.
On 80, practically impossible for most of us.

But even "best" is somewhat subjective. A dipole a wavelength up will have
a very low angle of radiation, almost horizontal. If you are interested in
working DX, this is exactly what you want.

On the other hand, maybe you are mostly interested in working people around
the state. In that case, you want your radiation at a sharper angle, so the
ideal case isn't what's best for you.

Happily, these things work reasonably well. The low angle of radiation is
fairly easy to achieve on 20, and 20 is hardly ever open to steep angles, so
if your band is 20, get that antenna up if possible, but for 20, "up" is
only about 30 feet - most of us can get close to that.

On 80, the band is almost always open to short skip, and the noise and
absorption are such that shorter contacts are a lot easier to achieve than
longer ones. So on 80, a higher angle of radiation is usually a better
thing, and it also turns out to be fairly easy to do, by putting your
antenna well below 130 feet.

You indicated limited space, but you didn't elaborate on what that means.
An attic dipole can do quite nicely on 20, and it's short enough that most
of us can fit it, or at least come close. Sometimes it takes a bit of
thought to get it clear of wiring, ductwork, etc., but people living in
private houses can generally manage it (not so apartment dwellers,
obviously).

Probably the next best thing to a dipole is a vertical. Verticals, however,
are somewhat tricky. They depend a lot on the sort of ground you have, and
generally you have no real way to test that other than to try. Depending on
your ground situation, you may need a fairly extensive radial system, which
may or may not be more of a problem than a big antenna, depending on your
situation. For some types of ground, it seems like no number of radials is
sufficient to make it work well. For others, just a ground rod is adequate.
Verticals are a common strategy for folks dealing with antenna covenants.
Once another spring passes the radial system disappears, and the antenna
itself can be hidden in a plastic flagpole.

A vertical has several advantages over a horizontal antenna for QRP
operation. Maybe not the most effective, but the least frustrating.
Generally, man made noise tends to be better received by a vertical, so the
noise level is higher, burying weaker signals. On the other hand,
interfering structures tend to be vertical, thus disrupting the vertically
polarized transmissions less than horizontally polarized emissions. The
result is that you can generally work anything you can hear with a vertical,
which makes QRP operation somewhat less frustrating. The catch is you don't
hear as much.

Any multi band antenna is a compromise, and while there are better antennas
than the simple, resonant kinds (beams, collinears, etc) they are all larger
if they are to be more effective. If you need a multiband antenna, the best
move seems to be to just get as much wire in the sky as you possibly can,
feed it with balanced line (lower loss), and use some sort of matching
network.

A word about balanced line. This stuff is a real pain, and although many
people swear by it, it's not a silver bullet. Balanced line is good if the
SWR at the antenna is horrid, and you can't put the matching network at the
antenna. Unbalanced line (coax) is much easier to work with, and at HF, the
loss is invisible, unless the SWR *at the antenna* is awful (5:1 to 10:1
depending on the band and length of run). However, if you have a random
wire that you are feeding on many bands, you can't put a fixed network at
the antenna, and the game of remote control of some sort of remote antenna
tuner becomes more of a pain than the balanced line, so this is a popular
approach for random wire antennas used for many bands.

You can also shorten a dipole or vertical with inductors or other
strategies, but these all reduce the effectiveness of the antenna. Still,
getting a shortened dipole up is a lot better than no antenna at all. At
the end of the day, you want as much metal in the sky as possible, and
usually as high as possible, and resonant beats nonresonant hands down.

Hope this helps...

72/73 de WB8RCR http://members.home.com/wb8rcr/index.htm
didileydadidah QRP-L #1446 Code Warriors #35

----- Original Message -----
From: "Dennis Ponsness" <wb0wao@hotmail.com>
To: "Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>
Sent: Friday, June 22, 2001 7:41 AM
Subject: Newbie Questions...

>
>
> OK... I am a "newbie" at the QRP game, so i have a couple questions that i
> would like to get some input on from you seasoned QRP ops..
>
> 1. What would you suggest for my "first" kit built QRP rig.....
>
> 2. What would be a good "limited space" antenna for QRP operation...
>
> TNX es 72
>
> Dennis WB0WAO
> _________________________________________________________________
> Get your FREE download of MSN Explorer at http://explorer.msn.com
>


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