Longwire antenna. Coil in parallel with var cap for matching circuit.

What is the likely Z range such a circuit might match?

"RB" wrote
Longwire antenna. Coil in parallel with var cap for matching circuit.

What is the likely Z range such a circuit might match?
-------------------------------------------------

Easy! Just say how long is the wire, at what height, at what frequency, and
the impedance of the ground connection, and lo-and-behold, all will be
revealed by a calculation which will involve arithmetic. And arithmetic is
an unwelcome subject on this newsgroup.

Not that the resulting value of Z = X + jY will be of much use to you. What
plans do you have to do with it?

In a fraction of the time expended in vague questions and obtaining even
more vague replies, why not erect the wire, as you propose, and connect the
capacitor in parallel to see what happens.

But you may save even more time by connecting the capacitor in series.

Good luck with your experiments.
----
Reg.

"RB" wrote
Longwire antenna. Coil in parallel with var cap for matching circuit.

What is the likely Z range such a circuit might match?

-------------------------------------------------

To put you in a good-enough ball-park, the input impedance of a vertical
wire, or an inverted-L antenna, is R + jX, where -

R = Square( 24 * L / Lambda ) ohms.

X = - 550 / Tangent( Angle ) ohms,

and where

L = overall length of the wire in metres.

Lambda = free-space wavelength in metres.

Angle = 2 * Pi * L / Lambda, in radians.

Or alternatively,

Angle = 360 * L / Lambda, in degrees.

For a thick lattice mast, change 550 to 350.

If you know what it is, add the ground electrode resistance to R. Otherwise
make a guess.

The foregoing good-enough approximations apply to overall wire lengths or
heights of 3/8 wavelengths or shorter.

Note that when L / Lambda passes through 1/4-wavelength, the reactance X
passes through zero and changes from negative (capacitative) to positive
(inductive). And R passes through 36 ohms as is common knowledge.

About the only practical use of the above pair of equations is if you intend
to follow by designing your own tuner, coil and capacitor values. Or
checking that a commercial tuner has an adequate range of operation. For
this a small computer program is needed unless you have hours to spend on
doing the unavoidable calculations.

But the real value of such simple equations lies in their ability to
precisely and very succinctly describe how antennas WORK. Far, far better
than thousands of vague words, spread over years, from agumentative,
disruptive, albeit well-intentioned do-gooders.
----
Reg, G4FGQ

Dear (I really mean it) Roy,

For my (unstated) benefit, you semi-seriously describe yourself as an 'old
wife'.

Now, I have NEVER described anyone as being an 'old wife'. I am too polite.
Although, I must admit, I do occasionly infer that such people do exist.
They are not objectionable

If you think the cap fits, then wear it. (Old English proverb.) But you
don't!

But for the benefit of anyone mildly interested, who may be unfamiliar with
the goings-on on this newsgroup, YOU are the VERY last person who I would
remotely associate with being an 'old wife'.

Just keep yourself familiar with 'arithmetic' and 'maths' will look after
itself. ;o)

History, Arithmetic and Elementary Statistics are the most neglected
subjects in English primary schools. They are easy and simple enough and
occupy little time. But what can be expected from ill-educated teachers? I
suspect the same applies in the USA.

I understand school teachers in the USA wear bullet-proof jackets. But, in
general, I am restricted to the BBC and the largely US-controlled unruly
English newspapers which concentrate on swasticas and the royal family,
nudes, the sexual habits of film-stars, cat-walk celebraties and overpaid
Manchester United football players. After all, everybody has a right to
make a living of some sort!
----
Just another advert from Reg, G4FGQ

On Tue, 18 Jan 2005 17:37:53 +0000 (UTC), "Reg Edwards"
wrote:

and overpaid Manchester United football players

Hi Reggie,

Just can't get Yanks (their goalie) off the radar screen, hmmm?

73's
Richard Clark, KB7QHC

RB wrote:
"What is the likely Z range such a circuit might match?"

A coil in parallel with a variable capacitor can be tuned to offset
reactance in the antenna. At resonance, a parallel circuit with a high Q
has a very high impedance.

Edmund A. Laport in "Radio Antenna Engineering" writes on page 310:
"Experiment has shown that the correct termination of a long-wire in
this manner (low-characteristic impedance wire to maximize antenna
current) requires a complex impedance and not solely a resistance. The
terminal impedance is of the type R+jX. The inductive reactance can be
obtained from the extension wire by making it longer than one-quarter
wavelength. This means that the correct resistance, obtained by
experiment, must also be located in the correct place along the wire,
and that the correct location is more than one-quarter wavelength from
the end of the wire. The resistance has an order of magnitude of 400 to
600 ohms in many cases, and 500 ohms is suggested as a starting place
for trials when the antenna consists of a single wire. The reactance
values required usually should be between j150 and j250 ohms, so that
with thin wire, the extension wire whould be somewhere around 105
degrees long."

Best regards, Richard Harrison, KB5WZI

I thank you very much for the kind words.

But on more than a few occasions, I've found that I firmly believe
something you have said with an air of confidence, even certainty, that
is a belief only of "old wives". Therefore it's an inescapable
conclusion that I am one.

Don't fret -- the title of "Reg's old wife" is one I wear with pride, if
a modicum of bemusement.

Roy Lewallen, W7EL

Reg Edwards wrote:
Dear (I really mean it) Roy,

For my (unstated) benefit, you semi-seriously describe yourself as an 'old
wife'.

Now, I have NEVER described anyone as being an 'old wife'. I am too polite.
Although, I must admit, I do occasionly infer that such people do exist.
They are not objectionable

If you think the cap fits, then wear it. (Old English proverb.) But you
don't!

But for the benefit of anyone mildly interested, who may be unfamiliar with
the goings-on on this newsgroup, YOU are the VERY last person who I would
remotely associate with being an 'old wife'.

Just keep yourself familiar with 'arithmetic' and 'maths' will look after
itself. ;o)

History, Arithmetic and Elementary Statistics are the most neglected
subjects in English primary schools. They are easy and simple enough and
occupy little time. But what can be expected from ill-educated teachers? I
suspect the same applies in the USA.

I understand school teachers in the USA wear bullet-proof jackets. But, in
general, I am restricted to the BBC and the largely US-controlled unruly
English newspapers which concentrate on swasticas and the royal family,
nudes, the sexual habits of film-stars, cat-walk celebraties and overpaid
Manchester United football players. After all, everybody has a right to
make a living of some sort!
----
Just another advert from Reg, G4FGQ