Jeffdeham wrote:

I was looking at the rainbow tuner which matches a
half wave end fed antenna to 50 ohms using a quarter
wave radial. I am guessing it
works like a transformer but not sure. Here is
the schematic;

Ant |----- quarter wave radial
| |
| GND
----------
| )
--- )
Variable )----- 50 ohm
--- )
| )
| )
----------
|
GND


If it does work like a transformer how does it do it?
How is the matching done?

Thanks for any help!

Jeff

The fed end of a half wave antenna appears as a high impedance.
The parallel resonant circuit (at resonance) also appears as a high impedance.
To match the low impedance of the feedline, it is necessary to
tap down on the coil (so as to feed between the tap and ground) -- like
an autotransformer.

Think of a half wave dipole -- you need good insulators on the end
of the half wave because the voltage there is very high, and the
current is very low. Uzing (pardon the pun) Z=E/I=High/Low yields
a high impedance.

This is a pretty simplified explanation, but if you think about
it it will work.

Hope it helps you to understand things a bit.

Irv VE6BP
--
--------------------------------------
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Beating it with diet and exercise!
297/215/210 (to be revised lower)
58"/43"(!)/44" (already lower too!)
--------------------------------------
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--------------------
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Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

Dale Parfitt wrote in message If it does work like a transformer how does it do it?
How is the matching done?
The antenna represents a hi impedance- in the 2K--4K range for HF and
wire as a radiator. The parallel tank L-C is also a hi impedance. The
coaxial cable then is tapped up from ground until the 50 Ohm point is
reached.

That makes perfect sense except if you have two high impedances in
series how does tapping the coil at a certain point give you 50 ohms?
What exactly is the process where by the impedance transformation
occurs?

Just trying to learn here.

Thanks for any help once again!

Jeff

Jeff,
Series? How about parallel.
'Doc

If the feedpoint resistance of the 1/2-wave vertical is
R ohms, then the input impedance of the coil tapping
point is R*(K squared) where K is the fraction of total
coil turns below the tap.

It requires only that L and C are resonant at the same
resonant frequency of the 1/2-wave antenna.

A roller inductor makes a nice job of it.
Disadvantage - the roller inductance L being fixed,
with C, limits the matching range to only 1 or 2 bands
before things become inefficient. For highest
efficiency L should be big and C should be quite small
like 10 to 50pF.

Solution - prune the antenna to an off-resonance
frequency. I have a program for it but never published
it because its too simple.
---
Reg G4FGQ.

Irv Finkleman wrote in message ...

Think of a half wave dipole -- you need good insulators on the end
of the half wave because the voltage there is very high, and the
current is very low. Uzing (pardon the pun) Z=E/I=High/Low yields
a high impedance.

This is a pretty simplified explanation, but if you think about
it it will work.

Hope it helps you to understand things a bit.

I have a much better understanding of how it all works now. I had an
idea that is how it would work but just couldn't visualize it. Been 27
years since electronics school and the old brain has gotten a little
fuzzy since then! 8-)

Thanks everyone for all your help!

73!

Jeff

Jeffdeham wrote:

I was looking at the rainbow tuner which matches a
half wave end fed antenna to 50 ohms using a quarter
wave radial. I am guessing it
works like a transformer but not sure. Here is
the schematic;

Ant |----- quarter wave radial
| |
| GND
----------
| )
--- )
Variable )----- 50 ohm
--- )
| )
| )
----------
|
GND


If it does work like a transformer how does it do it?
How is the matching done?

I'm not familiar with the rainbow tuner, but if I read your schematic right,
the tapped coil functions as an autotransformer. Autos are commonly used
as step-up and step-down transformers in power substations. A functional
equivalent using separate windings would look like this.


|
|
|
----------------
| )
| )
| )
----- )
----- ) ( ----------- 50 ohm
| ) (
| ) (
| ) (
-----------------------------------
|
GND




73 de KD5SZ

Bud Rogers wrote:



|
|
|
----------------
| )
| )
| )
----- )
----- ) ( ----------- 50 ohm
| ) (
| ) (
| ) (
-----------------------------------
|
GND


Gak. I screwed that up. Let's see if this comes out any better.

|
|
|
-----------------------
| )
| )
| )
----- )
----- ) ( ----------- 50 ohm
| ) (
| ) (
| ) (
-----------------------------------
|
GND


73 de KD5SZ

Jeffdeham wrote:

If you could answer yet another question. How does a autotransformer
work exactly? I can see how you can tap up and down the coil to hit a
certain point where you can a 50 ohm match. But when I look at the
schematic all I see is a coil in parallel with a variable cap. How
does the autotransformer in essence become a transformer? Or rather
how does it become the above schematic?

That amounts to two questions that aren't necessarily related. One is how
an autotransformer works, the other is how inductance and capacitance can
be combined to match an antenna to a feed line.

I'll take a stab at the first question. Think about how a transformer
works. If you place two inductances in close proximity, a magnetic field
induced in one will induce a current or voltage in the other. By selecting
the proper turns ratio between the two inductances, you can transform an AC
voltage or current up or down. You can transform high voltage/low current
to low voltage/high current or vice versa. Which is another way to say you
can transform impedance.

In an autotransformer the same principles apply, but a portion of the
inductance is common to both circuits. A voltage applied across the tapped
portion of the coil will produce a higher voltage across the whole coil. A
voltage applied across the entire coil will produce a proportionately lower
voltage across the tapped portion. The current in the tapped portion will
be the sum of the currents flowing in the two circuits. Not good if you
need isolation, but not an issue if all you want is to transform voltage,
current, or impedance. Does that make sense?

On the second question you might be better off to read the antenna matching
chapter in any Radio Amateurs Handbook. :}


73 de KD5SZ

Another good reference is at ARRL Site URL:
http://www.arrl.org/tis/info/pdf/9401070.pdf
Titled "Do You Need An Antenna Tuner"

73 From The Signal In The Noise
Caveat Lector Ya All
-------------------------------------------

If you could answer yet another question. How does a autotransformer
work exactly? I can see how you can tap up and down the coil to hit a
certain point where you can a 50 ohm match. But when I look at the
schematic all I see is a coil in parallel with a variable cap. How
does the autotransformer in essence become a transformer? Or rather
how does it become the above schematic?

On the second question you might be better off to read the antenna
matching
chapter in any Radio Amateurs Handbook. :}

Bud Rogers wrote in message ...

In an autotransformer the same principles apply, but a portion of the
inductance is common to both circuits. A voltage applied across the tapped
portion of the coil will produce a higher voltage across the whole coil. A
voltage applied across the entire coil will produce a proportionately lower
voltage across the tapped portion. The current in the tapped portion will
be the sum of the currents flowing in the two circuits. Not good if you
need isolation, but not an issue if all you want is to transform voltage,
current, or impedance. Does that make sense?

It makes sense now. That's the main question that I wanted answered. I
had a faint notion that was how it worked. You clarified it quite well
for me!

Thanks!

Jeff