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!)
--------------------------------------
Visit my HomePage at http://members.shaw.ca/finkirv/
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--------------------
Irv Finkleman,
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