[SMCARA] tuner questions

[Sam Leach]

Hi Allen,

Those are some really hard questions.  I doubt that anyone will be able to
just open up and lay out the answers.  If they can, my hat is off to them.

RF chokes come in many configurations.  For in circuit use, they usually
consist of many turns of very fine wire or one or more ferrite beads.  They
can also be made of ferrite rings, if you're choking off RF on the outside
of a transmission line.  I'd recommend the latest ARRL handbook for
familiarization with the various kinds of RF chokes and their application.
If we were setting at my dining room table, I could explain some of
them...but one would essentially have to "write the book" to do it here.

Question 2 is answered above...the first questions you need to answer is
"what is the choke for" and "where does it live in the circuit".

Your last question is entirely different animal.  At the feed point, you
want an antenna to appear as a resistive load...preferentially a 50 ohm one
(for coax fed antennas). Short antennas display some unknown resistance (if
they are short this value is usually low) and some capacitive reactance
(Xc).  The loading coil adds inductive reactance which cancels out the
capacitive reactance, leaving just the resistance (a purely resistive
load...which then may, or may not match your coax)).  You need to know how
much capacitive reactance you're cancelling.  Then, given the operating
frequency and inductive reactance required, there are formulas / tables for
determining the size of wire, number of turns and size of the loading coil.
Again, I'd send you to the handbook.  You might be able to determine the "L"
(inductance) needed in the loading coil by using an MFJ 259, or equivalent,
to evaluate the short antenna that you plan to "load", on your proposed
operating frequency...with a view to learning how much capacitive reactance
you're dealing with.  

Gentlemen, Allen has a worthy project here.  If someone has the background
and time, it would be great fun to work this through.  If I'm not
remembering this stuff correctly...please throw me and Allen a life ring!
The last time I worked with an unknown antenna element and tried to match it
to 50 ohms (non reactive) was at the old WKIK (1370 KHz AM) and probably 30
years ago.

73

sl



-----Original Message-----
From: smcara-bounces at mailman.qth.net [mailto:smcara-bounces at mailman.qth.net]
On Behalf Of mlandmann at juno.com
Sent: Thursday, December 29, 2011 22:25
To: smcara at mailman.qth.net
Subject: [SMCARA] tuner questions

I have a few questions that I thought someone could help me with.  Any help 
or suggestions would be welcome.

1. How do you determine the size of the coil, and the number of turns for a
RF choke?

2. How do you determine the turns on a balun for an antenna?

3. If you are building a loaded antenna, how do you determine the size of
the loading coil, and find out how much the antenna would be shortened?

Thanks,

Allen
KB3WEY

 


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