On Sun, 30 Jun 2013 10:58:08 -0700, Jeff Liebermann
wrote:

On Sun, 30 Jun 2013 07:24:34 -0500, John S
wrote:

On 6/27/2013 11:49 PM, Jeff Liebermann wrote:

On the short end of the scale, the 1/8th wave antenna at:
http://www.11junk.com/jeffl/antennas/Monopole/monopole_0_125/slides/monopole_0_125.html
shows 254 ohms, which will work with a 2:1 turns ratio transformer.

I don't think a transformer is a significant help. Without the
transformer the SWR is about 158:1. With the transformer, the SWR is
still up to about 61:1. That will probably kick in the SWR protection of
the transmitter.

John - KD5YI

Nope. A 2:1 turns ratio tranformer will provide a 4:1 impedance
ratio, not a 2:1 impedance ratio.

The required transformer ratio would be:
(254 / 50)^0.5 = sqrt(5) = 2.3
A 2:1 turns ratio xformer should be close enough.

Another way is to take the 2:1 turns ratio transformer, which has a
4:1 impedance ratio, and divide the antenna impedance by the impedance
ratio:
254 / 4 = 63.5 ohms.
Not exactly 50 ohms, but close enough.

Oops. My mistake. I couldn't recall if a 2:1 transformer referred to
the turns ratio or the impedance ratio. I've seen it done both ways
in other industries and transformer applications. I usually qualify
the label with either turns or impedance ratio but forgot this time.
However, skimming the available literature with Google, I find that
the common usage for RF xformers is the impedance ratio. Therefore,
your comments are correct and I should have specified a 4:1
transformer. Sorry(tm).



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558