[TowerTalk] 10M 4:1 "Transformer"

[David Gilbert]

Is it a folded monopole?  A 50 inch vertical section fed at the base 
looks to me (using EZNEC+) like it would have a native source impedance 
around 3.5 ohms, which a folded monopole with equal diameter elements 
would transform to 14 ohms (4:1).  If that's what you are doing, you can 
simply do a similar thing but with four elements to give you a 16:1 
transformation.   I'm pretty sure that would be more efficient than any 
kind of ferrite transformer.

There is about 300 ohms of capacitive reactance involved as well. How 
are you tuning that out?  I presume a coil.

73,
Dave   AB7E


On 5/21/2022 7:02 PM, Larry Banks via TowerTalk wrote:
> Hi All,
>
> I am designing a rather unique 10m FM vertical for my local repeater 
> net.  It has to fit into my attic, which is only 50" tall.  I have 
> this antenna designed, and it "should" work, but it has an impedance 
> of essentially 14 ohms.  (Typical for a shortened antenna.)  I want to 
> fabricate a 4:1 transformer to facilitate this.  There are many such 
> things available on the web, many of which I question. PLUS, I wish to 
> design and build this myself.
>
> I would appreciate any suggestions about how I should go about this.  
> Ferrite type and size, etc.  I have to admit that I have not yet done 
> an exhaustive search of the already available sources, especially all 
> of the great stuff from Jim, K9YC.  What I have already looked at is 
> lots of ads for questionable stuff.
>
> 73 -- Larry -- W1DYJ
>