[ARC5] Why an UNUN - and a drift into TCS territory

[Mike Hanz]

On 10/24/2012 10:29 PM, Laura and Jim wrote:
> I successfully run my TCS into a dipole in this manner:
>
> Crank the loading inductor to near zero turns, add a 365 PF series cap outboard, then add 1:4 autotransformer to feed the coax.
> Hot and ground go to separate posts on the autotransformer box, coax out the other side, T-network "tuna" to an NVIS dipole. I get good reports and the full 20W out.
>
> Now my ARC question: I have a 20 foot portable whip and 16 radials out on the lawn 30 ft away from the shack. What's the best way to feed the whip?
> a) Beefy cable from the TX antenna post to the whip feedpoint, separate ground wire out to the radials?
> b) Above mentioned UNUN (1:4) set-up and co-ax to the whip? Of so, what do I do at the end of the coax? Install another UNUN (4:1) to take the 50 ohms back to 12?
> d) None of the above (most likely............hihi)

The purist's answer is b).  That's what the shipboard and aircraft 
transmitters do, except that the antenna end is a variable tuner. What's 
the driving point impedance of your whip at the base of the antenna at 
the frequencies you want to work?  Is it *always* 12 ohms?  The unun 
will reflect the reactive component back down the coax, and you'll see 
losses from that in the coax.

On the other hand, see my previous note.  If you just want to get a 
signal out, you don't need to go to all that trouble...option a) is the 
way to go.

  - Mike