[Amps] Pi-L Switching Question

[Carl]

There are 3 schools of thought on that Jim:

1. leave open
2. short
3. Never tap a toroid

Perhaps you can test all 3 choices

Carl
KM1H


----- Original Message ----- 
From: "Jim Garland" <4cx250b at miamioh.edu>
To: <amps at contesting.com>
Sent: Thursday, January 31, 2013 2:24 PM
Subject: [Amps] Pi-L Switching Question


> I'm building a 160m/80m duoband amplifier with a Pi_L network and would
> appreciate comments on a issue pertaining to switching the L2 inductor (a
> toroid). On 160m I'll be using the full inductance (7.8 uH) and on 80m 
> I'll
> tap the inductor at 3.9uH (half the inductance). I plan to use a vacuum
> relay to switch in the 80m tap. So my question is whether it's preferable 
> to
> switch the inductor so that on 80m the unused segment is short-circuited, 
> or
> whether it's preferable to leave the unused segment floating?  If the
> former, there will be appreciable circulating currents in the shorted
> segment (i.e., heating and some loss), and if left open, then there will 
> be
> RF voltage induced on the unshorted segment.
>
>
>
> My instinct is to leave the unused segment open-circuited and place it at
> the downstream (antenna) side of the inductor, so that it's in a 50 ohm 
> part
> of the circuit. That would eliminate any circulating current and also keep
> the voltage maneagable. The only downside seems to me that I'd have to use
> an SPDT relay (with the common on the antenna), whereas with the
> short-circuited configuration I can use an SPST relay, and on the 160m 
> band
> the relay is out of the circuit. Now that I think about it, I probably 
> could
> use an ordinary open frame relay instead of a vacuum relay, since the RF
> voltage won't be more than a few hundred volts.
>
>
>
> So whaddya think?
>
> Jim W8ZR
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1430 / Virus Database: 2639/5572 - Release Date: 01/31/13
>