[Amps] L section of Pi-L filter

[Jeff Blaine AC0C]

To add to the recommendations, Pi-L is excellent.

On the toroid, 3 words of caution:

1. The power dissipation comments below are exactly right.  The toroid will 
heat up.  And you want to keep the heat down because the toroid 
characteristics change quite dramatically at higher temps.  So mount the 
toroid where A) there is air circulating around the toroid and B) do **NOT** 
block the center of the toroid with mounting hardware - the max heat point 
of the toroid will be in the center.

2. For the same flux limit reasons, dedicate a single toroid to each band. 
So that the turns can be equally spaced and spread around the entire 
circumference of the toroid.  I made the mistake of sharing the 40/80m 
toroid on one band - and when operating on 40m, the local heating under the 
40m part of the toroid is extreeme.  And the amp's efficiencies suffer as 
well for that.  Especially in RTTY service which is my favorite mode.  I 
will revise that design and dedicate a single toroid to each band.

3. Run the tank Q lower, rather than higher.  The heating and loss and 
voltage problems are quite a lot less with a Q=8 than a Q=12.  And the 
difference in 3rd order harmonic levels is almost zero.  On paper, higher Q 
is nicer, but in the practical real world, a lower Q has many benefits - 
easier tank construction, less heating, less arc potential - and (for 
contest cases) a lower Q allows a greater frequency change before touch-up 
retuning is required.

There are many other guys on the board with far far more experience than I 
and I defer to their comments.  But these items above are lessons learned 
the hard way with my first amp build.  Hope your build will be smoother than 
mine was!

Best of luck and happy new year.

73/jeff/ac0c


--------------------------------------------------
From: "Karl-Arne Markström" <sm0aom at telia.com>
Sent: Wednesday, January 06, 2010 1:16 PM
To: "Ulf Tjerneld" <star at frizon.org>; <Amps at contesting.com>
Subject: Re: [Amps] L section of Pi-L filter

> 1. Toroids have to be quite heavily dimensioned, as
> they are storing a lot of magnetic energy in a small space.
> At an L-network Q of 2 they have to store at least 2000 VAr in the toroid
> magnetic material, and the corresponding flux density is quite high.
>
> Even the largest iron-power toroids available may be marginal in this
> application, as the core losses for i.a. #2 material are in the 10's of
> watts range.
>
> 2. Using "reasonable" network Q:s, the current in the L-network part is
> about 80 - 90% of the current flowing in the Pi-part, so the same wire
> gauges are required.
>
> The "Pi-L Designer" utility by Jim Tonne is a very useful tool for output
> circuit design, together with the DL5SWB Mini Ring Core calculator.
>
> 73/
>
> Karl-Arne
> SM0AOM
>
>
> ----- Original Message ----- 
> From: "Ulf Tjerneld" <star at frizon.org>
> To: <Amps at contesting.com>
> Sent: Wednesday, January 06, 2010 7:33 PM
> Subject: [Amps] L section of Pi-L filter
>
>
>> Hi,
>>
>> Question 1: I'm considering using toroids in the L-section of my RF tank.
>> Are there any guidelines regarding suitable cores, wires and turns?
>>
>> Question 2: If I stay with a traditional air wound coil for the L-section
>> what would be a suitable wire diameter for a 1 KW amp? I'm pretty sure it
>> does not have to be the same heavy gauge as in the Pi-section!
>>
>> Thanks for any answer!
>>
>> 73's  Ulf /SM0NOR
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
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