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Re: [TowerTalk] tuners and power rating

To: towertalk@contesting.com
Subject: Re: [TowerTalk] tuners and power rating
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
Date: Mon, 29 Nov 2010 03:02:54 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

On 11/28/2010 3:26 PM, Danny Pease wrote:
> I have noticed several
Several? I don't know of any that are rated for "the legal limit" that 
can handle much more than medium power.
> so called legal limit tuners that cannot handle even
> medium power on 160 unless the load is very close to 50 ohms already.
Even the Palstar AT5K which is rated for 3500 watts under most 
conditions is derated to 1000 watts for a low impedance antenna on 160. 
However, if you raise the antenna to the point where the impedance is 20 
or 30 ohms it will handle the full 3500 watts.  The AT5K is a physically 
large tuner.
>   Also,
> note almost all tuners ratings are amplifier input, not power output.
I've never heard of any yet, except for the ones designed in the old 
days where the legal limit was 1 KW input.
The MFJ 989C will handle the full 1500 watts *into* the tuner as long as 
the load impedance is not so low as to be out of range.
Many adds are "input to the tuner" but I've not seen any modern ones 
that were rated input to the final.
>   They
> also do not specify at what mis-match the power rating is at.
MFJ and Palstar do give a power limit for an impedance range, but not 
specifics for each frequency, nor do we need such.
>   The ones I
> have seen arced are one of the capacitors or the tapped inductor switch. If
> you really want to test a tuner, try matching a 5:1 mismatch on 160 and then
> run 1500 watts through it.
As I run from one end of the band to the other I get SWRs far higher 
than that.
The 989C would arc running around 1200 Watts (Alpha 76A) into the tuner 
IF the antenna was about 2' too long to be resonant at the bottom end of 
160. Shorten the antenna to be resonant around 1.825 to 1.850 and it 
would cover the entire band.
I loaned the analyzer out, but IIRC running end to end on 75 gives a 
pretty high SWR. Much more than you want to run the legal limit through 
a voltage balun.  The current baluns 5 cores with 5 or 6 turns through 
them work very well.  The 160 antenna is going to get a 7 core, 6 turn 
coil of Davis BuryFlex(TM)within about a foot of the feed point.

> NG9R
>
>
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