Towertalk
[Top] [All Lists]

Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
From: Michael Tope <W4EF@ca.rr.com>
Date: Sat, 14 Dec 2013 13:28:26 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 12/14/2013 11:18 AM, Jim Lux wrote:
Interestingly, the output Z of your amplifier is probably not 50 ohms resistive in any case, but most autotuners don't "match to 50 ohms", they "match to minimum reflected power" or "match to minimum VSWR", so they take out the effect of changing amplifier Z too.


Jim, I am not sure I agree with your statement, at least not in the case of standalone auto tuner. While I am not familiar with ins and outs of practical auto tuner design, I do presume they use a 50 ohm directional coupler at the RF input to compute the VSWR presented to the RF source device. The tuning algorithm presumably aims to minimize this 50 ohm referenced VSWR (again presuming the coupler is 50 ohms) in the same way an operator aims to minimize VSWR on his VSWR meter when adjusting a manual tuner. I would argue that is for all practical purposes "matching to 50 ohms" or perhaps more correctly "matching as close as practically possible to 50 ohms". This WOULD NOT take out the effect of the changing output impedance of the amplifier.

If the auto tuner were part of the amplifier, then it would be possible for the tuner to using additional information (like drive power and DC current) to account for the changing amplifier output impedance.

73, Mike W4EF............



_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>