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

[Dan Schaaf]

Jim

I have an MFJ-998 Autotuner connected to the amp. Every time I change 
frequency I mouse click on the TUNE button on the screen, generate a carrier 
and the tuner does it's thing.

So, do I have distortion? Not sure, but the tuner does present a good match 
to the amp. I do ask on the air about my signal, SSB, CW and RTTY and always 
get good reports. Short of having a scope connected to the coax, I assume 
that all is OK. Gone are the days of the Yaesu Scopes for testing 
transmitter/amp output. I used to have one in line but never seeing anything 
obvious on my signal, I finally got rid of it.



Best Regards
Dan Schaaf
=================================
K3ZXL www.k3zxl.com
60 Meters www.60metersonline.net
=================================
NOBSKA www.nobska.net
Cape Cod Instruments www.gnm-inc.com
Tel: 941-766-0706
Fax: 941-766-0707
=================================

-----Original Message----- 
From: Jim Lux
Sent: Saturday, December 14, 2013 10:11 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

On 12/14/13 6:32 AM, Dan Schaaf wrote:
>
> Most SS amps, at least my Tokyo HyPower amp, shut down when SWR get to a
> point where 80 watts is reflected back into the amp. Does not matter
> what the SWR is. It matters how much power comes back down the line.
> Sometimes if you try to overdrive it, there will be a light flashing
> that indicates overdrive or another light for overvoltage. In any case,
> it wants the correct input and the correct SWR within 80 watt limitation.
>
Yes..
and at any point where the amplifier is not shutdown, does the
distortion increase?  I suspect that your amplifier (like many solid
state PAs) has two states:
Operating into a match that it thinks is acceptable - and distortion is low
Shut down due to reflected power, overdrive etc - no output, so no
distortion products to worry about.


What I'm getting at is Jim's assertion that a bad match leads to
distortion and splatter.  I think a load impedance that is beyond the
amplifier's capability to be adjusted fits this case, but not when the
amplifier is being operated within its limits.

With tube amps and manual tuning (characteristic of low cost amateur
equipment), there's little or no protection against improper operation,
and I can see someone having a terrible match, tuning up anyway, and
causing a problem.

But with a SSPA using automatic shutdown, maybe not.

I guess there are also poorly implemented automatic controls (e.g. a
clipper on the input to the amp)


_______________________________________________



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