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[AMPS] High-SWR protection

To: <amps@contesting.com>
Subject: [AMPS] High-SWR protection
From: bknox@rfapps.com (Bruce R. Knox)
Date: Mon, 30 Oct 2000 15:07:38 -0500
Hi All:

Our Website is NOT intended to offer a complete treatise on station and 
amplifier design.  There are lots of places (like this one, for sure!) to 
go to get all kinds of different information on how to do, or not do, 
things.  We build a product that is loved by most, and hated by a few 
(pretty much like a lot of products).  Yes, we have VSWR protection, and it 
is indeed driven by firmware in a PIC.  It takes about 400-600 milliseconds 
for it to respond (on purpose, see below.)  So, for instantaneous problems, 
it isn't going to do much.  But it will prevent long term (e.g. 20 minute 
keydown) problems, and will always let you know when you've forgotten to 
select the right antenna, icing, etc.

In terms of how you interrupt an amplifier:  many amps do not have 
sequenced relays.  We have had feedback from many users that they prefer to 
use the ALC approach.  Most use the PTT-block approach.  Most responses 
from users tell us they both work fine (within the limitations mentioned 
above).

So, Chuck, your "mindless parroting" wasn't so far off base.  I haven't 
looked to be sure, but my AL-1200 acts like it does not have sequenced 
relays (there seems to be just one T/R relay that is just marginally fast 
enough), so the ALC approach (and the PTT approach) might make sense for 
that application.  When I put a VFD or WinWatt between my 756PRO and the 
AL-1200, I get a momentary VSWR alarm as the AL-1200 T/R relay pulls in. 
 Because of situations like this, we have built in an intentional delay 
that occurs after we have sensed a problem and when we drop out the alarm 
relay.  Much like our "spike filter" that ignores the sometimes GIANT 
initial spikes that come out of a lot of modern rigs.  Those giant spikes 
cause the peak detectors to read sometimes hundreds of extra watts coming 
out of a 100 or 200 watt transceiver for some number of milliseconds. 
 Indeed, there are quite a few competing wattmeters out there that exhibit 
this flaw.  These are typical features of our design that focus our meters 
on OPERATING, not lab testing.  Sure, an "instant" VSWR alarm and display 
of a transient excessive initial peak would be great in a lab, but I can't 
see how you'd want to have those functions or indications while you're 
trying to run some rate (or check in to the Maritime Mobile Net for that 
matter).

We design our meters to offer the most performance for a reasonable amount 
of money.  You can't buy a Bird and a full set of slugs for $249, and even 
if you do pay all the extra money for a 43, peak kit and a full set of 
slugs, you won't get VSWR computation and an alarm.

To those who do not like our new sensor:  you can still buy the old one to 
use with the VFD.  It has mounting ears and is in a cast aluminum box. 
 Contact us if you would rather do that.  It just costs more (and works the 
same).  And to those that object to the "project box":  how much extra 
would you pay for a metal one?

Thanks for reading & 73,

Bruce R. Knox W8GN
RF Applications, Inc.

PS:  Yes, I know there's a mod for the AL-1200 relay.  Just need the time 
to do that!!!!!!!!!!

-----Original Message-----
From:   Chuck Counselman [SMTP:ccc@space.mit.edu]
Sent:   Monday, October 30, 2000 10:44
To:     amps@contesting.com
Cc:     W8JI@contesting.com; bknox@rfapps.com
Subject:        Re: [AMPS] High-SWR protection


Tom Rauch <w8ji@contesting.com> wrote:
>If the relays in the amp are properly sequenced, dropping the relay
>line won't hurt a thing.

That sounds right.  Thanks.  Sorry -- I was guilty of mindlessly parroting
the advice on RFApplications' website.  Perhaps someone from RFApplications
will chime in to enlighten us?


>I'd be reluctant to use application of ALC voltage as the sole means
>of shutting off the exciter, because ALC lines can be a bit slow and
>you also must depend on an external negative supply. Anything
>gets unplugged or shorted and you have no protection.
>
>If I used ALC, I'd be darned sure I also dropped the antenna relays
>during a fault.

Sensible advice.  Thanks again.

73 de Chuck W1HIS



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Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com


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