[AMRadio] New Topic - moniitoring modulation help please

John Coleman jc at pctechref.com
Thu Nov 14 16:34:12 EST 2013

You may want to look at " http://www.qsl.net/wa5bxo/asyam/aam3.html " .
Although the article was written about the difference between sine wave
audio vs. voice audio in AM and how the rules have changed for Ham Radio
some time back, it has the voltage vs. power and math to help you with your
question.  As for as the method of hooking it up. As small link of 4-5 turns
in the tank coil area of the XMTR has always been a favorite of mine but the
last hook up I had was an antenna wire in the trees outside to pick up the
transmitted signal.  I actually built a small tuner circuit with 3 switched
coils and a 450 uuf variable.  Each coil was link coupled to the small
sampling antenna and tuned to the operating freq with a knob.  I could
adjust the knob to set the level of received signal to the scope.  In my
case I was directly coupled to the defection plates of the scope tube.  But
it could be for any vertical input on any scope.  The sensing antenna DOES
need to be outside and away from the shack so as not to pick up any of the
driver RF.  It can be brought into the shack via small coax and connected to
the tuner I mention or directly to the scopes vertical input.  Be sure to
place some type of RF choke across the scope input to keep electrical 60 cps
stuff out and static DC at a minimal.

For another test set up using a meter for measurering PEP is a 6AL5 mounted
in the top box of a Heath Cantenna dummy load.  Very easy and fun as well as
extremely accurate.  Let me know if interested.

John, WA5BXO
-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bry Carling
Sent: Thursday, November 14, 2013 6:20 AM
To: amradio at mailman.qth.net; Brett Gazdzinski
Subject: [AMRadio] New Topic - moniitoring modulation help please

Brett's remarks were very positive and helpful... thank you!

I have a question for the seasoned AM ops here. 

Some of us (like me) need a little help.  I have read a lot of articles on
how one goes about proper moniitoring of our AM signals, but I have never
understood how to properly couple an RF sample into my oscilloscope and
determine the %mod. etc. 
I have also never felt comfortable trusting the mod meter designs I have
seen. Most seem to involve some magic comparator that I am supposed to trust
- like, "when this little light comes on you are at 85% mod and the other
opne tells you you are hitting 100%." 
How can I be confident?

Those of you who have actually DONE this and are confident with your method
and equipment please enlighten the rest of us with something simple enough
and reliable enough to motivate us to either build or buy the necessary
equipment to check our modulation. 

Anyone needing help - I have available one extra frequency meter with
digital readout to several places if anyone needs it.  It's a nice clean
Haltronix - good to 600 MHz very cheap to anyone in this group, or I am open
to trades,,,

Please elucidate fully or point us to web sites that will demystify the AM
monitoring process in easily uinderstood terms.  No deep calculus or
excessive formulas if possible (grin!)

I would not mind buying a commercial signal monitoring meter for my station
if it was reasonably priced. 

I think the real question some of us have is "How clean is my signal?"

I could be running 200% modulation and stays within a 6 kHz bandwidth  if
the signal is clean andI would be happy with that. I am thinking of my DX60B
(or FT857D) + TenTec 422 setup that will soon be pumping some tall ship
signals into the sky.

73 - Bry AF4K

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