[AMRadio] AMRadio Digest, Vol 118, Issue 6

Jim Candela jcandela at prodigy.net
Sat Nov 2 12:46:44 EDT 2013


Date: Sat, 2 Nov 2013 03:09:04 -0500
From: "Ross Stenberg" <k9cox at charter.net>

"The difference between 100 watts and 400 watts (just over the legal AM 
limit) is 6 dB or 1 S unit on a properly calibrated receiver."

        73 Ross K9COX

>>>I plan to get on and see what I can do with a hundred watts and a dipole.

Well said. The trick I find on 75M AM when running lower power is to do so in the late afternoon or early evening before the band goes long (gets noisy). Also sometimes good results can be had in the morning like 7 to 9 am or so after the long hall stuff goes away.

One important thing is that the "Tall Ships" by virtue of their strong carriers can get away with low average modulation, and even poorly equalized audio (way too much bass). When receiving these guys as heard across the country all that is left is an S9 carrier with only the occasional word legible. Then some guy from the same distance with nothing more then a DX-100 combined with a D-104, and the copy is 100X better despite the 6DB reduction in carrier strength.

Running low power AM on the low bands, to be successful, needs to be well modulated, and equalized to fit in the receivers 5-6 Khz bandwidth. This does not take hundred of dollars worth of audio processors, equalizers, etc. What it takes is a transmitter that can modulate cleanly up to 100% (or higher on upward peaks), a scope to monitor the RF envelope, and just a D-104 loaded into somewhere around 500K to 1.5 meg (depending on your voice, and preference). Then phase the audio polarity (try swapping wires on the D-104) such that any asymmetry in your voice is phased such that the downward modulation gets the lower amplitude. This allows you to boost the average (sometimes 2X) without negative peak clipping.

I have worked across Texas with a tweaked Retro-75 at 2 watts carrier. I did this several times at about 9 AM on 3880. The Retro-75 was peaking about 140% modulation, and the audio off the S0 carrier was still heard Q5. Boosting to 70 watts at 9PM and later, I could go across country. In the early evenings I sometimes get in there with my G-76 at 55 watts carrier. With a high average modulation, and good equalization, I 'm often louder (audio into receiver speaker) then the big dogs running 400 watts carrier...

When the band goes long, and local propagation is lost, The only ones who really do well are those running QRO, and are well modulated without an abundance of bass. Three guys stand out to me that always seem to do well, WA1HLR, WA1QIX, and K4KYV.


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