How to Choose the Right Transmitter

Posted on July 28, 2005

From time to time, a visitor will ask how to choose the right transmitter for a successful Part 15 AM BCB operation…

Since each operator will have different needs, interests, skills, budgets and content goals, I really don’t know if there is an ultimate answer. What follows is a general approach, and shared here for comments and suggestions.

I’d say there’s four areas to look at, power use, modulation, audio processing and RF buffering.

Power Use

Many folks use the power input limit criteria for compliance – that’s the 100mW standard that’s frequently kicked around.

That means you get 100 mW of power DC input into your XMTR, and it’s up to your XMTR to get the very most RF out of the 100mW DC in.

Good oscillator and buffer designs help this outcome a great deal, so you’ll want to find XMTR designs that get the most RF out of the power that’s dribbled in.

PLL or XTAL seems to work best for AM BCB, VCOs have never really made me too happy when you’re shooting for 24/7-365

So job #1 is getting the most carrier out of 100mW DC input. Some XMTRs are more effective at squeezing out every drop.

Modulation

Grabbing your audio input and effectively modulating your carrier is the next job your XMTR needs to do well. Some XMTR designs will be better at this than others, and the effectiveness of the modulation of the carrier will have a big impact on how happy you are with your station.

A short example – I had a PanAxis AM100, and I was very unhappy because it seemed to have a very short range. I contacted PanAxis and they gave me some modulation adjustment tips. I made the adjustments, and all of a sudden I could hear my XMTR a mile out. It seems the radio waves were actually going quite a distance, but the carrier wasn’t modulated well enough for my radio receiver to reproduce any audio – I was getting out, I just couldn’t hear me!

Adjustments aside, some XMTRs modulate better than others, and some modulation stages can swallow higher audio outputs – that makes your range seem greater because you can “kick over” the detector in receivers that are farther away.

Audio processing

The audio modulates better if it’s conditioned before it’s modulated.
First, the audio should be a fairly high voltage signal. I use an LM 386 low power audio amp IC in front of my XMTR, and it helps boost the audio from my PC sound card.

Next, the audio should be compressed. It helps make the station sound more like the other stations by keeping low volume content from dropping out because the amplitude isn’t adequate to modulate the carrier.

Finally, it should be limited so that certain loud content doesn’t swamp the modulation section and make that icky “it’s turned up too much” sound.

Some XMTRs do all of this internally. Some do none of it. Some do some. What you choose depends on what processing equip you might already have, the bandwidth and average VU level of your content, how much DC power your going to want to pump into the XMTR (All that processing needs addidtional input power), what your personal tastes might be and so on.

RF Buffering

The final stage of the XMTR should buffer the RF output adequately, so that minor changes in the environment around your ANT don’t negatively impact your oscillator and ruin the ANT/XMTR match, etc. Anyone who has fiddled with an unbuffered XMTR will know what I mean – it’s as bad as a VCO.

Here are some transmitter review sites that cover these issues. Always remember that “Net” means Not Entirely True, and draw your own conclusions!

http://www.vintage-radio.com/reviews

www.toddjenkins.com

http://www.geocities.com/raiu_harrison (old, but good)

www.oldtimeradioprograms.com

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