Why is it that all the major HF transceiver manufacturers set the AM modulation level so low in their radios?
Is 100% AM modulation not the standard in AM broadcasting?
Is 100% AM modulation not the standard in AM broadcasting?
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Why do HF transceivers have such low level AM modulation?
- Thread starter SMILEX2692002
- Start date
W8JI wrote:
Mike
I tried that battery trick. Works.HF SSB rigs on AM
Some newer HF transceivers are excellent on AM, with much less distortion and better fidelity than most older amateur AM rigs. The Ten-Tec Orion and the Yaesu FT-1000D are two examples of very good AM transceivers.
Peak envelope power, with 100% modulation, is four times carrier power. For 100% modulation in a 100-watt radio, a 100W PEP radio must run 25 watts or less carrier. With a 100-watt radio, peak power on voice peaks should be held to 100 watts or less.
I used an IC-751A or IC-706 ICOM on AM. The problem with that ICOM, like with many HF SSB radios, is it uses ALC to limit output power. Turning the output power down or mic control up will not increase the percentage of positive peaks. This is because the ALC system in most SSB rigs almost always detects peak power. If we adjust carrier power to 25 watts and try to modulate 100% (100 watts), the peaks cause the radio to reduce gain until peaks are back at the 25 watt carrier power level. The positive peaks stay at 25 watts or so...and the carrier drops to 7.5 watts when modulated!
The cure is to run the power level all the way wide open and apply an external stable negative voltage to the external ALC input. Adjust the external negative ALC until the carrier is 20-25 watts, and then the mic control until we have 100 watts on peaks using a good peak reading meter.
An external ALC carrier control can be a 9V battery across a 500k pot in a voltage divider. The positive battery lead goes to ground, and the ALC output comes from the pot wiper. P1 goes to the EXT ALC jack on the radio.
Remember to disconnect the battery when using other modes or when not using the radio!
To use this circuit, run the radio's normal power control wide open. Adjust the pot for 20-25 watts carrier. Adjust the mic control for 100% modulation, or 100 watts PEP (on a 100W radio).
Mike
There are two main reasons.
#1 Most operators don't know how to set up their radio for proper AM operation.
#2 When it comes to the CB world most operators don't understand what normal modulation is and are used to hearing audio from someone running 2 watts carrier swinging to God knows what.
Shadetree Mechanic
Delaware Base Station 808
I would use a scope or a monitor radio to make sure it doesn't sound like a ch19 radio. Isn't there a way to turn the ALC off?
Can someone provide a picture of this device constructed?
I would like to make one myself but would like to see one already made. I would hate to do something wrong and mess up my HF radio.
Excuse my lack of understanding the schematic. This is why I wanted to see one already made.
So the potentiometer has a positive connection and a negative connection.
The positive side of the 9 volt battery goes to the negative side of the potentiometer?
The negative side of the 9 volt battery goes to the positive side of the potentiometer?
I'm assuming the RCA cable has a positive wire and a negative wire.
Is the RCA cables center pin the positive? (male part of the plug?)
I guess what I'm asking is how do u connect the RCA cable correctly to the potentiometer?
The positive side of the 9 volt battery goes to the negative side of the potentiometer?
The negative side of the 9 volt battery goes to the positive side of the potentiometer?
I'm assuming the RCA cable has a positive wire and a negative wire.
Is the RCA cables center pin the positive? (male part of the plug?)
I guess what I'm asking is how do u connect the RCA cable correctly to the potentiometer?
Battery + to 1, - to 3
RCA tip to 2, sleeve to 1
RCA tip to 2, sleeve to 1
Long story short, a real AM transmitter limits mike audio when you reach the max modulation level.
All the ham HF radios use the sideband ALC in AM mode. When your audio reaches the limit, the entire signal *carrier and all* gets reduced.
Carrier and modulation, both.
Never have come up with a way to modify that kind of radio to leave the RF-signal level alone and control only the audio level in AM transmit. Would probably sound better.
Somehow I managed to miss the ALC/battery trick. Gotta try that.
73
All the ham HF radios use the sideband ALC in AM mode. When your audio reaches the limit, the entire signal *carrier and all* gets reduced.
Carrier and modulation, both.
Never have come up with a way to modify that kind of radio to leave the RF-signal level alone and control only the audio level in AM transmit. Would probably sound better.
Somehow I managed to miss the ALC/battery trick. Gotta try that.
73
I am able to get 150% positive modulation out of my ftdx1200 from a 15 watt carrier. It only makes 6k wide AM with a mic plugged into the front. Odd that it will do almost 4k on ssb and not make 8k on AM. No modifications except mars and the essb cheat code have been done to the radio.
Shadetree Mechanic
Delaware Base Station 808
I wonder if the accessory socket in the back would have some voltage to use instead of a battery? If using a battery it would be nice to have a switch to turn it off when not in use too. Would a wall wart be too noisy?