What is backswing?

[BlowinSmoke]

I think the Hammy AM'ers call it "downward modulation". If you look up that term you will get a better response from Google. The CB'ers call it "back-swing". Kinda like the CB box builders calling a Variable capacitor a "TUNER". Likewise, if you Google Variable capacitor you will get the definition of what is called a "tuner".

 

[LeapFrog]

I think the Hammy AM'ers call it "downward modulation". If you look up that term you will get a better response from Google. The CB'ers call it "back-swing". Kinda like the CB box builders calling a Variable capacitor a "TUNER". Likewise, if you Google Variable capacitor you will get the definition of what is called a "tuner".

Here is a serious example of downward modulation, I saw this while testing a mod out for a friend, the only way to combat this (besides reworking component values) was to lower the carrier, the modification is one for a 29 LTD, and I was trying to replicate his results. This test was a failure.

This was a good read: https://www.w8ji.com/amplitude_modulation.htm

 

[Limeybastard]

Here is a serious example of downward modulation, I saw this while testing a mod out for a friend, the only way to combat this (besides reworking component values) was to lower the carrier, the modification is one for a 29 LTD, and I was trying to replicate his results. This test was a failure.

This was a good read: https://www.w8ji.com/amplitude_modulation.htm

That is exactly what i was looking for cheers.

 

[StrangeBrew]

.... More power means more heat and a larger range of error.

For really big power you need to use a very large can, use peanut oil instead of mineral oil and throw a turkey in to help absorb the heat.

 

[543_Dallas]

Dry dummy loads are the better way to go and don't have to be expensive.

Take a couple of these bolted to a heatsink and maybe a fan if you want to do any extended testing.

http://www.ebay.com/itm/250W-100Ohm...621438&hash=item2ef5983898:g:oqQAAOSwPCVX~jRV


Also keep an eye out on ebay and the ham classifieds for retired commercial dummy loads. They are rated for continuous duty so no worries about transmitting too long. I have one rated for 5kw that cost me less than something from mfj.

 

[Paul-W5lz]

'Back swing' is applying more 'modulation' to a carrier than the total power of that carrier. The excess produces a 'negative' signal which decreases the total power of 'modulation' + carrier and so that output meter shows a drop in power... a 'backswing' of the meter's needle.

All of the above is an over simplification, it isn't scientifically accurate. It should give you the idea though. I don't have any way of drawing the wave forms to show that so some imagination would be nice.!

'Modulation' means a variable frequency signal (voice).
'Carrier' means a constant signal of a fixed frequency.
When that 'modulation' signal is combined/applied to the carrier signal it produces an Amplitude Modulated signal... AM -mode- signal.

Single Sideband signals are still Amplitude Modulated signals but it has the carrier and one sideband of that AM signal removed.

Nothing 'new' in this, just a different way of saying it...
Paul

 

[Paul-W5lz]

WET Dummy loads!
The fluid used in a wet dummy load is to dissipate the heat produced by that resistor in the dummy load. That fluid needs to be able to handle a lot of heat. So, a flammable fluid isn't a good idea, right?
Some of the commonly used fluids are mineral oil, transformer oil (not too common/cheap), and peanut oil. Pick whatever is easiest for you to get/$$$.
The two important factors are the stuff needs to be nonconductive, and can dissipate a large amount of heat. DON'T use motor oil, transmission fluid, etc!!
Paul

 

[543_Dallas]

Here's a wet dummy load that wont boil over, spill or stink up the shack.

http://www.ebay.com/itm/25-KW-Broad...938597?hash=item2a87247025:g:DuwAAOSw-0xYcTHN