[TowerTalk] Tower and antenna decisions

[Grant Saviers]

Jim,

You're the audio expert, but didn't Bob Carver figure out some clever 
variable supply voltage tricks with his big Sunfire amps?  I have two 
and my "Signature" stereo is rated 2 to 16 ohms (1250w/ch 4 ohms) and 
runs stone cold even when delivering big power.  No other amp I've tried 
had the headroom needed for my B&W 801Fs to generate max orchestral 
peaks at concert hall levels.

So isn't what you are suggesting been done before in the audio domain?

Grant KZ1W

On 10/26/2013 9:21 PM, Jim Lux wrote:
> On 10/26/13 9:05 PM, Richard Karlquist wrote:
>> On 2013-10-26 18:35, Richard Solomon wrote:
>>
>>> It may take some innovative fellow to come up with a SS Amp that 
>>> behaves
>>> better and my mention of LDMOS devices may be the way to go.
>>>
>>> Some of the newer LDMOS devices are quite robust, and seem to be
>>> oblivious of SWR (to a point).
>>>
>>> 73, Dick, W1KSZ
>>>
>>
>> The recent activity in the LDMOS arena with various manufacturers
>> trying to outdo each other on maximum SWR specs is not relevant
>> to this discussion.  This is because these specs are merely
>> survival specs for the active device.  It doesn't mean that the
>> device can deliver useful power into a mismatch.
>>
>> Unfortunately, there is no "innovative" circuit design or state of
>> the art device that will make a SS amp operate into a 2:1 SWR
>> efficiently.  If you want to deliver 1500W into any
>> load with better than a 2:1 SWR, you basically have to build an
>> amplifier that can deliver 3000W into 50 ohms, but only drive
>> it hard enough to get 1500W into 50 ohms.  It will also deliver
>> 1500W into any 2:1 SWR, but the gain will be somewhat higher or
>> lower than for 50 ohms, depending on the angle of the load impedance.
>
> It doesn't have to be able to deliver 3000 W to a 50 ohm load, it has 
> to be able to deliver 1500W to a 100 ohm or a 25 ohm load.  It depends 
> on what the limiting aspects are. One could build an amplifier that is 
> power supply limited but that can put 100V into a 50 ohm load, and 
> 140V into a 100 ohm load (for the same RF power), but that draws 70% 
> of the current at the high load impedance.
>
>>
>> The heat dissipation will basically double as well, and the power
>> supply will have to produce about 40% more current.
>
> That's not necessarily the case.. if the amplifier has variable bias 
> and drain voltages that adjust to what's needed for the match, you can 
> optimize the efficiency at all powers. A simple design with fixed 
> Vdd/Vcc, yes, you'll have a bigger power supply and efficiency will be 
> bad.
>
>
>  All that extra
>> heat will heat your shack, which is not good in the summer time.
>>
>> After considering the above, an outboard tuner looks like a much
>> better option.  Or use antennas that are well matched in the first
>> place.
>>
>
> Or a tuner at the feedpoint, or an antenna that is tunable.
>
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