[TowerTalk] Tower and antenna decisions

[Jim Lux]

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.