[TowerTalk] Tower and antenna decisions

Jim Lux jimlux at earthlink.net
Sun Oct 27 00:12:27 EDT 2013

On 10/26/13 6:35 PM, Richard Solomon wrote:
> The point I responded to was the statement that some amps fold-back with
> very low SWR's. Clearly the use of an antenna tuner was not included.
> The fact that most (not all) SS Amps have Automatic Antenna Tuners just
> reinforces my statement.
> 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).

It's not that the LDMOS device inherently tolerates bad swr, it's that 
when operated at maximum dissipation, it has lots of headroom on the max 
voltage and max current.   A 125V part operated with a 50V supply isn't 
ever going to overvoltage, even with a worst case mismatch.

> Clearly, the Tube Amp beats any of the SS Amps today, hopefully that will
> change in my lifetime.

The tube amp, with 2 or 3 manually operated tuning controls isn't really 
a fair comparison against a low cost SSPA.  The tube amp wont match a 50 
ohm load without touching the controls across 2-30 MHz, while the SS amp 
will.   Hams, in general, buy inexpensive amps without a lot of 
automatic protection or tuning.  Tubes, because of their physical mass, 
in a low cost design, are more tolerant of user errors than 
semiconductors in a low cost design.

Spend some money on a SSPA with suitable design margins and protection 
circuitry, and I think it will be quite competitive against a tube amp 
with similar protection and autotuning output.

You go out and spend a few 10k on a big SSPA from Amplifier Research


which is a nice 1000W unit for about $80k that will tolerate pretty much 
anything you care to hook up to it and works from 10kHz to 225 MHz.
It also only takes 1 milliwatt of drive, so it's got a lot more gain 
than the usual 10dB ham PA.  It has pretty bad THD, though, -20dBc.

So... bringing this back to antennas.. it's a system engineering and 
tradeoff thing.  You can build an antenna system which presents a good 
match to a 50 ohm source over a wide band, either by adding elements or 
adding an autotuner or making the antenna itself tunable. Or, you can 
build an amplifier which can match other than 50 ohms.

Part of this is driven by the FCC rules for amateurs limiting output 
power.  Putting the tuner inside the amplifier (e.g. the pi network in a 
tube amp) means you can run more power than putting the tuner inside the 
antenna, because any losses in the tuner are ahead of the power 
measurement point.

If you measured the power at the antenna feedpoint, claiming that your 
amplifier ended there, then that would be less important.

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