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Re: [TowerTalk] Tower and antenna decisions

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Tower and antenna decisions
From: Jim Lux <jimlux@earthlink.net>
Date: Sat, 26 Oct 2013 21:12:27 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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

http://www.arww-rfmicro.com/html/18200.asp?id=908

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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