[Amps] MOSFET amp filtering - was: auto-tune

[Jim Brown]

On Mon,12/12/2016 5:32 AM, Catherine James wrote:
> I hear a lot of recommendations here for antennas that are flat across the band and very close to SWR of 1:1 to keep solid-state amps happy.  That is completely unrealistic on the low bands.
>
> I have to re-tune my 160 meter dipole for even small excursions of a few tens of kHz.  A cage dipole would be broader-banded, but that is difficult to build and install, especially for a 270 foot long antenna that hangs in the trees.  I certainly couldn't put one up in the treetops where my current wire dipole lives, as it would get all tangled in the branches.

Excellent post, Catherine. You have hit the nail squarely on the head.  
However -- even a dipole at 100 ft is "low" for 160M, so it's both 
inefficient and radiates more at high angles than low.

A FAR better choice for something suspended between trees is a Tee 
vertical, where a flat top wire provides top loading for the vertical 
section, which does the radiation. Because antenna current splits 
equally left and right into the top wires, radiation from the top 
cancels, and you end up with a nice vertical radiator and a nice low 
angle of radiation. My primary TX antenna for 160 is a Tee vertical hung 
between trees, and I've broadbanded it by making the vertical section 
two #10 wires spaced by about a foot. This approximately doubled the SWR 
bandwidth.

Any end fed wire needs a counterpoise, either in the form of radials or 
some wire to carry the return current. If you don't have room for a 
radial system, K2AV's "Folded Counterpoise" is a very good alternative. 
Lots of ideas about 160M antennas in these slides for a talk I've given 
at Pacificon and to several ham clubs. http://k9yc.com/160MPacificon.pdf

AND -- there's another point that virtually EVERYONE who has commented 
in this thread seem ignorant of.  ANY distortion mechanism produces BOTH 
harmonics and INTERMOD. On SSB, it shows up as splatter, and on CW it 
shows up as clicks (remember that CW is 100% AM of a carrier by a 
rectangular wave, and the rise and fall of ANY rectangular wave consists 
of an infinite number of harmonics that excite IM.

An RF amplifier that is poorly matched to its load creates a lot more 
distortion. Yes, we can filter the harmonics, but we can't filter the 
IM.  This is NOT a matter of making the load EQUAL to the source 
impedance, but rather of providing to the output devices, solid or 
hollow state, the load into which they work most efficiently and are 
most linear.  I'm sure that most readers of this list are old enough to 
remember load lines. :)

>
> So much of discussion and recommendation around antennas seems to unconsciously assume that we are talking about the high bands.  I've lost count of the number of discussions where someone asked for a reco on an HF amp, and was told over and over, "don't start with an amp, improve your antenna system, put up a beam, etc."
>
> At this point in the cycle, I am spending more and more time on 160, less on 20 and 40, and essentially none at all on 10 and 15. Few hams can put up a beam on the bands below 20.  The longer the wavelength, the wider a given band will be as a fraction of that wavelength, and the less broad-banded the antenna will be without tuning. Tuning is a fact of life, and amps are more important on bands where the ham cannot have a rotatable directional antenna.
Again, exactly right. I have high fan dipoles for 80 and 40 (two at 
right angles to each other), and have switched stub matching networks 
for the CW and SSB bands.

> 160 has been amazing lately.

Yes, it has, although what you can work depends a LOT on where you live. 
Those near the Atlantic get nice openings to EU over an all water path. 
 From the west coast (I'm near San Francisco) we must go over the polar 
path. I haven't even HEARD EU in three years, and I have a 550 ft 
Beverage aimed to EU. :)  OTOH, I've been monitoring the JT65/JT9 
frequencies (USB, suppressed carrier frequency of 1838 kHz) for a couple 
of weeks, letting the decoder run all night. In about 3 weeks, I've 
decoded signals from more than 400 different stations, many from east of 
Chicago, a few from EU, VK, and BA.

>   10 doesn't appear to have opened at all for the contest last weekend, at least not here in New England.

That depends entirely on when you were in the shack. I worked 44 states 
with 1500W and a 3-el SteppIR. The only states I missed were DE (N3DXX 
was active, but not in the few hours the double-hop sporadic-E path was 
open between us), WY, MO, IA, NE, SD, and ND. I also missed the VE 
provinces east of VE3 and NT. I chose to only work  CW.  I also worked 
20 countries, but no EU, AS, or AF. DX was double-hop to the Caribbean 
and trans-equatorial to SA and OC.

> And 160 is where you really, really want an amp!

Yes. But having done WAS on 160M in a weekend contest, first with legal 
limit, then with 100W, I now need only VT and SC to finish it QRP.   
I've been at that for four years.

73, Jim K9YC