Generally, on most bands, we can "hide" in the band crud when the amp is
between -25 to -30db 3rd order. This includes bands like 40M at night, 20M
during a contest, 160M when the band is QRN noisy, or say 75M when the band
is long.
However, take 75M at late afternoon/ local sunset. Here on the east coast,
Eu is just coming in at S7 and the JA's LP are being heard just above the
noise floor. At the same time, the locals are high angle at S9+50 over. Even
a "clean" signal at -40db 3rd order will have S9 trash, 4kc up the band.
Takes out the weak DX.
I look at it this way - it's like going to a party with a cheap, out of
style, ill fitting suit. You worry about how you look and don't have a good
time. With a marginal amplifier on the hairy edge, you worry about who's
above you and below you, etc. Put on a good suit or clean amp - you can go
anywhere you want and have a good time without another thought... :-)
Speaking of amps, I've started building my new "-55db 3rd order" amp with
the 4CX-250 class A driver and 4CX-3000 final using 20 db of RF negative
feedback around this chain. The air sub chassis and grid circuits are
finished so far. We'll see how this works out. It will be a real thrill to
get on 75M DX window with this amp if it works as intended.
So, yes, I think there is a tremendous benefit to squeezing out an extra
10-20db of cleaner IMD, if possible. The improvement cannot be heard on
frequency, but your adjacent channel neighbors will love it when the band is
real quiet!
BTW, I am interested in what other guys have to say about Hal's question
concerning amplifier components affecting IMD. Aside from leaking or
arcing, what other factors affect capacitors and other amp parts to degrade
IMD?
73,
Tom, K1JJ
> Bill Turner wrote:
>
>> In other words, if an amp already meets FCC specs for IMD, what does
>> a five or ten dB improvement get you?
>>
>> It makes me think of Charlie Brown's statement: "It's like wetting
>> you pants wearing a dark suit. It gives you a warm feeling but nobody
>> notices." :-)
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|