Topband: SDR-1000 IMD-DR, blocking dynamic range, IP3, and IP2 measurements

Michael Tope W4EF at
Fri Feb 16 07:10:05 EST 2007

----- Original Message ----- 
From: "Tom Rauch" <w8ji at>
To: <topband at>
Sent: Friday, February 16, 2007 2:54 AM
Subject: Re: Topband: SDR-1000 IMD-DR, blocking dynamic range, IP3,and IP2 

>> This thread puzzled me as it seemed that Tom and Bob were
>> vehemently agreeing with each other. Let me explain:
>> a) What did Tom W8JI say when asked what was so bad about
>> the SDR-1000?
>> "It has essentially the same dynamic range regardless
>> of tone spacing"
>> b) What did Bob N4HY say when asked what was so great
>> about the SDR-1000?
>> "It has essentially the same dynamic range regardless
>> of tone spacing"
> It would be nice if we could and stick with facts.

Yes, I am all for facts. That's why I like being an engineer
and not a politician.

> 3.) What is not noticeable or even perceived as an advantage
> at one place under one condition can be a disadvantage at
> another. This is true with everything from antennas to
> preamps to radios to headphones.

That was sort of the point of my tongue and cheek post,
Tom. Your were saying that the SDR-1000 didn't meet
your needs because the wide spaced dynamic range was
inferior to a superhet design like the FT1000MP MK V
(on-order of  80 to 100dB depending on soundcard
and other factors). Bob was saying that the SDR-1000
is superior in some respects to the Orion since inside the
Orion's roofing filter bandwidth the SDR-1000 has has
better dynamic range (on-order of 80 to 100dB depending
on soundcard and other factors). I think both of the above
claims are probably true.

> 4.) Defending agreed upon facts with long subjective
> arguments is a waste of time. It's nice just to know how
> things work.

The problem I find with facts is that people often don't
agree upon them. I have a couple of engineers working
for me now on a project who are both much better
designers than I'll ever be, but I'll be damned it they don't
butt heads constantly each claiming their facts are right and
the other's are wrong -:)

> For example there is another tale going around that multiple
> phase controlled transmitters can be used to feed multiple
> elements, one example given is a four square. Yet a 4
> square, like any typical unidirectional array with multiple
> reasonably close spaced elements, has each element
> significantly coupled to the other through mutual coupling.

I know the guy who is working on that concept. He is a
lot smarter than I am, so I won't even begin to try to defend
what he is doing other than to say I think he is fully aware of
the challenges associated with such an approach (mutally
coupling, reactive power, etc).

> Why would I replace one amplifier running near capacity and
> a few coils, capacitors, and transmission lines with four
> amplifiers requiring up to 12 matching systems....with three
> of the amplifiers being heavily underutilized...when I could
> just use a $20 switch, use fewer cheaper components, and
> have the same result? If we ran all four amps into one
> feedpoint and then phased passively at the antenna, we could
> run around 2500 watts with significantly less hardware cost
> and the same pattern.

I think the idea behind the multiple phase-controlled
transmitter is to build a frequency agile adaptive system that
can be deployed quickly in a field situation with a somewhat
arbitrary arrangement of element spacings. Clearly this
approach suffers from the limitations you describe above,
which make it of dubious value for fixed installations. Whether
or not its supposed advantages in field environements will ever
overcome its disadvantages isn't clear to me. But you never
know until you try. Most R&D produces piles of useless
crap before it ever delivers anything tangibly valueable. Maybe
he'll get to something of practical value, maybe he won't.

> It's great to build really neat things, but at some time we
> have to understand how they fit the real world and that they
> will still never be all things to all people. We need to
> know there is no perfect free lunch, but there is always an
> imperfect more expensive one.

Yes, of course, but as enabling technology evolves, things that
were formerly impractical and overly expensive now start to
become practical and economical.

73, Mike W4EF...............................

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