[Premium-Rx] Mini-circuit amps

[Ben Dover]

>Date: Sun, 13 Nov 2005 04:00:35 -0600
>To: Karl-Arne Markström <sm0aom at telia.com>
>From: Ben Dover <quixote2 at ix.netcom.com>
>Subject: Re: [Premium-Rx] Mini-circuit amps
>In-Reply-To: <006801c5e82a$123598a0$bc01a8c0 at speedbox>
>References: <fc.2060bffa.30a80224 at aol.com>
>
>Hello Karl Arne.
>
>>>A word of advice regarding the ZFL-series and similar amplifiers.
>
>They are single-ended, so their IM2 performance is much worse than most
>receivers having sub-octave preselection. 
>
>My experience, earned the hard way, is that it takes a balanced amplifier
like the AM-109, 
>the HELA-10 or the circuit used in the R&S NV14 multicoupler to properly
handle the output signals from a 
>multi-octave gain antenna like a log-periodic or a rhombic.Amplifiers of
the ZFL class connected directly to 
>a large log-periodic convert the European HF spectrum to a mess of mixing
products even in the best of "Prem-RX's". <<
>
>I agree wholeheartedly!  
>
>My multicoupler presently uses a monster of an amplifier, the ZHL-6A-BNC;
a LARGE RF power amplifier
>with a decent noise figure. Besides having the amp available at the time,
my reasoning was that if the
>input signal has no prayer of driving the amp into the 1 DB compression
point, intermod problems are kept
>at bay. It seems to be working here.
>
>Second, gut instinct told me that driving a splitter requires a power amp,
not a voltage amp; this thing
>fills the bill quite nicely.
>
>I tried the AM-109 amp here, and was very pleased with it's performance,
but I rejected it for a couple of
>reasons. First and foremost, I only have one AM-109, and my multicoupler
is a dual channel unit (intended
>to be used with two identical T2FD antennas, one horizontal and one
vertical, for polarization diversity
>reception).
>
>The next reason for rejection the AM-109 is actually an extension of the
first; availability of the Anzac
>products seems to be difficult at best. Obtaining a second amp, or
replacements, at reasonable cost (read
>that as "I'm a cheapskate"!) can be a significant problem. 
>
>Finally, the AM-109 requires a split power supply (i.e., Op Amp style),
simply because it IS a balanced
>amplifier. Not a HUGE problem, but I felt it was an unnecessary irritation
and a big complication to my
>design.
>
>
>>> If the input spectrum is limited, the situation becomes much better. <<
>
>Once again, I agree; any time you limit the input spectrum you're going to
improve the overall noise
>figure of an amplifier based device, as well as limit the possibility of
intermod products.
>
>On the other hand, by doing so you limit the versatility and ease of use
of the gadget, while increasing
>it's complexity (i.e., you're making it bandswitching).
>
>My biggest concern, because of the huge headroom I designed into my
multicoupler, was not intermod from 
>HF signals, but from the intrusion of VHF signals at the input. To deal
with that I only used a 30 MHz
>low pass filter on the amplifier input in an effort to keep FM and TV
signals out of it. So far, I've 
>been marginally successful; we're still playing with it some. Then again,
besides the usual TV and FM
>stations to contend with (aprox. 15 - 20 miles away), I have one FM
station within a mile of me, a
>directional AM station, and one HELL of a hot police department base
station within .75 mile of me. 
>Since the design bandwidth of my multicoupler is 100 KHz - 30 MHz, the AM
is the biggest problem, but
>I am trying to avoid the use of a trap to suck it out (that would limit AM
BCB usefulness). The FM and
>the police radio will probably wind up being sucked out with series tuned
traps.
>
>
>73's,
>
>Tom, W9LBB
>
>