[Premium-Rx] Mini-circuit amps

Sun Nov 13 07:09:10 EST 2005

A receiving system using the T2FD has a "head start" IM-wise, as the average capture area for the
T2FD is about 25% (or -6 dB) of a dipole. 

This leads to a considerable advantage regarding the strongest signals
that have to be considered, and as its frequency response falls off quickly below the lowest design frequency,
it also helps to mitigate the AM broadcast problem.

I am well aware of the different conditions between Europe and the American continent,
the AM broadcast problem is for all practical purposes non-existent here, but on the other hand
there is a proliferation of sky-wave propagated SW-BC signals here.

If a spectrum analyzer is connected to a log-periodic beaming south or south-west,
at least 100 signals will be found night-time that are in excess of - 30 dBm, and perhaps 10 that are > -20 dBm.
Daytime they are somewhat fewer, but a few can be stronger.

Recently I have supervised repair work after a direct lightning hit to a large log-periodic array belonging 
to one of our customers, and made a few measurements to check the system performance.
These measurements indicated that the night-time noise floor at 6 - 8 MHz was around -110 dBm 
in an SSB bandwidth, and that the strongest signals were in the - 18 dBm range.

The levels are in line with the experiences from others, notably the spectrum occupancy measurements
made at Univ. of Manchester, and the data referenced to by G3RZP in a recent QEX article.

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Ben Dover" <quixote2 at ix.netcom.com>
To: <premium-rx at ml.skirrow.org>
Sent: Sunday, November 13, 2005 11:01 AM
Subject: Re: [Premium-Rx] Mini-circuit amps

>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
>
>

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