TopBand: Re:High input intercept devices

[YO3CTK]

Hi Tom,

The figures you quote are consistent with what I have measured in my
amplifiers using 2N5109. About 8dB gain and 1 to 50MHz bandwidth (with
correctly designed transformer), and input VSWR less than 1.5.

The reason of introducing a pre-amplifier into the receiving line is to
establish a low noise figure for the system. Although this may be not
important for 160m only, it is certainly a dominating factor when designing
a 1 to 30MHz receiver. I have no experience with loops, but I worked with
small HF receiving antennas (whips and similar) and they need a signal
boost, too.

73 de Mike, YO3CTK


-----Original Message-----
From:	Tom Rauch [SMTP:10eesfams2mi@mass1-pop.pmm.mci.net]
<mailto:[SMTP:10eesfams2mi@mass1-pop.pmm.mci.net]> 
Sent:	None
To:	yo3ctk@alltrom.ro <mailto:yo3ctk@alltrom.ro> 
Subject:	Re: FW: TopBand: High input intercept devices
> Date:	Thu, 09 Apr 1998 19:00:48 +0300
> From:	YO3CTK <yo3ctk@alltrom.ro <mailto:yo3ctk@alltrom.ro> >

> I'd say that one of the best solutions is to use a good old 2N5109 in
> non-dissipative negative feedback circuit (patented by Norton). About 25mA
> of collector current yields a +30dBm input IP3 with a very low noise and
> 50ohms input and output impedance. This arrangement has been used with
great
> success by Rohde&Schwarz in their EK070 VLF/MF/HF communication receiver.
> The amplifier use a specially wound trifilar transformer, which is easy to
> do after careful study of the patent.

Hi Mike, 
Ulric published the details in RF Design magazine back in the early 70's.
VK5KL and myself mailed some preamps back and forth using that design. I
used LT-1001 gold metalized CATV bipolars, and obtained a noise figure of
about 2 dB with near 30 dB IP3 intercept.
However, this low noise figure is totally unnecessary with a Beverage
antenna... even in a very quiet rural location. On six meters, with an array
pointed away from noise sources, it may be a good idea. But on 160 any noise
figure less than 20 dB is generally enough at night, so the extra work just
isn't worth the effort.
They do make excellent loop pre-amps.
73, Tom W8JI
w8ji.tom@MCIONE.com <mailto:w8ji.tom@MCIONE.com> 

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