Topband: Re; LF, MF Preamps

[Joe Reisert]

AMDX wrote:

>   The Nov 21 1994 issue of DXnews has an article titled "Ultralinear
>2N5109 and=20 2N3053 Amplifiers". Back issues or copies of the article may 
>still
>be available from the NRC. http://www.nrcdxas.org/catalog/  Articles A87, A88


Mike,

Thanks for the info.

I published two high dynamic range easy to build preamplifiers similar to 
what you described in Ham Radio Magazine. They were in my VHF/UHF World 
column in November 1984, page 97. Don't let the VHF/UHF throw you. Read on!!

There were two preamplifier designs in this article. The first used a 
standard feedback amplifier circuit with a 2N5109 transistor. It had a very 
high output intercept point (+26 dBm), a good input and output VSWR (to 50 
Ohms), a gain of about 14 dB and a low (4 dB) noise figure. It was useable 
from 1-70 MHz.

The second preamplifier (probably similar to what you described) used one 
of Dave Norton's low-noise wide bandwidth transformer coupled lossless 
feedback techniques but with readily available components for home brewers! 
It is the grounded base transistor type with an excellent input/output 
VSWR, about 9 dB gain and an output intercept point of +39 dBm!!! I used a 
binocular ferrite for the transformer. All details are in the article.

This second preamp worked from 1.8 to 200 MHz as is and could be improved 
at 1.8 MHz by using larger capacitors (0.1 uF versus 0.01 uF). For many 
years I used this same preamp on my Beverage antennas with great success. 
The only problem was that if it was left in line during the summer, 
lightning strikes would sometimes burn out the transistor even with a 1.6 
MHz high pass filter ahead of it! It was near impossible to overload it and 
the low gain (9 dB) and excellent 1.5 dB noise figure made it ideal for low 
noise antennas.

I would highly recommend this second amplifier for anyone on 160 that 
is looking for top-of the-line performance.

73,

Joe, W1JR



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