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Date: Sat, 6 Dec 1997 19:45:33 -0500 (EST)
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To: topband@contesting.com
From: tenex communications <vertex@execulink.com>
Subject: BEVERAGE PREAMPS
The Canadian postal strike is over-those of you who sent a request for the
info on the 73 Magazine preamp design should have it in about 2 weeks-they
say there is a backlog of 27 million pieces of mail! WOW-at the rate those
guys work you might not get the info until sometime in 1999!!
FURTHER INFORMATION ON CHEAP/SIMPLE PREAMPS
I had donated to me one of those Ramsay 1 mhz-1.3 ghz broadband jobs which
has about 10-20 db gain with a 3-5 microvolt db NF. I used one of these
years ago to beef up the front end of an aging transceiver, and it
worked....after a fashion. The gain was so great that it tended to overload
easily. But it did cure the deafness on 15 and 10 meters. On 20 and lower,
it required a bit of "taming".
I saw an earlier post asking if these Ramsay preamps would work with
Beverages etc. on topband. Well, they I found that they will...but you will
need a good tuned front end and most likely the BC band filter from an
earlier post by Frank, W3LPL. For those interested in trying them, the
model number is SA-7 and they cost about $8 in kit form. The design is set
up for low impedence input, so a 9:1 transformer in front should put you in
the ballpark for use with a Beverage, though not necessarily the best match.
Oh...and don't forget the grain-of-wheat bulb in series with the antenna
lead to protect you from Thor's revenge!
As I said, they work...not nearly as well as the FET preamp with biased gate
follower, but well enough to give an EWE or short Beverage a good boost. My
suggestion is to run this preamp from about 9V or less and so keep the gain
down to about 10 db or less. The only real limiting factor with this type
of preamp is the fact that the NF seems quite high. The FET preamps will
have a significantly lower noise figure, especially if you use a fairly hot FET.
ANOTHER CHEAPIE!
I ran across this one at a recent fleamarket. It is a single transistor
(2N2570??) made by a company called DIGITREX of Rochester, Mi. (I have no
idea if they are still in business). They made a wideband preamp called the
PA19 with up to 19 db gain from 2 to 200 mhz at a max. noise figure of 4.5
db. This unit is also set up for 50 ohm in/out, but as with the Ramsay unit
above, requires a tuned input and BC band filter to be effective for all but
those who live in the true wilderness. For someone not willing to build and
test a preamp, this would be a good starter, though you sure don't need or
want 19 db gain!
The purists say that a properly matched and fed Beverage does not need a
preamp, and I guess this is quite true. However, how about the IMPROPERLY
fed Beverage? I have one Beverage at present which is optimum (880
feet),properly matched, has good F/B but unfortunately is fed through about
200+ feet of CAC6 75 ohm TV coax buried in a conduit out to an adjacent
field. This antenna definitely requires some help with the really weak
ones(or is it just that my ears are aging?) A 10db preamp makes all the
difference, and helped in adding significantly to my countries worked total
over the past year. I have chosen to install mine at the antenna and keep
the gain to about 6-8 db (by adjusting the bias on the source of the
amplifier FET). I am one of those lucky enough not to be bothered by BC
interference.
So, for those of us who can't construct the "perfect" Beverage or are using
the EWE or other low-noise receive antenna, the above info on preamps may be
useful. I would like to hear from those who have had some experience in
using preamps with "imperfect" antennas, and I am sure some others on this
reflector would too.
73, Bill, VE3CSK
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