A well constructed beverage should rarely require more than ~ 10dB of
preamp gain. Many preamp users are mistakenly trying to get their S
meters to read the same as their regular TX antenna when in fact the goal
is improved S/N ratio. The combined directivity of a Beverage and the
10-15dB of peak signal strength loss as compared to many TX antennas
contributes to less RX overload problems. I use 3-10dB max of gain in a
tuneable preselector configuration. S Meter readings are not important to
me; digging into the noise is.
The QST Aug 88 preamp was aimed at snake ( NOT Slinky) and loop users
where very high gain is required. That preamp however is a very poor
choice for a good Beverage (never tried a EWE so no comment) since it:
A. has way too much gain
B. the MC1350 IC used has poor overload performance and was
developed as an AGC controlled IF amplifier.
C. reducing gain in that design can add up to 1dB to the RX noise
figure for every dB of reduction.
D. Since we start off with 40-45 dB of gain, a 20-35 dB reduction
can have a negative impact on weak signal performance.
E. A better use of that circuit would be to add a DPDT switch to
bypass the MC1350 entirely when not needed and just use
the ~ 10dB of gain from the MPF102 first stage. It would
then make it a better "all around" unit.
F. Remove the back to back diodes from the input to further
reduce IMD problems in strong signal
environments (particularly BC band). There are much better
ways to obtain front end protection.
I have built this circuit and still use with a loop; it does a good job
there. I have also sold over 200 kits over the years and am sharing here
many of the feedback comments that were received.
IMHO a low parts count or cheapest design may be self defeating;
particularly after spending all that $$$ on a new rig, or time on an old
one with PIN diodes, to wind up with a preamp that has worse performance
charateristics than the rig. Look very carefully at the design, specs and
source of the article; then pick what best suits your requirements. One
size may not fit all.........
73....Carl KM1H
On Tue, 7 Jan 1997 09:38:00 -0500 (EST) vertex@mercury.execulink.com
(Tenex Communications) writes:
>A short while ago, AA1K I think passed along some info about Beverage
>preamps and said he had an article on constructing one with a very low
>parts
>count. I think that was in June 73 Magazine p. 55. There are several
>other
>articles if anyone is interested; among them:
> QST April, 77; QST Dec. 77; QST Aug. 88
>
>I have built a few of the preamps from this last QST article by W1FB,
>and
>they work great. The parts count is much greater, but the gain is
>adjustable and they are a 2 band affair. I like this preamp, and
>have two
>operating now. If you don't want to breadboard, there is a board
>available
>from FAR circuits.
>
>I use a relay to switch remotely the band (80 and 160). On 80, this
>preamp
>is a real tiger with my NE 880 ft Beverage; on 160 it does a very good
>job,
>but the results are not as startling as on 80 where the long wire
>provides
>greater starting gain.
>
>The only problem I have encountered is overload, but if you keep the
>RF gain
>turned back this can be overcome nicely while still providing good
>gain.
>
>I have enjoyed the comments by all about whether to preamp or not to
>preamp.
>As far as I am concerned, a preamp is a necessity in my situation,
>where my
>receive antennas (both Beverages and ewes) is perhaps not the best
>(ground
>too moist for really good Beverage operation, not enough room in some
>directions), but I realize that preamps are not necessary in every
>situation.
>
>>From what I have seen, noise is not a problem with any of these
>preamps. I
>built one of the single FET jobs a few years ago as a broadband
>affair, and
>even with that one noise was not a problem.
>
>73,
>Bill, VE3CSK
>
>
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>
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