[TenTec] FW: Preselector

[Rick - DJ0IP / NJ0IP]

Hi Ed,

The Braun has 4 tuned circuits per band.
The amount of attenuation you can get depends not only on the number of
tuned circuits but also on the Quality of the tuned circuits.
The Braun uses "Topfkern" which is a technology that completely surrounds
the coil windings in ferrite.
I have no idea what the English term for Topfkern is.

Earlier I was quoting from memory (which is always dangerous at my age).
I said the ultimate attenuation was about 50 dB.
Now that I have re-read the manual, I see the ultimate attenuation is 40dB,
@ 10% away from the passband.

40dB attenuation will generally give you about 80dB to 100 dB attenuation of
intermodulation.
Intermod degrades much faster than the attenuation.  Something like 2:1 or
even 3:1.

As you saw in some of my demonstrations, 18 dB of attenuation using the
radio's built in attenuator ( which would be 3 S-Units), was dropping an S8
or S9 phantom to about S1 (42 dB to 48 dB drop).

Of course we don't know what the accuracy of the S-meters were on all of the
transceivers I tested.
Never-the-less, you can clearly see through my practical examples, just how
much a good preselector can help.

6dB attenuation (insertion loss) of a good preselector will get you 30 to 40
dB of attenuation; MUCH better than using an attenuator.

However most people do not need this if they are using one of today's top of
the field radios.
(See Rob's list for  this).

The only case that comes to mind is a multi-transmitter Field Day operation,
but in that case it is better to use the TX filters.  These work as a band
pass filter on TX AND they work as a band pass filter on RX.  These cost
less than what I paid for my Braun filter about 30 years ago.  (I paid 800
Deutsch Mark back in the early 1980s).


73
Rick, DJ0IP


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Edward A.
Feustel
Sent: Wednesday, October 23, 2013 11:29 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Preselector

Rick,

How many tuned circuits are in your home built preselector? At one point I
had an RME DB-22a preselector which had 3 tuned circuits with two active
stages: http://www.ohio.edu/people/postr/bapix/db_22a.html. It really helped
with a Hallicrafters SX-71 and an SX-28a, but would be overlarge for a
modern rig!  If I recall you get about 6 db attenuation per octave with each
tuned circuit (element?).
Now most rigs seem to use octave bandpass filters instead.

73,
Ed, N5EI