Newsgroups: alt.radio.pirate
Subject: Pt 2/3: FM-10 FAQ
Date: Wed, 18 Nov 1992 23:06:56 -0600

NOTE: This message was originally addressed to ALL
      and was forwarded to you by JOCK'S SLAVE
                          --------------------
I've tried this antenna and it works great!  It rivals my di-pole at home and
you can drive to a high, optimal location for your broadcasts.

You could also try a 5/8 wave length antenna, this would give you 2+db gain,
or almost 2x power gain on transmit.


Filter design for FM Radio Transmitters.
----------------------------------------
It is very important to have a clean signal,  the way 99% of all people
who get busted for illegal transmitting is that the people that live
around them complain about interference.  Do not do this, keep everyone
happy, including yourself, be clean.

When you amplify a signal, you get unwanted byproducts these are called
harmonics.  The show up at multiples of your starting frequency.  For
example if you amplify a 50mhz signal you may get echo's on 100mhz, 150mhz
200mhz, 250mhz...  If you walk on your neighbors TV, the local fire depart-
ment, or anyone else, you are going to get nailed.  If you are only on
the FM Band, you will hardly be noticed.

...
I have looked into the filter design more, I have come up with these two:
(basically scaled from the charts in the ARRL handbook).

Filter Design: 7 element Chebyshev
I run @ 88.1 so my 1st harmonic is 176.2
This filter seems ideal.

Fc               3db     20db    40db
85.8mhz         95.9mhz 116mhz  148mhz

              .132uh    .152uh     .132uh
        -----()()()-----()()()-----()()()-----
          |          |          |          |
<-50ohm   - 33pf     - 68pf     - 68pf     - 33pf  50ohm->
          -          -          -          -
          |          |          |          |
        --------------------------------------
                         |
                        ---
                         -


Filter Design: 5 element Chebyshev
Not as good as above, but thrown in anyway.

Fc               3db     20db    40db
81.8mhz         105mhz  147mhz  222mhz

             .128uh     .128uh
        -----()()()-----()()()-----
          |          |          |
<-50ohm   - 30pf     - 62pf     - 30pf  50ohm->
          -          -          -
          |          |          |
        ---------------------------
                     |
                    ---
                     -

The tuff part in the above is winding the coils. 3t of #12 wire 1/2" diam
should be about .12 uh. 4t is .17 uh. (but ugh, #12 wire is big stuff).
I may just make all the coils 3t, and bend them around a little while
watching the analyzer.  Maybe use var-caps too.  Well I will build this
weekend and try Tuesday.  I will let you know how it goes.

mycal


Very simple filter.
-------------------

                      .15uH
from fm10      >------()()()------> to antenna
                  |          |
                  - c1       - c2
                  -          -
                  |          |
                 ---        ---
                  -          -

      88mhz   102mhz     107mhz
c1     62pf     54pf      50pf
c2     62pf     54pf      50pf

its basically a scaled down version of the other filters.
it won't knock the harmonics down as much as the other two designs but
the good thing about this one is that it uses standard off the shelf
parts.  the inductor is one of those molded jobbies that looks like
a resistor so you can make this very small.

use the above cap values depending on which freq range you want to operate
at.  ie. if you run 87-90 use the 88mhz vals,  90-103 use 102mhz vals,
and above that use 107mhz vals.


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