Filters ?
Anyone know of a schematic for or an inexpensive output filter for an FM x-mitter?
My fundamental frequency is 107.9mhz and I would like to antenuate my 1st harmonic at 215.8Mhz which is the same as TV CHannel 13's audio (215.75 Mhz). I have a (bitchy) neigh-boor who complains about hearing me on CH 13 even though there is NO TV station on Ch 13 in my area! She also thinks that my signal is "damaging" her "antenna"!
Anybody have an idea?
You could try a quarter-wave shorted coax stub filter if you can increase your transmitter output power to make up for the coax loss. Go to Figure 6 at the bottom of this Web page for more information:
http://home.att.net/~weatheradio/HFtransmit.htm
Thanks for info. I won't raise my power any more than I'm at now...6-7 Watts! I get nervous the way it is!
On that same page, just a little above the "stub" info,
http://home.att.net/~weatheradio/HFtransmit.htm
is an easy schematic for a low pass filter. What I'm wondering is how do I find the values for the caps and # of loops in the coils?
1. If you're running 6-7 watts, you shouldn't have to worry about a bit more coax loss from a stub filter (and the extra line from filter to transmitter). I thought you were running a watt or less.
2. Regarding the LP filter in Figure 5:
a. I don't know what the proper L & C values would be for VHF.
b. Since your primary concern is second harmonic suppression, I think the coax stub would still be easier to do. The length formula is shown below Figure 6. You should be able to find the velocity factor for your coax online. Plug in 107.9 MHz to get your stub length, then use half that length for the coax between the stub and the transmitter. If you're using scrap coax and have to guess the velocity factor, I'd use 0.66 for solid plastic center insulation and 0.8 for foam center insulation.