Hi,
A 'stub' is a short, stubby, piece of coax cable, which you can easily assemble yourself.
It's basically a measured length of coax, with a short circuit or open circuit at one end, and joins to your main cable with a T connector.
The measured length is based on the frequency of the signal you wish to remove (filter out). the length is normally a 1/4 wavelength. Because when the signal has traveled down the stub, and been reflected by the short or open cct, it has then travelled exactly ½ wavelength, and cancels itself out.
The stub becomes a tuned filter to remove the unwanted frequency, without impacting on your normal operating frequency.
It has no impact on your operating frequency, as normally the frequency you want to remove is 2x (2nd harmonic) or 3x (3rd harmonic) of your operating frequency - ie: so far away that the filter charteristics of the stub don't impact on your operating frequency at all.
So if you operated on 107.5MHz, your second harmonic would be 215 MHz, which is within the ch9 TV channel.
215MHz has a wavelength of 1.39m (300 / f MHz = wavelength in m)
1/4 wavelength = 349mm (1.39m / 4)
You also have to take the velocity factor of the coax cable into account, so then multiply the length by the VF, which for RG213 is around 0.66.
So if you used RG213 your stub would be around 349mm x 0.66 = 230mm long.
Now that is all from memory - so someone please correct me if my memory is rusty :-)
It is normal practive to fit stub filters - we do it on our high-power broadcast sites as an effective means of reducing harmonic products whenever necessary.
Happy stub building :-)
Jochen
>>> radiopegasus@... 15/03/05 >>>
Please could our technical whizzes..morning Jochen...tell me what a STUB CUT ia...where do I purchase... or can it be easily assembled.
Thanks to Jochen.. Im aware of a cavity filter..
Regards Davewihamsteve" <kiwisteve@...>
>Reply-To: LPFM_Radio@...
>To: LPFM_Radio@...
>Subject: [LPFM] TV Interference
>Date: Mon, 14 Mar 2005 19:35:57 -0000
>
>
>Re Interference to TV from LPFM .
>I have experienced cases where the 88 end of the band causes
>interference to CH4 VHF and the 107 end causes interference
>to CH9 VHF . This is normally the second harmonic of the LPFM
>transmitter. Fit a stub cut for the freq you are on from RG213 coax
>cable and N connector,
>use an N type T connector to insert it in the feeder . Or if you have
>1 or 2 cavity filters they remove a considerable amount of second
>harmonic content . normally only those combinations of
>channels at very
>close range and in areas of poor TV reception or if the TV owner has
>a faulty antenna , feeder or mast head amplifier .
>
>SteveJ
>
>
>
>
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