[CQ-Contest] Stub and low pass filters

Marco Spalluzzi mspalluzzi at iol.it
Mon Feb 5 17:36:32 EST 2001

Hi guys, I have to report something that maybe can be normal for you, =
but it sounds strange to me.
I made some tests with a friend of mine using stubs and low pass =
filters, in order to reduce harmonics causing TVI.
The main problem in my area is caused by the 3rd harmonic when tx on 21 =
MHz, cause it falls exactly on the frequency where it's broadcastig one =
channel of the italian national tv.
Since we can't see if there's any reduction in the harmonic's amplitude =
when on 21, we decide to run a test on 28 MHz, and see what happens to =
the harmonic on 56 MHz, clearly spaced from 63 MHz on the spectrum =
We first use a stub, 1/4 wavelength at 28, shorted.
We cut it to the proper length to obtain a notch at 56 MHz (about 26 =
dB), then we placed it on the tx line.
SWR didn't changed, and that's correct.
The amplitude of the harmonic was the same. Reduction equals 0. It looks =
like it was not connected.
We connect it also along the tx line; nothing changed.
Then we connect it to the spectrum monitor: we saw a reduction of about =
7-8 dB of the tv signal at 63 MHz; using a better cable, or 2 stubs, we =
can sure make a more narrow and deep notch, but this is another story.
When transmitting, harmonic disappeared.
What a surprise: it seems to be that it's working, but only on the rx =
W H Y ?????

We made the same test using a low pass filter: on the spectrum scope we =
can see that reduction over 40 MHz is 80 (yes, eigthy) dB.
To be sure we placed it on the tv coax: every signal disappeared, and =
that's normal.
Then we placed it on the tx line: tvi reduction equals 0.
So, also in this case it seems performing very well, but only on the rx =
W H Y  ?????

If 28's second harmonic gives you troubles, you can solve the problem by =
placing the stub on the tv antenna receiving the channel(s) with =
interference (prior to mixing signals, or notch on higher harmonics can =
fall exactly on other channels!!!).
But when on 21, you can't place the stub on the tv side, or you'll =
simply null the tv signal!!!

The main question is: why are they working exactly as expected only on =
the receiving side?

Marco - IK2XSL.

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