[CQ-Contest] Bandpass filter

Timothy Coker n6win73 at gmail.com
Mon Jul 2 18:24:46 PDT 2012

If you are going to use two radios then bandpass filters (BPF) will be
needed to protect each of them when one or the other transmits, assuming
enough coupled power. Coupled power occurs with power output, antenna
polarization, and antenna proximity in mind. Bandpass filters pass whatever
frequency they are meant to pass and do nothing to stop in band
frequencies. I bought two sets of W3NQN filters for my home SO2R setup and
didn't fully appreciate why one would have a filter on the RX and the TX. I
just bought them because that is what so many experts told me was needed. I
now understand that say a 40m TX filter passes 40m signals to and from my
40m radio. If I have another radio on 20m then I will need a 20m BPF to
reject the 40m signals produced by my TX and pass any 20m signals to my
second radio.

My understanding is that the main function of my 40m BPF (when thinking of
my second radio on 20m) is to reduce the second harmonic on 20m produced by
my 40m TX. So if I am on 7.025 running, I will create a very strong second
harmonic signal on 14.050. Depending on my TX power, antenna proximity, and
antenna polarization, that signal might be enough to couple enough power on
my 20m line to fry my second radio front end. It may only be enough to
create a large amount of interference heard over a large swath of 20m
frequencies. I can further protect the radio and reduce the interference
(that big second harmonic produced on 20m) with a good quality BPF on the
TX... possibly by up to 80dB. The RX 20m BPF does nothing to stop this
second harmonic because it is a 20m signal which is passed by design of the
20m BPF... no different than everyone else's signal your trying to hear.

The place for the RX BPF is in reducing out of band signals that are strong
and might cause phase noise. Without the RX BPF you might be completely
fine or you may be so bad that your RX transceiver gets it's front end
fried. Every station is different, but it is safe to place a BPF on each
radio to prevent unwanted phase noise and to reduce TX harmonics. Then you
will also have the freedom to TX on either radio whenever you want, within
the limitations of your station design and whatever contest rules your
operating under.

George's (W2VJN) book, Managing Interstation Interference, is a great read
on this subject. It's available from Inrad directly. I finally spent the
money on it and consider it an invaluable resource for a SO2R or Multi-op
station designer.

Further details on your plans would be helpful if you want to proceed in
design and / or learning.


Tim / N6WIN.

On Mon, Jul 2, 2012 at 12:54 PM, Roberts, Will <Will.Roberts at pgnmail.com>wrote:

> If you are single op/1 radio and never listen on a band other than the one
> you are transmitting on, a bandpass filter isn't necessary. If you have a
> second receiver, then a bandpass filter could be helpful if you are trying
> to receive on a different band than you are transmitting on. For example,
> you are running on 40 meters and tuning for multipliers on 20m. If you had
> interference on 20m from your transmitted signal on 40, a bandpass filter
> could make a big difference in being able to hear 20m as you transmit on
> 40m.
> There is a lot of expertise here on the reflector for station design. It
> sounds like you are just asking fundamentally if it would help out in your
> situation.
> 73,
> Will AA4NC
> From: "James" <jms_k1sd at verizon.net>
> Subject: [CQ-Contest] Bandpass Filter
> To: <CQ-Contest at contesting.com>
> Message-ID: <000601cd57d5$62abcfd0$28036f70$@net>
> Content-Type: text/plain;       charset="us-ascii"
> If I operate single op and never transmit on two bands at once, is a
> bandpass filter necessary?  Is it helpful?
>      73 / James / K1SD / Rhode Island
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