[CQ-Contest] Bandpass FIlters

[Drew Vonada-Smith]

Joe,

You are correct that high SWR will cause high voltages that can damage a bandpass filter capacitor.  However, a decent filter is rated to 2:1 at least.  If your SWR is expected to be high, one workaround (other than the tuner) is to buy a higher power filter.  With the tuner you should be OK, if the tuner is AFTER the filter - not the one in the radio.  Just don't make an oops.

However, I fear this is not going to be your main problem.  The IC7300 is notoriously bad in a multi transmitter environment like FD, and your band filter will not help with in band noise that they create.  Nor will a roofing filter.  No filtering at the K3 can help, because the noise is *actually there*.  Depending on your circumstance, antenna spacing, band choices, etc, you may or may not be OK, but the 7300 is a bad choice for a FD radio.  Convince them to bring something better.  That does not necessarily mean expensive.  Some reasonable cost radios are good at this.  (TS590 is a great example).  

The IC7300 is a neat little radio.  But it is not one for a multi transmitter environment.

Contact me off list if you want to know what transceivers are good for this sort of use.  There are lots of them.

73,
Drew K3PA

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Message: 3
Date: Sun, 25 Apr 2021 08:33:17 -0500
From: Joe <nss at mwt.net>
To: "cq-contest at contesting.com" <cq-contest at contesting.com>
Subject: [CQ-Contest] Band Pass Filters
Message-ID: <65a4add2-1b11-d834-053f-7639fa59432f at mwt.net>
Content-Type: text/plain; charset=windows-1252; format=flowed

Our Field Day group has a set of these filters, like this 40 meters one,
https://www.dxengineering.com/parts/lbs-pb-f200-7

In the back of my mind I have two ummm "Specifications" That in the back of my mind are true, But I can not find any documentation to verify what I am thinking is actually true.

It all has to do with SWR.

1- To have the filters provide the ultimate best rejection they can provide the impedance must be as close to 50 ohms as possible.
Is this true? If it is, is there any published data showing how much is lost on performance due to "X" SWR?

2- This one is even more important. They are rated 200 watt units. "At
50 Ohms"
Now SWR that is NOT 50 Ohms Can make some pretty high voltages true?
And I am assuming these voltages are what gives each unit it's power rating.

Go too high and fry a cap.

Now Our Station, (The only CW station) will be running a K3 with the Roofing filters. My Partner in this station says,,,,,

Picking up a K3 tomorrow.
We will be using it at field day. It has a 400 Hz roofing filter, 100 Watts, internal tuner, USB interface, upgraded synth and the nice IO board.
Should be on the air for SST Sunday night.
Jim

 ?Now I know we will be clean on Transmit, To an OCFD.
That 400Hz roofing filter, How much will that protect US, see we will be surrounded by several IC-7300's

Would like to use the Band Pass filters to help but not sure if they can handle the not perfect SWR that a typical OCFD will give us.

I assume if we had to we can add an external manual tuner, to make the SWR flat then go to the filter and rig.
And the external manual tuner would also add in some band pass filtering effect too.

Joe WB9SBD







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End of CQ-Contest Digest, Vol 220, Issue 21
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