Re: [Elecraft_K3] Re: Integration of External Band Pass Filters

Randy Farmer W8FN

When transmitting on 20 meters and listening on 10 meters the receiver is seeing the 2nd harmonic energy from the 20 meter transmitter. A 10 meter BPF in the receiver input will provide zero discrimination against the 10 meter harmonic signal. You need a 20 meter BPF in the transmitter output to suppress the harmonic energy generated by the transmitter.

At the transceiver output, assuming a K3 with 100W output, the transmitter output is probably going to be around 0 dBm at the 2nd harmonic. (100W = +50 dBm with about 50 dB rejection from the transmitter LPF). Adding a 20 meter BPF at the transmitter output with another 45 dB of harmonic rejection (typical for the LBS filters) will drop the harmonic level to -45 dBm. With a reasonably small amount of coupling between the transmit and receive antennas, this may be enough filter rejection to make the situation tolerable. If you're using an amplifier then you'll definitely need a high power BPF at the amplifier output. Limiting the 2nd harmonic input to the amplifier won't do any good, since the amp itself will generate harmonic energy. And the filters will have to have an additional 12dB of harmonic rejection to handle the difference between the 100W transceiver level and the 1500W amplifier output.

I live on a very small lot and my antennas are closely coupled. I have found with my station that I need both a set of 200W BPFs at the transceiver output and a second set of high power LBS filters cascaded in the antenna line to make SO2R practical, even without an amplifier.

I'll be happy to send you a copy of a presentation I did at Dallas Ham-Com last year on SO2R RF Engineering if you're interested.

Randy, W8FN

On 1/15/2018 3:15 PM, aa3b.bud@... [Elecraft_K3] wrote:
Clarification to my last message:

"I transmitted a 500W CW signal on 20M and the K3 was receiving on 10M."

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