Connections to K3 ant 1 & 2

[Edward R Cole]

Tim,

What you ask is a common issue for moonbounce stations because QRO
levels are commonly used and sensitive preamps are connected to the
other side of the TR coax relay, so I am well familiar with the issue.

You should try to achieve having no more than +2 dBm come into the
receiver, so if you are running 1500w (~62 dBm) then you need an
isolation of 60-dB.  Most generic coax relays have 30-45 dB isolation
of ports.  Many manufacturers do give specifications of isolation and
max power per frequency (usually better specs at HF than 450 or 1200 MHz).

So what to do.  Measure the isolation is best, but you may not have a
miliwattmeter for doing this.  If you have a signal generator you can
use it to measure the isolation by comparing direct signal to
unswitched signal.  Adding a second coax relay in series with the Rx
side usually accomplishes sufficient isolation.  The second relay can
be a small low-power unit as it will only see maybe 1.5w if the main
relay has only 30-dB isolation.  Small power coax relays typically
have higher isolation specs.

In your station description a manual coax 2-way switch may be used
(assume it is only good for 30-dB isolation).  The unconnected side
goes to the amp not in use which probably has internal TR relays for
bypassing the amplifier when receiving.  If you insert a small relay
in that bypass line that will provide sufficient isolation.  You can
have that relay terminate to 50-ohms as a further protection from
high RF (but probably not necessary).  A small 12v relay probably can
be enclosed into either amp and controlled by the amplifiers TR
switching circuit.  Unless you operate QSK this should be a hardwire
control line from the exciter rather than RF sensed switching (on
CW/SSB this will lead to longer life of the relay and avoid relay
clatter everytime you speak or key).  At QRO one should not
hot-switch a coax relay, anyway.

73, Ed - KL7UW

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Message: 13
Date: Thu, 20 May 2010 06:18:37 -0400
From: "N3XX" <[hidden email]>
Subject: [Elecraft] Connections to K3 ant 1 & 2
To: "Elecraft Reflector" <[hidden email]>
Message-ID: <0E60319D4813430EB4CA8BD923C729B6@n3xx>
Content-Type: text/plain; format=flowed; charset="Windows-1252";
        reply-type=original

Hi to all on the list,

I just finished building my K3 kit a few weeks ago (4175).  All went well,
and the new xcvr is up & running with no problems.
The antenna setup at my qth is a single cable from the entrance panel at the
house going out to tower #1, through a remote antenna switch to select from
antennas on that tower or another connection going to tower #2.  At tower #2
is another remote antenna switch to select various antennas on that tower.
In the shack I have an amp for 160-10 meters, and an amp for 6 meters.
I would like to connect my single cable coming in from the entrance panel to
a 2 way coax switch,  with one of the ports going to my 160-10 meter amp,
and the other going to my 6 meter amp.  Then the 160-10 amp would be
connected to the K3 antenna connector 1, and the 6 meter amp would be
connected to K3 antenna connector 2.
Both amps will operate at the legal limit easily, and my concern is that
when xmitting through one of the antenna connectors & amp, I will put too
much rf back through the coax switch to the unused K3 antenna connector, and
possibly damage the K3.
I have been changing cabling manually but would really like to be able to
just operate a switch to make the change.
So, if anyone on the list doing something similar to what I have in mind, I
sure would like to hear about your setup, what kind of switch you are using,
how much port to port isolation is necessary to be safe, etc.

Tnx & 73,
Tim - N3XX

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