[Elecraft] {OT} - Variation on use of a coaxial switch (stan levandowski)

[Jim Brown]

The isolation provided by this switch will be really poor. In addition 
to the capacitive coupling of the switch itself, note that only the 
center conductor is carried between inputs and outputs, so that the 
chassis carries return current. This adds a lot of inductance to the 
path, and causes crosstalk between circuits.

The RIGHT way to do this is to provide a coaxial path, so that return 
current is confined to the coax shields.  If you look inside the Alpha 
Delta switches you see that they are built that way. There's not coax 
cable between in and outs, but the physical construction is such that 
the body of the switch forms the return path. Not perfect, but FAR better.

73, Jim K9YC

On Sun,1/1/2017 10:37 AM, John wrote:
> What Dale describes below is a very similar configuration to the MFJ switch
> I was given a year or so ago - an MFJ-1703.
>
> I just popped the lid off and took a photo of the insides for you:
>
> https://www.dropbox.com/s/h28jny6vrkj7rdj/2017-01-01%2018.28.00.jpg?dl=0
>
> The only difference being that it's basically a DPDT slide switch, rather
> than a rotary. Works just as well.
>
> At least, I think that's pretty much what he was describing anyway. Stick a
> dummy load on one output and your antenna on the other, and your two rigs on
> the two inputs.
>
> 73, and a firm left handshake,
> John (XLX)
>
> -----Original Message-----
> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dale
> Martin
> Sent: 01 January 2017 17:31
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] {OT} - Variation on use of a coaxial switch (stan
> levandowski)
>
> Stan,
> I haven't taken the time to read all the other responses, but have you
> considered building your own coax switch.
>
> Parts: 1 aluminum box, 4 SO-239's, 1 two-pole, multi-position rotary switch
> (you only need 2 positions, but more is okay), 100w dummy load
>
> Mount the SO-239's (J1-J4) and rotary switch
>
> Wire up the switch thusly:
> Pole 1 to J1 (KX2)
> 	Contact 1 to J3 (antenna feedline)
> 	Contact 2 to J4 (dummy load)
> Pole 2 to J2 (KX1)
> 	Contact 1 to J4 (dummy load)
> 	Contact 2 to J3 (Antenna feedline)
>
> When you switch one radio the antenna, the other radio is then put on the
> dummy load.
>
> 73,
> Dale, kg5u
>
>
>> ------------------------------
>>
>> Message: 3
>> Date: Fri, 30 Dec 2016 17:49:21 -0500 (EST)
>> From: stan levandowski <sjl219 at optonline.net>
>> To: Elecraft at mailman.qth.net
>> Subject: [Elecraft] {OT} - Variation on use of a coaxial switch
>> Message-ID: <1d09982f.1e408.15951ec5b8d.Webtop.41 at optonline.net>
>> Content-Type: text/plain; charset=UTF-8; format=flowed
>>
>> I would like to use a coaxial switch for an unintended use and I'm not
> sure if
>> it's a good idea in terms of RF isolation and potential equipment damage.
>>
>> I'm no expert in this area but I know this list is a wealth of expert
> opinion, so
>> here's my question:
>>
>>
>> I want to buy a two position coaxial switch. ?I want to connect ONE
>> antenna to it and switch this antenna between a KX2/KXPA100 (100
>> watts) and a KX1 (about 3 watts). ?The coax cable coming from this
>> switch to the KX2 is a short piece of RG8X and the coax cable going
>> from this switch to the KX1 is 25' of RG-174 (yes, I already computed
>> the dB loss and I can live with).?
>>
>>
>> Am I likely to cause any damage to to either piece of equipment?
>>
>>
>> Thanks,
>> Stan WB2LQF
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