[TowerTalk] Antenna switch isolation?

[TexasRF at aol.com]

Martin, the length of the coax center conductors showing outside the braid  
can act as small antennas and radiate signal from one center conductor 
through  the air to the other center conductor.
 
Shorter center conductors will make poorer antennas and reduce the  
radiation. Smaller relays will also have less radiation than larger ones.  Shields 
in a proper location can help. Making sure the center conductors are  
shorted to ground (shield) when open will help.
 
Some relays have a lead from the connection point to the actual relay  
contact. These leads can radiate and couple signal. One trick some times used is 
 to use a DPDT relay with the connection leads removed and then short the 
two  poles together. Input and output coax center conductors can then be 
connected to  the normally open contacts. This puts two contacts in series for 
increased  isolation using a single relay.
 
73,
Gerald K5GW
 
 
 
 
 
In a message dated 6/5/2010 11:49:30 A.M. Central Daylight Time,  
hs0zed at csloxinfo.com writes:

Some  interesting info here about how some of the commercial products are
built.  Pretty sure I must be doing something wrong here though. Okay this 
is
just  on the bench with a couple of pieces of coax and a couple of relays.
Relays  are single pole 30A  pcb mount type, encapsulated. Braid of the  
coax
cables joined together far ends of the coax go to sig gen and RX.  Feed
enough signal to get S7 on the meter with the centres joined and then  break
the centre of the 2 coaxes and connect to relay N/O contacts, braids  still
joined. Increase sig gen to get S7 again. See a fairly typical 40dB  at
14MHz. Add a second relay in series with the first with a wire between  the
two not more than an inch long. Still see about the same isolation.  Maybe I
need to move the relays further apart from each  other.

Interesting on this one was playing with a K3 this week and  noticed
something similar with the two antenna ports, S9 signal to the  radio was
still audible when switched to the unused antenna port. I think  they only
use a single relay in the antenna switching from Ant 1 to Ant  2.

Back to my switch project it seems clear that layout is critical as  may be
the choice of relay to minimise coupling. Whilst I think I have a  stray
coupling problem there also seems to be quite a few variables at  play.

Martin, HS0ZED

p.s. This switch box is for my quad  project, feeding the loops one at a
time.


-----Original  Message-----
From:  towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com]  On Behalf Of Martin Sole
Sent: 05 June 2010 12:21
To: 'Martin  Sole'
Cc: TowerTalk at contesting.com
Subject: Re: [TowerTalk] Antenna  switch isolation?

Thanks guys,

Just trying to stay sane, hi hi!  Missed a 0 off though, 1kw would be 100mW
at the other connections but  truly I think it can be discounted as said. 
Don
mentioned achieving 60dB,  I'm not an expert at how to predict or determine
quite where the RF might  go but I would be interested if there are pictures
of switches that can  achieve better than the ~40dB that I can get,
appreciated my test was a bit  'dirty' .

Martin,  HS0ZED

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