[TowerTalk] Antenna switch isolation?

[Jim Thomson]

Date: Sun, 6 Jun 2010 23:18:36 +0700
From: "Martin Sole" <hs0zed at csloxinfo.com>
Subject: Re: [TowerTalk] Antenna switch isolation?

Thanks Jim,

My original question, or one of them anyway, was do I need more isolation
than a single relay will likely provide? From your answer I'd say the answer
was no, not necessary. Now this is (slightly) at odds with what others have
either hinted at or in the case of DX Engineering it seems outright stated.

##  a DEAD SHORT  on ALL  UN-used ports is as good as it gets !  The isolation
is a non issue.   Any stray C  from a used port to an adjacent Un-used port
on either side is a  non issue. You can't couple power through stray C..then
into a dead short.... well you can... but it ain't going anywhere !  





The thinking being that all connected wires pickup signals, the question in
my mind is how relevant they are likely to be in the face of wanted signals
from a resonant antenna. So my 40m dipole provides more of the broadcast
signal that's on 7.4MHz than does my 20m quad, but with 40dB of isolation
will that be something that needs caring about?

##  When ur on 20m.... the broadcast station that's 60 db over S-9 on 
7.4 mhz  will be shorted out at the 40m relay..inside the box... so any stray
C is again, a non issue.  




Just writing this down as I think about it, let's say I want to copy a
signal at around S1 or about -100 as I seem to measure it, this on 20m with
a resonant antenna. The big broadcast signal on 7.35MHz registers about
S9(-73dBm) on the 20m quad and with the front end filters in the radio is of
no real concern. With the 40m dipole however the same signal is 60 over 9,
about -13dBm, with the 40dB through the out of circuit relay it will be at
about -53dBm, 20 over nine. In fact this is clearly still within the ability
of most modern radios to reject so should not impact anything much. Except,
does it impact the noise floor of the radio? Does it add, together with all
the other big signals in and around that area of the spectrum something that
could possibly degrade my already weak 20m signal? What about all the other
connected antennas adding their bit of noise from the area they are resonant
on, or nearly so.

##  all these sigs  from other bands  will be shorted out at the relay box! 
If ur really paranoid abt it... you could install a band-switched set of band pass
filters.  That will kill everything....except the band ur on.  The xcvr is probably
good enough anyway... so switched band pass filters  may well be a waste
of money.   





Just grounding the unused ports might be a simple enough idea to resolve any
concern for the above but with the quad that means ensuring the feed lines
to the various other driven elements are correctly chosen to ensure shorting
of the loop when on other bands (Maybe this helps contribute to the
rejection of unwanted signals) As Cebik stated the models show more
controled gain and F/B when the unused loops are closed. This means either a
lambda/4 length of cable with the far end open of a lambda/2 length if the
far end is shorted.   Though he goes on to suggest it is better to have each
feeder isolated from all others on both the inner and shield side of the
coax. So twice as much coax could necessary and certainly a more complex
switchbox would be required to meet these criteria.

## Interesting.  I never thought of what ur supposed to do with un-used
quad loops  from other bands.  If Cebik wants the un-used loops closed... then put
a NO spst relay  directly across all the loops feedpoints... and simply turn on only the
relay..for which band ur on.   Then you could get away  with using just SPST relays
in the remote switch box.   That gets messy though. You would require 12 vdc going
to each relay... on each loops DE. 




The real question remains, will it be noticeable, will it make a difference?
Maybe this is not quantifiable and I just need to build it anyway, maybe
both ways and compare. Since both AS and DXE take pains to strive for better
performance in his area than a single relay switchbox might provide I
suppose we should be thinking that it has an impact at some level. Is that
really so and will "I" notice it, hmm? Or is there more than a dash of
marketing over engineering going on? 

## The isolation issue is really only for 2 in 6 out box's.... like the AS
6 pack.. and 8 pack.   In that case, it's so you can stuff 2 kw up coax #1
while listening on a 2nd ant... with a 2nd xcvr... via coax #2.

## the simplest remote switch box is just a bunch of SPST relays...with COM
of each relay  going to the respective coax connector.. and the NO  contacts
of all relays going to the hot buss.    In that case... ALL the unused ants  will
have therir center conductor's  floating.  DPST relays could also be used.. with
all contacts in parallel.  That will quadruple the power rating.. and also provide
redundant contacts = better reliability.  You could also use 3PST relays.. and parallel
all contacts. 

## IF SPDT relays are used.... then it's easy to grnd out all the un-used ants. 
If DPDT relays, again.... power rating will quadruple... and  you now have redundant
contacts.   You could also use 3PDT relays.    AS uses  3PDT relays in all their 6 pack
box's. 

##  if you want it  100%  bomb proof,  use vac relays'.  They never go open, and WX
is not an issue. 

Later... Jim  VE7RF   







Regards
Martin, HS0ZED