In a message dated 97-08-12 10:26:31 EDT, CRB@knersus.nanoteq.co.za writes:
<< > This means that our coax leakage is at least *66 dB* stronger than
> the weakest signal the receiver can detect. Now add in coupling to
> cables from direct radiation from the equipment, antennas, and all
> other sources, and we're talking about mighty strong signals.
I submit that this factor is only important in co-frequency
situations, where antenna coupling would be much more significant
anyway. In any other situation (e.g. adjacent bands), the bandpass
filtering in the receive chain would serve to reject those signals.
Or am I mistaken?
Chris R. Burger
ZS6EZ >>
Absolutely correct, Chris, great observation. The ONLY emission that would
be that strong would be the main transmitted carrier. If one intends on
operating two stations on the same band 300 Hz apart, it might be
problematic!
Assuming the worst, and secondary emissions (spurs, parasitics, harmonics)
were - 30 dbM AND they occurred on an interfering frequency, they might be
at the S-5 or S-6 level. If offending parasitics are stronger than that,
there is an explanation other than leaky coax, if a good grade of coax has
been selected.
Which brings up several interesting points, why do we attempt to get over
100dB isolation with hardline, when our transceivers, amplifiers, SWR
bridges, and in some cases, even antenna tuners are leaking signals just 30dB
below the primary carrier. The hardline will definitely pass all that
garbage far more efficiently than coax without significantly improving the
power of the main carrier signal to the antenna.
Want to clean up interference between stations in a M/M setup? First, find
the cleanest transceiver you can buy from a standpoint of spectral purity and
receiver dynamic range. Second, isolate and tweak tuned inputs on amplifiers
(I seriously doubt that there is even 40dB isolation between bands there).
Next, ensure that SWR between transceiver and amp is as close to 1:1 as
possible. Use bandpass filters as necessary. Think seriously about your
choice of amplifiers. That SB-220 may be a bargain, but you'll find quickly
with an RF probe that it leaks RF like Swiss cheese. Use high power bandpass
filters and transmitting and receiving stubs as necessary. Finally, use an
RF probe to check every connector, the SWR bridge (I find some nice leakage
currents around my SWR bridge, but then again, its shielded in plastic!!!),
RF relays and anything else that might be in the RF chain. Once these things
have been accomplished and you feel you need that one dB that you might be
losing as a result of using coax, then get hardline.
Funny, if we thought of our transceivers, amplifiers and RF accessories as
transmission lines (and they very much are!) it might help us put this whole
thing in perspective. As Chris so wisely stated, that S-9 + 10dB signal
attributable to leakage between two adjacent runs of regular coax would only
occur only between stations operating on the same band, would probably cause
no more interference than any other signal on a typical crowded contest band.
The bottom line is that the RF chain is as weak as its weakest link. Using
hardline without first cleaning up everything ahead of it is like putting
racing tires on a Yugo!
73, Paul, KB8N
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|