[CQ-Contest] Bandpass filters for M/S or M/M environment

[jukka.klemola at nokia.com]

> -----Original Message-----
> From: cq-contest-bounces
...
> W0UN -- John
> Brosnahan
...
> Clean Up Your Signals with Band-Pass Filters  part 1 and part 2
> Ed Wetherhold, W3NQN    QST May and June 1998
> 
> In addition to the W3NQN filters I had a few ICE filters that 
> were used
> as benchmarks for the testing.  My specific notes are still 
> at my Colorado
> place--so I can't provide any additional information to what 
> was published
> in QST.  But the QST article has extensive information on the 
> performance
> of the filters.
...
> Each filter was designed to provide optimum rejection of the 
> immediately
> adjacent ham bands.  Performance remained quite good through low
> VHF but there were some series resonances (low rejection) in 
> the mid- to
> upper-VHF range as there will be with all filters using 
> real-world components.
> 
> 73--John   W0UN

In real-world there is a certain isolation between antennas. 

I have two towers and my isolation betwwn antennas is not too
many dozens of decibels.

ICE performance is enough for me.

If antennas isolation is at 20 dB level, or worse, like 10dB or
even less with tribanders many have, the band filter does not
have to be much better.

Also, relay system has an isolation, it can be like 25dB.
If the real-world antenna case is worse it does not have any
real life effect to system performance.


The filter selection should depend on the limiting factor, which is
the antenna system isolation.


One issue that does not come into mind when seeing blown filters
is the fact the filter takes only 200W.

If you have a 40m yagi and a tribander on same tower and you TX
1 kW to 40m antenna, induced power on the tribander can exceed
200W.


This can happen also with a 80m inv V - 40m antenna or what ever
combination.

Always use stubs in addition to band filters if you use amplifiers.
The stubs take the 6-10dB off the top and make the life manageable
for the band filters.


This power transferring phenomena can be measured by using a power
meter and dummy load connected to the 'passive' antenna line
while the other one is transmitting at full power.

You can always measure there is power.
Sometimes the result can be very surprising.


Remember, the victim is sometimes the receiving end, not always
the more-obvious transmitter in the real world of contest station
complexity.


73,
Jukka OH6LI