Momci, javio se Matej Ok1TEH koji prati ovu našu raspravu oko mikrovalova i poslao malo duži mail s molbom da ga ovdje objavim, a mislim da ga vrijedi pročitati. Na engleskom je, ali uz malu pomoć Dr Googla mislim da će biti zanimljivo
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Hi guys,
I'd like to write out few findings related to problem with 23cm QRM
from commercial or military towers and how to remove it. First you
must find out if the signal is in band or not by some spectral analyzer.
1) QRM by inband commercial signal
If the signal make problem in band, there is usually very little
what you can do.. One partial solution is to build up narrow dish
with high front to back suppression and use LNA with high IP3..
2) QRM by outband signal
If the QRM is caused by signal out of band, it's very important to
find out, how far (MHz) is the unwanted signal. At my home QTH in Prague
I had problem by DVBT2 tower operational some 300m far with few kW.
It was running DVBT some 300MHz below 1296 MHz so in this
case it was relatively easy to buy or make low ATT filter.
Making such a filter is relatively easy, we made such a filter
to suppress the mirror for the 28/1296 MHz transverter check out:
http://www.ok2kkw.com/00000104/preselector/bpf_23cm.htmI asked to Beert, PE1RKI, because he has very well done boxes and machinery
and at least it saved lot of time. In the case it was easy because the
source of QRM was several hundreds MHz away so the
cross attenuation was only of 0,2dB.
Much worse situation happed at our contest QTH JO60JJ,
as we suffer from radar from from 14,4km far Auersberg radar
which is operating around 1259 MHz with 2,5 MW in pulse.
As the result I asked to RKI to prepare special narrow BPF version
however due to very narrow bandwidth and very high suppression
it has relatively higher cross attenuation. It's because of problem
with Q at this high frequency. Luckily we found out that such a filter
at our case was working well even when it was placed between
LNA and transverter, so the higher attenuation was still acceptable
because the whole noise figure wasn't corrupted so much
(we use separated RX/TX path).
By the way if you have such a problem at 2m with FM BC,
check out
https://www.antennas-amplifiers.com/144mhz-bandpass-filter-bpfit's great BPF and I use it with good success..
3) QRM by usage of bad kind of LNA for contesting
For city environment with strong outband signal it's better to use
LNAs with very high IP3 like MMIC PGA103+ or some alternative.
Don't use 2 stage LNAs like VLNA from G4DDK or old LNA
with MGF1302, they are fine for EME, but not for city environment.
I had bad experience with LNAs from SSB electronics, little bit better
are HEMTs (Kuhne), better is PGA103+ but currently the best is probably LNA
from US4ICI with SKY67151, compared to PGA103 it has much lower NF
https://vhfdesign.com/lnas/lna-eme-sky67151-for-23cm-and-21cm-bands.htmlRemember for 23cm tropo contests that you really don't have to use 0,1dB NF LNA,
anything below cca 0,8dB is ok (because of high thermal noise from background)
4) Operation at on4kst chat
I read your discussion, It's not easy task.
At 23cm I used to ask to more people, because I use 2 or 3 (since June)
independent antennas, so I can turn them to more stations at the same time.
Sometime it's problematic that you ask somebody for chat and you are called
at random by some other local station, so sked partner has to wait and with
nowaday smaller patience he became to be angry, that he must wait for you...
I always try to do my best and ask stations only if I checked at Airscout
that airplane is already coming. But guys please release that the situation
in Middle Europe is far different to some DX area like 9A. We oftenly work
30 stations on 23cm during first 1 hour of contest so the pileup is big
however when you wait and ask some 9A station later like around 19utc
when you have more time for skeds, at the meantime he often go QRT
or try some other band.. An another thing is QRM situation, I often ask
the sked partner to move let's say to .310 or higher.
During some contests it's very problematic to find any free frequency
for CQ between 1296.150 to 1296.290 MHz...
In any case if you are fed-up with the too fast chat traffic, try use our VUSC
contest log because it has it's own KST chat client with very good
user text filters. If you are QRV 23cm, please write it at your name in
chat and if you are just listening, please select in KST chat that you are
"off the chat" so then is station marked with () and almost nobody will bother you.
At the end I must say that S5, 9A and YU operators are very well skilled
in CW/SSB operation so it's big pleasure to work you. And yes of course
please always send full rst+nr + loc code, the situation when
somebody send rst+nr only I don't like either hi. Last words: if you are small
stations with 10W, please realize that Airplanes are great help so even with
your small setup if you know CW, it has sense to try QSO up to 500 km,
just give it a try and please be patience hi.
73
Matej, OK1TEH
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Najbolje od svega mi je što je teško naći slobodnu frekvenciju i da postoji pile up na 23 cm, e da mi je to doživit...
Naprid naši!
73 GG
