The following discussion about how to eliminate QRM from 88-108 FM stations took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.
On 13-Jan-2001 SM5BSZ wrote:
When FM signals are strong enough to affect
the preamp something has to be placed directly at the antenna.
In principle one can make a band pass filter that is good enough
but in real life one often looses far too much on the noise
figure.
A more safe way is to add a notch filter.
< length A > rig-----------------|---------------|----------------------ant | | | | | |
Place two open stubs on the cable. The length A has to be 0.25wl
for 144MHz (electrical length)
The open stubs have to be identical and they should be open with
a length of 0.25wl at 100MHz (=0.36wl at 144MHz)
The two stubs affect SWR in an identical way but with opposite
signs so together they will make no change at all to SWR.
The cable of length A will see SWR so the losses in it will
increase, but considering the length losses are not severe.
(and you may make the .25wl section from better cable)
The open stubs will be a capacitive load at 144MHz.
If you use RG8 you can expect losses in the order of 0.5dB.
A better cable will give smaller losses in proportion.
You may also make the stubs from higher impedance. That reduces
the power into and out of the stub. (same voltage, higher Z). At
the same time SWR in the A section becomes smaller since the
reactive load seen at 144MHz rises proportional to Z.
I suggest high quality 70 ohm cable, the thickest you can find.
The double stub is very safe and I think it will cure the
88-108MHz problem in nearly all cases. If you need even more FM
attenuation than provided by the double stub you will most
probably be able to place it after the preamp without affecting
NF at all.
At the notch frequency attenuation will be extremely high, 80dB
or better, but the notch is sharp and 10MHz away you can expect
only about 15 to 20dB. If you make the stubs from your own air
insulated high Z (200 ohms or so) the notch will still be
extremely deep but it will cover much less bandwidth.
A single 0.25wl shorted stub for 144MHz will normally have no
effect at all for100MHz. It works only in case you was very lucky
placing it at the high impedance point with very high SWR at
100MHz. One of the advantages of the double stub is that you will
surely not have the bad luck to place both of them at very low
impedance points. Even if SWR is very high, at least one of them
will be very efficient.
On 14-Jan-2001 DD0VF wrote:
I just want to inform, that all my 88..108MHz
problems gone since i use a preamp with 7/8" input cavity.
The local tv/bc tx is appr. 1km away. My vhf-qth is appr. in the
same hight with the antennas of the tower, because of the lower
asl there.
There are 9 stations with a power between appr. 5 and 50kW. No
difference between IC275 IC756 FT736 (unmodified), the smeter
shows more than S9 qrm in several directions.
A single stub droped the qrm down to S2-3, but i stop this
experiments after i use the cavity-preamp. (I am sure the
dopple-notch, may solve all problems) btw: it's still a ordinary
MGF1303 inside and not a 1801.
On 14-Jan-2001 OZ5IQ wrote:
The problem is NOT what you think eventhrough
it look like a overdriven preamp. The problem is ALMOST the
mixers, who vy seldom do cope with great signallevels.
The BW of your preamp is in general great when using the std.
preamp with a GaAs-fet, and a single or two tuned circuits.
They will suppress the 100 MHz signal - but not that much, and
when coping with this level even your preamp will
"amplify" but with a lets say -6 dB gain. But your
MIXER in the stn really do suffer of this.It naturally helps with
a BP filter in the RX path - it lower the interferring signal
into your mixer. But DONT think, that the problem was/is your
preamp !
GD luck with it and remember ALWAYS to put a GOOD BP filter after
your preamp ! to still have the good NF.
I do normally this when designing preamps (I know they MIGHT SEE
strong signals) that set the HIGHEST ID and still keep the low
NF. Several fets do have a rather flat NF min curve. So use the
not 10 mA but 25 mA if this is actable.
After this find a GOOD BP filter such as the STORNO VHF 5 circuit
helix (-5.5db) but extremely sharp, and GD for you.
All the best - with your interferring signals and to
"kill" them from
On 14-Jan-2001 F/G8MBI wrote:
Where can I get one of these -6db amplifying,
high ID, 25ma preamps then ?
and do I need woofers and tweeters to go with it ?
On 14-Jan-2001 AL7EB wrote:
There was a great presentation at CSVHF last
summer at Winnipeg by Ralph Olds, VE5TR, on intermod causes and
cures. He pointed to the same wide bandwidth acceptance of
preamps and showed how input filtering the preamps had the best
cure. The problem for eme was the introducted loss and
coresponding rise in NF. So he concluded the next best
thing was preamp post filtering to keep out of band RF being
amplified and introduced to follow-on RF amps and mixers, as Kim
pointed out. Though I have not a pronounced observable
effect like Steffen, I recently decided to insert a DCI
144-146 filter before my second preamp. Have not raised
the system back into service so have yet to observe what it will
do for me.
Why did I decide to do this. At work, only 4.5 miles from
home, severe paging IM has impacted one of our VHF base
stations. A dual cavity filter was required. So, even
though I haven't observed a problem at home, I surmised that I
may still have my system noise level impacted.
These paging services run 1-2 kW ERP only few miles away.
You should see what they do on a spectrum analyzer veiw of the
spectrum noise floor over 20 MHz of the band. These same
pagers introduce a 250 mw noise signal [measured at the radio end
of the feedline] into one of my commercial repeaters located 300
feet from the paging antenna at the same site [thank god for
duplexers].
On 14-Jan-2001 OZ5IQ wrote:
>When FM signals are strong enough to
affect the
>preamp something has to be placed directly at the
IF you want to use a stub at the preamp front end, I would suggest
you to make it adjustable !.
You use NOT a fullu 1/4 L but lets say the half and mount a
adjustable cap. between the innerconductor and the screen of the
cable.
It funtions as well on "1/4L adjustable stubs on ie. TV.
receivers"
I am not aware how this behave during temperature changes ( in
mother nature )
(Note: I'm not showing any E-Mail
address here in order to avoid them from being collected by SpamBots. You can
possibly find the E-Mail addresses of the above OM at QRZ.COM.)
This discussion is still not closed. If you have any opinions or additional
related information you would like to be published here, just send
me an E-Mail
Read
other Moon-Net discussions
Home page
Site map
Radio Sherlock search engine
Terms of service
Privacy
Cookies