Eliminating QRM from 88-108 FM stations

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 )


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