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Low Pass Filter for 2 Meters VHF Radio

Discussion in 'Homebrew and Kit Projects' started by OA4AJP, Sep 6, 2012.

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  1. OA4AJP

    OA4AJP Ham Member QRZ Page

    Dear all:

    Are there schematics or sources to make a Low Pass Filter for 2 Meters VHF Radio ?

    I want to place the Low Pass Filter between an antenna and a Non Linear Amplifier.

    Regards.
    Alfredo Mendiola Loyola
    Lima, Peru
    OA4AJP
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd usually first confirm if there's really a harmonic problem. A low pass filter cannot improve IMD. And many Class C (non-linear) amps, used on FM mostly, have very good harmonic suppression and don't need a LPF.

    Have you measured anything to see what the amp is doing?
     
  3. OA4AJP

    OA4AJP Ham Member QRZ Page

    Last edited: Sep 6, 2012
  4. G0CWA

    G0CWA Ham Member QRZ Page

    Hi Alfredo

    It would be wise to put a filter after the amplifier as it does not look like there is any external filtering, I don't know any pre-designed circuit diagrams as such but this excellent design program will allow you to design your own. It is on the list of products.

    http://www.aade.com/

    Just make sure you use high enough capacitor operating voltages and a sufficiently heavy gauge of wire for the coils.

    Hope this helps you Nick G0CWA
     
  5. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Hello Alfredo, the graph shows the second harmonic is down 35 db and the 3rd harmonic is down a little more than 60 db. Although this isn't bad it's not good enough. So yes a low pass type of filter will work very well in this application.
    There are other calculators available and some sites have many like here; http://www.siversima.com/rf-calculator/skin-depth-calculator/. Although it appears to deal with microwave just put in your desired frequency on any valid RF frequency and it'll give you the values and often multiple values for different configurations.
    BTW don't forget the device you're interested in requires just 20 milliwatts to obtain about 8 watts out.
    Have fun
    73
    Gary
     
    Last edited: Sep 7, 2012
  6. OA4AJP

    OA4AJP Ham Member QRZ Page

    Dear Gary:

    What would be the correct cutoff Frequency for my 2 meters Low pass Filter 150 Mhz? (Butterworth)

    What is the bandwith for 0 attenuation on the ButterWorth Filter.

    Thanks.
    Alfredo Mendiola Loyola
    OA4AJP
     
  7. KO6WB

    KO6WB Premium Subscriber QRZ Page

    That's just a bit too close for a lowpass filter on 2 meters. It'll work but raising the cutoff frequency to about 175 Mhz would do well. There are no 0 attenuation filters as an insertion loss. They all have a bit of loss. This is quite often low enough that it can be disregarded. It'll depend on the design and the number of stages you use in your design. At a cutoff frequency of 150Mhz is represented by a point that is -3db from the reference at 0 db. So if you look back at the slope from the 0db reference level back to 148Mhz you will already be starting to attenuate the signal. Since the second harmonic of 145Mhz is so far away at 290Mhz then the cutoff frequency can be higher and give excellent performance.
    The bandwidth of a lowpass filter is usually from everything below the cutoff to the cutoff point. So a lowpass filter with a cutoff of 175Mhz has a 175Mhz bandwidth. That's in the perfect world and in actual operation there will be some minor spots where that simply doesn't apply. Your goal is to obtain enough attenuation at 290Mhz to reduce the second harmonic to a suitable level. The lowpass filter is supposed to work even better at the third harmonic and better still at the fourth harmonic. In the real application this is usually not completely true but it will be effective enough to be more than a usable application.
    A lowpass filter or any filter for that matter at the VHF range will not have what is called a "brick wall" cutoff. It will have a slope of varying degree and that is commonly called a shape factor. The slope at VHF is usually measured in the megahertz range.
    Hope this helps Alfredo.
    73
    Gary
     
  8. OA4AJP

    OA4AJP Ham Member QRZ Page

    Thank you for the information Gary.

    Can the Low Pass Filter change the impedance between my amplifier and my Antenna? (The amplifier output impedance is 50 Ohms)

    Is there any Harmonic calculator to get the second and third harmonic from a frenquency.

    In the AADE Filter Design Software, for the Butterworth Filter, is there any problem if I enter in the F2 field the value of the F1 field (Cutoff Frequency)?

    F2: StopBand Frequency.

    I have seen the if I change the F2 value the value of the Capacitors and Inductors don't change.

    Thanks.
    Alfredo Mendiola Loyola
    OA4AJP
     
    Last edited: Sep 7, 2012
  9. KO6WB

    KO6WB Premium Subscriber QRZ Page

    A variation of the low pass filter is the pi network. It has transformation capability but you're not trying to go from a high impedance tube circuit to a 50 ohms load. The RA08H1317M appears to me to have a 50 ohm output so there is no impedance transformation taking place.
    I'm not familiar with the AADE Filter Design Software and how it works, so I can't be much help with the perimeters and values it needs to calculate your LPF.
    For the Stopband enter the second harmonic of 290Mhz and the desired attenuation level it should present at that point. That should give you the values you're seeking.
    73
    Gary
     
  10. G0CWA

    G0CWA Ham Member QRZ Page

    Hi Alfredo

    Sorry I can't answer your question about the AADE package. I Just use it and have not had any problems with hitting the right designs. As you describe what you are entering you are describing a Perfect filter not allowing for the real world characteristics, but I may be mis reading what you mean.

    It is probably not changing the values as the filter base information you enter doesn't need to change them as they are near enough. The easy way to check, is to measure the VSWR into and out of the filter if this is OK you will have to assume everything is ok. Also check the harmonic levels both into and out of the filter using either a wave meter or GDO and check for a significant reduction.

    I agree with Gary about the "cut off" frequency values and type of filters to use.

    Hope this helps Nick G0CWA
     
  11. OA4AJP

    OA4AJP Ham Member QRZ Page

    What type of filter do you advice me, the ButterWorth or the Cebychev?

    I will use the filter with an APRS VHF transmitter

    Thanks.
    Alfredo Mendiola Loyola
    OA4AJP
     
  12. G0CWA

    G0CWA Ham Member QRZ Page


    Hi Alfredo

    I don't think it matters which type you use as long as it is lowpass, both do the same job, I have always used chebby ones personally.

    I don't know if Gary agrees, I'm hoping he does

    Hope this answers your question

    73 de Nick G0CWA
     
  13. G0CWA

    G0CWA Ham Member QRZ Page


    Hi again Alfredo sorry missed that question, there is no need for a calculator the harmonics are as follows depending on how you were draggged up in electronics.

    The "first" (also known as the fundimental) is 1x the frequency

    the 2nd harmonic is simply 2x the frequency

    3rd = 3x
    4th = 4x
    etc.

    The method I have given you is the answer to your question. Hope this helps you

    TTFN Nick
     
    Last edited: Sep 10, 2012
  14. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Hey, what happened to my lengthy well thought out posting? Really I did answer this a few days ago on the 9th.
    Okay, too late now to be much help but yes Nick I agree with you.
    What Alfredo needs to do is to get the attenuation value at 290Mhz and subtract the amount of attenuation the amplifier has already from the total attenuation desired.
    Have fun
    73
    Gary
     
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