Help:vhf front end

Discussion in 'Wireless & RF Design' started by hamopp, Sep 23, 2013.

  1. hamopp

    Thread Starter Active Member

    May 13, 2009
    Hi All.
    I have a circuit of a 2m rx front end which works well, the output is fed
    into a 9mhz if stage.

    What i would like, is to be able to have this circuit modified to work in the
    4m(70Mhz) and 6m(50Mhz) bands, I know that some components will have
    to be changed and i have marked the parts with a * .

    The front end coils in the 2m one are from Coilcraft (146-04J08SL), i also
    have these coils from them:
    2x 146-06J08SL 6.5t
    2x 146-07J08SL 7.5t
    2x 146-08J08SL 8.5t
    2x 146-10J08SL 10.5t

    data at

    Can anyone help with the values of the parts that need to be changed.

  2. KL7AJ

    Senior Member

    Nov 4, 2008
    Yes, you will have to change the local oscillator frequency, as well as the front end tuning.
  3. hamopp

    Thread Starter Active Member

    May 13, 2009
    Yes, i know.
    The whole thing is going to be used with a mcu and Si570 local oscillator so no probs there.

  4. bertus


    Apr 5, 2008

    There are basicaly two resonant circuits you will need to adapt:
    L201 / C203 and L202 / C204.

    You can use the formula for LC circuits for them:


  5. hamopp

    Thread Starter Active Member

    May 13, 2009
    That all looks like greek to me hi hi.
    Can you please explain in laymans terms as
    formula's like this are hard for me to understand.
    Maths is a sore point for me.

  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    If your going to halve the frequency of the receiver from 144Mhz to 70Mhz, you need to increase the inductance or capacitance to maintain the frequency resonance,

    so either double the capacitance or the inductance to obtain half the frequency of the original tuned circuit.
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    I would think that given there is a square root function involved it should rather be - "..... either quadruple the capacitance or inductance ..."
  8. Papabravo


    Feb 24, 2006
    Funny thing about RF circuits. You can't dependably adapt a circuit up and down the RF spectrum without knowing a bit more of the "maths" and the "physics" than is readily apparent. For example, an active device, like a mixer or a transistor, designed to work at 144 MHz may not work quite as well on 6M or 4M. You also need some test equipment to adapt a design and verify its performance. What exactly are you going to do if things don't work as expected?

    RF design is not for laymen. Hire someone who knows what they are doing. It will be less expensive and more efficient in the long run.
  9. vk6zgo

    Active Member

    Jul 21, 2012
    It seems you are a Ham--look around in your area for another Radio Amateur who can help you out.
    I don't think you are going to be able to make one unit work on multiple bands.

    Look around for designs for the bands you are interested in.
    If you find a similar circuit for one of the bands you are looking for,you may be able to use some of the parts of the 2m one.

    "Modification" from 2m to these other bands may entail that many new bits that it could take on the character
    of "George Washington's original axe"--------It's only had 6 new handles & 3 new heads!:D
    Papabravo likes this.