[FoxHunt] Moded ELTS, active atten?

[Kuon & Dale Hunt]

>de Rick K5RIC wrote:
>
>> Be careful attaching non LPER Antennas to your LPER DF unit.  When you are
>> in the DF mode DC  power will be sent up the coax, if you have a grounded
>> antenna, you my damage the LPER unit.
>
>Hi, Thanks we are aware of this. I would sort of expect the switched DC output
>on the ELPer to be currect limited to protect the drive circuitry. I havent
>checked, but isnt the DC(AC) present only in DF mode?


    The antenna switchbox requires a + voltage to aim the antenna in one
    direction and a - voltage to aim the antenna in the other direction.
    In receive mode there are three resistors in series between the
    + voltage source and the antenna jack, with a total resistance around
    15K ohms.  The resistor in the switchbox is 470 ohms.  This doesn't
    look like it should be a problem.



Is one side of the ELPer metter connected to common/case
>ground, or does  the meter "float" as Ive seen in other TDOA DF circuits?


   The meter floats.  (One side is connected to a reference voltage,
   and the other side to the output of the phase detector.)  I think
   the newer models may have an option for external meter.



>Does anyone have any circuit updates to the Active Attenuator circuit from the
>one in 11/92 QST? Im going to give this a try soon. Has anyone tried different
>type diodes, or even used a double balanced mixer such as an SBL(over kill?). I
>would also think you could use a different style oscillator such as a 74C04 and
>a 500 K or 1M ceramic resonator. Any thoughts?

    We use an SBL-1 or other DBM and a canned oscillator module.
    The balanced mixer helps reduce the intermod from off-frequency
    strong signals.  Applying DC in place of RF to the IF port of the
    mixer gives a range of on-frequency attenuation.  (Actually, I
    like to build my mixers by hand, using common 1N4148 diodes.  They
    give better attenuation characteristics, though with a higher
    minimum attenuation.)

    One recent project uses a TFM-2 mixer and 24MHz oscillator
    that allows me to hunt an ELT on a 2m only receiver.  The whole
    thing fits in a 1" x 1" x 2" box (with external battery.)


>One other trick we used to use was image reception...


    The simplest one I've found for sniffing around the ramp is a
    standard hand-held FM broadcast receiver.  Tune it around
    100MHz and listen for the ELT.  The old-fashioned kind with
    analog tuning dials were great, as you can add attenuation
    by just tuning off frequency.

     - Dale WB6BYU