A Wideband Low Voltage Offset Attenuator
A little background, first.
Anyone who's attempted VHF or UHF hidden transmitter hunts with a
standard hand held transceiver or scanner recognizes they're not
well shielded. When you get close to the hidden transmitter, the
signal begins to enter the radio even when you add attenuators between
the antenna and radio. The problem is that the radio's are not
well shielded because its not a requirement for typical use.
Afterall, the manuafcturer needs to keep costs low to
remain competitive. A strong RF source will blast right through
the plastic case and reach the detector circuits directly.
Your "S" meter is saturated, you've lost directivity from your yagi or
whatever you're using for a directional sensor.
On the otherhand, a well shielded radio has a related problem.
Usually it can be
used effectively with external passive
attenuators but many have found having a continuously
variable attenuator allows you to adjust the signal level as a manual
gain control so the "S" meter reads half scale. Then as you move
closer, the meter will move higher and you'll be able to bring it back
to the middle range by adjusting the manual gain to a slightly lower
level. Making note of the gain settings as you progress tells you
how close you're getting to the fox. With experience,
you'll be able to judge your distance at any time by observing the gain
setting. Of course, effective radiated power of the transmitter
can vary but I've found that it doesn't change that much particulary
when the power of the transmitter is stated beforehand and you take
that into consideration. However, most of us don't have well
shielded radios unless they're designed for activities like foxhunting.
A very clever solution developed many years ago has been to mix the
incoming RF signal with a LO (local Oscillator) to create the sum and
difference frequencies. The intensity of the output signal can be
well controlled (maybe a slight over statement) by the amount of LO
mixer. The very strong fox transmitter RF cannot get into
the receiver because the receiver is now tuned to the sum or difference
frequency and not the Fin. If the resulting mixing
products are too strong,
simply reduce the amount of LO signal and you can bring the "S" meter
into the mid range position. This solves the two common problems
found in handheld receivers listed above.
Most Amateur Radio Direction Finding (ARDF) is on 2 meters. A common foxhunting frequency is 146.565 Mhz. If our offset attenuator has an LO frequency of 2.00 Mhz then the output will be:
Fout1 = 146.565 +
2.0 = 148.565 Mhz
Fout2 = 146.565 - 2.0 = 144.565 Mhz
Now you'll need to tune your radio to one of these two frequencys to
hear the fox signal. Note that one is outside the amateur band
but many radios have receive capability for out of band signals.
Any other signals that may be residing on these frequencies will
probably get through w/o much attenuation and can be an interference
This is all common knowledge for foxhunters who have tried to use
handhelds for hidden transmitter hunts. What are some of the
My first OA worked great but had several undesireable characteristics:
I attempted to solve a few of the problems but theirs still plenty left if you want to try your luck. Referencing the items above:
The complete schematic can be found here.
All the circuitry below R2 potentiometer is for the oscillator and
power. R2 is available from Radio Shack and inexpensive.
D1 is the mixer diode with the LO injected via R1. The return
path is through R3. As the wiper of R2 travels more negative a
small DC bias is placed on the diode to help its conductivity.
When the wiper is in its low sensitivity position, the bias is removed
but a strong RF signal coming in will bias the diode with C1 holding
the offset voltage. Generally, its prefered to keep the
oscillator circuit ground separate from the chassis but you can try it
either way to minimize leakage of the LO.
No circuit board has been made but I'd be happy to give guidance for anyone who wishes to build it. With a directional antenna, a receiver and an OA you have all the equipment you need to be a serious foxhunter or interference sleuth.
|KA7CSZ looking for foxes
||Offset Attenuator velcro'd to
Yagi antenna boom