back to HTX-202 attenuator use

testing offset attenuators with the 202:


In hosting (and occasionally attending) fox-hunts over the years, I've amassed a large amount of gear, most of
which is for hosting hunts, including hunts where the participants don't have any of their own gear. (such as
Boy Scouts on JOTA days)

The main hardware I have are fox boxes and beacons, tape measure beams, HT-202 radios, and offset attenuators.
With over a dozen 202's, I run them on 6-AA battery packs with rechargeable batteries. The 202's are rugged,
easy to use, pretty easy to maintain, and are often cheap or free. Radios donated with the "Er1" error are
quickly repaired with the replacement of their coin battery and a reset.

One drawback of these radios is that, although they have an easy-to-read five segment meter, the signal range
the meter operates over is uncommonly small. This does have the advantage of increasing the precision of the
reading, but some sort of attenuator is mandatory. Offset attenuators are the choice for beating down strong
signals, but they also have an operational range that doesn't always overlap well with the direct meter values.

I've got four different offset attenuators that each have their own characteristics, which was making them a bit
confusing, so I decided to get some detailed measurements using an HP 8640B precision signal generator. Results
are on the sheet below. The most important numbers are in the second row, showing how strong a signal needs
to be to hit S-9 at full attenuation, and how weak a signal needs to be to hit S-1 at minimum attenuation.
This is the "efective range" of the attenuator/radio combination. Any closer and you can't get the meter to
drop below full-scale, and any farther away and you can't hear the fox at all.

If you switch back to the fox frequency (and either remove the attenuator or switch it into bypass etc) you're
then subject to the signal range of the radio, which will provide greater range but will just be stuck at full
scale when close. The offset-attenuator range often doesn't overlap the radio's unaided range, leaving a sort
of "donut" area around the fox where you're too close to use the radio directly and yet too far away to use
the offset attenuator. Ideally you want this donut to either not exist or be very small.

The other issue of course is how close can you get before the offset attenuator ("OA") is also full-scale.
These two factors define how helpful your gear is in a fox-hunt.

The 202's limited S-meter range (about 11dB) is somewhat countered by being able to adjust the OA's mix, but
the short meter range translates into more frequent adjustments during a hunt. It also often produces a thick
"donut of uncertainty" around the fox where the radio is all S-9's and the OA can't hear the fox, which can
be a real problem on a hunt.

To counter these limitations I'm taking on a two-pronged approach. First I'd like to increase the dynamic
range of the 202, although I may ultimately decide I want to keep the shorter range so as to maintain the
valuable high signal resolution it offers. The second task is to increase the range and sensitivity of the
OA's I'm using. I want the biggest dB range between minimum and maximum attenuation, try to get the minimum
attenuation as low as possible (so the attenuator never has to be connected or disconnected during a hunt)
and of course for the OA to offer as much attenuation as possible for when getting really close to a fox.

In reality, the last goal is the least important, because in most cases a fox transmitting a few watts won't
be able to overwhelm most OA's until it's only a few feet away. At that point, waiving the beam around starts
to have more of an effect on signal strength than does direction of the antenna. (when you're that close you
should stop using a beam and switch to a sniffer)

I was expecting the biggest take-away from this chart I made to be a validation that I needed to widen the
range of the 202's meter, but it provided much more useful information in comparing my four attenuators to
each other. I was a bit shocked to find my first crude OA offered the best attenuation, usable all the way
up to +11.5dB, although I think that's more due to it just having a very high insertions loss. (the smallest
signal it can detect is -50.5dB, making it useless for getting a bearing in the early game) Since the 202's
naked range is -100 to -111dB, this creates a giant 50dB donut of blindness around the fox. It does also have
the highest attenuation range of the four, at 69dB. (the printout says the delta is 39 dB, type-o)

The hybrid attenuator features four modes - pass-thru, -20dB, -40dB, and OA. Considering all the possible
configurations, this attenuator provides continuous coverage from -35 to -112 dB of signal strength. Probably
due to the rotary switch, it can't achieve as great of an ultimate attenuation though. In retrospect, the
rotary would probably have been better used in adding 20 and 40 dB of attenuation to the OA mode, rather than
to the direct mode.



HT-202 offset attenuator testing:



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last updated 10/29/2025 at 08:49:39 by make_www_index.command version 2025.06.28.A