[ARDF] VK3YNG Mk 4 vs. L-Per

[Dale Hunt, WB6BYU]

Hi Peter,

   I've used the L-Per for 10 years, teaching DF to
a Search and Rescue team.  I also have a number of ARDF
receivers, including a VK3YNG sniffer that is my main
competition receiver.  For our monthly mobile transmitter 
hunts we put out both a 2m transmitter and a practice
ELT, so I get to hunt it fairly often.  So perhaps I
am in a reasonable position to compare them.

   First, I have an earlier version of the sniffer.
I don't remember exactly which one, but it doesn't have
the squelched FM mode.  I had a problem when I first
tried to hunt my ELT with it because there wasn't enough
filtering on the RSSI pin.  The change in signal strength
during the ELT sweep confused the auto-ranging circuitry
so it didn't always switch steps properly.  I solved this
by using manual attenuation steps (though I could never
remember exactly how this worked, and usually managed
to change the frequency a couple times before I
got it figured out.)  I understand that Bryan has fixed
this problem in later units.  Since then I have turned
down the modulation level in my ELT and haven't had any
further problem, but I can't be sure that I may not
encounter a poorly-adjusted one in an actual hunt.

    I have great respect for the original L-Per receiver,
which is one of the most sensitive that I have ever used.
For driving around town where the roads are at right angles,
hainging the antenna out the window to give a FORE/AFT
indication simplifies hunting since you don't have to turn
the antenna (except by turning the car.)  Using the 243
antenna is handy - especially when I am driving through the
fog all by myself trying to DF at the same time.  Overall
I think the L'Per with the LEFT/RIGHT antenna is capable of
more accurate bearings than a VK3YNG sniffer with a 2-element
quad.  With the sniffer I can hear differences of 1/4dB or
better, which is about +/- 15 degrees with a 2-element quad
(or perhaps +/- 12 degrees with 3 elements, vs. +/- 20
degrees for a 3-element yagi.)  The switched pattern DF 
on the L-Per is supposed to be good to 5 degrees,
but that assumes a good signal and a skilled operator.  
I've often seen bearings off by far more than that. When
the L'Per is used with a quad the bearings won't be
quite so precise, but I don't have one handy to measure
the minimum detectable response on it.  (I suspect it
is around 1dB, but could be half a dB depending on the
meter sensitivity.)

   While the switched pattern DF is good for sharp bearings
on a clear signal, it will get confused in the presence
of reflections or multi-path.  The standard solution is to
make sure that there are only two meter crossings while
turning 180 degrees, and that moving sideways doesn't
change the bearing significantly.  If these tests fail
then you shouldn't trust bearings from the site.  That
doesn't help much, however, when you are sniffing around
inside a metal hanger for an ELT.  In that case using
a 243 MHz yagi (on a dual-band L'Per) may work much
better, or even putting on a rubber ducky and looking
for where the signal is strongest.  Actually a 2-element
yagi for 121.5 MHz with 7" spacing can give a pretty
good account for itself, though it is still a bit on the
long side.  Actually, such a yagi will work with either
the L-Per or the VK3YNG sniffer for close-range work.

My preference for close-range work is a 732 MHz yagi
with 6 elements or so - it is small and light with a
good directive pattern.  A "DC-to-daylight" receiver
tuned to the 6th harmonic of the ELT (or the 5th
harmonic of a 2m transmitter using same antenna)
allows hunting indoors better than anything else I've
found, but may only work over a few hundred feet.
Another inexpensive option for close-in work is a 
simple crystal set for 121.5 MHz - no controls to
worry about or battery to wear down, and simply 
point the antenna for the loudest signal.  Mine is
good for about 50 yards on my practice beacon, 
which should be fine for a small airport - just walk
down the row of hangers until you hear something
and follow the antenna to the source.

   But back to the question at hand.  I think the
L'Per has a slight edge in sensitivity, though I haven't
had them both on the bench at the same time to measure
it.  The Mk4 is still pretty good, however - adding a
low noise preamp (0.6dB noise figure) only increased
the MDS by 1dB.  For initial signal acquisition I carry
a 6-element fold-up yagi for additional gain, which 
will work with either set.

   For ease of use I'd say the VK3YNG has an edge,
though both sets have drawbacks for untrained
operators.  Sometimes I've gotten the VK3YNG sniffer
into a strange mode by pressing a button accidentally,
and while the default frequency can be set to 121.5, 
figuring out how to change it to a practice frequency may
not be easy for someone who hasn't used it a lot.
(A keypad cribsheet would be a good idea.)  But the
automatic attenuation and audio readout make it very
easy to use otherwise.  The L'Per requires training in
how to tell if the bearing is reliable and in remembering
to turn down the RF gain control, but the controls are
perhaps easier to figure out how to use.  Certainly for
use in the bush the VK3YNG with a tape measure yagi is
easier to use than the switched-pattern L-Per antenna,
though the latter, being higher in the air, may give
better readings in weak signal conditions.  Of course
the L'Per can be used with an exernal yagi as well, or
a switched-pattern antenna can be made with tape measure
elements.

    In my years with the SAR team we practiced once a
year at best, and most members didn't get enough hands-on
time with the receiver to remember how to use it when
we actually had a search.  The biggest problem was not
turning down the RF gain control to keep the meter on
scale.  Generally this can be solved by fixing the
audio gain at maximum, which forces them to turn down 
the RF gain control or it is too loud.

   One annoying (to me, anyway) quirk of the Sniffer is
that the AM modulation doesn't change loudness with
signal strength the way you would expect for an AM
receiver.  (This is due to the logrithmic nature of 
the RSSI output used as a detector.)  Most folks won't
care, since the signal strength indicates in tone mode
instead.  This is a personal annoynance since I know
how a real AM receiver is "supposed" to work.

   So in summary I find the VK3YNG sniffer very useful
for hunting ELTs.  If I had both there may be times when
I use the L'Per, but once set up the Sniffer is easy for
relatively untrained folks to use and walk right up to
the transmitter.

   Hope that helps.  I'll be happy to answer further
questions off-line.

     - Dale  WB6BYU


Peter Devanney wrote:
> 
> Hello Group,
> 
> First of all, if this is not the appropriate forum, please accept my apologies for the QRM, let me know and I will respect the groups wishes. If it is, I have a few questions on the relative merits of DFing with the  YNG Mk4 Sniffer and the L-Per (the xtal version, not the new one) on AM (121.5)
> 
> In addition to some foxhunting, I do a fair amount of training with our local volunteer air search and rescue team in pursuit of (mostly) training practice beacons (ELTs).
> 
> We use the L-per with the quad antennas on the truck. On foot we use the big cross.
> 
> I have read many accolades for the VK3YNG sniffer in foxhunting and ARDF work. I am curious if there are any on the list with experience DFing ELT's with this device. If so, would they be willing to comment on the relative advantages and disadvantages versus the L-per. I am trying to find a more portable set-up for foot work DFing ELTs.
> 
> Finally, out of curiosity,  has anyone used (or heard of someone using) the Mk4 sniffer from an aircraft?
> 
> Thanks and 73
> 
> Peter
> VA3YOW
> 
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