[RFI] MFJ-856

[Dale Svetanoff]

Pete and All,

I recently purchased an Icom IC-91A, a 2m/70cm dual bander, and it has air
(and even SW band) AM capability.  I plan to use it as Pete describes the
next time I have a power line-related RFI problem.

I do have a word of caution regarding use of HTs (or even mobile/base rigs)
for tracking RFI sources: it has been my observation that the S-meters on
many of today's rigs have limited to very limited dynamic range.  Back in
pre-historic times (the 50's and 60's), many receivers had S-meters which
actually required 50 microvolts to = S9, and the " xxdB over 9" part of the
meter scale were actually fairly accurate.  (I can attest to the meter
performance of my Collins S-Line receivers, as well as my Hammarlund
HQ-110.)  My modern rigs have S-meter performance all over the map.  I have
one rig that has only slightly more than 30 dB dynamic range between S1 and
40 over 9!!!

I have not yet measured the meter's dynamic range on the '91A, but will do
so prior to using it as an RFI tracker.  Due to the limitation of dynamic
metering range on so many rigs, it is a good idea to have an external
attenuator available when using one of these radios as an RFI detector. 
Mine is a Kay Elemetrics attenuator, fitted with BNC connectors, and it has
about 60 dB of insertable attenuation (available in steps via toggle
switches).  That should be enough to keep even a very strong signal within
the range of the '91A's metering circuit.  Incidentally, the IC-91A
includes an internal attenuator to reduce front end overload.  It is
selectable via the menu system and provides "about 10 dB" of attenuation
(according to the manual).  

One more caution with use of external attenuators: since HTs and most other
rigs are transceivers, the attenuators can be damaged if you accidentally
transmit into them.  Be sure to set the rig on its lowest power setting
before connecting the external attenuator.  Yes, I know that most rigs will
not transmit at all if you are using the radio outside of the ham bands,
but you never know when the unexpected can happen.  Toasted attenuators are
not useful.

73, Dale
WA9ENA     


> [Original Message]
> From: Pete Smith <n4zr at comcast.net>
> To: <rfi at contesting.com>
> Date: 4/23/2012 6:38:39
> Subject: Re: [RFI] MFJ-856
>
> OTOH, for less money, you can buy a  shirt-pocket handi with 2-band 
> transmit capability and AM receive capability on the aircraft band.  I 
> built a 136-mhz moxon (from QST} out of hardware store aluminum rods, 
> and it works fabulously well, particularly when using the narrow rear 
> null to pinpoint a source.  Even if you can just isolate the source to a 
> particular pole, the power company guys will be very grateful.
> 73, Pete N4ZR
>
> On 4/22/2012 8:13 PM, Dave Harmon wrote:
> > I have both the MFJ units....with and without the yagi....they work well
> >
> > Dave Harmon
> > K6XYZ[at]sbcglobal[dot]net
> > Sperry, Ok.
> >
> > -----Original Message-----
> > From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
> > Behalf Of Patrick Dyer
> > Sent: Sunday, April 22, 2012 12:02 PM
> > To: RFI at contesting.com
> > Subject: [RFI] MFJ-856
> >
> > Is this MFJ-856 worth a second glance for DFing RFI ?
> >
> > For that part of the spectrum I'm currently (since 2003) using the R,
DE,
> > D1, and D2 of an 11-el 144-MHz Cushcraft with an Icom IC-A4 (108-137 MHz
> > airband xcvr).  The 7-MHz difference seems to have minimal skew to the
> > pattern (using an old Precision E-200-C signal generator as a test
source in
> > the yard).
> >
> > However, the MFJ-856 has a signal-strength meter, and the 3-el antenna
boom
> > length (with the rcvr on it) may be easier to manage/handle than what
I've
> > been using for nearly a decade.
> >
> > Before I got the IC-A4 (originally to track the MUF of Es events above
the
> > FM broadcast band using the VOR/ILS navaids) I had been using a rather
> > decrepit (1971) Radio Shack JetStream mini (slightly retuned to
> > 144 MHz) for RFI chasing.
> >
> > 73, Pat - WA5IYX
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