Cal,
I once compared my FT-897 (I think similar to the 817) to using a
transverter in front of a few different generations of better radios. Here
is an excerpt from that:
"In VHF/UHF receiving, a primary goal is being able to hear very weak
signals, and frequently this needs to be done in the presence of very strong
signals on that same band. With that as the objective, I have attempted an
analysis of the situation.
For reference data and background info, I have used the test results
published by the ARRL in their product reviews, their explanation of these
tests, and the recent presentation by Bob Sherwood to the Boulder, Colorado
Amateur Radio Club in February. (see http://www.sherweng.com/)
Dynamic range measurements characterize a receiver's ability to function on
one frequency in the presence of one or more unwanted signals on other
frequencies. There are a number of different types dynamic range parameters
and effects. I will focus on the so called close-in third order dynamic
range. I enjoy contesting and other signals close in frequency can be a real
issue for us on the East coast. The further-out measurements for third order
dynamic range have been shown to be inadequate for real-world crowded
conditions, and that is why the ARRL had started to do the 5 kHz spacing
measurements a few years ago. If a different parameter is of interest to
someone (receiver blocking, for example) a similar analysis could be
performed.
I will examine 3 situations:
1- a Yaesu FT-897 being used on 2M
2- a good transverter in front of a previous generation IF radio
3- a good transverter in front of an IF radio having a true "roofing
filter"
First let's look at the FT-897 on 2M. In my experience, it is quite good at
hearing weak signals. During contests, it was OK, but I did have to steer
clear of the strong locals by quite a bit when I used it there.
The ARRL data for the 897's noise floor (aka minimum discernible signal, or
MDS) is -140 dBm, which converts to 0.02 uV in 50 ohms, and they further
listed the 12dB SINAD sensitivity as 0.2 uV. That shows why the FT-897 hears
weak signals well.
The close-in (5 kHz), two-tone, third-order IMD dynamic range of this radio
on 2M is 64 dB.
Now 64 dB above 0.02 uV is 32 uV. An accepted "standard" for S9 is 50 uV, so
this means that some combination of nearby signals slightly under S9 would
start to cause some undesirable effects in the FT-897 receiver. (As an
aside, the ARRL measured that it takes only 2.2 uV to read S9 on the radio's
meter, which is quite generous, so 32 uV would have read S9++ on its
bargraph display.) This result is consistent with the contest performance
that I had with this radio, namely I had to stay far away from other strong
signals.."
73
Chet, N8RA
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of Cal Cotner
Sent: Friday, December 11, 2009 2:32 PM
To: vhfcontesting@contesting.com
Subject: [VHFcontesting] 6M HB9CV Antenna & FT-817 2M Overload
Hi all,
.
.
Also, does anyone have comments on the receive overload performance on
2M of the Yaesu FT-817. When mountain topping In two past VHF contests I've
had a lot of trouble on 2M with possible overload of mine. I suppose It
could be splatter from high powered, multi-operator stations sending
automated CQs. I've not experienced this on 6M. Again, any comment would
be welcome.
73,
Cal K4JSI
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