Thanks to Ken N4UK for letting me put this on
my page for all of us to read!
6/17/97
OK, I admit it. I'm a weak signal junkie.
I'm addicted to it. I know I'm not the only one with this
addiction. Gimme a DB and I'm happy. Take one away and a nervous
sweat breaks out. How did this all begin? I'm going to tell you.
I'm also going to let you know about the obvious and the not so
obvious things you can do to improve your weak signal receive
capabilities. Sit back, grab a cold one, light one up and read
on.
A lesson we must all learn...
My obsession with trying to obtain the best
weak signal receive performance all began a few years ago during
the September 1992 VHF QSO Party. I was operating right smack in
the middle of the high power corridor in Eastern FM19 near
Lancaster, Pennsylvania. It was during this effort that I
learned first hand about phase noise and the nightmare it can
put you through. My receiver's noise floor increased 2-3 S units
anytime any of the high powered locals transmitted many KCs
away. The pain was excruciating! I have been at it ever since...
On to Dixie Land...
Living in a townhouse in Maryland did not give
me much of an opportunity to experiment on the VHF bands but a
job transfer to South Carolina was a blessing in disguise. Now I
had the space and quiet to see how many DB I could squeeze out
of my VHF/UHF receivers! The quest had begun.
The obvious things you should look for and do...
I know many of you are aware of the essentials
you need to do to minimize losses on your receive systems but
I'll briefly talk about them for the benefit of the newcomers to
weak signal VHFing and there may be something even the
experienced operators may have overlooked.
Be all that you can be...
1- Get an excellent transverter and HF rig combination. A
great transverter is only great if the I.F. rig is also great.
Using a phase noisey I.F. rig without good QRM fighting features
defeats the whole purpose of going to a transverter. Likewise,
using a crummy transverter with an FT1000MP doesn't make a whole
lot of sense either. A high dynamic range and 3rd order
intercept point are other features to look for in an I.F. rig
and transverter. A noise blanker that works is essential! All
the great features of your transverter and I.F. rig are null and
void if noise keeps you from hearing weak signals.
2- Use the best feedline you can afford. Use half inch or
better for 144, 222, and 432 use 7/8th hardline or better for
432 and above. A half db loss or more with other types of coax
is unacceptable on the weak signal bands!
3- Make sure the connectors on your hardline are correctly
installed. Improperly installed connectors add losses and could
eventually lead to total receive system failure or, worse yet,
equipment damage. If the N connector body (not PL259!) spins on
the coax it has NOT been properly installed. If the center pin
sticks out beyond the collar it has NOT been properly installed.
If the center pin is set too low in the body it has NOT been
properly installed. You can improve your receive capabilities
anywhere from tenths of a db to a DB or more when going from an
improperly installed connector to a properly installed one.
4- Get the biggest antenna(s) with the most gain and best
pattern you can put up and put it as high as you can get it. The
two best reasons to do this are increased gain on receive and
the ability to null out qrm, noise, etc. from unintended
directions.
5- Use a low noise figure receive preamp on 2m and above and
most especially on 432 and above.
6- Find the sources of nighborhood noise and do whatever you
can to get rid of it. Pester the power company, take your
neighbor to lunch (or dinner if she's really good looking!),
kiss whatever ass you need to kiss to get rid of that noisey
power line, doorbell transformer, electric fence charger, etc.
Just because the noise comes from a direction that you
hardly ever turn the antenna towards doesn't mean that you
aren't getting noise in your receiver when you're beaming
elsewhere. The tiniest amount of noise adds to your receive
losses!
7- Use the best coax for jumpers and make sure the
connectors are properly installed.
And now the not so obvious...
1- Buy yourself an RMS voltmeter. My HP 3400A has been an
enormous help in discovering what makes my receiver quieter and
what makes it noisier. I have seen them for sale fairly cheap at
hamfests. Plug them into your headphone jack, keep a reference
level and work your way from there. If your I.F. rig has a
constant receive audio low level output then you're even better
off yet!
2- Set your transverter I.F. Rx level gain correctly. Most of
my levels are set so that there is a barely perceptible increase
in noise when I turn on the transverter. IGNORE your I.F. rig's
S meter!!! The S meter circuitry is not where it is supposed to
be in the RF chain when you use the HF rig as an I.F. rig. An S
meter reading is worthless when your transverter's Rx I.F. level
is properly set for maximum signal to noise. You are hurting
your receive capabilities when you adjust the I.F. gain for
maximum s meter movement. Too bad that transverters don't come
with S meter circuitry and a real Signal level meter. I know;
old habits are hard to break. Work at it! Properly setting my 2m
I.F. Rx levels improved my S/N ratio by a full db.
3- Weak signal receive equipment and data equipment are like
oil and vinegar. They just don't mix! Most computers and other
equipment using digital circuitry emanate noise. Seperate your
receive system coax cables, equipment, power cables, power
sources, etc. from your computer, monitor, serial cables, packet
TNC etc. My noise floor decreased a full db on 222 and 2m by
rerouting the computer and associated cables further away from
the coaxes and preamps. It was barely perceptible to my ears and
I never even noticed it but the AC voltmeter tells the truth and
nothing but the truth. If worse comes to worse turn off all
digital devices while you are chasing weak signals.
4- Periodically monitor the reference level on your A/C
voltmeter. An increase in noise can mean that the new serial
cable you routed in the shack is poorly shielded or that a power
pole down the street has started to act up. You can now act on
it before that 1500 mile meteor scatter schedule time arrives.
My reference level on my 2m receiver is -10dbm. I use the
constant level output on the Icom IC740; this way I don't have
to worry about adjusting the volume control to the exact same
setting each time. The noise floor varies from a wintertime high
of -12dbm to a summertime, high humidity day, -20dbm, an 8db
swing! This is a great way to know how good your weak signal
receive opportunities are on any given day.
5- Determine if the noises on the bands are in band or due to
intermod from out of band, high power sources.(TV, FM broadcast
stations, etc.) Get the appropriate filters to take care of the
problem. Remember that the DB insertion loss of the filter can
be nullified by the DB (or better) decrease in your noise floor
when the intermod products are removed from the weak signal
band. The best ham products on the market to remove the crud are
notch filters from PAR Electronics and the band pass filters
being sold by DCI in Canada.
6- Get rid of those cheap power cubes! When the transformer
inside of them gets hot they get noisey. Do yourself a favor and
use a good heavy duty power supply for all your 12v accesories.
If you absolutely have to use a power cube for, let's say, 9VDC
then be extremely rating conservative! If you need 9VDC at 20ma
then use a power cube rated at 9VDC at 200ma or better. The
combined noise from 5 hot power cubes can easily add a db of
noise or more and, worse yet, you'll think you have line noise
somewhere in the neighborhood when the problem is right under
your nose.
7- Keep cooling fans away from audio circuits. They have a bad
habit of inducing noise into your receive audio circuits. Noise
is noise no matter where it comes from!
Either to the poor house or the nut house...
My station is still a work in progress. I'm
constantly trying to make the receive systems better. Whenever a
new neighborhood noise becomes evident I sometimes consider
throwing in the towel but wind up fighting back with full force.
Nothing is worse than taking a step forward and then being
kicked two steps back through no fault of your own. It's become
an obsession I tell you. I spent last weekend playing with
preamps and listening for weak signals instead of contesting.
The most exciting thing that happened all weekend was hearing a
weak cw signal from K3YTL in FN11 CQing on 222 Mhz. I'm one step
closer to the worst kind of lower signal-to-noise ratio fanatic.
I hear that some of those guys have wound up in permanent
residence at the local funny farm! I'll eventually wind up
sitting next to those guys at a hamfest reminiscing about "the
good old days" when you could hear a pin drop on any of the
bands. Yes, someday I'll eventually become a full blown EMEer...
73 Ken, N4UK
EM84xp South Carolina
This site is owned by Dave Booth KC6WFS.