Improve Your Weak Signal Capability

Ken Rameriz, N4UK


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...

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 decibels 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 VHF operation, and there may be something even the experienced operators may have overlooked.

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 coax 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 directional 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 undesired directions.

5- Find the sources of nighborhood noise and do whatever you can to get rid of it. Pester the power company or do what else is needed to get rid of that noisey power line, doorbell transformer, mobile phone charger, electric fence charger, electric shaver 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!

6- Use the best coax for jumpers and make sure the connectors are properly installed.

7- Try phase cancellation for minimizing or eliminating noise.

8- Go around the house with a AM VHF portable radio, and listen for any noise that is present. Take notice of where the noise is strongest and weakest and move your DX antennas to the quietest spot. The other option is to move noise sources away from the antennas

9- 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. Separate 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.

10- 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.

11- Determine if the noises on the bands are in band or due to intermodulation 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 intermodulation 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.

12- 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!

13- Keep the receiver's noise blanker permanently switched on.

14- Try vertical or horizontal polarization, depending which provides the best signal to noise ratio. Usually man-made noise is predominately vertical.

15- If your VCR, TV, or computer is radiating noise, try winding the power cord around a ferrite rod. Keep the rod as close as practicable to the output of the appliance.

16- Before you decide to rent or purchase a house, try checking out the general ambient noise levels with a portable AM VHF radio.

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