1

Stop Jamming Yourself and Let the Weak

Signals Through!

Rob Renoud, K3RWR

Introduction

Every EME operator knows that the signals encountered on the EME bands are the epitome of “Weak

Signal” communications. That being the case, the EME community pays particular attention achieving

low noise figures in our preamplifiers, selecting antennas and arrays with high gain to temperature (G/T)

ratios, and installing low-loss feed lines to maximize receiver and transmitter performance. Likewise,

we continually tweak our receivers and software in search of every additional dB of system signal gain

in order to maximize our signal-to-noise ratio (SNR). While most of our efforts are aimed toward

maximizing signal, the numerator of the SNR equation, the noise contribution to the SNR equation, the

denominator, is equally important. Each dB of noise reduction offers the same benefits as an additional

dB of signal gain with regard to SNR. This paper highlights the steps taken at K3RWR in identifying

and mitigating numerous obvious, and some not-so-obvious, noise sources both internal and external to

the station. The positive results encountered at K3RWR were significant enough that other VHF and

UHF station operators may find these actions useful as well.

Background

The K3RWR EME station was built from the bottom up specifically for 2 meter EME operations.

Starting with a blank slate, station design and hardware selection were based on recommended best

practices as gleaned from the weak signal and EME communities. There were three general design

goals:

(1) Implement dual polarization receive and transmit capability;

(2) Provide capability for unassisted operation by using MAP 65 software to both display and

decode signals across the 2 meter EME spectrum;

(3) Provide the capability to be competitive in two major yearly EME contests: The DUBUS

144 MHz Digital EME Championships and the ARRL EME Contest

Everything came together in the spring of 2012 when the station came on the air. The basic station

configuration was as follows:

Antenna: 4 x 2.6 wavelength cross polarized Yagi array

Transmitter: TS-2000 transceiver driving a Henry 1.5 kW amplifier

2

Receiver: 2 x SDR-IQ software defined receivers

Software: Linrad, MAP65 and WSJT

I worked numerous stations without difficulty from the start and soon thereafter felt confident enough to

undertake the 2012 ARRL EME contest. At the completion of the contest I was pleased to end up in the

middle of the pack having worked 39 stations during the contest.

After analyzing the results, however, I felt that I simply was not decoding or working the number of

stations that I felt the station was capable of working. I had certainly worked every station I heard!

Additionally, W3SZ had worked 125 stations during the same contest period! Signal reports from

stations worked were good so I felt pretty confident that the software and hardware sides of the station

were working pretty well. At this point my engineering background took over and pointed to excessive

noise as a likely culprit with regard to the degraded station performance. The several noise sources I

identified and mitigated over the following year are detailed in the following sections. How much

improvement did I experience? I’ll share my before and after results based on two years performance in

the ARRL EME contest in the conclusion section.

Digital Pager Transmitter Noise

The first thing I uncovered was broadband noise from several digital paging systems located on a FM

broadcast tower approximately 1/3 mile north-east of my station. Digital Pager noise appeared in the

form of random increases of the 2 meter background noise level lasting for several seconds or more.

The broadband noise intensity and frequency of occurrence varied dramatically by time of day, day of

the week and position of the Yagi array. Often the noise was almost undetectable, but it could also be

overwhelming at times. I initially did not consider pager transmitters as the source of the noise. I was

seeing and hearing none the traditional paging tones often associated with pager interference. Also, my

2 meter FM transceiver continued to work as advertised with no noted interference.

The source of the noise became immediately apparent when I finally conducted a spectrum sweep of my

QTH. The results are shown in Figure 1. The spectrum sweep clearly showed three digital pagers

operating in the 152 MHz band with signal strength of more than -10 dBm! The pager signal timing

also directly correlated with increases in the 2 meter background noise. The spectrum also showed

considerable high-level RF activity both above and below the 2 meter EME Center Frequency of

144.130 MHz. While the digital pagers transmitted none of the tones frequently associated with pager

systems, the digital transmitters obviously put out significant broad-band noise.

Considering the number and intensity of signals both above and below the EME center frequency, I

implemented a brute force solution consisting of a pair of 4” cavity band-pass filters on each of the

receive-line coaxes coming down from the tower. The cavity filters, shown in Figure 2, were found on

EBay at a reasonable price. The filters were designed to cover 148 – 174 MHz but were easily tunable

down to 144 MHz. The pass-band was adjusted to pass only the bottom 300 KHz of the 2 meter band to

eliminate as much noise as possible.

3

Figure 1 – Sweep of VHF Spectrum at K3RWR

Figure 2 – Band-pass Filters Placed on Each RX Line

4

I did try the popular PAR Electronics VHFDN152 VHF Notch Filters. While the PAR filters

significantly reduced the pager noise, they did not entirely mitigate it under the worst conditions.

Additionally, I also replaced the two WA2ODO-type preamplifiers on the tower with ARR

preamplifiers. The ARR preamps appeared to handle the high ambient signal levels better with no

overloading or gain compression observed.

Figure 3 shows the MAP65 waterfall before and after inserting the cavity filters in the RX line under

high pager related noise conditions. The results are definitely dramatic with the dark red showing

extremely high background noise during pager transmissions as compared to the much quieter blue!

One can see short periods of blue background between the bright red pager activity sessions before the

cavity filters were installed. No pager activity was detectable in the receive signal once the filters were

installed.

Video Monitor Cable & Video Mode Noise

I run six video monitors from two computers in my shack. All the monitors and computers are used

during EME operations. It became apparent during my search for noise sources that several of the video

monitor cables were themselves contributing significant interference. Upon investigation, it was

determined that the VGA video mode cables were the worst offenders followed closely by the HDMI

video mode cables. Those monitors running DVI video mode and cables definitely generated the least

noise. Needles to say, I easily solved that problem by switching all my monitors over to DVI video!

While I did not quantify the actual levels, the associated noise reduction was definitely noticeable in

increased receive performance. This was any easy fix and my first case of self-jamming was mitigated.

Figure 3 – MAP65 Waterfall Before & After Insertion of Cavity Band-pass Filter

5

LED Video Monitor Noise

WARNING: Not all LED computer monitors are created equal!

During my initiation into the realm of weak signal EME communications, I became aware that my

background noise level increased by some 3 to 5 dB during EME operations as the antenna approached

the horizon. I attributed that increase in noise level to being in the “urban environment”. While

wishing for a quieter environment near the horizon, I felt there was really little I could do about it and

delayed pursuing possible sources for some time. The noise was pretty much the same regardless of

antenna azimuth but was definitely prominent below 15 degrees of elevation.

One day while working stations at moonset, I accidently dislodged the power cord from one of my prize

24” LED video monitors. The background noise level on MAP65 immediately dropped by 2-3 dB!

Further investigation showed that the two identical 24” LED monitors, which were specifically

purchased because of the low-noise footprint of LED monitors, were sourcing a total of 4-6 dB of noise

when the antenna array was below 15 degrees of elevation! Figure 4 shows the MAP65 Waterfall with

the 24” monitors both on and off.

The RX noise level is 29.7 dB with the monitors on and drops to 25.7 dB with the monitors off! I

reluctantly traded these very nice monitors with my XYL and received up a pair of smaller, but very RF

quiet, monitors from her in exchange. She also graciously offered not to use her new monitors while I

am operating EME. This again was an easy and effective fix. Another case of self-jamming resolved!

Figure 4 – MAP65 Waterfall with Monitors On and Off

6

Internet Router & Ethernet Cable Noise

Joel Harrison, W5ZN, recently published the paper “Slaying the Grim Reaper of Internet Router Noise

on 6 Meters” which is available on his website: http://www.w5zn.org/. In the paper he documented that

his noise floor dropped 10 dB when he turned off his internet equipment at the W5ZN QTH. His paper

prompted me to do the same test at my QTH. Like Joel, I was surprised to see approximately 50% of

my many birdies completely disappear and my noise floor drop by another 4-5 dB!

My internet service is provided by my local cable company. My home router supported numerous

connections for two home offices, the K3RWR station, and multiple TV sets via a wired Ethernet

network that runs throughout the house.

Solving the Ethernet RFI problem at my QTH involved migrating as many internet devices as possible

from wired to wireless connectivity. The one device which could not be transitioned to wireless, a

network printer, was mitigated by installing toroid cores, as described in detail by W5ZN, at each end of

the CAT5 cable running between the router and the printer. The results were immediate and definitely

worth the time and effort! Another case of self-jamming resolved! Note: I am unable to provide any

before vs. after comparison figures for this paper as I no longer maintain any significant wired Ethernet

capability at my QTH.

Power Line & Impulse RFI

While I have not been troubled by power line or impulse RFI at my current QTH, I did experience high

levels of both on 6 meters at my previous QTH located in a busy urban area. The noise and RFI was

often severe enough to completely prevent any operations on 6 meters including CW, SSB and digital

modes.

I did find a solution that significantly mitigated power line & Impulse (ignition) noise RFI to the point

that I successfully competed in and placed first in category in a number of meteor scatter contests over

several years. The solution consisted of using a software defined radio (SDR) receiver along with the

excellent noise reduction algorithms implemented in SDR software programs such as Winrad and

Linrad. In my case, I used an SDR-IQ receiver along with a down converter and the Winrad radio

control software package. Figures 5 and 6 are before and after examples of monitoring the W3APL

beacon under high power line noise conditions. In Figure 5, with the noise blanker off, the noise level is

-95 to -100 dBm and the W3APL beacon is virtually obscured by the noise. In Figure 6, with the noise

blank on at the 10% level, the noise level has dropped to -115 dBm and the W3APL beacon is clearly

visible in the pass band and 100% usable.

7

Figure 5 – Winrad Display with Noise Blanker Off

Figure 6 – Winrad Display with Noise Blanker On

8

Tools for Identifying and Tracking Noise Sources

Over the course of my hunt for and elimination of RF noise sources, I found the most useful tools were:

1) a spectrum analyzer, and 2) a small 2 meter Yagi and battery powered 2 meter receiver.

The spectrum analyzer allowed me to capture and categorize very strong RF signals of interest as was

the case with the digital paging systems addressed earlier.

Figure 7 shows the handheld 3-element 2 meter Yagi and Kenwood TH-F6 transceiver, operated in the

SSB receive mode, that I used to pinpoint and trace noise sources throughout the K3RWR QTH. It was

an easy matter to find “hot” items and then confirm them as the source by turning them on and off while

listening the noise background through the receiver. I also ran the audio output of the TH-F6 into a

laptop running MAP65. This made it easy to quantify the noise level by observing the changing

background noise level figure displayed on MAP65.

Figure 7 – 2 Meter Yagi and TH-F6 Used to Track Noise Sources

Results at K3RWR

The results were absolutely astounding! After systematically identifying and mitigating multiple noise

sources, both external and internal to the station, the number of signals and successful decodes increased

dramatically across the entire EME spectrum. In 2013 I again participated in the ARRL EME contest.

This time I worked 112 stations vice the 39 worked in the previous year! Nearly tripling my score

validated the improvements made to the station over that year!

9

Conclusion

Hopefully this presentation will help those owning and operating weak signal stations to diligently

search out and eliminate sources of noise at their stations. Based on my experiences, I encourage you to

“Stop Jamming Yourself and Let the Weak Signals Through”!

It will be my pleasure to address any questions you may have regarding the steps taken and work

performed at K3RWR. Also, thank you to W5ZN for his paper on identifying and dealing with Internet

Router Noise.