Welcome to Episode 415 of Linux in the Ham Shack. In this episode, we welcome back Michelle Thompson, W5NYV, from the Open Research Institute. We had an interview about the ORI and its affiliated projects early after the inception of the institute back in 2018. This update involves some longstanding projects, grants and new affiliations of the ORI including the P4DX project, M17, AmbaSat, Aqua Phage and more. We hope you enjoy this update episode and have a great week.

73 de The LHS Crew

After being absent from 2m EME (moonbounce) for the past couple of years, I decided to spark-up again this spring to see if my simple system was still up to the task.

Pretty much every month, during the moon’s sweep through its northern declination, I get several days with moonrises right out in front of the house overlooking Georgia Strait. This is the large body of saltwater separating the British Columbian mainland (and the rest of North America) from Vancouver Island to my west.

Having an ‘over the ocean’ moonrise offers several advantages for me as it pretty well guarantees an extra 6db minimum of system gain (both on transmit and receive modes) and provides a noise-free environment for the antenna to look into.

Because of this advantage I’ve been able to get away with a very minimal system consisting of a single 9el Yagi and a small FM ‘brick’ amplifier which yields around 120W of output. The antenna is tower-mounted at 60’ and controlled in azimuth only. Without being able to track the moon as it rises, the Yagi is broad enough to give me about 2 hours of moon-time on each session before I start to lose signals. With most EME stations using four or more Yagis and high power, most of the heavy-lifting on my two-way work is being done by the other station. With the extra sea-gain here, my single 9el Yagi performs more like an array of four similar Yagis.
 

There always seems to be new stations to work whenever I get on the band and this spring was no exception. All told, I had 20 contacts, with 12 being new 'initials', bringing my total initials count to 130. The remaining 8 contacts were with stations I have worked previously. I was also able to add 2 new states, New Hampshire and Wisconsin, bringing my 2m WAS total to 30.

Conditions were poor to average, with one day in particular being excellent, when at one point I had a pileup of three callers!

Most of the stations contacted are always surprised to learn of my small system and comment that my station is the smallest one they have worked. I have worked a couple of two-Yagi stations over the years with one of them being worked several times.

Here are the cards that have arrived so far for this spring’s session:

If you haven’t given single-Yagi EME a try I would encourage you to test it out as you might be surprised at your results. Even without the added sea-gain, many of my contacts were loud enough to be easily workable with 6db less gain ... and there are dozens of big capable stations out there just waiting for new initials!

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In this episode, Martin (M1MRB) is joined by Chris Howard (M0TCH), Martin Rothwell (M0SGL), Ed Durrant (DD5LP) and Bill Barnes (WC3B) to discuss the latest Amateur / Ham Radio news. Colin (M6BOY) rounds up the news in brief and in the episode we feature have a double feature Hubs, Clubs and Virtual Meetings USA Style and Gateways On The Air Event

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When discussing signal levels and power output, hams like to say things like:

Using higher power isn’t important because it only gives you one additional S unit

and

You’ll lose some power in the coax but you won’t even notice a few dB

These statements are often true and at the same time may be completely wrong. I’ve noticed that radio amateurs pushing the limits of their station pay close attention to every decibel they gain or lose. This is especially true at VHF/UHF frequencies where signals may be weak. A dB here, a dB there, the next thing you know it adds up to something big!

Definitions

First, let’s make sure we have a few definitions right. The decibel (dB) is defined as the ratio of two power levels:

dB = 10 log (P2/P1)

One decibel corresponds to a 26% increase in power level. A well-known rule of thumb is that doubling the power corresponds to a 3 dB increase. Similarly, chopping the power in half drops the signal level by 3 dB.  A 10 times increase in power is 10 dB. (Voltage can also be used to calculate decibel relationships but to keep it simple, I’ll just use power.)

The S Unit is normally defined as a 6-dB change in signal level, which is a factor of 4 in power. (Your S meter may or may not actually follow this rule but that is a topic for another day.)

Power Level

Let’s compare a few different power levels to get a feel for how decibels and S units behave. Let’s use a 5 watt QRP level as our reference power. If we crank up the power to 100 watts, we  have 10 log (100/5) = 13 dB increase in power level. This is slightly more than two S units (2 x 6 dB), so we would expect the S meter on the other end to read 2 units higher.

Now suppose we kick in our linear amplifier to produce a 1 kilowatt RF signal. This power level is 10 log (1000/5) = 23 dB higher than the 5 watt signal, or roughly four S units.

Now if our QRP signal was a solid S9 to start with, adding another 23 dB on top of it may not be that significant. The station can be heard at S9 or can be heard even louder at S9 + 23 dB. Except when there’s a pile of stations all calling that rare DX…then the loudest station tends to be heard. Crafty operating skill and good luck may overcome the power difference.

But consider the other extreme. Our QRP station is being heard right at the noise floor on the receive end. The two stations are struggling to complete the contact and the propagation path degrades by 2 dB. Now the QRP station is below the noise and uncopyable. We increase our power to 100 watts and gain 2 S units…still not very strong but the ability to receive the signal improves dramatically. Crank it up to 1000 watts and you gain another couple of S units and the copy is quite good. The key point is that changes in signal level matter most at the margin, when you can just barely copy the signal. (By the way, there is nothing wrong with running QRP…many ops enjoy the challenge of making contacts with low power.)

At the receiver, our ability to recover the signal is determined by the signal-to-noise ratio (SNR). A higher noise floor at the receiver means it will be more difficult to hear the signal coming in. The type of modulation being used may also make a big difference. Good old CW and the WSJT modes use a narrower bandwidth and will get through when wider-band modulation (SSB, FM) fails. In all cases, a stronger signal works better.

Antennas

Antenna systems also increase our signal level…and they do it for both transmit and receive. I recently did some comparisons of VHF antennas from a SOTA summit. My 2m Yagi antenna has 6 dB of gain (referenced to a dipole) and my comparisons showed that the performance of this antenna was good enough to pull some signals out of the noise to be solid copy. This occurred when the other station’s signal was right at the noise floor (using my lower gain antennas) such that the 6 dB improvement had a significant impact.

Sometimes hams will say that VHF is just line-of-sight propagation and that the signal level doesn’t matter much. This is partially true but often we are stretching for contacts beyond line-of-sight. Take a look at this article: The Myth of VHF Line-Of-Sight. This is another case where we are operating on the margin and every dB matters.

Feedline loss can cause us to lose decibels, which impacts both transmit and receive performance. If your coaxial cable is short, then the losses may be negligible. Increasing cable length and increasing frequency produce more loss. For example, 100 feet of RG-8X has only 1.1 dB of loss at 10 MHz. Increase the frequency to 146 MHz and the loss jumps to 4.5 dB, using the Times Microwave cable calculator. That means 50 watts of power at the transmitter turns into 17.7 watts at the other end of the cable. Using LMR-400 coax reduces the attenuation to 1.5 dB.

Summary

You can choose to ignore small changes in your signal level. A dB here or there may not make a big difference with casual ham radio operating. But these losses tend to add up and may become significant. Most importantly, just a few dB may be the critical difference between making a radio contact or not, when operating at the margin.

The post A Decibel Is Still A Decibel appeared first on The KØNR Radio Site.

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Extra Class Exam Questions – Part 15.
E4A Test equipment: analog and digital instruments, spectrum analyzers, antenna analyzers, oscilloscopes, RF measurements, computer-aided measurements.
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