All posts by David Honey

Podcast about Understanding Receiver Specifications

Thought the following Podcast might be interesting to members:

https://www.hamradioworkbench.com/podcast/understanding-hf-receiver-specifications

Eric Swartz WA6HHQ, co-founder of Elecraft, joins us to guide our understanding of RF receiver performance specifications. Eric introduces us to common receiver specs such as Sensitivity, Noise Floor, Dynamic Range, Intermod Dynamic Range, Phase Noise, and RMDR. He tells us what they mean in real-world receiver performance terms, how they are tested, and whether it’s better to have a higher or lower number in each one. It’s also the final episode before the 2018 Hamvention and George and Jeremy share our plans for our Booth, special offers, and exclusives for show listeners who stop by booth 3104 in Xenia.

David M0DHO

RSGB Commonwealth Contest (BERU)

The RSGB Commonweatlh Contest is on next weekend, starting at 10:00 Saturday 10th and ending at 10:00 Sunday 11th. This is a CW contest.

Details of the contest including rules can be found at http://www.rsgbcc.org/hf/rules/2018/rberu.shtml.

If you are not normally active in this historic contest then please consider taking part. The Commonwealth Contest (where CQ Beru is used to solicit QSOs) is a great opportunity to work Commonwealth DX stations without the normal EU wall of contesters to contend with. Often the DX stations can be worked at marginal signal levels which would be quite impossible in the big international contests. For stacks of background information see Bob G3PJTs excellent website.

UK HQ stations

This year we will have seven RSGB HQ stations using the regional variations of G6XX the RSGB Contest Club callsign. This is following clarification from Ofcom that a Club Callsign can be active from multiple UK regions simultaneously. See QRZ.com for the history of G6XX.

 Chris is also using GB5CC under the new rule that other HQ stations may be active to celebrate or commemorate events of significance to the Commonwealth. This will be to recognise the forthcoming marriage of Prince Harry and Megan Markle in May, unless Chris can suggest an alternative justification.

X8.2 Solar flare

I was on 20m working EU and DX stations using FT8 this afternoon. Just after 16:00, I noticed all signals faded quite markedly and within a minute or two. It was sufficiciently ominous for me to check on http://dx.qsl.net/propagation/index.html and http://www.swpc.noaa.gov/products/goes-x-ray-flux. A couple of minutes later, I saw an X class flare reported. That explained the HF fadeout. About 5 minutes later I saw it was rated as X8.2, only a little weaker than the X9 flare we had on the 6th September. It will be interesting to see if the CME was Earth directed, and if so, how much aurora we’ll get for VHF propogation fun.

David M0DHO

Overdriven audio on FT8, or other data modes

This evening I came across a terrible signal on 15m FT8. The audio from a Cuban station sounded distorted. I won’t name and shame the station. The audio fundamental was about 600Hz, and the waterfall showed strong signals at 1200Hz and 1800Hz matching the 2nd and 3rd harmonic of the audio signal. This caused significant QRM across a significant part of the FT8 segment. Almost certainly the station was overdriving the audio, either causing distortion from their soundcard, and/or distortion in the rig’s modulation chain.

There are several things you can and should do to avoid this, especially when using significant power and/or gain antennas:

  1. Don’t use full output from your soundcard. Set at no more than 50% volume level. Play a pure tone at that level and listen to it on the soundcard output as you adjust the level.
  2. Some rigs, such as the Elecraft K3, have a dedicated data audio input and data mode that removes audio processing like compression or other ALC artifacts. Use this if possible.
  3. For rigs without dedicated data modes, disable all audio compression and set the RF power to a desired amount, say 25W, and start with a low audio signal so the RF power is much lower than that. Then increase the audio level using the soundcard volume and rig audio level  until just below the desired RF output while keeping the sound card level in the range 10% to 50%.
  4. Use an audio fundamental of at least 1400Hz. If necessary, tune the rig to adjust the dial frequency. For example, say you are using FT8 on 20m where the band segment starts at 14.074Mhz. You see a free frequency on the waterfall of 400Hz, and you want to start calling there. If you transmitted a 400Hz tone, then the 2nd, 3rd and 4th harmomics at 800Hz, 1200Hz, and 1600Hz will all be in the audio passband of the rig. Hence the rig will transmit the desired signal at 14.074400, as well as undesired signals at 14.074800, 14.075200, and 14.075600. To avoid this, retune the rig 1kHz lower, to 14.073Mhz and use an audio frequency of 1400Hz. This will generate the same RF for the audio fundamental, but the 2nd and higher audio harmonics will be mostly outside of the rigs audio passband and will be suppressed from the RF output.

Best regards,
David M0DHO

New FT8 data mode versus JT65 and JT9

I’ve been trying JT65 andf JT9 for about 5 weeks now. I thought I’d give FT8 a try. This is a new data mode designed by Joe Taylor, K1JT, and Steven Franke, K9AN. You can find more information at http://www.arrl.org/news/ft8-mode-is-latest-bright-shiny-object-in-amateur-radio-digital-world.

One of the strengths of JT65 and JT9 is that they are very sensitive, JT9 being slightly more so than JT65. However, each transmit or receive period is a minute. So it takes at least 4 minutes to complete a QSO. In many cases, the signal to noise ratio of stations is well above the minimum for a successful decode. So FT8 was designed for that, using 15 second periods rather than 1 minute. So one big advantage is that in many cases you can complete a QSO 4x faster with FT8 than JT65 or JT9.

Unfortunately, there’s a downside. Some JT65 and JT9 decoders, such as those in JTDX, are very good at decoding multiple replies to your CQ, even those on the same frequency (to the nearest Hz). It does this through multiple decoding passes where decoded stong signals are then deducted from the recorded data to expose weaker signals beneath. However, the same does not currently appear to be the case with the FT8 decoder in WSJT-X 1.8.0-rc1. So while you can see a reply on the waterfall, you often get no decode. This is particularly the case if your signal is strong at long distance and attracts multiple replies. Even with single replies, often decode fails despite a visible trace on the waterfall.

For working DX on 80m, I’ve found JT65 and JT9 is much better than FT8, being able to decode signals only just faintly visible on the waterfall. So which data mode you should use depends on what you are trying to achieve. For faster QSOs with decent strength signals where multiple repies are not the norm, then FT8 is a better choice. But for working DX with marginal signals, where your patience might be better rewarded, JT65 and JT9 might be better.

David M0DHO