Working Bee, 2021-03-06

Saturday was a beautiful day for getting things done, and saw a “mini” working bee at the club, with a few of us gathering to complete a number of tasks. There on the day were:

• VK2AUJ Yves
• VK2FWOO Glen
• VK2JNA Dom
• VK2SKY Richard
• VK2THT Sean

And then there were TwoThree…

(Correction: we have not two, but three APRS IGates now; I forgot about VK2MB-1, which is in its eighth year of operation. See Repeaters, Gateways, etc.)

In the previous post here, we told of the club’s new Receive-only IGate, which relays APRS traffic heard on the thirty metre band to the APRS Internet servers. In turn, APRS-IS feeds Google Maps APRS and similar sites.

Processing audio signals from HF USB transmissions must take account of stations that are slightly off frequency. After a month on the job, it became clear that the Raspberry Pi Zero, the heart of the IGate, was struggling to keep up with the workload of running multiple signal decoders to pick up off-frequency stations.

Thanks to a Raspberry Pi 3B+ donated by John VK2ZWD, the HF IGate is now performing much better, running nine signal decoders (while the Pi Zero struggled to run just three). Thanks John!

The Pi Zero has been given the easier job of two metre IGate: VHF FM signals are not as fussy about the frequency as sideband. This new IGate will eventually listen for APRS traffic in parts of the Northern Beaches not covered by existing digipeaters and IGates; for now it’s sharing the receive antenna used by the repeater audio streamer.

New IGate antenna

The 30m IGate has been using a 40m antenna as a temporary measure, so part of the working bee was to give it a proper 30m band wire dipole. Yves and Dom took on this job. Dipoles are normally easy to build and tune, but this one proved to be the exception to the rule, with the SWR unwilling to come down below about 3.5:1, and with an unexplained “dip” down around 5MHz. Trimming the antenna didn’t seem to help much. Investigations are continuing, but it looks like the problem is a combination of trees and getting the antenna high enough. (Update: on the following Saturday, Yves identified and repaired a dodgy connection, so now the SWR is a much healthier 1.2:1. He also reconfigure the dipole as an inverted L; a few days of data from Google Maps APRS shows the IGate is hearing local stations up to 40km away, plus distant stations 500-700km away, in VK3 and VK4. Further data may prompt other changes.)

We live to serve…

On the IT side of things, Glenn and Yves reorganised some of the cabling in the server room, and Glen performed some much-needed configuration updates to handle the new IGate and other services. He also investigated the power usage of various instruments and appliances in the clubhouse. Keeping the club’s power bill from upsetting our Treasurer too much!

No grounds for complaint!

Finally, Sean performed maintenance on the club grounds, mowing the lawn, and generally keeping the place looking respectable. Thanks Sean, your diligent work keeps the landlord happy, which keeps the club happy!

Animation of coverage (1-6 March 2021) of VK2MB-1 & VK2MB-2, last slide shows coverage of VK2AUJ-1 in Rhodes. Note that VK2MB-2 was only commissioned on the 6-Mar-2021.

Next video shows coverage of VK2RAG-1, VK2MB-2 (commissioned 6-Mar-2021), VK2MB-1, VK2AUJ-1, VK2RHR-1, VK2AMW-1 1-Mar to 15-Mar-2021. Note that RAG, RHR and AMW are digipeaters, so VK2MB-1 and -2 may appear on the maps as iGates. The http link at the top of the screen shows which station is currently displayed on the video.

Next video shows coverage of VK2MB-1 for 1-21 Mar-2021 and VK2MB-2 for 6-21 Mar-2021. Note: VK2MB-1 aerial is single folded dipole at the top of the 100′ tower at the VCS, same height as the VK2RMB repeater folded dipole array. The VK2MB-2 aerial is a commercial quarter wave on 4 radial on a 10 m pole at top of the hill behind the Girls Guide Hall.

Dipole Update

Richard SKY and Yves returned to the Club House on Saturday 13-Mar-2021 to investigate the 30m dipole issues. Went back to basics and plugged the antenna analyser directly to the aerial using a short patch lead – bypassing the feedline to the shack. As we had to bring down the centre plate, we decided to make the aerial “vertical”, basically an elevated L with an inverted V top (see below) … because that’s where the support points were!

All the sudden the aerial came good… very good – see below.

However, another reading before leaving the site gave us a return to the previous 3.5-5:1 SWR. It turned out the dipole wire connecting to the SO239 casing had an intermittent “air gap”. This was resolved by tightening the nut. Adding washers and waterproofing the termination will be done at our next visit, and the aerial may return it’s original “horizontal” setup … unless this elevated L with inverted V cap is proven to be great.