Wednesday, 19 June 2019

Fiberglass pole/mast repair

Many hams buy sets of these military surplus 4' fiberglass poles for use as portable radio masts. They are lightweight, easy to deploy and store well in the back of the car. But there's a reason that they are surplussed out from the military; they are mostly split at the bottom.

Typical split in a fiberglass pole
Over time these splits will get larger and larger. Eventually the pole will fail under stress and the mast will collapse. This will most likely happen right when you are passing that vital piece of information to the Net Control station after that tornado tore through your neighborhood and you are the only station left on the air.

My set of poles suffers from this problem. So I have made some minor repairs to mitigate the inevitable loss. I have a bracket that holds the bottom of the mast. It's about 8 inches tall and so can well support the worst damaged pole. I always use the worst damaged one at the bottom.

The worst pole gets put at the bottom where there is most support
Then using a hose clamp I secure the other poles such that they can no longer splay out. Do not tighten the clamp too tight as you'll misshape the pole. You simply want to add to the structural integrity. If the cracks are too bad you can drill a hole right at the top of the crack to prevent it from moving further up the pole. I stole this trick from the Liberty Bell 😁

Add a hose clamp to the damaged pole
Finally, assemble the poles in the usual manner. 

2 "repaired" poles seated together
In addition to the repairs I added an eye bolt to the very top pole so that I can attach an antenna wire to it.

Eye bolt installed for wire antenna. HSMM-Mesh adapter also installed.

Wednesday, 5 June 2019

500kbps packet radio

Packet radio is pretty much all but dead here in the US but it seems not to be that way in France.

I recently saw a message on the AMPR mailing list about a 500kbps modem that someone in France had designed. The design files and software were put up on the Hackaday website. The design uses a handful of development boards cobbled together and has an Ethernet interface for the client computers to connect to. 

So I decided to have a go at this project. I ordered some PCB's from JCPL and searched the usual online suppliers for all the other parts. It took about 6 weeks to get everything together but for about $65 per modem I was able to build 2 and test them across my workbench.

One of the modems under test

I've been playing with them for a few weeks and am very impressed with them. I can sustain my MMDVM hotspot across them as well as attach a WiFi access point and make a WiFi Calling cellphone call through it too.

The downside for us here in the US is the bandwidth and speed. The FCC says that we can only use a symbol rate of 56kbaud and a maximum bandwidth of 100KHz for data modes on the UHF 70 centimeter ham band (where these modems operate). Currently the modem software will only allow links of 200kbps in 270KHz and so do not conform to the rules. 

Interestingly, this is not the only data mode to be found on the 70CMs band. Digital ATV has been around using the DVB-S standard for quite some time here. The ATV crowd erroneously reclassify their data (up to 2mbps!) as pictures/video rather than the data that it actually is. In this way they fool the FCC into allowing the mode without a rule change. So maybe that's how we do it for Packet Radio? Reclassify our data transfers as "image". After all, web pages have pictures in them. :)

Thanks to F4HDK for his design.

The network that supports the modems under test