Going underground with 2m FM!

A meeting of interests over past weeks meant that I had to build a moderately-directional antenna for 2m FM, to be used with a Baofeng UV-5R III handheld.

The interest is in trying to add some evidence to the expected existence of a shaft, roughly 70 feet (21m) deep, that was dug to assist the driving of a drainage adit for the mighty Penrhyn slate quarry, for many decades the largest slate quarry in the world. It was also the site of what remains the longest-ever dispute in British industrial history, a strike lasting between 1900 and 1903.

Penrhyn old quarry, ca. 1970s, before work stopped and water flooded the lower levels.

 

Penrhyn old quarry today. Image: RCAHMW.

Started around 1849, the adit is 1.9km long, with a fall (slope) over that distance of only 3.7 feet (1.1m). It was driven by Mormons who had previously worked in the coal mines of South Wales. They took 2 and a half years to complete their amazing work, and then left Wales for Salt Lake City in Utah, a focus of Mormon culture.

A remarkable (and long!) feat of human effort - and a fascinating journey through time and slate beds!

 

The direction of drill holes, when surveyed fully and plotted out, provides compelling evidence for the existence of shafts - which were expected to be necessary in the contract for the works - in that there are slate arches from which the direction of driving the adit goes off in opposing directions. 

Shoving a mobile phone camera above one of the arches showed some, but not especially clear evidence of a void above one of the arches. So I thought that, in order to conclude several months of work down there, I'd try using VHF handhelds to see if the signal received on the surface might vary when a transmission underground passed under the putative shaft.

Arch number 9, steel-lined and potentially problematic for 2m radio!

 

Unfortunately, the best and most easily surface-located position of a shaft has a steel-lined ('tubbed') roof. This, its concave form, and its radius, means that it is almost perfectly suited to bounce any signal sent from within the arch right back to where it came from, and attenuating anything getting through!

Luckily, another proposed shaft location has a slate arch, so this will likely be much better to try, albeit more difficult to locate on the surface to position a receive antenna. We'll see if it works in a few days' time.

Easton and Amos (1859) water pressure engine, using only a head of water as an energy source, to pump water from levels 137 feet below. This emptied its water into the drainage adit.

 

Due to size, weight and robustness considerations (underground activity is very harsh on any equipment taken in), I opted for a 20mm PVC tube-based Moxon. Moxons are a bit fiddly to make, but not too bad.

There are a lot of plans online for this antenna. Sadly, a number of them haven't paid attention to what they were doing, with all sorts of wrong measurements, some not making mathematical sense, written-up for all to be confused!

Simple, a bit fiddly to make, but cheap and effective!

 

I went back to basics, and used an online calculator. This proved accurate when using enamelled hard-drawn copper wire of about 1.2mm diameter. If you use plastic-coated stranded wire, your measurements may well work out differently due to velocity factor variations.

So, if you want to save some time, and you are looking for FM-range 2m operation in Britain, then here are the measurements that worked:

Long arms for radiator and director, two of each per element = 318mm

Short connectors between element centre junction boxes and handle junction box = 80mm

Radiating element, total single wire length needed = 91.2cm - cut this in half for the dipole elements.

Reflecting element, single wire length needed = 99.6cm.

I made small loops in the ends of each wire so that a thin cable tie could be used to secure and adjust the radiator-director separation. This was 22mm in my case - a bit of trial and error is likely to be needed for each build with this dimension, which is why cable ties are so useful.

 

Very pleasing outcome, plotted by the RigExpert AA230 Zoom analyser.

After a bit of messing-about making sure it was all fairly symmetrical, and winding an air common-mode choke, the analyser saw below 1.2:1 SWR across the FM part of 2m, and below 1.3:1 for the entire 2m band, with a real impedance of ~43 Ohms, and an imaginary component of +2.1 Ohms at 145.500MHz.