A few weeks ago, I posted an article on an RF choke I had built. I was very happy with the results when using it in the shack on my Cushcraft trapped vertical antenna. Much reduced noise levels, and also slightly improved reception reports when using WSPR and FT8. Not sure exactly why that was, but less RF in the shack allowed better manual ATU tuning I suppose, and also less noise being transmitted with the data signals, I'm going to surmise. So all good and I was ready to run some more objective tests. Then came the antenna change to an End-Fed Half-Wave. Apples should be compared only to apples, so I cleared the partial observation fluff from my head and decided to start any comparative tests from scratch using alternate fruit.

The antenna is not properly installed yet - it's awaiting installation of a second supporting mast, but I couldn't resist seeing if it was useable and it absolutely is. Much better results than the vertical it has replaced. But there was a new problem which I didn't really expect. A heap of RF in the shack. Just what my RF choke was built to avoid. Disappointing. All my USB peripherals began playing up - a sure sign of unwanted RF. I'm not too concerned at this point, as the antenna is going to be raised much higher, and I'm only getting problems when using more than around 60W on bands where the SWR is less than ideal. Still, the plan was always to mount this choke outdoors, and then to potentially install a second one at the transceiver end of the coax as well. This is what my guide and mentor Ian GM3SEK recommends anyway, so I'm just going to push on with my plans at this point.

So step one is to mount this enclosure outside. I had a Male-Female-Female-Male PL259/SO239 join in my feedline anyway, where some idiot cut through the coax with a pair of edging shears earlier this summer, and I didn't want the choke too close to the antenna so this seemed the perfect place to insert it.

Off to the workshop to find, cut and fold a suitable 2mm sheet of aluminium and form it into the above bracket. M5 x 16mm stainless button-head Allen bolts screw straight into to mounting holes on the rear of these excellent Fibox enclosures. The holes are untapped, but M5 bolts self-tap a perfectly adequate thread in the housing and would tighten up way more than the (approx. 3Nm) torque that I used to solidly secure the mounting plate to the back.

One other modification I made to the enclosure before relocating it outdoors was to add a really neat little pressure compensation valve to the underside of the unit. It only requires an 8mm hole.

I tried hard to find these two-way pressure valves in the UK, but to no avail. I had to source them from HF Kits in the Netherlands which is fair enough, but the £13 UPS shipping cost was tough to swallow. They did arrive within a couple of days however, and as I felt bad about the postage being more than the parts, I ordered loads of other HF goodies to make myself feel better...

The valves (part number RND 455-01100 if you want to try and source them elsewhere yourself) are there to solve the potential problem of the pressure differential between the inside and outside of the IP66/67 enclosure as the internal coax and toroids heat up and cool down with normal use. If you just punched a hole in the enclosure, it'll just fill up with spiders and moisture over time, so I've been told.

Another little job completed, and on to the next one.

Berni M0XYF


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