MA5B work carried out, now resonant on 20m and 10m

Jim 2I0SBI and Pat EI2HX cleaning the trap assemblies 

With a good weather forecast for the first half of Saturday I put a small team together to take down my MA5B minibeam and try to ascertain why it does not seem to want to work on 20m and 15m. Pat EI2HX and Jim 2I0SBI arrived between 10 and 11 am and we got to work.

The first task was to take the mast off the wall brackets and lean it over enough for me to be able to remove the minibeam from atop the extension roof. This only took a few minutes. The MA5B is light enough that I could hand it down to the two lads.

The matching network was in good condition

We decided to concentrate our efforts on looking at the matching network and disassembling the driven element. It turned out that the matching network seemed fine. There were no obvious signs of any water ingress although there was a small amount of surface rusting on the core of the coax in the box itself. This was easily scraped away with a Stanley knife. I resealed the lid on the matching network box and put amalgamating tape around it.

The driven element provided a few surprises. We found that the trap in the "hot" leg of the driven element (labelled MT1) had suffered a drastic melting of the insulator. We are not entirely sure how this might have affected the performance of the antenna because the 2mm-thick aluminium wire wound around the insulator is actually connected to the casing of the trap also.

The burnt trap insulator before cleaning

Nonetheless Pat EI2HX cleaned up the burned and melted section of plastic and then put some tape around a section of wire leading to the trap connection to ensure that it was insulated as it would have been when the trap was brand new. The trap on the cold leg of the driven element was in pristine condition. A decision was quickly made to swap them around, so that the trap in good condition would now become the "hot" trap and the damaged one was transferred to the cold leg.

In addition to the above we found that the connections on the driven element had all suffered from corrosion, which was evident from the amount of white powder we found when disassembling the trap joinings and the element connections. All of these were cleaned up with emery paper and wire brushing. This took quite a bit of time because all the joints had been well sealed with tape and were also clamped with jubilee clips. But having three pairs of hands made light work and in no time at all the driven element had been given a thorough overhaul.

This work even included cleaning the nuts and bolts and washers where the coax joins the two driven element sections. We decided it was best to make a good effort so that the antenna might give a good few years of use if needed. After all it has been at this QTH for a couple of months but only getting very limited use.

Yours truly reattaching the minibeam to its mast

With everything reassembled and all the joins tightened up with clips and sealed over with tape we finally hoisted the antenna back up onto the roof and reattached it to the mast. However, a quick test revealed very high VSWR on 15m and 20m. So I took a decision to measure the distance between the elements just in case this had been interfered with during correction the work. Indeed it was found that the director was a quarter of an inch too close to the centre element. So this was adjusted and another test revealed a flat SWR on 14.040 and a flat SWR on 28.010. Indeed the SWR is only 1.5 on 28.500 so the antenna is resonant throughout a good portion of the 10m band. The bandwith on 20m, however, is very narrow, with the SWR going up to 2:1 on 14.005. Nothing that the tuner in the FT-1000MP won't handle. The VSWR is still very high throughout 15m though, so some further examination of the measurements on the antenna will need to be undertaken.

However, for now some of the problems with the minibeam have been resolved and it is now resonant on 20 metres CW where it had a high SWR before. I will put the beam through its paces in the coming days and will report on progress here.

In the meantime, a big thanks to Pat and Jim for the help.