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Stationmaster MK1 5/8 conversion on 6m

24 January, 2016 - Reading time: 3 minutes

Having already had limited success with converting the Stationmaster MK1 to 6m, I decided to try something different... turning it into a 5/8 vertical.

I modelled a 5/8 in MMANA. The antenna models with a lower radiation angle then a 1/4 GP or a 1/2 vertical, resulting in slightly more gain. 5/8 of a wave is 0.62WL. Some claim that the optimum length is 0.64WL. MMANA says that over a Real GND the radiation angle is only 0.2 degrees lower - something you won't see in the real world in my opinion. Perhaps I did something wrong?

Radial lengths are a 1/4 at the frequency of interest.

The calculated results are quite good. X is brought close to zero by introducing some L in the form of a coil at the base. I hoped to use the original SM coil... So I plugged some rough figures in. The magic number is apparently close to 0.545uH.

I had an old Stationmaster base, and found some aluminium lengths of tubing. Some radials were made out of 10mm tube and attached to the base of the "C" bracket.

I then tried the original coil, but results were erratic and the antenna was not able to be tuned. Instead I wound a new coil on a 20mm PVC tube using 2mm enamelled wire. 

I tried tapping the coil. I started roughly 5.5 turns down as can be seen by the solder blob. The antenna had a good SWR curve down at 51MHz. This is too low, therefore I went up one turn where results were much better near my desired frequency (53.7MHz).

-14dB return loss bandwidth measured 1.6MHz.

1MHz bandwidth sweep. I then did some comparisons with a 1/2 flowerpot antenna which I currently use at home for 6M. The 1/2 wave is about 5m AGL with about 10m of RG-213 coax. The 5/8 was tested 1.8m AGL with about 15m of RG-58. Received signal levels from VK7RCH 6m were made, and the 5/8 appeared to be 2.2dB better.