I was very aware that the presence of nulls in my 136kHz loop was limiting my ability to have QSOs with some of the operators, particular towards the continent. It was not readily feasible to rotate the loop to change the situation, and it was hoped that the vertical, if it did not increase performance in the directions already covered, might fill in these null gaps. I also had a lot of encouragement from other operators to at least give a vertical a try. Some were rather more blunt than others, and there still seems to be a sense of disbelief that I am able to get out at all with a loop antenna, such is the scorn these antennas are held in some quarters!
Thus it was that in summer 2001 I worked out a suitable vertical design and set to work
to construct it. Progress was rather slow because of other commitments, but by the
autumn it was complete and I was able to have some QSOs.
|The vertical at the G3YMC QTH is based on my old Butternut HF2V 40/80m vertical. This forms the vertical section of the antenna, and is free standing at 10m high. Additional top loading increases the capacitive impedance. It is resonated at the base with a 6.5mH loading coil, and impedance matched to the 50 ohm coaxial input using a matching transformer wound on a 58 mm OD 3C85 core ferrite ring.|
The Butternut as used on 80/40 is of tapering aluminium tube construction, and quite flexible in its upper sections. It is mounted on a ground spike and insulated from this with a composite plastic base insulator. It amazes me that this insulator supports the whole antenna, but it has survived many a storm in the 15 years it has been up and apart from a bend in the direction of the prevailing wind, is in remarkable condition. Existing loading coils for 80/40 are mounted at the 1m level, with a further insulator at that point. These coils are retained, in series with the main loading coil, but are relatively small (c 100uH). The 80m matching capacitor and and its coupling strap have been removed.
Calculations showed that I would need some form of top loading in order to resonate the
antenna with a reasonable size loading coil. The bare Butternut has a self capacitance of
around 90pF and would require 15mH for resonance, an unrealistic value. However it was
feasible to attach a 12m horizontal wire at the top and this would increase the self
capacitance. Theory indicates around 5pF per metre for such a top loading wire, however
the exact figure depends on many conditions. It was thought that the Butternut was too
flexible for just this single wire, so a couple of short back guy wires were also used.
Sloping loading wires are not normally recommended however as they can reduce the
effective height, so these were deliberately kept short. Initial measurements found that
6.5mH were required for resonance, giving an effective capacitance of 210pF. It seems in
my case I am getting rather more than 5pF per metre!
It was decided to construct the loading coil on plastic domestic 2 gallon buckets - these have tapered sides with a mean diameter of 230mm and a usable winding length of 220mm. Using solenoid2.exe it was readily established that 135 turns of 16/.2mm pvc insulated wire would fit on a bucket and have an inductance of 3mH and a series resistance of 3.8 ohms - conveniently the amount of wire required is just under one 100m reel. So just over 2 'buckets' of inductance would be required (Like one used to measure capacitance in Jars, you can now measure inductance in Buckets!). There seemed little point in using thicker wire to reduce the series resistance, as this was expected to be swamped by the ground losses.
Two 'buckets' were constructed. One of these was tapped every ten turns to provide a
means of coarse adjustment, the other was untapped. In order to increase the inductance
of the series combination the two buckets were loosely coupled by placing the upper
bucket slightly inside the lower one - by moving the bucket in and out of the lower one
the combined inductance could be varied. In conjunction with the taps on the bucket, the
correct inductance could be easily obtained. The buckets were held in position with a
compressed wad of paper - Heath Robinson approach.
The output stages were redesigned to include a two stage low pass filter. At the same
time the opportunity was taken to include a modified driver stage. The circuit of the
modified driver and output stages are shown here. After these
modifications had been done a reasonable (though far from perfect) match was obtained,
and the output waveform was a far better sinewave. As a bonus, it was found that the
output power was increased to a remarkable 80W.
I have had a few QSOs with the new antenna, and have been able to obtain comparative reports. In directions where the loop performs well, ie to the north and south, signals are either very similar on the vertical or noticeably down. The relative signal reports seem to correlate very well with the difference in the receive signal from that station. The only cases where the vertical performs better than the loop are those in the null of the loop. G3OLB in Devon is quite weak on the loop, and no QSO has been possible with Tom on that antenna. However on the vertical he is several s-points stronger and a satisfactory 449 QSO was achieved with no difficultly.
The tuning of the vertical is very broad, which is to be expected in light of the rather high earth losses. No retuning is necessary to operate over most of the cw section of the band. However the matching varies considerably from day to day. Droop in the top loading wire is one factor (this needs to be regularly retensioned), but there also seems to be leakage across the base insulator - matching is worse in the early morning when dew is present, and improves as it dries out. Until I have a satisfactory method of adjusting the tapping of the matching transformer it is not easy to get round this, and I am only able to operate the vertical at certain times.
The performance of my vertical is clearly being hampered by a high earth resistance. It
would seem difficult to overcome this, but I shall be trying various things over the
coming months. In the meantime I shall continue to have QSOs and continue to do
comparisons. However initial results indicate that the loop remains my best antenna.
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