The scrounged 27MHz antenna, dismantled
I am a keen short wave listener, and enjoy listening to QSOs between other radio communication enthusiasts particularly on the 40 metre band (7MHz) as this band usually shows some activity right around the clock. I use a Yaesu FRG-7 as a receiver - not exactly state of the art nowadays, but still a good little radio nonetheless, despite the drawback of having a wide IF filter which increases noise.
The antenna system is a very important component in any radio station, whether it be a receive only setup or one equipped and licensed to transmit, therefore I decided to turn my attention to the kind of antenna I was using and how it could be improved.
The old antenna was very crude indeed, being nothing more than a wire let out of a downstairs window, up the outside wall and anchored into an upstairs window. It was brought to some kind of resonance by a small coil of wire into which a short piece of ferrite could be inserted. By carefully adjusting the position of the ferrite core, the signal meter could be peaked.
This arrangement, whilst enabling me to receive radio signals had many drawbacks. For example, although I was able to obtain a peak on the signal meter, this doesn't necessarily mean the antenna is truly resonant or that it is providing the correct output impedance to maximise signal strength. The proximity of the building must also have had a severe de-tuning effect on the antenna and would have caused a great deal of signal loss.
Finally, and certainly the most important factor in pushing me to rethink my antenna was the high level of interference induced into it from the mains wiring - it seemed as if just about every electrical appliance had some effect, the fluorescent strip light in the kitchen being by far the worst offender. There was also an unknown source of what can best be described as computer data interference. In trying to track down the source of this I was surprised to discover other significant sources, such as some of the kids' games, televisions, the broadband ADSL router and phone chargers. However, the source under investigation has still not been located - it may be coming from an adjoining neighbour's house.
The obvious and only solution to all this was to reposition the antenna at the end of the garden, as far away from domestic interference sources as possible. The new antenna was to be a quarter wave vertical, this time with a proper earth system as it was going to be constructed not only for efficient receive performance, but also with transmitting in mind as I am contemplating taking the Radio Amateur Examination and don't really want to redesign the antenna yet again. I wanted to do the job just once, and try to do it reasonably well.
As the idea was coming together in my mind, I started to think of the various components I would need in order to build this new antenna. The neighbours on one side had an old broken CB antenna in their garden - an end-fed dipole mounted on an aluminium mast so I approached them and managed to scrounge it. The antenna was made of several interlocking aluminium tubes totaling about 18 feet in length, and the aluminium mounting mast was about 3 metres in length. Everything was dismantled, and the main components seemed to be in a useable condition after a bit of a clean-up.
The entire assembly was to function as a single antenna, from the base of the support mast on up the CB antenna bolted to it. To achieve this, I needed to electrically connect the CB antenna's bracket to the CB antenna itself as the two were insulated from each other. To do this I simply drilled a hole in the bracket and bolted on a wire which was to be attached to the main element using a jubilee clip, thus very simply bypassing the old balun mounted in the base of the bracket.
Now, a full size quarter wave antenna would need to be about 10 metres long, and the components I had totaled only 8 metres - somewhat too short to resonate on the required frequency and therefore I would have been unable to get a useable SWR should I ever wish to use it to transmit.
There were two options open to me: Either add some more length to the antenna or incorporate a loading coil to bring the antenna to resonance. Adding extra sections would have been the ideal solution, but that would mean an even longer antenna, with a greater possibility of complaints from locals or wind damage, plus I didn't have the components in my possession. However, I did have the necessary components to construct a loading coil, so this is what I opted for.
Next I had to determine a way of fitting this loading coil somewhere along the antenna's length. After much searching around I found an old wooden broom handle which happened to be a good fit into the first and largest aluminium section of the CB antenna. Wood would not normally be a good choice for a coil former as it would absorb moisture, changing the loading coil's characteristics, but I decided the best way to deal with this would be to impregnate it with melted wax to seal it. I used an old baked bean tin in which to melt some candles and when the wax was good and hot I immersed the ends of the wood, allowing the wax to soak up into the end grain. I also liberally painted molten wax all over the remaining length and used a paint stripper to heat up the wood so that the molten wax soaked well into the surface.
The next job was to wind on the loading coil. I had a fair quantity of 2mm diameter enamelled copper wire kicking about so decided to use this to create the coil. The number of turns was initially going to be guesswork with a view to correcting it if and when the opportunity arose to transmit, but fortunately I discovered a little DOS program (vertload.zip) designed to calculate loading coil characteristics for vertical antennas, and after carefully inputting all the required parameters it informed me that the loading coil, using 2mm diameter wire and wound on a 25mm diameter former would require 30 turns.
I wound a total of 32 turns onto the wax-impregnated broom handle section. There is a fair possibility that the loading coil characteristics are not exactly spot-on, but there is room for adjustment to the antenna by altering the overall length by sliding aluminium sections in or out as necessary to achieve the lowest SWR, or possibly removing some turns from the coil if the antenna resonated at too low a frequency.
The appropriate section of aluminium tube was cut in half to enable to wooden handle to be inserted between the halves, and slots cut in the ends of the tubing to allow everything to be securely clamped up using jubilee clips.
All the components of the old antenna with new loading coil were assembled and these were then secured to the mounting pole using the original bracket, creating the new antenna.
The completed vertical element now needed to be mounted securely to a suitable support, and in this case I was able to use an old concrete fence post which was very solidly embedded in the ground. One problem which needed to be overcome was that the antenna needed to be insulated from the mounting point and the ground, and the picture below illustrates how I managed to do this.
Stainless steel jubilee clips were used to fasten the antenna to the post, and thick sections of plastic were used to insulate the antenna from the concrete, and the clips from the concrete and the antenna. One half of a plastic wire reel was used at the base, which provided a good insulation from the ground below it. As the post was set in the ground at a bit of an angle, I had to space the base of the antenna out somewhat from the concrete post to make it upright, and for this I used a block of wood which had been boiled in wax to make it impervious to moisture and rot, as with the loading coil former.
No vertical is complete without a decent ground plane, and unless care is taken in this area the results can be disappointing as the earth resistance represents signal loss.
The first step was to create an earth bonding point, and for this I used a 60mm length of copper pipe which I was fortunately able to drive into the soft ground to its full length. The top had been cut into 4 sections, each having a hole drilled to facilitate the connection of radials and feeder, as shown in the above photo.
Multi-strand wire was obtained and the insulation removed, ready for being inserted into the ground. This was an easy enough operation, simply using a spade to cut a slot into the turf wide enough to allow the bare wire to be pushed into it, the slot being pushed closed again by foot pressure. RF currents tend to travel in the upper surface of the soil so there is no advantage in burying the radials too deep - 2 inches or so is fine, and the use of multi-strand wire aids flexibility and increases the surface area in contact with the soil. Once buried, the ends of the radials were then attached to the bonding points.
Technically the more radials you can install the better, but all too often the available space forces a compromise and this was the case in my garden as I was only able to lay 3 reasonably long radials, with a few much shorter ones. Despite this, it seems to function well with a low earth resistance.