The physical length of any HF antenna is critical to achieving resonance and good performance on a particular band. Dipoles need to be half a wavelength long, locating these long antennas can be an issue, especially when we are aiming for the 80 metre band or lower.
However, we can reduce the space needed to a quarter wavelength if we decide to use an end fed antenna. The end fed antenna is essentially half a dipole, it uses a good earth connection to complete the arangment.
Below is one example of how to build an 80 metre band long wire antenna for a house with a smaller garden.
To identify the correct length I used the fantastic Antenna design page created by Martin. E. Meserve: http://www.k7mem.com/Ant_End_Fed.html. For the 80 meter band, the ideal length is 21.9 metres.
Feeder and UnUn
The antenna is fed by low loss coax into a 9:1 UnUn. One terminal of the UnUn is connected to a good ground and the other to the antenna wire. The UnUn is located away from the house. Low loss coax is used due to the distance between the radio and the UnUn.
I used a length of copper water pipe as the earth spike. A standard earth connector tab was used to make the connection. Counterpoise radials can also be added to improve antenna performance.
I later added Coax ground points along the feeder line. I used Coaxial Lightning Arrestors to achieve this.
For this type of antenna, an ATU will be required. I used an Auto-ATU, however, because this is a mono band design a simple manual tuner may be better. I found that my ATU was able to find a 1:1 match easily.
To fit the antenna in to the room available the antenna wire (or at least the para cord supporting it) is strung over the roof of the house. This was achieved by attaching fishing line to a tennis ball and throwing this over the roof. Paracord was then attached, then the insulator and the antenna pulled over. This paracord was pulled taught and tied off on a cleat.
The UnUn is fixed to a pole at the furthest point from the house. The antenna wire is connected to the UnUn along with the earth connection and feeder coax.
With the use of an ATU (antenna matching unit) a 1:1 match was easily achieved. The 80 metre performance was great, far better than trying to tune up a 40 dipole to work on 80. Also, this antenna is quite, very quiet. This is probably due to the antenna being fed away from the house. Good news!
If the antenna is close to the house, it's important to keep the power down. You'll need to think about this if your going beyond QRP levels.
This antenna is currently only useful on 80 metres, but if traps were used then the antenna could become multi-band...Sounds like a fun project for the winter. Also, this would make the antenna shorter which could be useful to help keep the antenna away from the house.
Hight is also important. Using a longer pole for more height at the fed end of the antenna improves performance and helps move the antenna away from the house.
Overall I was very happy with this antenna. The antenna worked well on 80 metres and just fitted into my back garden, this would not have been possible with a dipole design.
However, the proximity to the house was an issue, power had to be kept down to keep things within limits. Being mono-band is restrictive, this type of antenna may not be practical for any length of time.
I would recommend this antenna to anybody struggling to fit an 80 metre dipole in their yard, especially for temporary low power operations.
However, a better solution may be a multi-band inverted L antenna as these are physically shorter, keeping the antenna away from the house and this my be more useful over time.
A fun project with many more tinkering possibilities for the future.