How can I use multiple HF bands with one antenna?
Question:
As the aerial (a long wire) is not adjustable don’t you lose a lot of power moving off its resonant frequency? I ask as I was also wondering how one can use a transceiver on different bands with an adjustable aerial.
Answer:
It’s common for those who use the HF bands to want to be able to switch between the bands, especially as bands “open” and “close” at different times of the day – but as you ideally need to have an antenna of the correct length for each band, what are the realistic options? Well, there are 3 options – some are more practical than others:
1. Have multiple antennas, and switch between them
If your garden permits, you could have multiple antennas set up, potentially one for each band that you want to work, and in the shack, a manual switch that connects your transceiver to the appropriate antenna.
This is obviously the ideal technical solution, but for most of us, it’s just not possible to have multiple antennas in the garden
2. Use antennas that are designed to be used on multiple bands
There are some antenna designs that allow operation on more than one HF band:
- Some do this automatically – an example of this would be a “trap dipole” which uses RF “traps” to stop certain frequencies, limiting the physical length they can travel (for more, see trap dipole). Another example is the CobWebb design, which looks like a rotary clothesline, but through clever design, supports typically 5 bands
- Some feature a motorised component to physically alter the antenna length on command (for example, the Yaesu ATAS 120-A, or antenna systems from SteppIR systems)
- Some require physical adjustment to the antenna – moving a connector, adding another section or through use of a coil.
The above options are all pretty efficient, but can be a) expensive, or b) require a suitably-sized location for the antenna
3. Use a single antenna and an ATU
For many, the compromise is to get an antenna that works well for your normal band of choice, and then use an ATU (Antenna Tuning Unit) to “match” the antenna to get the lowest possible SWR
The down-side is that using an ATU to match whatever antenna you have to the band you want to use will reduce the performance of the antenna, but the upside is that you can use multiple bands with a single antenna cheaply and easily
