Multi-band antennas - convenience over performance?
Is your multi-band antenna compromising performance in favour of operator convenience? Have you chosen to forfeit efficiency in order to provision for operating on one or two additional bands? If you have I’d suggest you reconsider your antenna policy and adopt a ‘less is more’ approach, one that is sadly often overlooked these days.
When it comes to radiating a good signal on the HF bands less can often be more. By working on fewer bands (less) and by having dedicated resonant antennas cut for those bands you can often radiate much more of the transmitter power delivered to the antenna system.
So how is this achieved? Simple - by limiting losses within the antenna system itself. In many ‘magic’ multi-band antennas (the likes of which you often see on eBay and, sadly, sold by reputable amateur radio dealers) the majority of any power loss occurs within the coaxial feedline.
Unfortunately too many operators are sold on the idea of ‘a low SWR is all you need for an efficient antenna’. Sadly this is far from the truth because an apparent low SWR, as measured at the shack end of the coax, only masks what may be a very high SWR along the length of the coax…and coaxial cable incurs huge losses at high SWR.
Remember, the low SWR reported on your antenna tuner in the shack is the SWR between your rig and the tuner. This needs to be low to allow many modern radios to deliver full power into the antenna system (where the antenna system is the tuner, inductors, baluns, coils, traps and other radiating elements plus the feedline). How much of the power delivered to the antenna system that is actually radiated as RF (rather than converted to heat) depends on how much of that power is not lost within an inefficient antenna system.
So, if you’re finding that your wizzy-woo multi-band ‘magic’ antenna isn’t working as well as you might have expected, why not try a simple resonant dipole for 20m and another for 40m (space permitting). This will allow you to work a lot of DX, day and night as band conditions permit, and you can be pretty sure that rather than just warming your coax you’ll actually be radiating most of the power your transmitter is delivering.