50cm Antenna for 28MHz.

My blogging colleague, PE4BAS, recently posted a very interesting piece about his early ideas for a QRP contest.

The sting in the tail is the maximum permitted radiator length: 50cm!  The band can be any (not just 28MHz, as I wrongly indicated earlier).

I sent a comment that this was something I would definitely take part in.  As Bas says, the ideas are at an early stage, and there is plenty about the rules to iron-out first.  I completely get that.

I wondered about a magnetic loop, first of all.  These are very efficient, typically in excess of 95% at higher frequencies with 'microbore' copper tubing. 50cm is probably possible as a radiator length, but it's so small that it won't be efficient at all, in all likelihood.

My small 4-loop array which I tested against (and beat) a large delta loop at 28MHz earlier this summer.  It's 45cm across, but each radiator is (pi x d) longer than that.

 

I then wondered if Bas would allow a 50cm diameter loop?  If not, and if I then limited the radiator length to 50cm, how about when I make a parallel array of them?  Is it then a size of no more than 50cm in any dimension?  

I completley accept this goes a little against the spirit of what Bas is trying to do; it's not in any way a criticism, just an interesting reflection of what might be possible - and what might not.  But if the idea gets a lot of support - and I'm sure it will - then people will look for ways to get a good signal whilst exploiting any weaknesses or oversights in the rules.

But then, if we are going to limit ourselves to what Bas originally seems to have had in mind - a 50cm vertical antenna - then we are going to need to use loading. 

The MRW-27L (27-28MHz) 'Super Gainer' (probably not) vertical.  But it seems to include a lot more wire than that, in the form of closely-wound, very fine wire coils, making it longer than 50cm in reality.

 

Adding such a loading coil inevitably means adding quite a decent length of wire, but compressed into a small length.  Do we add the total length of the wire to the calculation, or just the physical space it occupies?  With such a small antenna limit, I think we have to accept that the overall physical size of the radiator has to be kept to within 50cm, where the coil's physical length, rather than the coil wire's length, is the measured value.

As always, coming up with new rules means there are many ways, certainly when it comes to antenna designs, in which things can be overlooked!  I've got a working and fairly efficient model made up at 28MHz, but the lower bands need some thought, too.