11 Jun 2013

Efficient electrically short antennas

Way back in my early professional life I was responsible for the design and implementation of a tiny 70cm antenna that had to fit inside the Pye PF8 UHF handportable transceiver. The mechanical design team had left virtually no space for the plate antenna we had intended, so yours truly had to come up with a solution that both fitted and worked efficiently. I managed it by changing the plate antenna to a skeleton plate antenna. In the end it worked very well but the tuning was VERY sharp. Reference here should be made the Wheeler and the Chu limit that defines the bandwidth that an efficient electrically short antenna can achieve.

Basically the thing to remember is this: the smaller the antenna is the higher the Q, and the narrower the bandwidth is, for efficient operation. This is why magnetic loops can work well if designed with a very very high unloaded Q and why they have a very narrow bandwidth if working efficiently. It also explains why a short loaded antenna will have a narrower bandwidth if working efficiently.  Dr Underhill has argued that a small magnetic loop actually violates the Chu limit.

There is a good analysis of small antennas at http://www.m0rzf.talktalk.net/RobCentral/E.S.Antenna.html which goes into some of the maths and looks at various electrically small antennas. Some are "snake oil" of course, but some well designed ones do work with the caveat of narrow bandwidth e.g. well designed magnetic loops.

What is the interest in all this for me today? Well, I am still thinking about HF operation at the new QTH and how to meet my goals of a small, low visual impact antenna, that works well. One thing in my favour is I tend to operate on quite small parts of the HF bands e.g. around the WSPR/PSK frequencies or around the QRP frequencies. As long as I am  prepared to stay close to certain frequencies like 28.1246MHz I should be able to design a very small efficient antenna. I already know that a magentic loop antenna works very well indeed and would certainly meet my goals.


Anonymous said...

Hello Roger.
I noted your interest in Mag-loops in this and previous Blogs.
I have used such a loop (commercial) for several years and found it to be very good for my CW QRP. I advocate vertical mounting.

Much of the literature tends to suggest that their radiation pattern is essentially omni-directional, except for two small nulls in the axis of the loop. From my experience this description is questionable as I have found that my loop is much more directional than is generally recognised (along the line of the loop). If you are going to undertake serious low power and WSPR experiments I would strongly advise that you consider some means to rotate your loop. Having the loop pointing towards the signal source can make a significant difference (between decent copy and no signal at all).

All the best with you house move Roger.
John - GW3OIN

Roger G3XBM said...

Yes I agree John. Both my wire loop for 136/472kHz and the small copper pipe loop for 20-10m were definitely directional in the line of the loop. I had in mind mounting a copper pipe loop in the loft where I could easily turn it if need be.

Anonymous said...

I have used my mag-loop in the loft of my bungalow without any problem. I have not encountered any EMC issues, even though there are some telephone lines not too far from the loop - that said I have not used in excess of 5 watts to the loop.
I will be interested to hear how you get on.
John - GW3OIN.

Anonymous said...

EFHW, not strictly an ESA but unobtrusive, easy to hang, and works pretty good.