Hi all
I’ve recently been experimenting with Yagis for 2M and “discovered” something that has probably been discovered before (!), but I can’t find anything in the literature.
One of the issues with a Yagi is matching to 50 ohms, as the driven element impedance is too low if you just have a dipole. There are various solutions ranging from a folded driven element, to coax baluns, delta or gamma match. These all tend to be a bit unwieldy for a portable setup, and may be difficult to tune especially with a home built one.
In MMANA, I was playing around with a design based on a DL6WU short boom antenna, with either 4 or 6 elements. As expected, the feed point impedance came in around 20 ohms.
I then experimented with MMANA’s optimization tool for a while and got a completely unexpected result. By optimizing with a target of 50 ohm feedpoint impedance, but allowing free positioning of the elements along the boom, the result was like this (4 el version vertically polarised at 4m agl) :
The first “director” is very close to the driven element but is not electrically connected
The results for this simulation are
SWR - 1.32:1 @ 50 ohm (actually 66R + 2jX)
F/B ratio - 14dB
Forward gain - 11.3 dBi
Which is about where I’d expect to be with a 3-element conventional Yagi with z-match
The az and el plots look nice too
I have built a 6 element version and then reduced it to 4 element to make it easier to handle. It works !
So, after this marathon post, the question is : is this (a) new (b) crazy or (c) something everyone but me already knows about ?
Cheers
Rick
