TECHNICAL

"Hen-Tenna" for 4 m
Published on 25 May 2011 by David G4UNB

Having read about the "Hen-Tenna", an unusual rectangular antenna design for the high HF and low VHF bands, I wanted to try one of these on four metres.

The Hen-Tenna is a form of resonant skeleton slot antenna

This design produces a bi-directional beam (at right-angles to the plane of the antenna) with about 3 dBd gain, but with horizontal polarisation when mounted with its long axis vertically, or vertical polarisation when mounted "on it's side" (similar to a skeleton slot antenna).
This configuration provides advantages if you're looking for a horizontal beam with a small turning circle, or a vertical antenna to fit inside a low loft-space!

Construction

The Hen-Tenna consists of an oblong shape, being one half-wavelength long and one sixth of a wavelength wide. My prototype is built from 15mm copper pipe, using push-on corner pieces which are very convenient and save time in construction, but if you do use these corner-pieces, you should note that they add about 3/4" each on the length, so bear this in mind when cutting the pipes to length.
The antenna is fed 1/10th of a wavelength from one end, via two arms which match to 50-ohm co-ax. I used hard-drawn copper wire for these, mounted on a non-conductive plastic support. These two feed points need to be moved along the copper pipes to achieve the best matching, so rather than solder or bolt them onto the pipe, I used hose-clips for ease of adjustment.
The co-ax is soldered to the two centre ends of the feed arms (about an inch apart). I made the initial part of the feeder in the form of a short patch-lead, and then attached the main feeder length using co-axial adaptors, so that I could disconnect the feeder to move the antenna without disturbing the soldered joints. I did find however that I needed to keep this joint away from the copper pipe, to stop any interaction and ensure a good VSWR.
I have found it quite easy to achieve an SWR of slightly less than 1:5.1 over the whole band.

One very important point is that the Hen-Tenna must be mounted on a non-conductive mast or pole. I used a 38mm plastic pipe with a piece of wooden dowel pushed up the inside for extra strength. I cut a 15 mm groove in the top of the pipe mount and simply pushed the 15 mm copper antenna pipe into it, and then fixed the bottom half of the antenna pipe to the plastic mast with strong tape (cable ties would be just as good).

G4UNB's plumber's delight hen-tenna prototype mounted for vertical polarisation

Testing on-air

I have mounted my Hen-Tenna to give me vertical polarization, as most of the contacts I speak with are also running verticals, but the Hen-Tenna can be mounted to give either vertical or horizontal polarization...the choice is yours!

Testing from my home QTH in North Manchester with a station about ten miles away, I got significantly better signals when using the 'Hen-Tenna' mounted about twelve feet off the ground than with my quarter-wave vertical in the loft (about 10-12 feet higher).
I have read of other tests which suggest that the Hen-Tenna can be expected to outperform a 5/8 vertical (but it should be remembered that the Hen-Tenna is bi-directional) and has an approximate gain of about 3 dBd...about the same as a two-element yagi.

I've also helped another amateur to build his own Hen-Tenna, and tried mounting this for horizontal polarisation, where we achieved a VSWR of 1:1.

Footnote

Apparently 'hen' is Japanese for 'weird': this antenna was developed by three Japanese radio amateurs in the 1970's, and they couldn't work out why it worked so well...so they called it 'weird' hence the 'Hen-Tenna'!
It may be weird...but it could also be wonderful as well!