LINEAR-LOADED VERTICAL

While my first 160M antenna, an inverted "L", worked satisfactorily, it was not as effective in working DX as I had hoped.  While I do have the room here for a full-sized 1/4 wave antenna, I was not inclined to go that route for "aesthetic" reasons.  So, in 1993 I modified my 80/40 vertical for 160M, based on an article in ON4UN's book LOW BAND DXING (II-p.36-37).  Of course, with a much shorter antenna, the efficiency would not be as high, but it seemed that the antenna should perform somewhat better than the inverted "L".

Adding several 135 foot radials, bonding the ground system to the pasture fence and underground water piping, and building a new matching network produced an antenna which was considerably better than the inverted "L".  Immediately my country total began to climb on Topband...the effort had certainly been worthwhile! My most memorable moment using this antenna was working VK0IR, the Heard Is. expedition while running just 500 watts!

ANTENNA
                                                                       Fig.1-basic layout of the tower with dimensions

Note that the top set of guys is bonded to the tower.  When I first began to run the linear, I experienced severe arcing from the tower to this insulated set of guys.  Bonding eliminated the arcing and provided an added benefit of  wider bandwidth between the 2:1 SWR points.  The slider across the linear loading "trombone" was adjusted for lowest SWR.  With the dimensions shown, the 2:1 SWR bandwidth is about 80 khz.  I found that spacing of the linear loader was not critical, and I ultimately set it at 1 foot between the elements and 1 foot from the tower.

The size of the top hat was dictated by two things: the ability of the top mast to support weight without a third set of guys and the material I had on hand.  As a rough approximation, for each foot of top hat diameter you add two feet to the effective electrical height. Thus, had I been able to construct an 8 foot hat, I would have been able to add 16 feet to the effective height instead of 8 and so increase the efficiency of the antenna somewhat.

The linear loader is not a lossless device; nothing can take the place of height in making an antenna efficient.  Employing this method was preferable to using a loading coil at the top or using a larger loading/matching coil at the bottom.

Analysing this antenna through ELNEC has shown it to be about 58% efficient (compared to a full-size 1/4 wave vertical).  This seems fine for an antenna only 71 feet high which is not objectionable to my XYL.

TUNER SCHEMATIC
                                                           Fig. 2-linear-loaded vertical 3 band tuning system

I built this tuning unit to take high power. The relays are large open-frame types used, in an earlier military life, with open wire feeders. The contacts are 25 amp!  The variables are  rated for 3500 VDC and the complete unit is built on plexiglass and enclosed in a weatherproof stainless steel box. Relay coils are decoupled with RF chokes and bypassed.  Even running one KW output there is no RF leakage to the relay control circuitry.

On both 80 and 40 meters as well as 160, this antenna worked extremely well;  I was able to work nearly 200 countries on both 80 and 40 with casual operation, many at 100 watts output.


Last modified on August 30, 2008 by VE3NH