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Windom Over a House Roof ?

  

ALTHOUGH I HAVE NO EXPERIENCE WITH

A WINDOM OVER A HOUSE ROOF,

I DO HAVE EXPERIENCE WITH A DIPOLE OVER A HOUSE ROOF.

 

THIS PAGE IS A LOGICAL PROJECTION OF THAT EXPERIENCE

TO THE WINDOM (OCF) ANTENNA

 

(This entire page is an ASSUMPTION)

First: A Dipole Over a House Roof

 

In 1980 while working in Bremen Germany on a temporary assignment, I rented a room in a private house, WITH permission to put up an antenna.

 

I had a double-dipole for 80m and 40m that I had used at several QTH’s and it had always worked just fine;  good SWR, good performance, etc.

 

Basic antenna details:  No balun.  40m legs suspended about 40cm (16 in.) below the 80m legs, separated by short pieces of wooden dowels.

 

When I first put the antenna up at this temporary location, one leg of the antenna ran over the house.  Part of the 40m and quite a bit of the 80m dipole were only about 1.5m (5 ft.) above the roof.

 

The SWR was crap on both bands.

 

Shortening and lengthening the antenna did not help.  It moved the point of resonance, but did not improve the SWR.

 

I concluded it must be the influence of the roof.

 

So I did the following:

  •  Returned both legs of both antennas to their normal calculated length.
  •  Made sure the leg connected to the shield was the side over the house.
  •  Measured the resonant frequency and minimum SWR for each band and recorded it.
  •  Beginning with 80m I adjusted just the length of the leg over the roof
    • (always start with the lowest band),
    •  As I shortened the antenna length in small increments, I watched the SWR come down.
    • I continued shotening until the SWR reached its typical value of about 1.3:1.
  • Then I repeated this procedure for 40m.
  • When I was finished, the SWR was fine and the antenna worked like it used to.
  • Voila!

 

Lesson learned.  Houses do influence antennas.


The reason I placed the shield side over the house is, I figured without a balun, more of the power would radiate from the leg that was in the clear and less power would radiate from the leg over the house.

 

In the case of a symmetrical dipole, the  house was capacitive-loading one side of the antenna and I compensated each band by shortening the dipole on just that one side.

 

Next: A Windom Over a House Roof

 

It is assumed that a Windom will be affected by the house roof, just as the dipole was affected.

 

It is also assumed that the capacitive loading from the roof will affect each band differently. Therefore each band will need a different amount of adjustment.

 

Although I was able to adjust each band separately on my Double-Dipole, there is no separate band-by-band adjustement on a Windom.

 

CONCLUSION:  It is fair to conclude that since we cannot compensate individual bands on the Windom, its SWR may be worse than normal on some bands.

 

DANGER:  Any time you have high SWR on a balun wound on a toroid, it heats up much faster than it does when working in a well matched load.  Manufacturers rate their toroids for use in matched conditions, NOT in conditions with bad SWR!

High SWR can cause your balun to burn up!

 

 

AGAIN, THIS IS JUST MY THEORY ON WHAT YOU WILL EXPERIENCE.

I HAVE NOT TRIED THIS MYSELF YET!

BUT IF YOU ARE IN THIS SITUATION AND CAN'T GET THE WINDOM TO WORK RIGHT,

MAYBE THIS IS THE REASON WHY.

 

FEEDBACK WELCOMED !