[TowerTalk] 80/40 coax trap dipole design

[Jim Brown]

On 12/15/2010 6:21 AM, DJ7WW wrote:
> It will not work on two bands without a trap

As others have observed, that is simply false.

There are four basic requirements for an antenna to "work."

1) It must accept power from the transmitter, which infers some 
reasonable match to a transmission line, and of the transmission line to 
the transmitter.

2) The power coupled to the antenna must cause current flow in a way 
that generates a relatively strong field in the desired direction(s).

3) The antenna, by virtue of its position and orientation, must interact 
well (or at least not badly) with the earth.

4) The loss resistance must be small in comparison to the radiation 
resistance.

Traps, inductors, etc., when they work, help with #1 and #2, and they 
don't make too much of a mess of  #4.. W8JI has observed that certain 
trap designs, as well as Bazooka designs, are bad with respect to #4.

  There are an infinite number of combinations of traps and inductors, 
and, as has been observed in another post, ANY inductor will have some 
stray C that can contribute positively or negatively or not at all on a 
desired frequency.  Part of the design of antennas that use inductors 
and traps is understanding those interactions, and managing them in a 
way that makes the antenna work on multiple bands.  By virtue of the 
current distributions at any given frequency, any antenna will have 
multiple lobes and nulls that vary with frequency.  The traps and 
inductors VARY that current distribution as a function of frequency, so 
they can move those nulls and lobes around.

The loaded 80M antenna that Barry at Hypower sells provides a very good 
match and works will on all of 40M, provides an acceptable match over 
part of 80M and works over all of 80/75 with a decent tuner, and also 
"works" in some directions on 30M, 15M, and 10M.  As installed in 
Chicago, that antenna worked very well into EU on 30M, but was on 30M to 
the south and west.

73, Jim Brown K9YC