Many thanks to Dale (N3HXZ) who shares the following guest post:
Portable Operation Using a Vertical Antenna Without a Counterpoise Wire!
by Dale (N3HXZ)
In a previous article that Thomas was kind enough to post, I ran experiments to determine if the counterpoise wire orientation of the Chameleon MPAS LITE antenna system in the vertical configuration in any way shaped the antenna pattern. The configuration of the antenna is shown below.
The system consists of a 17’ telescoping whip antenna, a matching transformer, a 50’ coax cable with an in-line RF choke, and a counterpoise wire (60’ long, but unwound to only 25’ for use in the vertical configuration).
Subsequent studies I performed have reinforced my earlier conclusions that the positioning of the counterpoise wire in this setup does not appreciably shape the antenna pattern. One of the comments from my post came from Stephan (HB9EA). He commented on the fact that the coax cable is indeed a counterpoise via the coax shield and that in fact my system set-up had two counterpoises.
It was Stephan’s comment which led me to contact Chameleon Antenna to discuss counterpoises with the MPAS LITE antenna in the vertical configuration. I communicated with the Director of Research and Development group at Chameleon and found out some interesting information that I wanted to pass on.
He mentioned that the system can be used with the coax cable acting as the lone counterpoise as long as the coax cable is at least 25’ long, and preferably 50’ long. The coax should be spread out and not coiled along any portion of its length. It was stated that the counterpoise wire provided with the product should be used with short-length coax cables.
With regards to a counterpoise orientation shaping the propagation direction, he stated that it is generally accepted that using only the coax as a counterpoise tends to have the antenna pattern slightly favor the direction in which the coax is laid out. Adding the counterpoise wire in a direction opposite to the coax would tend to balance out that pattern. Having the coax and counterpoise wire at some angle to each other introduces variables that make it difficult to determine the antenna pattern.
My initial study had the counterpoise wire oriented at an angle to the coax cable. I mentioned that my initial study showed that orientation of the counterpoise wire did not seem to appreciably shape the antenna pattern, and he agreed. What is unknown is the overall degree to which the coax cable shapes the antenna pattern with or without use of the counterpoise wire. This is an area I am currently investigating.
Operating the MPAS-LITE vertical configuration with only the coax cable as a counterpoise was an intriguing thought.
To test it out I teamed up with Jim (KJ3D) for a SOTA activation on W3/PT-003 (Seven Springs). We both deployed the MPAS-Lite with only the supplied 50’ coax cable attached. Coax cables were laid out in a straight line. One cable was oriented southeast, the other one oriented east. We operated on 10 Watts in CW mode. The activation successfully worked all bands from 40M to 10M. I operated an Elecraft KX2 and Jim operated a KX3. The internal tuner of both rigs easily obtained a 1:1 match across all bands.
A map of the QSO’s is shown below. Note that despite the coax cables facing to the southeast and east, we easily worked stations in the opposite directions. Hence any minor shaping of the antenna pattern by the coax orientation does not appear to impede successful communication in all directions.
In summary, the MPAS-LITE antenna system in the vertical configuration utilizing the 50’ coax cable with an in-line RF choke as the single counterpoise has been demonstrated to perform well in the field.
For portable operation where fast deployment and simplicity is paramount, using the furnished coax as a counterpoise is a quick and dirty way to get the antenna erected and on the air with reliable communication capability. Eliminating the counterpoise wire allows for quicker set-up and tear-down, and one less thing to put in the backpack!