Chameleon MPAS Lite: Using the coax shield as a counterpoise and how this might affect directionality

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!

13 thoughts on “Chameleon MPAS Lite: Using the coax shield as a counterpoise and how this might affect directionality”

  1. Wow–thank you for sharing this, Dale.
    You’re right if taking the full coax cable, it appears there’s no need to use a separate counterpoise for SOTA/POTA ops. I’ll give this a go myself.
    And as you discovered, any directionality doesn’t get in the way of making contacts from all directions.
    Thank you for sharing!

  2. Thanks for the interesting article, I assume you have the feeder orientated with the choke at the radio end when using it as a counterpoise?

    I generally prefer wire antennas for portable operation, but I’m glad to have my MPAS Lite available as my bad weather antenna. I’ve used it for several SOTA & POTA activations in 30mph+ winds and rain and its never let me down.

    However for hiking with the antenna I no longer use the supplied 50ft RG58 or the conterpoise wire – its just too bulky and heavy, and I find the RG58 can become a bit of a handful in cold weather.

    Instead I prefer to use multiple non-resonant shorter radials and for my MPAS Lite my current preferred configuration is 8 x 3.75m radials made from PTFE covered 22AWG wire (a lucky eBay purchase) configurred as 4 pairs with 2mm bullet connectors so I can easily deploy however many I need. Using multiple shorter radials is easy to throw out on a summit and takes up less room. I use orange coloured wire to avoid any trip hazards. Weight just under 200g for the radials, with a total wire length of 30m.

    And for the feeder I use 5m of RG174 with an inline common mode choke (feeder wound on a FT114-43 core before fitting the connectors).

    These changes keep the total MPAS Lite antenna system down to just under 1.5kg and it also packs more easily. Still a bit on the heavy side, but as I said I don’t worry about what the weather thows at it!

    73 Jonathan

    1. I don’t see the choke in the photo above and he said he used it in that configuration so I think the choke is by the radio. I would love for him to confirm this. I have been using the choke by the antenna and using a wire door screen a connected to the spike for a conterpoise.

      One problem I have had, is I have had my antenna ( the full MPAS micro), blow over in the wind twice. It may be a result of where I operate which is right at the edge of San Francisco Bay and it’s sometimes very windy in the afternoon. I had my husband make a nice plastic piece with three holes for guy lines. It slides over the top of the lower section of the antenna and it stopped from sliding all the way down by a zip tie that is pulled against the antenna. Since I have done this I have had no problems. I love reading these articles and comments

  3. I have a question… If mounted higher, with an ‘elevated radial’, instead of a counterpoise, how would the antenna radiate?

    I’ve alway seen elevated radials as a means to provide some gain on an antenna. e.g. At home, I use my 10 meter telescopic SpiderBeam mast to suspend a 20m vertical wire. And with a one or two elevated radials at the feedpoint, pointed South, or East, ‘aim’ Southeast.

    What is the difference between a counterpoise and an elevated radial?

    1. Rand :

      A single elevated wire radial will radiate and will impact the directionality of the vertical; it is part of the antenna. This creates what is essentially an L-shaped dipole. If it is a single elevated wire then the gain will be concentrated in the direction that it points. If there are two or more elevated horizontal radials and the currents in them are perfectly balanced then the antenna pattern will be omni-directional.

      Wires on the ground essentially act as the plate of a capacitor and couple the antenna to ground and help to reduce ground losses. They have less impact on the radiation pattern.

      As a rule of thumb a short vertical will benefit more from many shorter radials on the ground, than a few longer ones. With a shortened vertical most of the ground current is concentrated closer to the base of the antenna so reducing ground losses there with more short radials will improve antenna efficiency.

      For portable operation with a short vertical it always comes down to efficiency vs convenience. Go with whatever setup works well enough for you.



      1. Thanks Michael… That’s what I thought.

        I have a orange fiberglass dowel marker, I bought at Home Depot. It’s about a meter long, and pointed to shove in the ground. It works great.

        There is a reference to the use in elevated radials in an old HF Manpack Antenna ‘Shootout.” on a Yahoo group. It seems to be the benchmark they used to evaluate the vertical antennas of the time.

        Thanks again for your insights.

    2. Hi Rand,

      The difference between a counterpoise and an elevated radial is:

      A counterpoise, single or multiples are simply laid on the ground ,during temporary installation, or underground for permanent use.

      With an elevated radial, the vertical become a half wave dipole, which means the impedance is low and this suits the transmitter well.

      It also means that the far end of the sloping wire (counterpoise) is a high impedance. Because of this, the end is at a high voltage and should not connected to ground.

      It is recommended that it is attached to an non conducting post at about 2 ft height.

      The vertical I use portable is an Outbacker Perth, AND MY NON CONDUCTING POST IS SIMPLY A 2 FT LONG BBQ FORK . at the end which hold my wire winder in place.

      This way it becomes a simple vertical dipole.

      The support I use for any vertical like my outbacker, or buddistick or simply a hamstick is the CHA-UCM from Cameleon Antenna, a super support, high quality which fit on anything up to 3 inches thick.

      I learned that it is not all pik-nik table which have a thickness of about 1 inches, sometimes it happened to me to find one at 2 inches or so.

      So That’s what I use for park on the air, and I always adjust my antenna with an AA-54 antenna analyser.

      For other bands, I if I want to change them, I adjust my wire with the help of different coloured of electrical tape anf bingo I always have the right lenght.

      By the way This idea came from:

      All Right Reserve , 2010

      Hope it helps, try it you’ll like it

      All the very best , 72/73
      Mike VE2TH
      The QRP’er for 60 years.-

      1. Hey Mike…

        I don’t often deploy a verticals, but if I do it’s at the QTH, and the simple vertical dipole as Michael (VE3WMB) noted above.

        In the past, I’ve helped other Hams deploy there various verticals, and with enough radials, they really can send a signal!

        The Chameleon MPAS is a high quality and expertly engineered antenna system.

        I just prefer simpler wire antennas; a doublet, dipole, or end-fed wire. Simple & cheap, yet if deployed properly, very efficient as well.

        Thanks for your comments,

  4. To reiterate one of the previous comments, when using the coax as a counterpoise I assume you connect the inline RF choke at the radio end instead of the antenna end, as you normally would with the MPAS Lite deployment? Thanks for the interesting post. 73 Ken

  5. For most of my portable activations, I use a vertical antenna with only one counterpoise. I typically do not see directivity because of that. I have a lot of activations on my website with QSO maps. Nice write-up.

  6. Hi all,

    Thanks for this interesting article!

    This topic sparked a conversation with a friend: We we were discussing what the exact effect of a choke at the feed point of the antenna is. For the purpose of simplification we are assuming a single counter poise wire is present here.

    1.) On the one hand side the choke would of course block common mode current from flowing back to the TRX and also prevent the coax from radiating.

    2.) On the other hand we are wondering if the shield of the coax would still be able to act as an additional counterpoise with the choke at the feed point present?

    We heard a theory that the outside layer of the coax shield would be blocked for RF by the choke but the inside layer of the coax shield could still act as a counterpoise in spite of the choke?!

    If anyone has more knowledge on this or could point me to more information that would be awesome.

    Thanks es vy 73 de Leo

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