Updated: Aug 7
Yes is the answer!!! Then why do our antennas come without one?
Let me try to explain.
Our antennas are broad band transformers (multi-band EFHW) and they use the coax as a counterpoise for the antenna. In the image below you will see there is no counterpoise and no common mode choke at the bottom of the antenna.
What is happening here is the outside of the coax shield is acting as your counterpoise. This is not perfect, but it works. Trust me I've worked over 150 DX entities with a very similar setup.
Just a small thing, for 20w+ on transmit we would recommend a Common Mode Choke (CMC) as close to transceiver as possible.
This CMC will have two effects. Firstly it will stop any return current coming back down the coax to the transceiver (at QRP the return current is insignificant). Secondly, a CMC will reduce the amount of RFI your coax will pickup. This will help with reception, especially if you are in or near a built up area.
We sell CMC's in the shop. If you are using one ensure the toroid is at the transceiver end, not the antenna end when you use it.
However, lets assume we have an EFHW antenna with a built in Common Mode Choke (CMC) and the feed point. Like in the drawing below.
What will happen here is the CMC will isolate the outer coax from the antenna. This is fine, no more return current going back along the outer brain of the coax. However, now we do not have a counterpoise for the antenna. Therefore it will not work, there is nothing for the antenna to push against.
We can cure the problem easily by adding a small counterpoise between the CMC and Transformer on the earth side. The counterpoise will need to be about .05L in length (at the lowest frequency of operation). To build a EFHW Broad Brand Transformer with an inbuilt CMC it will look like the diagram below.
Above on the left we have coax coming into a CMC (12-14 turns around a mix 43 toroid) we then have the earth lead for grounding. Followed by a capacitor to help with the higher bands and then the 49:1 mix 43 transformer. This should work. So, I thought I would make one. Test it and then send it out for review.
Here it is the prototype antenna.
Apologies for the spaghetti soldering, if I ever make these commercially I would take a slightly different approach.
I have tested the design with 3.2k resister across the antenna and ground post. The next step will be for me to test the radiated power of 5w at my home QTH, with and without the counterpoise to see what happens. Then I will asked Craig from M0SZT to play with the antenna in the field so he can see how it performs in real life.
Below is a plot from my AA54 with the 3.2k ohm resister is very typical of the 49:1 transformer without a CMC.
It's unlikely I will put these into production, unless someone asks me to make one. But I will let you know how it performs.