Thursday, August 25, 2011

Half-wave 27MHz Antenna as Broadband Vertical

Years and years ago, a friend of mine gave me a (brand new) half-wave CB antenna. Well, I have never used it, for the reason of not being QRV on 11m. For 10m I had my double bazooka, so there was no need for it. Now that it has been sitting in a corner for a while, I thought, maybe it could be useful for something else. Some else as in broadband vertical...

Now, let's have a look how those things are built:

aerial schematics
This drawing is not made by me, however, it nicely sketches what the following photographs of my very own version of that antenna show.

all still in one part

taken apart
What we have is the cavity and the transformer that was sitting in it. The air core transformer has got a winding ratio of 7.5:2. I figure, using a ferrite or iron powder toroid would improve performance on lower frequencies.
The cavity measures 35mm in diameter, having a depth of 24mm. This clearly offers enough space to house a smaller toroid transformer.

At this place, I would like to remind you of the BB7V (Diamond) having an UnUn transformer with (resistive) termination shunt. This vertical would be 6.7m tall. The regular 11m half-wave vertical could possibly be stretched to a tallness somewhat beyond 5.5m. Close enough to me.

Now let's think... for QRP work a T80-2 would make an ok UnUn. With some luck, transformers with a T130-2 core could possibly be squeezed in the cavity, if made carefully.
Reconsidering the BB7V's terminator, would I put one in there? Only if I would be using this aerial for QRPp only. The base of the CB-aerial is made from plastics material, which is not able to conduct any heat. Hence, heat created inside the cavity would never be dissipated. For QRSs/QRPp, the resistive terminator is a very appealing option, in particular since for such power levels, a T50-2 transformer would offer sufficient empty space for the terminator.
A T130-2 would possibly be good for a regular 100W rig. However, in a worst case, 50W of heat have to be dissipated, hence, some cooling of the termination would be required, therefore, this is a no-go. Some adjustments and selective use of bands would still be an option for such an arrangement.
Personally, I am very tempted to try a version of a T68-2 transformer (not sure about termination yet), in combination with my IC-703. This is somewhat of a compromise...
Should I ever again get into QRPp/QRSs/WSPR/WSJT/ROS actively, my preference would be the small transformer&terminator option.

Additionally, I figure, it could be of use to add an additional Al-pipe of about one meter to the top of the vertical. This will result in a total length of about 6.4m and a further distance to a quarter-wave on 20m, without getting too close to a quarter-wave on 30m.

There you have it, a new life for a cheap half-wave 11m vertical.