I had an email from, Francesco, IK0FUX, who was interested in the antenna I was using for my 10m QSOs. So I thought I’d just put it here for everyone to see.
It’s a cheeky little number made from 75 ohm coax. I got the design I an email from the RSGB a few years ago and it has been my travelling antenna since then. I recently discovered that I could also use it at low power on 12m as I captured a cheeky RTTY QSO with another DXpedition – I don’t recommend this – my K3 was tolerant – your mileage may vary as they say. I’ve taken the liberty of copying some parts of the RSGB article below.
Anyway, here is the design. Apparently it’s called an RFD (Resonant Feedline Dipole) and it consists of a quarter wave wire connected to the inner of a 1/4 wave of coax and fed through a choke at the base of the coax. The current on the inner conductor of the coax continues right on up the quarter-wave wire. The current on the inside of the shield now goes back down the outside of the shield.
Now, we want it to stop after going a quarter-wave down the shield.
To do that we put a high impedance choke in its path by winding the
coax into a coil. This does not affect the signal coming down the inside of the cable, but it stops the flow on the outside of the shield. You can also use a choke from G3TXQs web site.
The quarter wave wire cut for 28.5MHz should in theory be 2.63m long. In reality, and if we are using PVC-coated wire (which has a lower
velocity factor) you should really cut it at about 2.5m. You may find that you need to fold it over a little to bring the low SWR point to where you want it in the 10m band once it is installed.
We then solder the quarter wave wire to the INNER of the coax feedline, leaving the outer braid connected to nothing.
Now, we measure 2.5m down the coax and start to create our choke. At this point get hold of a cylinder with a diameter of about about 4.25 inches (112 mm). I used a plastic container I found in the kitchen. Now lay some pre-cut eight-inch strips of gaffer or duct tape on the container with the
adhesive side pointing outwards. You now start winding your choke onto the former – keeping it nice and neat with no space between the turns. In total you need five turns and then you can tape the whole thing neatly together using the gaffer tape. I also superglue the turns together since I found tape tends to stretch and move in high temperatures.
I use this antenna on a 12m spiderpole to elevate it as high as possible, in 9H I use it on top of out block of Flats on an 8m pole. It should have a very low takeoff angle on 10m and I’ve had success working VK long path from C6 (his claim – I guess he has a beam 😃). Either way, signal reports are consistently good but better than the signal reports I can give from it which indicates it may be a little deaf. However, it’s an excellent, robust, portable antenna and if you seal the coax ends then it should not degrade in the weather it’s moisture ingress and the like.
Good luck and Merry Christmas!