This morning, I removed from the loft, a pair of nested dipoles for 40 and 20 meters fed with one coax feeder. The 40 meter dipole was loaded with some coils so that I could fit it in the attic.
It gave me 40, 20 and 15 with a push because although a 40 meter dipole should give you a resonant 15 meter antenna, in this case – with the coils for 40 – it mucked the maths up and caused the ATU some trouble. On 40m, it worked a treat.
If anyone is interested in making up the “shorty forty”, you may find my experience of interest: Each coil was made from 3 meters of hard drawn BT downlead, coiled around a PVC plumbers pipe of around 25mm diameter and about 30 turns or thereabouts. The actual spec from the dipole centre was as follows:
- Dipole centre to coil: 3.7 meters
- Coil to end of dipole leg: 3.3 meters
- = 7 meters each leg + length of the coil
I replaced the nest was replaced by a half-size G5RV in a “lazy” inverted V configuration, tucked up in the rafters. Good for running a second radio and it gets me on 10m as well as the WARC bands.
Incidentally, when I was testing this original dipole, I started off with 8 meters for each leg and ran it through the MFJ analyser. It was resonant on 6.66 Mhz. An old favorite pirate band that some people may remember. Adding the coils raised the renonant freq from 6.6 to 8.5, so cutting a meter off each side to get the whole thing working for the amateur bands was necessary.
So there you have it, a 45 foot long 40m band dipole.