Quote:
Originally Posted by Kymmy
Not entirely true, the thickness of a transmission antenna can increase the resonant bandwidth so a thicker dipole might have more bandwidth than a thinner dipole but this bandwidth is in relation to forward and reflected power (otherwise known as SWR measurements).
This is though totally different to bandwidth of a carrier medium like co-axial cable of which he is getting mixed up with.
|
Practicalities seem to be the biggest problem with thicker elements, it's not much of a problem for VHF or UHF but to make a large increase with for example a top band dipole would be impractical, hence the use of birdcage for HF. I did play around with a prototype VHF 30-90MHz Fractal antenna a few years ago, that had a nice wide bandwidth, reasonably good efficiency and low VSWR across the band, unfortunately the design wasn't mechanically practical for the intended application.
The biggest limitation on bandwidth with coax is generally the absorbing properties of the dielectric as frequency increases. The formula to calculate the loss at a particular frequency when provided with the actual loss at a lower frequency is generally logarithmic until this point is met, then it becomes very unpredictable. The use of a foam dielectric in cables such as LDF4-50a and LDF5-50a (There are more modern types and this shows my age) allows them to be used generally at a higher frequency than the PTFE dielectric types.