Quote:
Originally Posted by Escapee
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.
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Ecoflex 15 inparticular which has a gas rich dielectric compound is usable up to 6GHz and there's a more recent plus version which can run up to 8GHz.
I use it on my 2m and 70cm arrays which is probably a bit of overkill but it's mechanically very nice and flexible thus easier to route and work with than hardline
