I am in the process of planning a multiband vertical antenna and have a few questions I would be grateful for any advice on.
So I plan to have 1/4 wave elements for bands 20-10m with a common feedpoint and shared radials, something akin to the commercially available DX Commander antenna, whereby elements self select on the basis of desired impedance. I would like to include 40m also , but there is one question I have regarding this:-
So I have found a supplier here in the UK that can supply a 6m long fibreglass pole/tube. I think this will be a good solution as unlike the telescopic poles sold by many ham radio outlets, this will be the same diameter throughout the whole length (around 45mm with a wall thickness of 2.5mm). My plan is to have around 50cm of the pole below ground, sitting in a sleeve made from a length of PVC pipe concreted in place. So with this in mind, the pole will be long enough to support elements for each band from 20 to 10m using plastic plates as guides for the elements (basically a copy of the DX Commander antenna). My idea for 40m is to make the element linear loaded so the wire extends up the pole, back down and then back up in such a way that it is the desired length. The question I have is will this linear loaded element present near enough a 50R impedance, I will tune all the elements using an antenna analyser, but I just want to know if a 50R or close enough will be easy to achieve? My aim is for the whole thing of each band to be tunable with my rig's (ft991a) internal tuner. The other option I am considering for 40 is to make the element an inverted L, which would be ok but not as neat mechanically as the linear loaded version. I realise that an inverted L might enable some local contacts due to the horizontal section, but I primarily am interested in making DX contacts with the antenna, I have an OCFD which certainly for 40m works well for NVIS contacts due to its limited height. Can anyone tell me which of the two variants (inverted L or linear loaded vertical) is likely to yield the best results for DX? Or if the difference is unlikely to be noticeable.
Many thanks for your help.