HI all i'm wanting a 27 mhz ground Indepenendent antenna as i can't get enough good metal ground plan on my ute and getting bad swr .
Solution is actually very simple and you already have it. Use the coax connected to the antenna as a RF ground.
When an installation has a piss poor RF ground the antenna system is trying to use the coax as one to try to compensate - it's the whole reason the old wives tale of "you have to cut your coax to 9,18,27 or whatever feet to get a low SWR" came into being. Your coax forming part of the RF ground is generally a VERY BAD THING. You get common mode RFI, lots of noise and interference.
HOWEVER if we know that the coax is being used as a RF ground we can make it work to our advantage and we can use a solution to deal with the downsides.
So how to achieve this. A fooking good RF choke. So what you want to do is to measure back from the antenna mount an electrical quarter wave length of coax which we're going to deliberately allow to be used as a RF ground. So it'll be 234/freq (in MHz) x velocity factor of the coax so 234/27.5 x 0.66 (RG58) so 5.6ft. That's the bit we're going to deliberately use as a RF ground.
We now need to put in a RF choke at that point so we're not getting common mode RFI and all the problems that come with it on the rest of the coax. We want to make a temporary one at first as we may need to do a bit of tweaking with the length of the coax to get the SWR lowest. So we'll do it by making a coil of 5 turns of the coax at that point with a 4.25" diameter centre. If the SWR is still high once you've tried tuning the antenna you can unwind the coil and start winding it a few inches nearer or further away from the antenna, re-adjust and retest and move again if necessary. You may find you need to lengthen that bit of coax by a couple of feet before winding the choke if your coax has a velocity factor of 0.85 which is the other velocity factor sometimes found in RG58. So don't get disheartened if you lengthen the bit from the mount to where you start winding the choke a few times and it's not working well - its most likely the VF of the coax is 0.85, not the most common 0.66.
Now once you've found the best length to have between the RF choke and the antenna mount you can either make a better one by wrapping 8 turns of the coax around a FT240-61 ferrite at that point or cut the coax there, fit a PL259 on the end and use a high quality commercially made RF choke. Or if that coil you made doesn't bother you just tape it up somwhere.
