In my quest to use radio on my way home ( I can access dead repeaters for the first 5/6 miles and hear a 70cm Hubnet repeater most of the way but quite scratchy in parts due to the rolling countryside) I’m now looking into building an improved antenna ( just a 1/4 wave at the moment) to improve the scratchy parts and see if I could make it usable for QSO’s. I have looked at trying to convert base station/portable antenna designs to use on a mobile mount (3/8ths) ,most of these seem to use the Jpole type matching arrangement which unfortunately makes the antenna longer and adds to the complexity of mounting it and I’m guessing putting the matching section horizontally against the car roof would cause problems so my question is this, If I build a 5/8th wave antenna then add a second element above using a 1/4 wave phasing section will this majorly affect the tuning compared to the original 5/8th wave ( experiments showed the 5/8th wave had no benefit over the 1/4 wave ( static tests only) ? My plan was to use an old CB antenna ( Valour half breed) rewind the coil to create the matching section then a straight 5/8 th wave element the whip above that bent to form the phasing section and then the second 5/8th wave above that. This would mean I could make the whole antenna in one piece making it mechanically tough for mobile use. The option which I tried yesterday was to connect into HUBnet with my phone and Echolink ( Not really what I want to do, keep it radio !) but even then mobile signal cuts out at multiple points ! As said before I can hear the Hubnet connected repeater ( GB3HN) all the way home but rough in places. I am trying to get this to work first rather than buy a dedicated mobile radio only to find out I have wasted my money, In the spirit of experimentation I am keen to Homebrew as much as I can !
Thanks all, A rather long way of saying can I just plonk a second element on the top !
There are a range of “white sticks” made by Diamond and other commercial companies that do what you are suggesting, more or less.
They start with a quarter wave radiator, add a coil which is there as a phasing element to give a 180 degree phase shift and it connects to a half wave element. Further half wave elements are added each fed by an associated phasing coil.
They are made from a continuous length of about 3 mm diameter rod. To provide stability and weather protection they are housed in a fibreglass tube which is painted white.
Apart from the clever design and knowing how many turns make 180 degrees, they have managed to get a reasonable SWR at the feed point. A quarter wave might look like 35 ohms at resonance and the collinear may double this to 75 ohms so the SWR magnitude still looks good.
How much gain do you get?
Well depending on your installation a 5/8 may give from 1 to 3 dB gain over the quarter wave. A quarter wave feeding a half wave will be much the same.
I think anything much longer than a 5/8 will be a mechanical challenge for mobile ops.
Going from a nudge bar mount to a roof mount will do a better job filling in the holes. But of course some gain always helps.
I have a couple of 2 m 5/8 whips somewhere. They have been used as a 50/144 MHz dual band antenna. I had a dozen or so 8,000 km contacts on 6 m and 1,400 km contacts on 2 m with them.
All due to conditions and being on air at the right time.
My two-penneth is to give the existing 2m quarter-wave vertical a better ground plane. Ron suggests you have the antenna on a nudge-bar (bumper). If that’s the case, put the antenna in the centre of a METAL roof and you will see a big improvement. Not only from increased height but from a better ground plane.
I already have the 70cms 1/4 wave in the middle of the roof , I was originally using a 2 meter 1/4 wave but this gave very poor performance and switching between one or the other made a world of difference although both gave a good match.
In the UK we have to do a calculation as regards the Electromagnetic field around our antennas , if you have under 10 watts ERP you are fine but other wise you have to know the safe distance for other people around you (you yourself are exempt) this makes it fairly easy to breech this regulation ( at least in traffic or areas where pedestrians can be), I think a lot of operators either do not realise this or choose to ignore it. As I said originally I’m not far off being able to achieve operation for the full distance of my 25 mile journey on 4 watts so an extra 3 db or so would most likely be enough. According to the RSGB calculator I can do up to 18 watts out with the 1/4 wave or 7 watts with a antenna with 6dbi gain and stay within the regs. That said the compliance distance is quite small with higher power so once I’m in the countryside I could use higher power anyway. I’m also a cheapskate so doing it with the antenna would be the cheaper option.
