Pollywog Posted April 20, 2014 Share Posted April 20, 2014 Evening all, I've been issued a VHF radio by my 4x4 response group but I'm in the position of being the only soft top in the group and as such am having issues finding a decent place to mount the antenna. I have the radio wired in to the auxiliary battery and at present have the antenna mounted through an existing hole in the wing originally meant for a wing mirror. The radio picks up 2 of the nearest repeaters with the engine off but as soon as I start the engine I lose all reception. I think things are made worse with the fact that being a lightweight the wings are 6" lower than standard and have the steep side of the bonnet to contend with and possible interference from the engine? Any ideas on where I could mount the antenna for better signal? I was wondering about using the lightbar on the roof, but that would have no ground plane so perhaps make a few radials from some wire? Or alternatively another roof bar mounted behind the lightbar with a flat metal disc mounted on it to form a ground plane? Quote Link to comment Share on other sites More sharing options...
western Posted April 20, 2014 Share Posted April 20, 2014 mount a metal [steel or aluminium] plate on the roof bar as the ground plane then attach the antenna to that, there are artificial ground plane mounts available from places like 4x4cb.com or communicationeleven or thunderpole that would probably work as well. Quote Link to comment Share on other sites More sharing options...
supaimpy Posted April 20, 2014 Share Posted April 20, 2014 Why not fit an FFR aerial box on the front wing then mount the aerial to that? Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 21, 2014 Author Share Posted April 21, 2014 I was looking at the FFR mounts but other than raising the antenna a few inches I don't see how it would help with the ground plane? Looking at the light bar I can possibly get a disc with an 8" radius under the antenna, the antenna is 16" so I don't think that's going to really be enough either? I tried mounting the antenna on the bonnet this morning using one of the spare wheel mount holes and still had the same issue it works fine with the engine off but won't transmit or receive with the engine running. The size of the bonnet should of provided a good ground plane, I'm guessing the closeness of the antenna & coax to the engine is the issue but could I be missing something else? Quote Link to comment Share on other sites More sharing options...
GW8IZR Posted April 21, 2014 Share Posted April 21, 2014 The antenna is almost certainly a 1/4 wave long which suggests your operating at VHF high band possibly 170MHz or thereabouts. Your ground plane needs to be about that same 16'' diameter. It would be useful to get confirmation of the frequency. The good news is its not life or death critical that you achieve that 16" or bigger ground plane. Middle of a hardtop is perfect but if you can mount it to a hood stick or roll cage with that 16" in at least two directions it will be good enough. To improve on that connect a couple of wires to the hood stick and run them at 90 degrees to the sticks, those four wires and the increased height will significantly improve transmit / receive performance. The whip length should be adjusted for best VSWR in its final position- ask some local radio amateur to help you set it up, they should be happy to help. Need more information on this no transmit / receive when the engine is running as that sounds like a separate issue - what exactly are the symptoms? Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 21, 2014 Author Share Posted April 21, 2014 Hi, Yes it is a 1/4 wave antenna from co-star http://www.co-star.co.uk/prod/779/vhf-antenna-14-wave-antenna.html & one of their M8 bases http://www.co-star.co.uk/prod/376/m8-panorama-antenna-base-universal-modular.html I'll ask about the frequencies. I'm thinking a plate on the light bar is definitely the next thing to try, I can get the full 32" plane along the length of the bar and possibly get half the plane in the other directions. I re-mounted the antenna on the wing this afternoon and went for a drive around, out in the middle of open common land I can get a signal in all directions parked up with and without the engine running and also in 2 directions when moving so I think the problem is definitely the ground plane coupled with the antenna being mounted so low and a bit of added interference from the engine. Quote Link to comment Share on other sites More sharing options...
GW8IZR Posted April 21, 2014 Share Posted April 21, 2014 If the engine is a TD5 then interference is a big issue, anything like a 2286 petrol to a TDI and everything between shouldn't be a problem. Petrols may need a bit of suppression and resistor plugs etc but nothing that can't be overcome. Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 21, 2014 Author Share Posted April 21, 2014 It's a 200tdi, I was wondering would having the radios power leads running alongside the vehicles main power feeds cause interference? Also something else bugging me whilst I have you; I've just noticed the installation instructions show that the mic hanger clip should be earthed, what is the reasoning for this? Quote Link to comment Share on other sites More sharing options...
western Posted April 21, 2014 Share Posted April 21, 2014 my 110 is a 200tdi, power wires for my ham radios & MSA rally radio come from battery via fuses in both wires, no interference problems mike clip just needs screwing to dash, no need for it to be earthed. Quote Link to comment Share on other sites More sharing options...
GW8IZR Posted April 21, 2014 Share Posted April 21, 2014 A 200 or 300 tdi should be no problem at all, one of the reasons I went TDI was no injector (RF) noise. Cable routing should not be an issue as there is no noise to induce into your wiring. If you can hear a low level but high pitched whine that varies with engine revs you may have some noise from the alternator but its very distinctive, you haven't mentioned it so its most likely not there :-) Ideally the radio power should be fed direct from the battery with a fuse in both +ve and -ve line. Sometimes the microphone clip is used to 'close' the tone signalling receiver gate. On most equipment that is done with a magnet or contact. Some equipment detects the ground connection to do this. Instal instructions tend to be generic so it may not apply to your setup. Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 21, 2014 Author Share Posted April 21, 2014 Thanks for your help guys, yes the power cable has fuses in both wires and is wired direct to the aux battery. I'll get the light bar mounted and cut some steel plate tomorrow. Quote Link to comment Share on other sites More sharing options...
western Posted April 21, 2014 Share Posted April 21, 2014 as a matter of interest what make & model is the transceiver you have fitted ? as I have a VHF high band rig to fit, had it about 4 years & haven't got around to it yet Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 21, 2014 Author Share Posted April 21, 2014 It's an Icom IC-F110S 1 Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 23, 2014 Author Share Posted April 23, 2014 Had another play this afternoon... 34x16" plate so I have the full plane in 2 directions and 1/2 in the others, It's certainly a much better reception from the repeater and it now acts the same with or without the engine running. I'll go with this for now and see how it performs on Sunday when I'm able to test it communicating with the other guys in the team. Thanks muchly for your input Quote Link to comment Share on other sites More sharing options...
western Posted April 23, 2014 Share Posted April 23, 2014 you might get away with fixing the plate inside under the hood, the material covering it shouldn't make any difference to signal strength/quality & it'l lok more discrete too. Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 23, 2014 Author Share Posted April 23, 2014 If it performs well this weekend I'll tidy it up a bit with a coat of black and add some trim on the other edges. If it could go below the canvas I guess something else to consider would be a larger plate mounted on the center hoop, or better yet spanning between the center & rear hoops, I'm not overly keen on making a hole in the canvas to pass the antenna through but I guess if I put a properly fitted eyelet in to stop it tearing..... something else to keep in mind, thanks. It'll be much easier during the winter when I have a truck cab or full hard top on. Quote Link to comment Share on other sites More sharing options...
Pollywog Posted April 28, 2014 Author Share Posted April 28, 2014 I'm pleased to say it worked very well and I'm pleased with the reception I got from it, as well as a paint it also needs stiffening as it causes a lot of buffeting noise above 40mph. Quote Link to comment Share on other sites More sharing options...
Snagger Posted April 29, 2014 Share Posted April 29, 2014 You don't need to attach the antenna directly to the ground plane as long as the plane is earthed, so you can fit it inside the rag top without any cutting. Quote Link to comment Share on other sites More sharing options...
Pollywog Posted July 14, 2014 Author Share Posted July 14, 2014 A quick update on this, the antenna works very well mounted in this way and I was very happy with the signal I was receiving BUT the drumming of the plate when traveling at anything above 40mph is quite deafening, not only that but it has also taken it's toll on the light bar having caused both of the bars mounting plates to fracture. The hard top will be going on soon so I will have all winter to come up with an alternative arrangement for next spring & summer. Quote Link to comment Share on other sites More sharing options...
TSD Posted July 14, 2014 Share Posted July 14, 2014 When we made the fibreglass roof for my Ibex (ok miketomcat made, I only made a mess), we embedded about 30 feet of fine stainless steel mesh tape (used for emc shielding) in the glass to act as a ground plane. That seemed to work pretty well. At VHF, a grid of conductors of 25mm or less should form a pretty good imitation of a solid sheet, so maybe using weld mesh or even chickwire instead of a solid sheet would help? It probably has higher wind resistance, but shouldn't 'drum' so audibly. Quote Link to comment Share on other sites More sharing options...
GW8IZR Posted July 14, 2014 Share Posted July 14, 2014 If you weld ( or clamp) a lug onto your light bar about 18" from one end and fit the M8 base to that you would probably not notice a huge difference in performance. It would be a compromise but most likely ok. Quote Link to comment Share on other sites More sharing options...
cactus Posted July 14, 2014 Share Posted July 14, 2014 I've a steel plate between the hoodsticks so it's hidden from view... A Mag-mount will grip through the canvas and the coax passes through the Door-shut to the radio. Mine's been inuse for 3 years now without any problems... Quote Link to comment Share on other sites More sharing options...
Pollywog Posted July 15, 2014 Author Share Posted July 15, 2014 Thanks for the further suggestions guys. I did wonder if a mag mount would be able to cope with having canvass between it and the ground plane I've heard that some don't stick so well even when straight to steel but I guess that all comes down to the cheapness of the mount. I'm tempted to go mag mount for the hard top anyway; bond a small steel plate to the roof instead of drilling holes. Quote Link to comment Share on other sites More sharing options...
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