I’m after some advice or thoughts on vehicle antennas. Not strictly SOTA related, but there is a wealth of knowledge to be tapped here!
I’ve been using mag-mounts for a while on the occasion I do mobile operations from my pick-up, but I might move onto a screw-driver style HF antenna, in place of multiple whips. To that end I’m looking at more permanent installations for mounting antenna’s. Many pick-up owners use mounts attached to the tailgates, but I want to avoid doing that, as I need to travel with the tailgate open at times.
I have factory fit roof rails and I’m considering using this clamp on the rail. The obvious issue is that I’m not longer coupled to the body and using it as a ground plane (the roof rails are coated in plastic and are pretty unlikely to have a good electrical bond to the roof anyway). The point at which I’m wanting to bring the coax into the cab is on the rear face, behind the rear seats. I’m thinking about using a panel mount double socketed SO239 connector like one of the two following styles:
By cleaning off the paint around the connector, I could get a good bond between the case and the body. I’m more likely to use the second one, as it’d be a more robust installation and easier to seal with silicone. I’m not to experienced in antenna physics, but I’m wondering if this would still result in a good ground plane from the body of the cab, if it’s electrically connected about 1.5 meters way from the base of the antenna, effectively through the coax braid and a connector? Would a separate earth braid work better, although there is nothing near by for me to ground too near the rails.
The only other location I’ve considered is at the edge of the bonnet near the windscreen, as I could use one of these mounts (grounded through the mounting pins and I would use a proper earth braid to an earth point about 6 inches away on the bonnet hinge mount). I’m less keen on this, due to having the antenna in the edge of my vision.
Do people thing my idea about grounding through the panel connector would work well? Any other idea’s I may have missed in my googling?
The problem with roof rail mounts is the 2.1 to 2.2 m height limit in covered car parks and refiling stations. You can simultaneously take out their fluro lights and your whip.
I used a bonnet mount successfully for many years. Put it on the passenger side if you are hypersensitive about a whip within your sightline.
Of course that means the first overhanging tree you pass could take the whip out.
Hi Dave, I would expect there would be issues with radiation off the feedline and some average performances from using 1/4 wave antennas on an ungrounded installation. My current vehicle has metal roof racks and 1/4 wave style antennas seems to work OK from them. Recently I have been using some ground independent antennas on the roofracks and they work well RF wise although if I forget and go into an underground carpark, I am reminded with a metallic clatter as the antenna clangs onto the carpark roof (they are pretty flexible so no real damage). I also have a couple of antennas on the bullbar (elevated feed on UHF and a dualband SG7200) so they are the right size to get into underground carparks without hitting the carpark roof too often. Previously I have run antennas on bonnet edge mounts - they also work fine and if the right size, also avoid hitting underground carparks.
The above are mainly for VHF/UHF (80MHz commercial and VHF highband/UHF antennas. Personally I would steer clear of putting an HF antenna on the roof of the vehicle - bulbar mount it (if you have one) or make up a bracket coming out of the front of the chasis/frame and mount it there - yes there will be some directionality and it may be a small impediment to your vision, but performance will be far better than roofrack mounted with a poor ground.
In the UK, nearly all petrol stations have 4m clearance or more. I’ve lifted the suspension by 2" and use off-road wheels and all terrain tyres, so there are already some car parks I’m too high for.
Unfortunately we can’t use proper bull bars in the UK, as they present too much danger to pedestrians, especially as the need to defend against road kill on animals is relatively low, otherwise that would be a go-to spot.
I was hoping beyond hope to try and use a screwdriver HF antenna, and I’ll keep dreaming unless someone has tried something similar already.
I have a Nudge Bar fitted to my Toyota. It’s more pedestrian friendly than a bull bar but gives a reasonable ground connection. I run a dual band VHF/UHF whip on one mount on the bar and have a mount for Hustler whips on the trailer tow bar at the other end. I occasionally use a mag mount on the roof for UHF CB.
I recommend a tri-pad magmount for vehicle roof HF antennas. I’ve used this one with HF monoband whips for decades and more recently with my Chameleon MPAS Lite (5.2m long) whip.
Excellent RF coupling to the vehicle roof for ground plane. Very strong magnetic grip – in fact you need a technique to yank the magmount off the roof. With its thin rubber(?) underside mats, no marks on car paintwork.
I’ve never operated HF whilst driving but occasionally I’ve driven at 70mph with the antenna on roof – no problem except for overhanging branches and low arches.
Hi Dave, no dramas. Key takeaway for HF is that the roof rack mount envisaged most likely unsuitable. Ron’s comments about earthing absolutely agreed with. Also if you have or can get a nudge bar approved by the vehicle inspection people that would provide a suitable mounting point.
If your pickup is an open bed, possibly look at fashioning a bracket in the bed (I have seen Codan 9350 auto tuners setup like that - despite close proximity to the ute body, they seem to work OK). Or like earlier comment, see if you can find something on the front from the chassis or frame that you can extend a bracket out and up to provide a base to mount the antenna to.
Dave, there’s nothing to stop you adding a grounding strap from the roof bar mount to the roof/body. You want a short and flat strap, such as used for the -ve battery connection. You can buy such straps from car spares shops for a few pounds. Trim to length and add some heatshrink covering. You will need to drill a hole in the roof to bold the strap. The hole will need waterproofing.
When you sell the vehicle, remove the bolt and strap and fill the hole with a bung and sealant. If you are creative you may be able to locate the bolt under one of the bars. You may remember that many ex-business cars in the 90s would have had a mobile phone aerial fitted and replaced with such a bung when sold. I had 3 cars that had had such fittings back then.
So simple! That is certainly do-able. The body is already less than pristine as its been used as an off-roader and drilling a hole or two isn’t something that worries me; I’ll look at how easy it is to get behind the roof lining and install an earth stud or two Whilst I’m at it, I could feed the coax through the roof under the roof bar using some sort of gland, possibly one of those fittings used in campervans to pass solar cables into the living space.
Thanks Andy!
It isn’t, I have a fibreglass canopy installed at the moment:
The canopy leaks a bit (not badly, but it is a nuisance. I’m planning to replace it with a mountain top roll tonneau cover next year, which you can put bars across the bed (similar to roof bars), on which I’ll mount a clam-shell roof tent. The tent will overhang the end of the bed by about 500mm (which is legal!), but this would render any mounts placed on the tailgate unusable.
The nudge bars available in the UK for my Navara don’t extend far enough forward to clear the bumper, and I suspect any home made bracketry which did would come under scrutiny at it’s next inspection.
I have a triple mag-mount on the cab roof at the moment (you can just see it on the above photo) which I use for the HF whips and a 2m/70cm antenna I have, and have just put up with putting it through the off-side door and closing the door on the coax, which has pinched it and will eventually kill either the coax or the door seal.
I might have a go with my triple mag-mount and see if I get a good bond..
Thanks for everyone’s thoughts! I suspect I’ll go with Andys suggestion of an earth stud under the rail and look to feed the coax there too, assuming I can get access behind the roof lining! A youtube video suggests its just a case of removing a rather large amount of trim. Do-able, but I’d best start doing it first thing in the morning on a day with nothing else planned!
After several hours of research and frustration with the international market, I’ve ordered a solution.
I already have a Diamond K-400 mount but I’m not convinced it’ll do my bonnet any good with such a short lever-arm (aero load would be acting to pivot in the worst direction, putting high load on the bonnet lip).
Frustratingly, you easily can buy ready-made bespoke antenna mounts for my vehicle model, predominantly intended for CB in the outback but could easily be modified for HAM antennas, which bolt to the bonnet hinge brackets and peak outside the bonnet. These seem to be about £50-£75 to buy a pair, and would cost another £50 to ship them (from the retailers site - I’m sure you could ship it for less).
Several hours of googling later and I find what appears to be the perfect solution for a reasonable cost. You can get these work-light mounts designed for my Navara from Ali Express, which fix to the bonnet bracket and could easily be adapted for antennas, if I reuse parts of the K-400 mount to get the antenna vertical. Just over £46 delivered, and forecast 14th-21st September.
There are earth points very close to where these mount, so a short run of earth braid should get a good ground. The time spent googling has paid off, without the need for pulling out the headlining of the cab and drilling holes for earth studs and passing coax to use the roof rails as a mounting locations.
I throw up some photo’s once I have them installed.
The work-light mounts arrived a few weeks ago, but my truck has been a labour of love the past month; I ‘popped’ the bumper off to install some rated recovery points and a bash plate under the engine to find someone had done a bodge-job repair on the bumper previously. A very long story made substantially shorter, I’ve replaced the bumper and spent a long time getting the recovery points and bash plate installed due to a thousand small issues, each of which were over-come and just one or two would have been fine, but I’ve ended up spending the last 4 weekends getting it all done.
Anyhoo, the brackets have been installed and antenna mounts installed! Apologies for the night-time photo’s but they’re good enough;
(I’ve composited two photo’s together as I couldn’t get the camera to focus on both mounts)
I’ve hooked up by VNA and the 6m/2m/70cm whip tunes up well on 2m and 70cms (SWR of 1.1 @144mhz rising to 1.25 at 145.5mhz). My HF whips are struggling to tune up however. For my 10m and 40m whip I can’t an SWR of less than 3 . Both mounts are grounded with a big chunky braided connection to both the bonnet and then the main body. I’m hoping that the issue is that the truck is parked on my drive and the close proximity of the house and some tree’s is affecting it, but I’m also aware there is a couple of inches between the mount the and metal of the car body. I’ve double checked with my mag-mount direct into the VNA and it’s no different. I’ll not get chance to ‘field test’ for a few weeks.
I do have a question though; As previously mentioned, the So-239 connectors on the mounts are grounded to the truck body with earth braid. If I were to throw up a mast and put out a dipole or some other balanced antenna and connect it to the SO-239 connector on the mount, will I get any undesirable effects from the grounding to the body? My gut says no, but I was wondering if anyone already has experience around this issue?
I’ve still got a couple of hours of labour to finish the radio install, but I’m going to try the ATAS-120A with m FT-891 once it’s been plumbed in to see how well that tunes up. Worst case is I have a triple mag mount that couples well enough for the ATAS-120A to tune up, when positioned on the roof, and the back-up would be to put the mag-mount out once I’ve stopped and plug the mag-mount into the above SO-239 (keep the mag-mount RG58 from getting pinched and my door seals intact!)
Dave, considering one of your HF whips, presumably the VNA is telling you whether the antenna seems too ‘short’ or too ‘long’ for the band in question.
My hf whips all adjusted to 1.8:1 or better. The main problem was narrow bandwidth.
Your HF whips with 3:1 min swr could need some attention. Is it resistive or complex impedance at min swr?
Say your whip shows up as 150 ohms resistive, then I would be looking for losses in the assembly of the whip as that’s too high imo.
Check your grounding straps in case there is a high RF loss there.
If it’s around 15 ohms resistive then the whip is probably good and you need a small LC network to match better.
Of course you could accept the small loss in the coax and use a small tuner in the cab to give the rig a nice swr. Something like the ATU100 from Banggood might do.
The VNA is giving live SWR (and other data, but I only had SWR on last night as a first look). I could get down to 3:1 by adjusting the lengths of the whips, which are all adjustable, but narrow-band (as mentioned by Ron VK3AFW).
I’m going to take a deeper look later tonight or tomorrow, when I get a free hour or so.
I’ll take a closer look later today or tomorrow and add more metrics to the display on the VNA (which I do calibrate every time I change the sweep frequency- a common problem people trip themselves up with these devices).
I had delivered today a milliohm meter and I’ve already had chance to go out and take a look with it. I bought it to check the return on the 12Vdc feed I’ve put in the cab. I used 6 gauge cable through the bulkhead for +12Vdc, but I’m using the body to return to 0V. I wasn’t convinced I had a good bond, as my multimeter showed 0.2 ohms (but it not that accurate with small resistances). The new 4-lead tester showed I had a pretty good bond back to the battery terminal of 2.4 milli-ohms, which I’m happy with!
I also still need to check the bond between the chassis and the body, incase there is something funky going on there. The guy who fitted my tow-bar electrics complained he couldn’t get a good bond using the chassis for the 0V return, so it needs checking out.
I also checked the bonding of the antenna mounts to the body, and both are less than 2 milli-ohms. I already have an ATU100, but would prefer not to use it if I can avoid it. Especially as I’m planning to use the ATAS-120A for mobile* operations.
* I don’t plan on actually using the HF rig whilst driving, just parked up to do BOTA or POTA activations.
That certainly sounds like you have lowered your ground resistance, making the antennas more efficient, but that also lowers the feedpoint impedance further below 50 ohms. (One would guess around 50 / 3 = 17 ohms or so.) A matching transformer, or a shunt coil connected from the bottom of the antenna element to ground, should take care of that.
That’s a common problem with short HF mobile whips.
Had some time today to work on the antenna mounts again.
Turns out that the off-side mount has an issue with the coax feeder somewhere. When I was doing the HF rig testing a few weeks ago, I assumed that it was an antenna/ground issue. The HF rig tunes up the ATAS to 1.5:1 with no hesitation on the near-side mount, which I previously used with the VHF rig (which also had no issues; my vhf whip is 1.3:1 ).
I suspect I have either kinked the coax putting though the bulkhead or it might have absorbed moisture as the off-side mount was purchased about 2 years ago and has been in the man-cave (fancy shed) at the bottom of the garden for 2 winters…
I am heading out next weekend for just over 2 weeks and I won’t be able to sort this before then, but at least I can do both 2m and HF from the car, just not at the same time! I’ve got an A/B switch I’ll slave in temporarily to connect both rigs the the near-side mount, rather than constantly moving the connector between the rigs. I’ll get a new run of coax when I get back and hopefully that will be the end of it!
I initially checked the grounding on the SO239 connectors. Both the near-side and off-side mounts were bonded well to the chassis, with a resistance of < 4 milli-ohms (measured with a kelvin-bridge type milli-ohmeter.
I’ve since rechecked and nothing has changed. I’ve also tested the centre and braid of the coax for continuity, and there is no connection. Unfortunately I don’t own or have access to a Megger or other insulation resistance test meter to check if the insulation is breaking down under load..
At this point I’m happy to throw £13 at it and get a cheap new SO-239 banjo fitting and slave it into the mount and run the coax back to the rig through the open car door and see if that proves the current coax is the issue. It’s from Amazon, so if it doesn’t solve the issue then I can return it with ease! If it works then it is relatively easy to replace the original feed once I’m on holiday, now that I’ve done the hard work of getting it through the bulkhead once!
Whilst I’m at it, I could feed the coax through the roof under the roof bar using some sort of gland, possibly one of those fittings used in campervans to pass solar cables into the living space.
. Both mounts are grounded with a big chunky braided connection to both the bonnet and then the main body. I’m hoping that the issue is that the truck is parked on my drive and the close proximity of the house and some tree’s is affecting it, but I’m also aware there is a couple of inches between the mount the and metal of the car body. I’ve double checked with my mag-mount direct into the VNA and it’s no different. I’ll not get chance to ‘field test’ for a few weeks.