CB, Ham & Communications etc

[The Homba]

Thanks for the reply guys , i checked the conections and no short , also checked with a tester and there is a conection between the conector (outer part ) and the earth of the body . but i dont think it great as i use a earth wire for the lamps to the battery and i think the only conection from the lightbar to roof is from the bolts , will a piece of welding wire be ok for using instead of the whip ? will also try an earth strap from lightbar to roof.

cheers Iain

[RPR]

Hmmmm, Works for my 35watt 2m ham radio dual band antenna & my MSA Rally radio gives me a good SWR reading on both units.

I'm not trying to set myself up as radio guru here. Your set-up on an aluminium roof shouldn't work. But I'm glad to hear that it does. Effective ground plane should require sufficient ferrous metal surface. Aluminium and/or birmabright and its realted products, should be insufficiently ferrous to provide ground plane. Some additional resources on the issue:

Ground Plane

Ferrous Surfaces for Ground Planes

[Lars L]

"For a Body mount antenna other metals may be used as it does not have a magnet that needs to stick to it."

From Richard's second link.

Me no guru either... And that's the way my antenna has been sitting for years, in the middle of the roof, and the function of my CB has been very good with long enough reach.

Now it has been moved to a bracket on my roofrack and has yet to be tested for longer distances.

[errol209]

Aluminium doesn't work as a ground plane. Not sufficiently conductive/magnetic or something else.

Don't automatically believe everything you read on the web! Aluminium is perfectly good, as 100% of Sky satellite dishes, the Lovell main dish at Jodrell bank and the Forum's collective experience with CB, Ham and airband radio attached to land rovers (and their roofs) suggests. Just as an aside, the electrical conductivity of aluminium alloy is 5*10^-8 ohms/m, whereas iron is 10 and steel is 15. Most high voltage overhead cables are aluminium over a steel core, because copper is four times denser than ally and therefore the cables are lighter. Lastly, the ground plane exists to provide an eletromagnetic reflector, so anything that conducts will do really, like sea water, chicken wire, or even the ground ...

But aluminium is no better than fiberglass or wood. You need iron/steel.

Only if you need to fix the antenna with a magmount, as the text of your link says.

1 square foot seems to be about the minimum it takes. A light bar won't do it either because it generally offers insufficient surface area.

Any flat conductive surface will help matters (see above), but the science suggests even a 101 ambulance roof isn't big enough to achieve a 100% effective reflector in the 27MHz area.

[pete3000]

I'm not trying to set myself up as radio guru here. Your set-up on an aluminium roof shouldn't work. But I'm glad to hear that it does. Effective ground plane should require sufficient ferrous metal surface. Aluminium and/or birmabright and its realted products, should be insufficiently ferrous to provide ground plane.

FAIL.

Most Amateur antennas or components of are made from Aluminium, Copper Stainless, Silver, Gold. The giveaway as Errol says is the metal part. It doesn't even have to be a continuous sheet (i.e a roof) A discone or web is plenty. So if you have a grp roof 4,6,8 or 12 radials will suffice if they are at least a 1/4 wl.

Pete

[ThreeSheds]

IMHO the only problem with an aluminium ground plane when used with a bolt-through mounting is the significant chance of bimetallic corrosion.

I used a through-body mount on the aluminium roof of my RRC for my 2M amateur radio in October last year and it is still fine - I just lathered the thing with waterproof grease all around the hole.

Roger

[MrKev]

The second link you cite contradicts your claim.

"A Ground Plane is a flat surface of ferrous metal (galvanized, steel, etc) that a Magnet Mount or Body Mount antenna uses as an integrated part of it. For a Body mount antenna other metals may be used as it does not have a magnet that needs to stick to it."

i.e. it needs to be magnetic if you plan on sticking a magnet to it (duh) but otherwise, it just needs to be electrically conductive.

I'm not trying to set myself up as radio guru here. Your set-up on an aluminium roof shouldn't work. But I'm glad to hear that it does. Effective ground plane should require sufficient ferrous metal surface. Aluminium and/or birmabright and its realted products, should be insufficiently ferrous to provide ground plane. Some additional resources on the issue:

Ground Plane

Ferrous Surfaces for Ground Planes

[Bull Bar Cowboy]

Slightly OT, but can anyone explain to me why people fit a foot square of steel to antennae which are mounted in the middle of an aluminium LandRover roof, or even why the roof bar attached to 2tonnes of LR underneath it isn't enough for a CB ground plane?

Do you want the quick version or the full half hour ? …….. here is the quick version…..

A Land Rover (90 / 110) is considered to be one of the better vehicles for mobile antenna performance

A mobile antenna in its simplest form is just one half of a dipole on its end. As with a ground mounted vertical antenna the earth (or the ‘ground plane’) takes the place of the "missing" half of the dipole. The capacitance between the vertical radiator and the ground plane allows the necessary return path to the transmitter. The more loss in the return path, then the poorer the antenna performance will be.

With vehicle mounted systems the antenna length does not always correspond to the calculated quarter wave length (or other sub multiples thereof) as the ‘quality’ of the ground plane significantly alters the tuning impedance at resonance. If the ground plane was a perfect quarter wave length of the antenna frequency then ideally we would get a perfect 50 ohm resistive match at the antenna feed point. However, the quality of the ground plane can introduce capacitive and inductive losses which in certain circumstances can make the feedpoint impedance vey reactive.

OK …… so where did the 12 inch square come from ?

Well, nowhere really !. In the UK the traditional PMR (once known as Private Mobile Radio & now known as Professional Mobile Radio) band was a transmit frequency from the mobile in the band of 171M (166Mhz receive – two frequency semi duplex). A quarter wave at 171 Mhz is <> 17 inches and the recommendation for a good ground plane on glass fibre roofed vehicles was an 18inch square of aluminium with the minimum size being 12 inches .

The antenna feed point impedance is the generally considered to be the resonant frequency of the antenna (although it needn’t be). This is what you are measuring when tuning for minimum voltage standing wave ratio (VSWR), as the absence of any reflection would indicate a perfectly matched transmission line to the antenna feed point.

For lower frequencies (i.e.27Mhz) the antenna at a quarter wave would be too long to be practical (109 inches). To overcome this issue the vertical section is electrically modified (inductance & Capacitance) to reduce the overall length of the radiator to manageable proportions. Hence, antennas at bottom loaded or centre loaded and in very exceptional circumstances top loaded.

It should never be assumed that the polar diagram (radiation from the antenna) of any mobile installation is a perfect circle (often referred to as a doughnut because it is 3 dimensional). The position of the antenna over the ground plane and other metallic obstructions on the vehicle will considerably alter the radiated pattern of the antenna. His is of particular interest when considering fire appliances and other specialist vehicles.

Mounting an antenna on a light bar is OK, provided the light bar has a metallic contact the body of the vehicle and the antenna is mounted in the centre of the light bar. However, interference from the light bar motors can be an issue, but this can often be resolved with a little capacitance in parallel with the motor brushes.

When tuning a mobile installation for minimum VSWR don’t worry too much about getting a perfect match (i.e VSWR of below 1.1:1). When the vehicle is in motion the proximity of the antenna to other vehicles and objects (buildings, hedges, trees etc) will alter its feed point impedance considerably therefore when static in open ground the VSWR should be less the 1.8:1 (most professional installers will aim for 1.5:1). The design of the mobile transmitter will cater for these reflections. The output stage constantly monitors the reflected voltage and uses it to lower the power output from the final transmitter stage to keep the device heat dissipation within a respectable range (this functionality is often referred to as ALC or auto protection).

In terms of mobile antenna efficiency the quarter wave whip mounted in the centre of the roof rules the roost, followed by mounting to either front wing or boot …….. then we have the much less efficient Magnetic mount , followed by the very inefficient low profile antenna (buses & Trains), and lastly comes the abomination known as the glass mount.

[ThreeSheds]

Good write-up, BBC

[MrKev]

When you are talking about light bar 'motors' it sounds like you're talking about an whirly thing, probably in the centre of the roof, rather than the bridge things carrying some spots over the front windscreen.....?

Mounting an antenna on a light bar is OK, provided the light bar has a metallic contact the body of the vehicle and the antenna is mounted in the centre of the light bar. However, interference from the light bar motors can be an issue, but this can often be resolved with a little capacitance in parallel with the motor brushes.

[The Homba]

Hi guys thanks for all the replys , i think i should have said in the first post that it is the light bar at the front with 4 spots on it and not a lightbar/beacon . i welded a tab to the back and mounted the antenna there. going to get a lone of a nother SWR meter as i tryed with a piece of wire and it still goes off the scale started with it the same size and in the end removed about a foot and still the same .

cheers Iain

[Happyoldgit]

We seem to attract quite a few queries and requests for info regarding CB, amateur radio and other communications gear so I've started pulling and merging all the various threads together here.

[L81ker]

Hi Y'all

for a pseudo-technical answer about needing a ferrous (magnetic) ground plate to help get your aerials tuned properly.

Note there is a big difference between a ground plane and a large steel plate.

A radio wave has two parts, the electrical wave, and the magnetic wave. Aluminium, not being too good in the magnetic side of things, makes great aerials for TV etc because these TV ones are based on dipoles. Your springer type CB aerial is a "short vertical" (not a dipole) using the coil as the shortening device.

A vertical needs either a good ground or a ground plane to provide the 'other half' of the dipole, and a big steel plate helps fool the system into working because of that big magnetic component.

If you have good 'earth' straps instead, be careful because these might well be carrying radio signals and acting like the other half of a dipole inside the cab.. watch out for 'rusty bolt' effect, and RF burns.

4 or 8 lengths of copper wire all around 104" long (they must all be the same length) and secured as straight as you can get them to form a star shape fanning under the aerial and connected to the SHIELD (ground) side of the coax with the far end NOT connected to anything but taped up, inside the roof will provide a really good electrical ground plane.

See this link for the idea, or fit this actual device under your aerial

http://www.cbradiostore.com/servlet/the-157/Mobile-CB-Antenna-Radial/Detail

You can try this in the garden using e.g. a piece of strong cardboard to mount the aerial on a wooden table, with the 4 or 8 wires under the cardboard but on top of and hanging down the sides of the table.

Some trucks use a second shorter but tuned CB aerial on the truck mirror, pointing downwards and only connected to the shield (ground) so that their aerial is a true dipole - because of fibreglass or other modern materials cabs are made of.

This is a CB dipole but its too big for a lanny! http://www.nevadaradio.co.uk/cb/cb-antennas/cb-base-antennas/sirio-sd-27

If you want help, feel free to PM me.

Kind regards

Clive in Suffolk.

[M005]

Rigid aerial

Looking for an alternative to the usual short springer aerial.

Am thinking of one of these MOUNTS on the spare wheel bracket on the back of the Disco, as a bit more of a descrete install that a huge springer hanging off the top.

[Happyoldgit]

I've used those mounts in the past and they do the job they were designed for. Can't think there is much else to say about them.

[M&S]

My mate recently fitted one to the roof of his Disco. It certainly doesn't feel like it will take as many knocks as my stinger. Also because it's bright and shiney it does stand out anyway. I think the top section will stay bent if it got folded under trees. Looks better on top of a taxi - but as said above, it does the job so...

As it's quite short fitting to the rear wheel carrier will affect the range also (in a forwards direction at least), and I bet it will still whip about and hit the back door.

I have a black 3ft stinger in the centre on the roof with a dome surface mount and don't think it looks bad at all http://www.4x4cb.com/public/item.cfm?itemID=701

Can be quickly unscrewed if required.

I think this one is a better option for a descreet look if it's just vehicle to vehicle http://www.4x4cb.com/public/item.cfm?itemID=707

Anyway, just my opinion

[simonr]

Avoid! I bought one for my van (being fairly compact). Although the whip is stainless, the rest is made of lightly chromed rust!

The CB works better - no I'm not joking - with the aerial removed. The spring rusted through and the aerial fell off within 18 months. I didn't notice until I saw it. The reception was just as good after as before!

Si

[disco_al]

got a similar one off ebay last year, in black, and it's definately resilient to knocks and stuff. only thing is if you hit a low branch at speed on the road, it will whip back quite a lot. although if it's going on the back it won't really matter.

can only really comment on the rubber duck ones from years ago, and they were pretty naff, only good for short range comms so probably be ok for laning.

[white90]

Tried a few now and the best has always been the Trucker 80

although I have broken 3 and had one stolen

the stainless mount has survived albeit a bit passed its best now after the weekend being punched through the roof.

It is still there although the antenna isn't

I only really use the CB for the PA function so an alternate mounting point is being considered.

[Retroanaconda]

I have a big springer one, and also a 'Rubber Duck' one which is only about 15" long and more discrete.

The PSM-1 mount is good, very strong and does the job.

As an aside, Jim at 4x4cb is a very nice chap to deal with

[landy andy.]

I have one still in it's wrapper if you want it Pm me

Andy

[white90]

If M005 Passes I'd be interested in the mount

[M005]

PM Sent Andy

[RaDi90Hybrid]

I have a big springer one, and also a 'Rubber Duck' one which is only about 15" long and more discrete.

The PSM-1 mount is good, very strong and does the job.

As an aside, Jim at 4x4cb is a very nice chap to deal with

I have a "rubber duck", excellent at short distance and being 15" is less susceptable to damage. I agree, Jim is brilliant, when I was trying to decide on the mount options one evenning he was still answering e-mails at 11 O'clock at night!!!

[discomikey]

i had one,6 ft tall on magmount, musta bin 20 yrs old. it snapped under a tree. probably went brittle though