cb antenna

[rocky]

Hi,

Is it possible to fit twin antennas to a cb and if it is, will it increase signal distance?

Regards,

Rocky

[CwazyWabbit]

It is possible but they need to be at least 9 feet apart or it will make the reception worse. Have a look at this thread http://www.thunderpole.co.uk/forum/viewtopic.php?f=18&t=2315 if you want a bit more on the subject, the last post looks to be copied from a book and covers some of the the tech detail (I think )

copied here for convenience

'Co-Phasing Your Mobile Antennas ("Twin Truckers")

Co-Phasing antennas simply means taking two identical antennas, mounting them on the vehicle and feeding them in-phase. One of the biggest misconception of radio operators is what kind of effect this has on the radiation pattern. Most people think that after you Co-Phase two mobile antennas, your signal will be strongest in line with the vehicle body (meaning the signal is strongest down the road straight in front of you and straight behind you also. This is the theoretical effect that you would get from co-phasing two omnidirectional antennas. However, to realize this effect you need to satisfy a couple of requirements. For one, a good earth ground with long (over a wavelength or so) radial wires is required. Secondly, at CB frequencies the closest you would be able to place these antennas are about 18 feet apart. Since it is impossible to satisfy these requirements, the effect of co-phasing is seriously diminished. Unfortunately, even the "Radio Shack Antenna Book" states that co-phasing two mobile antennas will produce a two directional signal.

So then, is there any advantage to co-phasing two mobile antennas? Why yes, there is. Before we noted that the radiation pattern of a single antenna is "pulled" where there is the most metal vehicle body. You can see the pattern is not perfectly omnidirectional like we would expect it be. As we travel down the road, you will notice signal fade ("flutter" or "waver") from this uneven radiation pattern. Co-phasing two antennas will even out the pattern irregularities. Instead of making the pattern more two directional, it will make it more omnidirectional. Do not expect more "gain" from two antennas. Figure 3 shows how co-phased antennas clean up the radiation pattern. Read the section "Co-Phasing" for instructions on how to make a harness to feed your co-phased antennas. It best to get them as far apart as possible. The best way would be to mount one on the front bumper in the center and one on the back bumper in the center also. Most people think this looks silly (me included!) and mount one on each side of the vehicle'

*That post quotes 18feet but others quote 9 and I'm reasonably certain I've seen 9 quoted in other places

** Original source of that post with pics http://www.signalengineering.com/ultimate/mobile_antennas.html

Shows better coverage but NOT greater distance

Edited August 26, 2011 by CwazyWabbit

[CwazyWabbit]

btw why do you want twin aerials? Is it for looks or do you need more range?

[Farmerfred]

Been a long time Rocky since I was into CB radio but to answer your question twin antennas would not significantly increase signal, either in transmitting or receiving. The most efficient way to gain an optimum signal is to have a good 'ground plane', an area below the antenna that 'reflects' your transmitted signal, idealy the largest area of ferrous metal plate that is practical with the antenna positioned right in the centre to give you uniform reflection, positioning the antenna to one side or in one corner gives you a more directional signal output, signal being 'thrown' towards and away from the largest area of 'plane'. A good quality antenna is quite vital, one with 'virtual' length of a 1/4, 1/2, 3/4 or 'full' wave, a full wave is the length of the actual radio signal which, on the 27MHZ band is 11 metres and the wave length of the antenna must be an exact fraction of this to work properly in order that the power that is put out through it when you transmit is not 'reflected' back to your equipment and melt it. Aluminium is not a terribly good conductor for a ground plane but if you earth the antenna from the base, on which the outer braiding of the coaxial cable is connected, somewhere to the chassis you will get better conductivity and a better ground plane, the steel roof on a Disco is good. Remember also to 'tune' an antenna with an SWR (Standing Wave Ratio) meter for optimum power output, the reading should be no more than 2 on the lowes channel and the same on the highest channel, lower is better, high reading on low channels, lower or shorten antanna, high reading on high channels, lengthen antenna. That's the basics. 10/4 good buddy!

[rocky]

Thanks for the reply,it was very informative. There is a defender about a mile away from me which has twin aerials on the back. I wondered if there was any benefit from having the two (it does look balanced).

The reason I am going to fit a cb in my 90 is because the road I have to travel to work has virtually no mobile phone reception. It's a bit remote and gets badly snowed over in the winter. There are wood lorries and gamekeepers about that have cb's plus a few homebases.

I have decided on fitting a "Lafeyette venus" as the box can be easily hidden, just remove the mike which has all the controls.

Regards, rocky

[Mark]

It might also be worth saying that there are a number of electronic ground plane products available, for plastic, canvas, or possibly aluminium roofed vehicles. never tried one, and would probably take some advice, but since no-one has mentioned them yet I thought I would...

[rocky]

Thanks for all the replies. I used to have a cb back in the early eighties when I used to commute from Aberdeenshire to Leicestershire. I found it invaluable for getting round traffic jams and getting road conditions in winter.

I realise the major problem with defenders is the lack of ground plane, but hopefully I'll get round that by running an earth to the chassis.

Regards, rocky

[MrKev]

Aluminium roofs make adequate ground planes, the biggest problem is making a secure electrical connection to it such that it doesn't corrode. Running a wire off to the chassis doesn't really help, in fact dangling a piece of wire 2.75m long (ideally as straight down as possible, but I realise it's gonna need a bend in it somewhere before you get to the floor) from the earth of the antenna connection that goes no-where will probably have a better effect.

Kev

[Frax]

Why 2.75m long, is there a reason for the length and direction of travel.

Regards

Frax

[nicksmelly]

2.75 is 1/4 wave of 11m (27mhz)

[Happyoldgit]

Even way back in the pre 27/81 days I never had problems with inadequate ground planes on hardtop Land Rovers. Back in those drill happy days as long as it had a decent shock spring I tended to slap the antenna in the middle of the roof as possible. That said I remember whacking K40's and the like on the roof more than once even though they would get the hell beaten out of them [and the roof] on branches etc.

Twin mirror mount antennas on articulated trucks maybe but no real advantage other than being a liability on something the size of a normal Land Rover.

[MrKev]

Yes, 2.75m is 1/4 wavelength for CB and that is why.

One type of aerial is where 1/4 wavelength goes straight up (from the centre conductor), and the same straight down from the outer. It's called a dipole, and it's a fundamental building block for many kinds of aerial.

A reasonable compromise is to have just the top half, and have a large conductive (i.e. any kind of metal) surface - a 'ground plane' horizontally, and extending 1/4 wavelength in every direction (i.e. 2.75m in this case). Clearly the roof of a Landrover is not 5 1/2m across, so this is another compromise.

Furthermore, partly for an old legal reason, and partly for size, the whippy bit at the top can be loaded up with a coil (the black lump you often see somewher along then length, the at the base of a springer) to artificially shorten it without affecting it's electrical appearance, so far as the transmitter is concerned. It will affect the range, as there's less metal up there.

As Mark touched on, the same principle can be applied to the ground-y end - a loading coil ("artificial ground plane") can be used to compensate / eliminate the requirement for the large ground plane requirement.

If you choose to use a magmount, then of course the surface has to be ferrous, but otherwise, it's generally best to just make a sound electrical connection to the roof, ideally with the whip in the centre and make do.

Kev